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Abstract and Figures

An air-breathing electric propulsion system (RAM-EP) ingests the air of the residual atmosphere through an Air-Intake and uses it as propellant for an electric thruster. Key component of the system is the Air-Intake, which has the task of collecting atmosphere particles and directing them into the thruster, accompanied by compression. Studies have considered different configurations, and have shown the feasibility of the device. Within this paper an overview of the current Air-Intake designs is given. Results of DSMC simulations, performed with our in-house code PICLas, are presented and compared to those from the respective publications. Moreover, the influence of simplifying assumptions, such as free molecular or hyperthermal flow, is shown. Additionally, a simple analytical model based on transmittances and the balance of particle flows is derived, applicable for the analysis and further possible optimization of a generic Air-Intake design. The model is compared to the results of DSMC simulations and a sensitivity analysis of the basic parameters is performed. Regarding the assumption of hyperthermal flow, results shows that part of the particles will interact with the lateral structures of the Air-Intake and therefore will be scattered loosing their macroscopic velocity. Moreover, the implementation of straws into the Air-Intake simulations needed particular attention. A deeper investigation over their single transmittances has been done. Through the balancing model a sensitivity analysis over their L/R ratio has been performed and results show how a low ratio would lead to higher densities and collection efficiencies.
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Air-Intake Design Investigation for an Air-Breathing
Electric Propulsion System
IEPC-2015-90524/ISTS-2015-b-90524
Presented at Joint Conference of 30th International Symposium on Space Technology and Science,
34th International Electric Propulsion Conference and 6th Nano-satellite Symposium
Hyogo-Kobe, Japan
July 4–10, 2015
Francesco Romano, Tilman Binder, Georg Herdrich, Stefanos Fasoulas§
Institute of Space Systems (IRS), University of Stuttgart, Stuttgart, 70569, Germany
and
Tony Sch¨onherr
The University of Tokyo, Bunkyo, Tokyo, 113-8656, Japan
An air-breathing electric propulsion system (RAM-EP) ingests the air of the residual
atmosphere through an Air-Intake and uses it as propellant for an electric thruster. Key
component of the system is the Air-Intake, which has the task of collecting atmosphere
particles and directing them into the thruster, accompanied by compression. Studies have
considered different configurations, and have shown the feasibility of the device. Within this
paper an overview of the current Air-Intake designs is given. Results of DSMC simulations,
performed with our in-house code PICLas, are presented and compared to those from the
respective publications. Moreover, the influence of simplifying assumptions, such as free
molecular or hyperthermal flow, is shown. Additionally, a simple analytical model based
on transmittances and the balance of particle flows is derived, applicable for the analysis
and further possible optimization of a generic Air-Intake design. The model is compared
to the results of DSMC simulations and a sensitivity analysis of the basic parameters is
performed. Regarding the assumption of hyperthermal flow, results shows that part of the
particles will interact with the lateral structures of the Air-Intake and therefore will be
scattered loosing their macroscopic velocity. Moreover, the implementation of straws into
the Air-Intake simulations needed particular attention. A deeper investigation over their
single transmittances has been done. Through the balancing model a sensitivity analysis
over their L/R ratio has been performed and results show how a low ratio would lead to
higher densities and collection efficiencies.
Ph.D. Student, Institute of Space Systems (IRS), romano@irs.uni-stuttgart.de.
Ph.D. Student, Institute of Space Systems (IRS), binder@irs.uni-stuttgart.de.
Head Plasma Wind Tunnels and Electric Propulsion, Institute of Space Systems (IRS), herdrich@irs.uni-stuttgart.de.
§Head Department of Space Transportation, Institute of Space Systems (IRS), fasoulas@irs.uni-stuttgart.de.
Assistant Professor, Dpt. of Aeronautics and Astronautics, schoenherr@al.t.u-tokyo.ac.jp.
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IEPC-2015-269ISTS-2015-b-269
IEPC-2015-269ISTS-2015-b-269
Nomenclature
Ain = Air-Intake Front Surface
Aout = Air-Intake Outlet Surface
Ap = Geomagnetic Index
BM = Balancing Model
DSMC = Direct Simulation Monte Carlo
ECR = Electron Cyclotron Resonance
El. = Element
F= Thrust, Force
F10.7 = Solar Radio Flux at λ=10.7cm
h= Altitude
kB= Boltzmann Constant
L=Length
mp= Particle Mass
˙mthr = Mass Flow to Thruster
navg,ch = Average Number Density in Chamber
nch = Number Density in Chamber
nin = Number Density in Inflow
ntot,in = Total Number Density in Inflow
˙
Naccel = Accelerated Particle Flow
˙
Nin = Incoming Particle Flow
˙
Nthr = Particle Flow to Thruster
˙
Nout = Outlet Particle Flow
pch = Chamber Pressure
pin = Inflow Pressure
R = Radius
Rel. Dev. = Relative Deviation
RAM EP = Air-Breathing Electric Propulsion
S/C = Spacecraft
Tch = Chamber Temperature
Tin = Inflow Temperature
Twall =WallTemperature
vch =VelocityinChamber
vin = Inflow Velocity
vout = Exhaust Velocity
vz=VelocityinzDirection
α= Accommodation Coefficient
Γ = Mass Flux (e.g. by Thermal Effusion)
ηc= Collection Efficiency
ΘClausing = Backflow Transmittance following Clausing’s Assumptions
Θfast = Transmittance for Fast, Unscattered Particles
Θintake1= Transmittance for the Intake in Inflow direction
Θintake2= Transmittance for the Intake in Backflow direction
Θout = Transmittance for the Outflow
Θscattered = Transmittance for Scattered Particles
χ= Aspect Ratio
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I. Introduction
Very low orbits are of great interest for many scientific, civil and military purposes. Recently ESA’s
mission GOCE ended, it provided detailed information of the Earth’s geomagnetic field by orbiting
as low as 229 km using ion thrusters to compensate the drag. The amount of propellant on board is a
limiting lifetime factor for such a mission, in particular if the S/C is orbiting very low around a planet with
atmosphere. The atmosphere is indeed responsible for the drag, which slows down the S/C and reduces its
total mission lifetime. It is also a limiting factor in terms of costs, as more drag to be compensated for a
longer time means an increased amount of propellant to be carried on board, which again increases the total
mass. The lifetime of a S/C orbiting in LEO can be significantly increased by the application of an efficient
propulsion system capable to compensate the drag.
The basic idea of an Air-Breathing Electric Propulsion System, shortened RAM-EP, is to use the air of
the residual atmosphere as propellant and to process it through a device for generating thrust. This will
decrease, ideally nullify, the on board propellant requirement and will generate thrust to partially or fully
compensate the drag. A conceptual scheme of the S/C is shown in Fig. 1.
This paper will focus on the device needed to efficiently collect and drive the atmosphere particles to
Incoming flow
Flight Direction Solar Array
Solar Array
Air-Intake Exhaust
S/C Core
Figure 1: Air-Breathing Electric Propulsion S/C Concept.
the thruster, called the Air-Intake. The investigation on an Air-Intake is part of a Ph.D. program at IRS,
focused on the use of a small inductively heated plasma thruster based on IPG6-S for an Air-Breathing
Electric Propulsion application,14.13 Emphasizing its crucial design, recent studies involved ESA,4BUSEK
Inc.8and JAXA,5,7that proposed and studied different possible design configurations. The outcome of
these studies are sustained by DSMC simulations and experimental activity on ground. In the following,
an overview of these designs is provided, together with results of DSMC simulations, performed with our
in-house code PICLas.10 Additionally, a simple analytical model based on transmittances and the balance of
particle flows is derived, applicable for the analysis and further possible optimization of a generic Air-Intake
design. The model is compared to the results of the DSMC simulations and a sensitivity analysis of the basic
parameters is performed.
II. Assumptions and Considerations
The design of an Air-Intake for an Air-Breathing Electric Propulsion System is a challenge and depends
on many different factors. The requirement is an efficient collection of the particles encountered by the S/C
in order to feed the thruster. The ratio between the collected particle flow ˙
Nthr and the incoming particle
flow ˙
Nin is named Collection Efficiency, see Eq. 1. In order to have a highly efficient device, it is required to
keep ηcas high as possible.
ηc=˙
Nthr
˙
Nin
(1)
A consideration on the need of an Air-Intake should not be avoided. The flow the Air-Intake encounters
will be highly rarefied and, therefore, it is not possible to follow intuition, which is strictly connected to
our experience on Earth where continuum flow sovereigns. For different flow conditions, the presence of an
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Air-Intake that works perfectly in continuum might worsen and even become counterproductive, providing
to the thruster less particle flow than without an Air-Intake. The incoming particle flow to the intake is
defined by the open front area of the intake Ain and the free stream conditions (e.g. number density nin
and velocity vin)as: ˙
Nin =ninvin Ain (2)
Multiplying the particle flow ˙
Nwith the average particle mass mpresults in the corresponding mass flow
˙m=mp˙
N. With the assumption that the entire collected mass flow ˙mthr is accelerated by the thruster, the
produced thrust can be calculated as in Eq. 3:
F=mp˙
Nthrvout =mpηc˙
Ninvout =(mpnin vinvout )(ηcAin)(3)
Here, vout is the exhaust velocity out of the thruster. A deeper description of the influencing factors for the
flow into the propulsion device will be further described as part of the balancing model in Sec. IV. In the
context of the design of an Air-Intake for an Air-Breathing Electric Propulsion System two important points
have to be considered. On the one hand, for maximizing the thrust, not only the efficiency of the thruster
itself has to be taken care of, but also a sufficient amount of mass flow has to be provided. Eq. 3 suggests that
the area of the intake Ain should be as large as possible to collect the most amount of mass flow, however,
the S/C front area also determines the drag. On the other hand, when considering the feeding system to the
chamber as part of the Air-Intake, the collected gas has also to be fed at a sufficient pressure and, inside an
ionization chamber, the neutral gas should remain as long as possible for an efficient ionization process. For
this, the particles have to be slowed down while increasing pressure. Therefore, a direct flow of free stream
particles into the chamber would not be desired.
III. Air-Intake Review
A. Basic Concepts
The basic concepts for an Air-Intake design are hereby described and shown.
A first logical configuration can be a short cylinder with the cross section of the entire S/C, followed by
a simple entrance cone as shown in Fig. 2a. The cross section cone could converge directly to the size of
the propulsion system or, alternatively, to a feeding system allowing the other S/C subsystems to be placed
behind the Air-Intake.
(a) Air-Intake Simple Cone Concept. (b) Air-Intake By-Pass Concept.
Figure 2: Air-Intake Concepts
However, the approach of using a simple cone is not the best one as the flow is not in the continuum
regime and, thus, dominated by collisions with the walls rather than of inter-particle collisions. This basically
prevents the particles from reaching the end of the cone, which has also been verified through preliminary
Direct Monte Carlo Simulations (DSMC), emphasizing the importance of the right wall model. In case of
diffusive reflections (the most realistic case1), most of the particles will be scattered into a random direction
when hitting the wall. As the solid angle including the cone exit represents for the most reflections only
a very small part of the entire half space, that comprises all possible target directions, nearly all particles
are reflected back into the flight direction. In the case of specular reflection, the situation might be only
improved together with a small cone angle. The basic problem arises that the short cylinder between the
front collection area and the cone does not have only a high transmission probability for the incoming
particles, but also for the flow that is scattered back (the backflow) and, basically, only the particles which
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were already directed into the projection of the thruster entrance section will enter into it, resulting in a low
collection efficiency ηc.
The performance of the Air-Intake can be improved by providing a design with a high transmission probability
for the incoming flow, and trapping it by a lower transmission probability for the backflow. The key concept
is that the free stream condition has the nature of being a collimated, hyperthermal flow, as the velocity of
the S/C is large compared to the thermal movement of the atmosphere according to the kinetic theory of
gases. Thus, it is less obstructed by lateral structures, such as grids or tubes, than the scattered backflow
is, which has only thermal movement when diffusive reflections at the walls are assumed. Based on this,
a long duct instead of a short one can be better for the collection. According to the study of BUSEK,8
particles will impact at the end of a long duct on the inclined walls gaining random velocity and, due to
the already existing compression, they might be subjected to collisions with the further incoming particles,
creating an even higher compression at the end of the duct. The actual influence of this “collision cascade”
will be analyzed as part of the DSMC simulation in Sec. V. The difference in the transmission for inflow
and scattered backflow might be utilized more efficiently with the introduction of a honeycomb structure of
small straws at the entrance of the Air-Intake.
Another configuration is the by-pass design, see Fig. 2b, which can be additionally combined with the
honeycomb approach. The flow enters from a ring and reaches the inclined surfaces at the end of the duct.
These will act as reflectors/diffusers for the particles which will be then subjected to multiple reflections due
to the presence of the satellite core, as shown in the Fig. 2b. The satellite core provides a sensible position
for most of the other S/C subsystems.
B. Literature Review
From literature review the most detailed Air-Intake studies for Air-Breathing Electric Propulsion applica-
tions are those from ESA,4BUSEK8and JAXA.5
1. ESA
The study from ESA4considers a mechanical Air-Intake as shown in Fig. 3: At the inlet section, a grid is
positioned to stop the incoming particles, a long duct is following to get the required steady pressure level
at the end of the device, where a cone shape finally drives particles into the gridded ion thruster GIT. There
are important considerations about the fact that over a certain length of the Air-Intake, there is no more
improvement in the pressure at the end and, moreover, that a concave or divergent shape at the end does
not improve the flow.
Figure 3: Air-Intake and GIT from ESA.4
2. BUSEK
The BUSEK Inc.8studied the MArs Breathing Hall Effect Thruster (MABHET), a S/C with an Atmosphere-
Breathing Electric Propulsion System. In this design, the Air-Intake is a long tube of 3.7m length and0.6m
diameter, possibly with a honeycomb structure in the front composed of many straws as described in Sec. A.
The Air-Intake is designed as a long tube in order to achieve a higher density region at the back part due to
the presence of an assumed “collision cascade”. A total pressure increment of 100 was observed in DSMC.
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Examplary designs are shown in Fig. 4a and Fig. 4b.
The design of the Air-Intake led to the following observations:
A long duct allows a compression zone to form at the back, due to collisions between incoming and
already trapped particles (“collision cascade”);
Straws fill the inlet, with a certain length, placed in order to block the reflected backflow;
Main influencing parameter is the ratio of the collisional mean free path to the tube dimension;
Small inclination of the Air-Intake surfaces will more likely lead to specular reflections at the wall, as
the parallel component of the velocity is still much higher than the perpendicular.
(a) Air-Intake, DSMC results from BUSEK Report.8
(b) Air-Intake, general concept from BUSEK Presentation.9
Figure 4: BUSEK Design
3. Fujita, JAXA
Fujita’s study5considers a by-pass-like design in which the air flow enters through a ring as shown in Fig. 5.
The particles reach the back of the intake hitting a 45steep surface, the diffuser/reflector, and are afterwards
reflected on the back of the satellite core and to the thruster’s acceleration grids. Again, such an annular
intake represents a structure with different transmission probabilities for the inflow and the backflow, when
the effective diameter of the open cross section is small compared to the length. An ECR device ionizes the
particles in the part behind the satellite core (the ionization chamber) that are afterwards extracted through
accelerating grids. As difference to the BUSEK design, grids are at the outlet, and the whole thruster is
already included in the design and, therefore, it is not possible to precisely distinguish between Air-Intake
and thruster. In 2012 a new paper from JAXA7dealing with the development of this Air-Intake has been
published. The main design of the Air-Intake is kept with one important modification, that is the addition
of straws inside the ring-shaped inlet region, similarly to the BUSEK design.
From this brief review, the following main points can be summarized:
A long, annular inlet duct will allow compression at the end of the intake;
Straws at the entrance section let incoming particles going through, but block the backflow;
A conical shape at the end of the intake can drive the flow and scatter the particles into the end of the
inlet where they can be driven by other matters to the thruster.
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Figure 5: Air-Intake from Fujita’s 2004 paper,.5
IV. Balancing Model
A. Introduction
In this Section a simple, analytical model for the evaluation of a generic RAM-EP Air-Intake configuration
is presented. The generic design becomes obvious when comparing the introduced designs, where an intake
section collects the particles with free stream conditions and guides them into the propulsion system. In the
context of this model, the intake section is followed by a chamber section in which it is assumed that all
particles have already gone through wall collisions and, thus, have only a thermal movement with regard to
the wall temperature left. By this, the only particle flows directed out of the chamber are due to thermal
diffusion. One flow back through the intake with its desirably low transmittance probability, and another flow
through the outlet. The representation of the outlet flow strongly depends on the specific configuration. For
the JAXA’s design, it is the flow passing through the thruster grids, increased by the acceleration provided
by them. In general, a feeding system and the thruster itself follows. By balancing these particle flows, the
conditions in the separate sections can be estimated.
B. Assumptions
The basic assumptions for the analytical model are following the nomenclature in Fig. 6:
Twall
Free Stream Condition
pin,n
in,T
in,v
in
Intake Control Volume, Chamber
pch,n
ch,T
ch,v
ch
˙
Nin
Ain
Θintake1,˙
Nintake1
Θintake2,˙
Nintake2
Θout,˙
Nout
(˙
Naccel)
Aout
Figure 6: Balancing Model Scheme.
Ain and Aout are the respective cross sections for the inflow and the outflow representing those of the
chamber section. The parameters of the incoming flow are known: number density nin (or pressure pin),
flow temperature Tin and free stream velocity vin.
Θ is the transmittance into a specific direction through a single structure, indicated in the subscript, and
is the fraction of particles which pass through the exit section against the amount of particles which passed
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into inflow direction through its start section, as part of them is partially scattered back while crossing the
volume. The transmittance can represent any intake, including a case with straws. This is also used for the
outlet, for the JAXA case with a transmittance for the acceleration grids. For a general design, it represents
the feeding system and thruster.
All in all, three transmittances are to be set: one for the incoming flow, one for the backflow coming from
the chamber (accounting for two values through the intake part), and a third for the flow through the outlet.
Based on these transmittances, the respective particle flows can be defined. ˙
Nintake1is the flow of particles
passing through the intake section from free stream into the chamber section, ˙
Nintake2is the backflow that
goes back to the free stream after having reached the chamber section, and ˙
Nout is the net outflow.
The basic hypotheses of this model are the following:
Free molecular flow;
Single species;
Ideal gas;
Complete diffusive accommodation, α=1;
Fixed temperature, Tch =Twall;
No macroscopic velocity inside the chamber, vch =0m/s.
The general particle flow into the direction of xiis defined as in Eq. 4, where nis the number density,
¯vxiis the averaged flow velocity into xiand Ais the passed area perpendicular to ¯vxi.
˙
Nxi=n¯vxiA(4)
Relying on this equation, the particle flow ˙
Nin into the Air-Intake which can be collected at most is simply
the one using the free stream conditions and the open area.
Regarding the actually collected flow, the following can be written:
˙
Nintake1=˙
NinΘintake1(5)
Based on the for-mentioned hypotheses, the macroscopic velocity of the collected particles will be brought
to zero and a superposed backflow will not influence the inflow as it is a free molecular flow.
Starting from the temperature of the particles inside the chamber, the thermal mass flux Γ, defined in Eq. 6,
according to [1, p.151], can be calculated.
Γ(n, T )xi=nmpkBTch
2π=mpn¯vxi(6)
Therefore, it is possible to apply Γ to determine backflow and outflow in the chamber of the Air-Intake
as following:
˙
Nintake2=Γ(nch,T
ch)
mp
AinΘintake2(7)
˙
Nout =Γ(nch,T
ch)
mp
AoutΘout +˙
Naccel (8)
To this, the continuity equation, see Eq. 9, can be applied which states that the net particle flow through
a control volume, in this case the chamber section, having certain number of outlets and inlets must be zero.
˙
Naccel is the accelerated particle flow which will be actively extracted by the thruster. This value strongly
depends on the operation point of the specific thruster and its respective acceleration process and is, amongst
others, also a function of the ninside the chamber. It is expected that a minimum nis needed inside the
chamber for the thruster ignition. Therefore, the focus is at the situation before ignition, ˙
Naccel =0.
˙
Nintake1=˙
Nintake2+˙
Nout (9)
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July 4–10, 2015
The assumption of a nullified macroscopic velocity in the chamber allows to rewrite Eq. 9 as in Eq. 10:
˙
NinΘintake1=Γ(nch ,T
ch)
mp
(AinΘintake2+Aout Θout) (10)
Therefore Γ can be extracted and, thus, the density nch inside the chamber from Eq. 6 results in:
nch (nch,T
ch)2π
mpkBTch
(11)
The pressure can be calculated by applying the ideal gas condition as:
pch =nchkBTch (12)
It is therefore possible to analytically calculate the parameters inside the chamber section, when input
conditions and transmittances are given. Based on this, efficiencies for the number particle flow, pressure
and mass flow can be extracted. The approach will be verified through DSMC simulation in Sec. V.
The number density inside the chamber is the value which results together with the other gas properties in a
thermal effusion out of the chamber equal to ˙
Nintake2and the effusion part of ˙
Nout. In a dynamic view, this
values rises until the balance is reached. Considering also the extracted flow ˙
Naccel, it is expected that the
actual nch will decrease after ignition, since the thermal effusion required for mass balance would be smaller.
Collection Efficiency ηcin Eq. 13, pressure ratio in Eq. 14 and number density ratio in Eq. 15 are important
values for the evaluation of an Air-Intake. With regard to Eq. 1, ˙
Nthr can be seen either as ˙
Nintake1or as
˙
Nout. For consistency with the respective publications of the analyzed designs and for a simple comparison
with the DSMC simulations, the latter has been chosen.
ηc=˙
Nout
˙
Nin
=Γ(nch,T
ch)
mp
AoutΘout
˙
Nin
=Θintake1AoutΘout
AinΘintake2+Aout Θout
(13)
pch
pin
=mp˙
NinΘintake1
AinΘintake2+Aout Θout 2π
mpkBTch
Tch
Tinnin
(14)
nch
nin
=mp˙
NinΘintake1
AinΘintake2+Aout Θout 2π
mpkBTch
1
nin
(15)
V. Air-Intake Simulations
In this section, the DSMC simulations performed with our in-house code PICLas are presented. The
consideration of a honeycomb structure of small straws at the entrance required an elaborate modelling
approach which is additionally described.
A. PICLas
PICLas, developed at the IRS and the Institute of Aerodynamics and Gas Dynamics (IAG) of the University
of Stuttgart,10 was employed for the following simulations. The code is a coupled, three-dimensional particle
method based on unstructured, hexahedral meshes and includes, besides general routines for particle tracking
and particle-wall collisions, a Direct Simulation Monte Carlo (DSMC)1as well as a particle-in-cell method,
with additional modules for Low Diffusion and Fokker-Planck models.11
In the context of this paper, only the DSMC part was used. A pairing scheme including the Natural-Sample-
Size method with the variable hard sphere model was applied for inter-particle collisions. Internal degrees of
freedom of molecules were taken into account while neglecting chemical reactions because of the relatively low
temperatures in all simulations. By switching the pairing scheme off (corresponding to collision probabilities
of zero) and, thus, using only the tracking, wall collisions and analysis routines, also ideal, free molecular
flows were simulated.
In general, symmetry was exploited by simulating only one quarter of the domain. Diffusive reflection with
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July 4–10, 2015
full accommodation was assumed at walls, whereas specular reflection for the planes of symmetry. All
other boundaries were open with a defined transparence (i.e., crossing particles are deleted with the given
probability, otherwise reflected) with an optional inflow from a virtual buffer layer filled with particles of a
Maxwell-Boltzmann distribution corresponding to the respective inlet condition.
B. DSMC Simulations of the Reference Cases
1. JAXA Air-Intake
Based on the Fujita’s 2004 study,5DSMC simulations have been performed for comparison with their
paper results and to verify the balancing model. The input values are the same as reported in the paper5
(atmospheric model MSISE-90), as well as the geometrical data. In particular, two different geometric
configurations are considered, with a form factor of χ=10.0 for a short intake optimized for low altitudes,
and χ=20.0 for a longer intake optimized for higher altitudes. The form factor χ, see Fig. 7, is defined as in
Eq. 16. Two altitudes have been simulated, 140km and 180 km, the highest with both geometries (χ=10.0
and χ=20.0) and the lowest with a different outlet transmittance, given by the acceleration grids of the
assumed thruster (Θout =0.1an
out =0.2).5
χ=L1
R2R1
(16)
Figure 7: Air-Intake Geometry,.5
Additionally to the MSISE-90, the NRLMSISE-00 atmosphere model was used for comparison. It is
the most advanced regarding the composition of the atmosphere at low altitudes.12 The differences of
the two models are, in our simulations, the inclusion of additional species ( He and Ar at 140 km, also
Nat180km),alowerTin and a slightly lower ntot. Input parameters for the model have been set for
01/01/2014 at hour =1.50, lat. 55.00, long. 45.00 in average solar and geomagnetic activity defined by
F10.7=F10.7avg = 140 and Ap = 15. In Tab. A1 of the Appendix the input parameters are shown.
Concerning the short Air-Intake, χ=10.0 at 140 km using NRLMSISE-00 model, DSMC simulations have
been run also with the hypothesis of an ideal free molecular flow, that means considering only collisions
between particles and walls. By this, the contribution, if there is any, of inter-particle collisions inside the
chamber can be evaluated.
The results of our simulations are compared to the Fujita’s from 2004 in Tab. 1 in terms of the averaged
value of the total number density ninside the region behind the satellite core. Results have, except for
the χ= 10 case at 180 km, a deviation of less than 10% from the reference values. Therefore the results
are, at least, in the same range and verify the general simulation set-up. The main reason of the deviations
is assumed to be the ambiguity of a defined chamber pressure. Furthermore, not all assumptions of the
reference are known.
A more elaborate presentation and discussion of the results is given in Sec. VI.
2. BUSEK Air-Intake Simulation
Based on the BUSEK design,8DSMC simulations have been performed to cover a wider range of Air-Intake
configurations. The considered design is supposed to operate in a low Mars orbit and is a 3.7m long tube
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
Table 1: Fujita’s 2004 Air-Intake DSMC Results Comparison
Θout nch ,from
5navg,ch , PICLas Rel. Dev. Note
km - - m3m3%-
140 10.0 0.1 7.24 ×1018 7.77 ×1018 +7.32
140 10.0 0.2 4.82 ×1018 4.95 ×1018 +2.70
”” ” 4.40 ×1018 8.71 NRLMSISE-00
180 10.0 0.2 1.11 ×1018 9.32 ×1017 16.04
180 20.0 0.2 1.37 ×1018 1.237 ×1018 9.71
”” ” 1.01 ×1018 26.28 NRLMSISE-00
Table 2: BUSEK Air-Intake DSMC Results Comparison
ηc,from
8ηc, PICLas Rel. Dev. Note
%%% -
28 23 17.9 With Collisions
<19 17 - No Collisions
20 - With Straws, With Collisions
<19 15 - With Straws, No Collisions
with a 0.6 m diameter terminating with a conical surface converging on a 0.14 m diameter exit to fed the Hall-
Thruster. As complete data is not available, nhas been extracted from a given plot showing the compression
efficiency ηcat different products of number densities and intake diameter. A value of ntot =5.3×1017 m3
was used for the simulated intake diameter of 0.6 m. Remaining flow condition are set to vin =3.5km/sand
Tin = 250K, the latter represents a common value for the temperature of the upper Mars atmosphere.6As
simplifying assumption only CO2was considered (that is present 96%) and the internal degrees of freedom
of the particles were neglected.
The main physical principle pointed out by BUSEK is the pushing of the incoming particles on the already
trapped particles by inter-particle collisions. To quantify this effect, those collisions have been switched off
for comparison, analogously to the JAXA cases. The main results from the simulations are briefly presented
in Tab. 2 and compared to the available data from BUSEK,8that is an ηcof 28% for the simulated point
and 19% for the lowest included nwhich is, therefore, interpretable as an upper limit for the simulations
assuming free molecular flow. The results are here also in the same range as those from the reference which
verifies the general system set-up, but emphasizes the availability lack of the exact reference assumptions.
In Tab. 2 there also already simulations with included straws, these will be further described in Sec. VI.
C. Consideration of Straws inside the Air-Intake
JAXA7and BUSEK9presented an Air-Intake with a honeycomb straw structure inside the inlet. The
function of the straws, as already explained, is to minimize the backflow and, therefore, increase pressure
and density at the end of the Air-Intake and ηc. The main assumption for the straws, in both studies, is that
the incoming flow, as result of being hyperthermal, is well collimated and no interaction between particles
and straw walls will arise. That means the particles will keep their macroscopic velocity until they scatter
at the surfaces of the end of the Air-Intake.
1. Implementation in DSMC
Due to the high computational resources required for the actual inclusion of an honeycomb straw structure
into the DSMC domain, a simplifying modeling approach has been used within the simulations.
For this, single straws have been simulated, represented by simple cylinders open at both sides with an in-
coming flow along the direction of the axis. To simulate the Air-Intake completely with straws, the velocity
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
distribution at the outlet of a single straw has been extracted and used as input for the Air-Intake simulation
itself. The velocity distributions show two main peaks, one corresponding to the high, macroscopic velocity
of particles that went through the straw without wall collisions, and another for the particles scattered along
the inner wall, with a macroscopic velocity of zero. The number of fast and of scattered particles are ex-
tracted from the simulations and the respective transmittances, Θfast and Θscattered, used for the following
Air-Intake simulations were included.
As there will be also a backflow from the downstream part of the intake through the straw, a transmit-
tance for this flow is needed. The corresponding problem is an already well-known subject of gas kinetics,
mostly connected to the work of Clausing2who dealt with free molecular flow through cylinders in terms of
transmission probability. The Clausing Factor, referred as ΘClausing, depends only on the L/R ratio of the
respective cylinder and is valid under the following assumptions:
Ideal free molecular flow;
vin =0;
Tin =Twall;
fully diffusive reflections at the walls, α=0.
Precise solutions of the Clausing equations were calculated by Cole3for discrete L/R ratios. These solutions
were used, rather than the long tube approximation, in which the Clausing factor is approximated as in
Eq. 17, due to the significant error the latter produces in the range of our interest (L/R < 100), as shown
in Fig. 8. For example, the relative error is 40% for L/R = 10 and still of 5% for L/R = 100.
ΘClausing =8
3
R
L(17)
L/R, -
10-1 10010110210 3
Transmittance, -
10-3
10-2
10-1
100
Transmittance vs. L/R
Clausing Equation vs. Long Tube Approximation
Clausing Equation
Long Tube Approximation
Figure 8: Comparison of Precise Clausing Factors Against Long Tube Approximation.
2. Verification of the Approach
To verify the applicability of the for-mentioned approach for the inclusion of straws into the Air-Intakes
simulation, additional DSMC simulations have been performed for a geometry including only the annular
intake section of Fujita’s Air-Intake, by which, the required transmittances could be extracted for a simulation
including only the part after the satellite core. This has been chosen as the effective diameter of the ring
open cross section is small compared to the length, similarly as for a single straw and, thus, it shows a similar
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
compression ability. Furthermore, the general approach can be demonstrated in the following.
The intake section to be modeled is simulated with open boundaries at both ends, i.e., all particles crossing
these areas are deleted. However, their velocities during the crossing are saved, which enables the evaluation
of the velocity distribution. This probability density function of directed velocity,however, does not represent
the function inside a given volume, as every particle crossing an outlet was saved and, therefore, fast particles
were preferred against slow velocities which could have started also from a larger distance. The velocity
distribution at the end is assumed to be Bi-Maxwellian, including the fast, unscattered particles with vin
and Tin on the one hand, and the scattered particles with Twall on the other hand. Based on this, the only
values to be extracted are the flow rates of fast and scattered particles that flow into the domain. These are
the corresponding transmittances Θfast and Θscatter ed multiplied by the incoming particle flow. An accepted
error is based on the fact that particles with a large lateral velocity will be scattered inside the cylinder with
a higher probability and, thus, the outflow is not supposed to follow an ideal, Gaussian distribution into
those corresponding directions.
Fig. 9 shows the corresponding data for the simulated case including both inlet and outlet section. A,Band
Care the areas given by the integrals over the individual distributions, respectively of the scattered backflow
A, the scattered flow Band the fast/unscattered flow C. As the sum of all 3 areas (A+B+C) represents
Nin
fast is calculated as C/(A+B+C)an
scattered as B/(A+B+C). The same was conducted for
an inflow without macroscopic velocity, giving only the areas Aand Band a ΘC lausing of B/(A+B). These
three transmittances, shown in Tab. 3, were applied as input to the simulation including only the chamber
section as mentioned above. Results showed no significant difference to the simulation including both intake
and chamber section, as it will be shown in Sec. VI. Therefore, the extraction of the approach has been
verified and can be applied for the straw implementation.
vz, m/s
-4000 -2000 0 2000 4000 6000 8000 10000 12000
f(vz), -
C
B
A
Figure 9: Extracted Velocity Distribution, Fujita’s design inflow including only the Annular Intake Section.
Table 3: Annular Intake Section Transmittances.
El. ΘClausing ,(A
scattered,(B
fast,(C)
O 0.212 0.289 0.392
N20.212 0.276 0.466
O20.212 0.253 0.512
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July 4–10, 2015
3. JAXA Straws
In 2012 a new paper from JAXA7dealt with the improvement of Fujita’s design, in particular a model for
laboratory testing was developed with the addition of straws in a honeycomb structure, and tested with an
atomic oxygen flux generator. Since the geometry of this particular Air-Intake is not available, its straws
with a L/R = 20 have been applied to the Fujita’s design for the altitude of 140 km, χ= 10. The atmospheric
model applied is the NRLMSISE-00. The transmittances extracted from our simulations are summarized for
each species in Tab. 4.
Table 4: JAXA Straw Transmittances Results.
Element Θfast Θscattered ΘClausing
Ar 0.381 0.293 0.109
He 0.064 0.288 0.109
N20.307 0.315 0.109
O 0.204 0.334 0.109
O20.333 0.308 0.109
4. BUSEK Straws
In the BUSEK presentation on MABHET,9straws are mentioned to be used in the Air-Intake for the fore-
mentioned reasons. Detailed information on their geometry is not available and, therefore, a rough estimation
from the schemes in the report8and in the presentation9was done. The estimated values provide an aspect
ratio of L/R 35. The extracted transmittances are presented in Tab. 5.
Table 5: BUSEK Straw Transmittances Results.
Element Θfast Θscattered ΘClausing
CO20.100 0.306 0.067
VI. Simulation Results and discussion
In this section, the results of the Air-Intake simulations for both Fujita/JAXA and BUSEK cases, also
with the inclusion of straws, are presented and discussed. Averaged, macroscopic values inside discrete cells
are extracted for the total number density nand velocity vzinto inflow direction zfrom lines along the
Air-Intake, as it is shown in Fig. 10 together with contours of total nfor the JAXA intake with MSISE-90
model, χ=10.0an
out =0.2 at 140 km (“chamber” illustrates the averaging volume for the calculation
of nch). For the BUSEK cases, center lines have been extracted.
A. JAXA Air-Intake
Fig. A1a in the appendix, shows vzand nalong the intake for the reference cases at 140 km. nincreases
when getting closer to the chamber while the vzdecreases, which is also expected as the part of scattered
particles increases. Higher density and lower velocity are shown in the case with a lower transmittance of
the outlet grid, indicating a higher compression. In the plot the result including only the chamber section
for the modeling of the velocity distribution is also shown, and very good agreement is visible, therefore,
verifying the extraction approach of the velocity distribution.
Fig. A1b shows the results for the Air-Intake at 180 km with both atmospheric models and both χ. Red and
black lines are using the same atmospheric model but a different χ, the short Air-Intake optimized for 140km
provides indeed a smaller nwhile keeping the velocity higher than in the case with the long Air-Intake of
χ=20.0. The slightly lower nof the NRLMSISE-00 model compared to the MSISE-90, produces the lower
nin the Air-Intake of the same size, the different Tin probably influences the shape of the curve itself. Tab. 6
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
Figure 10: JAXA intake with MSISE-90 model, χ=10.0an
out =0.2 at 140 km.
shows the results of the simulations performed with the NRLMSISE-00 model, illustrating the influence of
inter-particle collisions and straws inside the intake. In particular, the extracted average number density
over the chamber volume navg,ch is shown. Starting from this value the pressure and density ratio are derived
and, moreover, ηcis also included. The line plots of the Air-Intake are shown in Fig. A2 in the Appendix.
Results indicate that the use of straws, in this particular geometric configuration, will provide less nin the
chamber, thus, density and pressure ratio and, therefore, also ηcwill be lower. In particular, from Fig. A2,
the nprofile shows an higher value at the entrance, which decreases towards the chamber. The opposite
behavior is obtained without straws which shows the additional compression by the straws but, as a matter
of fact, only at the inflow part of the intake. Lower performance of the Air-Intake means that the presence of
straws is counterproductive for this particular geometry. Regarding the effect of the inter-particle collisions
in the case without straws, both simulations have very similar results which shows the good approximation
of an ideal free molecular flow for this specific case. With collisions, a slightly lower nis achieved inside the
chamber, possibly due to the fact that the particles represent an obstacle for the incoming flow.
Table 6: DSMC Results for JAXA’s design: Influence of inter-particle collisions and straws inside the intake.
Θout nav g,ch ηcpch/pin nch /nin Note
km - - m3-- -
140 10.0 0.2 4.40 ×1018 0.49 29.4 56.9 NRLMSISE-00, With Collisions
”” ”4.64 ×1018 0.50 31.0 60.0 NRLMSISE-00, Without Collisions
”” ”3.61 ×1018 0.40 24.1 46.7 NRLMSISE-00, Straws, With Collisions
B. BUSEK
Fig. A3 in the Appendix shows nalong the center line (zaxis) of the BUSEK Air-Intake design. The rapidly
decreasing nat the end of the intake is due to the assumption of a completely open outlet8with no backflow.
In the real case, the thruster systems would follow, creating also a backflow. The DSMC results of the
simulations without straws (black and blue lines) show that nsignificantly increases and reaches a region of
nearly constant state at the back, if collisions are taken into account. When collisions are neglected, that
region is missing and, instead, an almost linear increase of nalong the Air-Intake occurs with a maximum
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
less than the maximum value of the case with collisions. This shows that the assumed cascade effect actually
exists, in which incoming particles collides with the trapped particles and form a region of higher pressure at
the back of the Air-Intake. Concerning the application of straws to this design, Fig. A3 shows a slightly lower
n, but based on the higher nin the front part it is evident that particles are actually trapped inside the Air-
Intake. Conspicuous is the existence of a region of lower nin front of the honeycomb outlet with increasing n
into both direction for the collisional case, while, without collisions, nis monotonically decreasing, similar to
the JAXA case with straws. This can be explained by the collisions of incoming particles with the trapped
ones, representing an obstacle in the case with collisions. This can be also seen in Fig. A4 that shows the
velocity along the zaxis. In the cases with collisions, there is a jump in the velocity at the entrance - the
particles already inside are pushed into the intake.
All in all, results show that colliding particles will actually create a region of higher nat the back of the
Air-Intake, and that this principle can be used to generate a region of higher constant ninside the Air-
Intake. However, an actual improvement by the existence of straw cannot be observed which motivated a
more detailed analysis of their influence.
VII. Application of the Balancing Model and Straw Sensitivity Analysis
A. Comparison of the Balancing Model with DSMC
In the following, the Balancing Model is verified for exemplary cases analyzed by DSMC. They were:
1. Fujits’s design, MSISE-90 model, χ=10.0an
out =0.2 at 140 km, with collisions;
2. Fujita’s design with NRLMSISE-00 model and straws, with collisions;
3. BUSEK design without straws (with and without collisions);
4. BUSEK design with straws (with and without collisions).
Same inflow conditions as for the DSMC simulations were used. As the transmittances are different for each
species, the balancing model was applied for each species and the resulting nand ˙
Nare added together
for calculating the total values. The cases with straws represent an intake section including two separate
parts - the straws and the remaining intake without straws. Therefore, Eq. 18 is applied for combining two
transmittances ΘAand ΘBto a single value ΘAB,withAfor the straw values and Bfor the values of the
remaining free intake (a ring for the Fujita/JAXA and tube for the BUSEK case).
ΘAB
AΘB+
N
n=0
ΘA(1 ΘB)(1 ΘA,Clausing )[(1 ΘA,C lausing)(1 ΘB ,Clausing )]nΘB,Clausing (18)
Similar to the straw implementation in DSMC, transmittances were divided into ΘAB,scattered and ΘAB ,f ast,
their sum was used for the transmittances inside the balance model. The values were taken from DSMC
simulations (ΘBfor the JAXA cases from calculations of only the intake ring) or, where applicable, as known
Clausing Factors. The transmittances are summarized in Tab. A2 in the Appendix.
As Tab. 7 shows, results agree very well with DSMC results in terms of nch and ηc. Compared to the DSMC
simulations assuming free molecular flow (corresponding to the same conditions as for the Balancing model),
the relative error is less than 10%, in the Fujita/JAXA case with straws even nearly exact for nch and 1.3%
regarding ηc. For the BUSEK case, the values are also compared to the simulations without collisions and
are still in the same range. Regarding the increased discrepancy when compared to DSMC with collisions,
this is because their effect is that the incoming particles literally push the already trapped particles further
into the Air-Intake, therefore increasing ηcand n. In the Balancing Model, this cannot be taken into account,
as it assumes free molecular flow.
The Balancing Model seems to be a very good approximation for the DSMC results and it also confirms the
reduction of ηcwhen including straws inside the intake section. However, most of the used transmittances
have to be calculated by DSMC at first and, thus, this does not represent a general approach for evaluating
any arbitrary configuration. For this, the transmittances need to be calculated directly from a given geometry.
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July 4–10, 2015
Table 7: Results Balancing Model, BM , vs. DSMC Results
Case nDSMC
ch ηDSMC
cnBM
ch ηBM
cErr. nch Err. ηc
m3% m3%%%
JAXA 4.95 ×1018 48 5.34 ×1018 51 +7.9 +5.8
JAXA with straws 3.61 ×1018 40 3.61 ×1018 40 0.0 -1.3
BUSEK (without collisions) 6.1×1019 17 6.43 ×1019 18 +5.4 +4.7
BUSEK (with collisions) 7.5×1019 23 ””-14.3 -21.4
BUSEK with straws (without collisions) 6.5×1019 15 5.93 ×1019 17 -8.8 +7.8
BUSEK with straws (with collisions) 6.5×1019 20 ””-8.8 -16.7
B. Straw Sensitivity Analysis
DSMC simulations on various straw geometries with macroscopic vin >0 have shown that a certain fraction
of the particles does not collide with the walls and leaves the straw with inflow condition, while the remaining
part is scattered, and has only a thermal movement corresponding to Twall left. As this problem is not covered
by the well-known Clausing’s assumptions and equations, it is of interest to get a wider idea of how straw
geometries and flow conditions influence the respective transmittances. Therefore, a sensitivity analysis was
performed by DSMC simulations varying the following parameters:
Tin and Twall;
incoming flow velocity, vin ;
particle mass mp, different species;
Land R.
The results of the sensitivity analysis are shown in Tab. A3 in the Appendix. By comparing Cases 1 and
3, both having the same L/R ratio but different radii, it is expected that for the same ratio, the same Θ’s
follow. Additionally, Cases 1 and 2 show that if the ratio Twall/mpis kept constant, the transmittances do
not change. Cases 4 and 5, and 10 and 11 illustrate that Twall does not have any influence and Cases 7 and
8 that Θ’s do not change for a constant vinR/L. All in all, it can be assumed that in the considered range
of parameters, the driving non-dimensional value is Xfrom Eq. 19.
X=kBTin
mp
vin
L
R(19)
The ratio of the square-root and vin can be interpreted as the one of the lateral, thermal velocity to the
axial, macroscopic velocity. Plotting all points of Tab. A3 in terms of Θ(X) results in Fig. 11. It can be seen
that the transmittances correlate well by the use of the polynomial fits given in Eq. 20 for the scattered part
and Eq. 21 for the fast part. Based on this, different values of L/R can now be analyzed both for JAXA
and BUSEK cases. L/R values with ΘClausing given by Cole3were used, in particular the Xrange was kept
the same as for the correlation. The length is kept constant (corresponding to different straw radii) which
enables the use of the same transmittances for the remaining intake part without straws (B from Eq. 18).
Θ(X)scattered
2=0.041447835X30.2850944924X2+0.5808664686X0.031410537 (20)
Θ(X)fast
1=0.0532264802X3+0.405367838X21.0704252233X+1.0533360985 (21)
In Fig. 11 represents Θ3the total inflow transmittance, while the Clausing factor ΘClausing represents
the backflow transmittance. The former depends on both the geometry (L/R) and the inflow conditions
(Tin,mpand vin ), but the latter only on L/R. By merging both dependencies to the single value X,
Θ3is now dependent on solely Xand ΘClausing additionally on the geometry or the inflow conditions.
Therefore, ΘClausing is shown for a set of three different inflow conditions representing most of the range of
the sensitivity analysis and, thus, the considered RAM-EP cases. Fig. 11 shows that, independently from
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
the actual flow condition, Θ3decreases always slower than ΘClausing for low L/R ratios, but faster for large
ratios. Therefore, it is preferable to choose a design with low L/R ratios, as an high transmittance for the
inflow, together with a low transmittance for the backflow is desired. Moreover, it can be seen that for a
fixed X
Clausing increases with Tin and decreases with mpand vin , while Θ3remains constant. By this,
it can be assumed that, e.g., lighter species are collected less efficiently than heavier ones, as it is also shown
in Fig. A5. Fig. A5, A6, A7, and A8 in the Appendix show the resulting dependence of nch and ηcfrom
L/R for the separate species of the JAXA case and three different intake lengths for the BUSEK case. They
show that, for the considered range of parameters, the Air-Intake is supposed to be most efficient for small
L/R, corresponding to a simple grid. To verify this conclusion, DSMC simulations for shorter geometries
should be performed which would additionally include the influence of inter-particle collisions.
Figure 11: Generalized Cylinder Transmittances
VIII. Conclusion
This paper focused on the Air-Intake which collects and drives the atmosphere particles to the thruster,
in an air-breathing electric propulsion system. Recent studies have been reviewed, in particular the focus
has been on the Fujita’s design, its improvement from JAXA and the BUSEK design. Their early results
have been verified through our DSMC code.
Moreover, the introduction of straws and the influence of inter-particle collisions inside the Air-Intake, that
could improve the collection efficiencies, have been analyzed. In order to simulate the presence of straws, a
method of extraction of the velocity distribution at the end of a single straw to be inserted as input condition
for the Air-Intake has been verified and applied.
A sensitivity analysis on the straws in terms of various input parameter has been performed and showed
the dependency of the transmittances results. The velocity distribution shows that the particles will always
interact with the straw and, therefore, only a part of the flow will reach the end with a macroscopic velocity
while the rest will be only moving due to thermal diffusion.
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July 4–10, 2015
In particular for the Fujita’s design, the introduction of the straws of JAXA studies has shown a decrease
collection efficiency ηcand a smaller total number density nch inside the chamber than in the case without.
The application of a newer atmospheric model showed slightly lower atmospheric nand lower Tin.
Regarding the BUSEK study, the principle of a collision cascade at the end of a long tube shaped Air-Intake
that generates a compression has been verified. In particular a cushion of almost constant nis present in
front of the end of the Air-Intake and its length is increased by the addition of straws, although with a
decrease of n.
A balancing model based on separated particle flows and transmittances has been introduced and compared
with the DSMC results which have shown a good approximation in the results, however it must be sustained
by the calculation of transmittances a priori. With this model a sensitivity analysis over the L/R ratio of
the straws has been performed and applied to Fujita’s and BUSEK Air-Intakes.
This was additionally sustained by the development of a correlation between geometry/flow data and the
respective transmittance. Ideally, it should be as high as possible for the inflow, and as low as possible for
the backflow in an Air-Intake. The results show indeed, that a lower L/R ratio is preferable as it provides
higher ηcand nch. It suggests that a grid, rather than a honeycomb structure of straws at the front of the
Air-Intake, shall be used to form a higher performance Air-Intake.
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Appendix
Table A1: Inflow Conditions for Fujita/JAXA cases.
hn
tot,in Tin vin nN2nO2nOnAr nHe nNAtm. Model
×1016 ×1016 ×1015 ×1016 ×1013 ×1013 ×1012
km m3Kkm/sm
3m3m3m3m3m3-
140 8.73 682.07.82 5.68 3.84 2.67 - - - MSISE-90
”7.73 580.57.82 4.36 3.79 2.98 8.68 0.29 - NRLMSISE-00
180 1.75 968.07.80 0.90 0.47 0.80 - - - MSISE-90
”1.38 731.47.80 0.53 0.29 0.815 0.45 4.12 9.30 NRLMSISE-00
Table A2: Balancing Model Results.
Straw Ring/Tube
Case Θscattered Θfast ΘC lausing Θscattered Θfast ΘClausing Θintake1Θintak e2
Fujita/JAXA, O 1110.289 0.392 0.212 0.681 0.212
”, N21110.276 0.466 0.212 0.742 0.212
”, O21110.253 0.512 0.212 0.765 0.212
Fujita/JAXA, O 0.330 0.200 0.109 0.255 0.472 0.222 0.401 0.078
”, N20.330 0.200 0.109 0.255 0.472 0.222 0.488 0.078
”, O20.310 0.330 0.109 0.202 0.597 0.222 0.500 0.078
”, He 0.290 0.064 0.109 0.243 0.265 0.222 0.256 0.078
”, Ar 0.290 0.38 0.109 0.182 0.642 0.222 0.533 0.078
BUSEK 1110.265 0.447 0.162 0.712 0.162
0.307 0.099 0.067 0.240 0.507 0.181 0.315 0.049
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Table A3: Extracted Straws Transmittances from DSMC Simulations.
Case L/R vin Twall Tin mp/El. Θfast Θscattered Note
No. -km/sK K ×1026 kg --
110 7.80 300 300 2.66/O0.613 0.192 R=R1
210 7.80 600 600 5.31/2×O0.613 0.192
310 7.80 300 300 2.66/O0.616 0.192 R=R1/2
410 7.80 150 150 2.66/O0.722 0.139
510 7.80 600 150 2.66/O0.721 0.139
610 15.60 300 300 2.66/O0.803 0.099
710 3.90 300 300 2.66/O0.326 0.311
820 7.80 300 300 2.66/O0.325 0.311
920 7.80 300 300 2.66/O0.324 0.311 nin n140 km, Collisions
10 10 7.80 600 600 2.66/O0.475 0.256
11 10 7.80 150 600 2.66/O0.474 0.256
12 14.89 7.80 300 300 2.66/O0.450 0.265
13 4.44 7.80 300 300 2.66/O0.825 0.088
14 60.63 7.80 300 300 2.66/O0.05 0.25
15 34.48 7.80 300 300 2.66/O0.141 0.323
16 34.48 3.50 300 250 7.31/CO20.100 0.306
17 20 7.82 300 580.5 6.34/Ar 0.380 0.293
18 20 7.82 300 580.5 0.665/He 0.063 0.281
19 20 3.91 300 1451.25 6.65/10 ×He 0.063 0.281
20 20 7.82 300 580.5 4.65/N20.307 0.315
21 20 7.82 300 580.5 2.66/O0.204 0.334
22 20 7.82 300 580.5 5.31/O20.333 0.308
(If not mentioned, inter-particle collisions are switched off, assuming free molecular flow.)
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July 4–10, 2015
z, m
0 0.2 0.4 0.6 0.8 1 1.2
vz, m/s
0
100
200
Fujita Air-Intake 140 km, χ=10.0, MSISE-90, v z and n along a line
vz, ΘOut =0.2
n, ΘOut =0.2
vz, ΘOut =0.1
n, ΘOut =0.1
vz, Chamber , ΘOut =0.2
nz, Chamber, ΘOut =0.2
n, m -3
×10 18
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
(a) Air-Intake, DSMC results at 140km for JAXA Design, MSISE-90.
z/L, -
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
vz, m/s
0
20
40
60
80
100
Fujita Air-Intake 180 km, ΘOut =0.2, v z and n along a line
n, m -3
×10 18
0
0.5
1
1.5
2
vz, χ=10
n, χ=10
vz, χ=20
n, χ=20
vz, χ=20, NRLMSISE-00
nz, χ=20, NRLMSISE-00
(b) Air-Intake, DSMC results at 180km for JAXA Design, MSISE-90, NRLMSISE-00
Figure A1: Air-Intake, JAXA results, MSISE-90 and NRLMSISE-00.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
z, m
0 0.2 0.4 0.6 0.8 1 1.2
vz, m/s
0
100
200
Fujita Air-Intake 140 km, χ=10.0, ΘOut =0.2, NRLMSISE-00, v z and n along a line
vz
n
vz, without collisions
n, without collisions
vz, straws
nz, straws
n, m -3
×10 18
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
Figure A2: Air-Intake, DSMC results, JAXA Design with straws NRLMSISE-00.
z, m
00.5 11.5 22.5 33.5 4
n,m -3
×10 19
0
1
2
3
4
5
6
7
8BUSEK Air-Intake Mars, n along the center line
with collisions
without collisions
straws with collisions
straws without collisions
Figure A3: Density Along Center Line, BUSEK Design
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
z,m
00.5 11.5 22.5 33.5 4
vz,m/s
0
20
40
60
80
100
120
140
160
180
BUSEK Air-Intake Mars, v z along the center line
with collisions
without collisions
straws with collisions
straws without collisions
Figure A4: Velocity Along Center Line, BUSEK Design
L/R,-
6 8 10 12 14 16 18 20
ηc,-
20
25
30
35
40
45
50
55
60
65
70 Collection Efficiency, JAXA Air-Intake, χ=10.0 over L/R straw ratio
O
N2
O2
He
Ar
Figure A5: Balancing model applied to the JAXA Design, Collection Efficiency over L/R.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
L/R,-
6 8 10 12 14 16 18 20
nch/n in , -
10
20
30
40
50
60
70
80
90
100
110
nch/n in ratio, JAXA Air-Intake, χ=10.0 over L/R straw ratio
O
N2
O2
He
Ar
Figure A6: Balancing model applied to the JAXA Design, Number Density Ratio over L/R.
L/Rstraw ,-
0 5 10 15 20 25 30 35 40
ηc,-
10
15
20
25
30
35
40
45 Collection Efficiency, BUSEK Design, over L/R straw ratio
Ltube,after straw =3.2 m
Ltube,after straw =1.5 m
Ltube,after straw =6.0 m
Figure A7: Balancing model applied to the BUSEK Design, Collection Efficiency over L/R.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
L/Rstraw ,-
0 5 10 15 20 25 30 35 40
nch/n in , -
100
120
140
160
180
200
220
240
260
280
300
nch/n in ratio, BUSEK Design, over L/R straw ratio
Ltube,after straw =3.2 m
Ltube,after straw =1.5 m
Ltube,after straw =6.0 m
Figure A8: Balancing model applied to the BUSEK Design, Number Density Ratio over L/R.
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Joint Conference of 30th ISTS, 34th IEPC and 6th NSAT, Hyogo-Kobe, Japan
July 4–10, 2015
Acknowledgments
F. Romano gratefully thanks the Landesgraduiertenf¨orderung of the University of Stuttgart for the finan-
cial support; the authors acknowledge Mr. Yasuyoshi Hisamoto for the fruitful exchange of data regarding
the japanese studies.
References
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... The work of Di Cara et al. [4] concluded that the complexity of an ABEP system was advantageous at altitudes below 250km compared to conventional electric thrusters, based on the mass of conventional propellant required for a worthwhile mission duration in a dawn-dusk Sunsynchronous orbit (SSO). Romano et al. [5] outlined a calculation of drag for finding required thrust, with a simplified drag coefficient value that was constant with altitude to reduce uncertainty. Singh [6] and Andreussi et al. [7] introduced a more complete drag calculation, including lateral surfaces such as solar arrays, which are typically assumed to be aligned parallel to the incoming airflow. ...
... Atmospheric properties in VLEO are dependent on solar activity, with a general increase in density at a given altitude for higher solar activity, and this is modelled via the 10.7cm solar radio flux index F10.7 and magnetic Ap index. As common in literature [5], F10.7 = 140 and Ap = 15 values are used for average solar activity. Atmospheric properties vary significantly with geographic location and time of year, as well as solar activity. ...
... The global intake collection is calculated from the transmission probabilities of the major constituent intake flows, which typically requires costly Direct Simulation Monte-Carlo (DSMC) simulations. To avoid introducing uncertainty, this study uses a typical η c range of 0.3−0.5 as modelled in previous studies [3,5]. ...
Conference Paper
An air-breathing electric propulsion spacecraft uses air in the upper atmosphere as propellant for an electric thruster. The thrust produced compensates drag, allowing a long mission duration in very-low Earth orbit. This study derives a drag-compensation analysis which links the required thruster figures of merit to feasible operating altitude. The orbit of an air-breathing spacecraft, with thruster performance based on a gridded-ion thruster tested with atmospheric propellants, is propagated with time. The unavoidable orbital eccentricity and atmospheric variability results in an unstable altitude profile, which prompts the introduction of a thruster control law that demonstrates a long-term stable altitude profile in a Sun-synchronous orbit.
... = Temperature = Density = Number density 10.7 = Solar radio flux index = Magnetic index = Onset airflow velocity = Thrust = Thruster specific impulse / = Thruster thrust-to-power ratio = Thruster total electrical efficiency 0 = Sea-level gravity acceleration 1 PhD Student, Surrey Space Centre, m.tisaev@surrey.ac.uk. 2 Lecturer in Astrodynamics, Surrey Space Centre. 3 Senior Lecturer in Propulsion, Surrey Space Centre, a.luccafabris@surrey.ac.uk. 4 Electric Propulsion Engineer, SITAEL S.p.A. 5 Electric Propulsion Engineer, SITAEL S.p.A. 6 Senior Electric Propulsion Engineer, SITAEL S.p.A. 7 Electric Propulsion Technical Manager, SITAEL S.p.A., tommaso.andreussi@sitael.com. = Power factor for non-thruster sub-systems 0 = Solar irradiance at Earth = Solar panel efficiency = Solar panel assembly efficiency = Solar panel area = Solar array area = Spacecraft cylindrical side area = Spacecraft aspect ratio = Intake frontal area = Spacecraft length = Intake diameter = Knudsen number = Specular reflection fraction = Molecular speed ratio = Specific gas constant = Drag coefficient = Surface angle to onset airflow = Intake aspect ratio = Intake collection efficiency = Mass flow rate = Intake compression ratio = Orbit radius = Earth gravity harmonic (¯and 0 ) = Semi-major axis (mean and initial) = Mean Earth radius ...
... Di Cara et al. [4] concluded that an ABEP system was advantageous at altitudes below 250 , based on the mass of propellant required for a conventional electric thruster to achieve a worthwhile mission duration in a dawn-dusk Sun synchronous orbit (SSO). Romano et al. [5] outlined a calculation of drag for finding required thrust, using the simplifications of a constant drag coefficient with altitude and neglecting the drag of spacecraft surfaces aligned parallel to the incoming flow, such as the solar arrays. Singh [6] and Andreussi et al. [7] introduced a drag calculation without these assumptions, with the latter presenting an approach for optimising the thrust-to-drag ratio of an ABEP system and including initial considerations of the on-orbit spacecraft behaviour. ...
... The total intake collection is calculated from the transmission probabilities of the major constituent intake flows, which typically requires costly DSMC simulations. To avoid introducing uncertainty, a typical range of 0.3 − 0.5 is modelled here based on DSMC studies in previous literature [3,5]. ...
Conference Paper
Full-text available
A spacecraft using air-breathing electric propulsion operates in very-low altitude orbits, collecting air in the upper atmosphere as propellant for an electric thruster. This provides thrust to counteract drag, thereby removing the need for onboard propellant storage and the limit this imposes on mission lifetime. This study presents an analytical model based on drag-compensation by linking the figures of merit required for an air-breathing electric thruster to the achievable operating altitude. The analysis serves as a first-order design tool for a feasible air-breathing system, predicting a minimum required specific impulse of approximately 3000s and thrust to power of 8mN/kW for typical spacecraft properties. The on-orbit behaviour of such a spacecraft with time is also analysed, using a thruster model based on test data of a gridded-ion thruster operating with atmospheric propellants. Orbital propagations with time reveal that the atmospheric variability and unavoidable orbital eccentricity result in a naturally unstable altitude profile. This motivates the introduction of a thruster control law, which demonstrates a long-term stable altitude profile in a Sun-synchronous orbit at altitudes between 160−180km above the mean Earth radius.
... An analysis of ABEP was presented by Romano et al. [6,7] and Romano [8] at the University of Stuttgart from 2015. This study presented the variation of atmospheric properties with solar activity and calculated the drag for specific sizes of spacecraft to obtain an estimate of the thrust required. ...
... The nominal orientation of the satellite is assumed to be with the intake perpendicular to the onset flow and the solar arrays parallel to the flow vector, as shown in Fig. 1. The ratio of intake diameter to length of the satellite body is defined as the spacecraft aspect ratio , which was estimated in the ≈ 1 order of magnitude from previous literature [3,6]. The orientation of the intake would be maintained perpendicular to the onset flow to maximise intake performance. ...
... The friction drag coefficient was found using the Blasius laminar flow solution for a parallel flat plate, as per Eq. (6). This method avoids introducing uncertainty, with the laminar approximation being valid as ≪ 10 5 -10 6 where transition to turbulent flow begins [22]: ...
Article
Full-text available
Air-breathing electric propulsion (ABEP) enables long duration missions at very low orbital altitudes through the use of drag compensation. A system-level spacecraft model is developed, using the interaction between thruster, intake and solar arrays, and coupled to a calculation of the drag. A quadratic solution is found for specific impulse and evaluated to identify the thruster performance required for drag-compensation at varying altitudes. An upper altitude limit around 190 km is based on a minimum thruster propellant density, resulting in required values of Isp>3000s and T/P>8mN/kW for a realistic ABEP spacecraft. The orbit of an air-breathing spacecraft is propagated with time, which highlights the prescribed orbit eccentricity due to non-spherical gravity and therefore an increased variability in the atmospheric conditions. A thruster control law is introduced which avoids a divergent altitude behaviour by preventing thruster firings around the orbit periapsis, as well as adding robustness against atmospheric changes due to season and solar activity. Through the use of an initial frozen orbit, thruster control and an augmented T/P, a stable long-term profile is demonstrated based on the performance data of a gridded-ion thruster tested with atmospheric propellants. An initial mean semi-major axis altitude of 200 km relative to the equatorial Earth radius, a spacecraft mass of 200 kg, Isp=5450s and T/P=23mN/kW, results in an altitude range of around 10 km at altitudes of 160–183km.
... Although the discrepancy is small, the results show that the presence of intermolecular collisions leads to a slight reduction in the performance of the system. This suggests that the incoming flow molecules collide with those already trapped at the back of the intake and increase the backflow through the lateral ducts [17]. ...
... To increase the accuracy of future simulations, further empirical data should be collected within the specific geographical and temporal envelope of interest for the mission. To optimise the performance of the ABIE intake, the addition of a honeycomb duct at the inlet could be implemented, as this has been shown to act as a molecular trap, thereby reducing the backflow of the incoming flow [17,18]. Furthermore, given the importance of the gas-surface interaction model chosen, materials that lead to specular collisions should be investigated and adopted as DSMC simulations of specular intakes yielded intake collection efficiencies of up to 94% [18]. ...
Article
Full-text available
The Air-Breathing Ion Engine (ABIE) is an electric propulsion system capable of compensating for drag at low altitudes by ingesting the surrounding atmospheric particles to be utilized as propellant. The current state of the art of the ABIE performance is evaluated via Direct Simulation Monte Carlo (DSMC), due to the rarefied nature of the atmosphere in Very-Low Earth Orbit (VLEO). Nevertheless, the scarce availability of relevant simulation methodologies in the literature limits the repeatability of such numerical studies. Therefore, this paper proposes an independent methodology applicable to the modelling and simulation of Atmosphere-Breathing Electric Propulsion (ABEP) intakes that aims to validate the ABIE DSMC results retrieved from the literature. This is achieved by investigating the ABIE intake collection efficiency and compression ratio through the open-source solver dsmcFoam+ and by assessing the results against the available RARAC-3D DSMC data. First, the variation of grid transparency is discussed and compared between both solvers, yielding a mean percentage error of 2.97% for the compression ratio and 2.06% for the collection efficiency. Second, the absence of intermolecular collisions is verified for which errors of 1.61% for collection efficiency and 3.49% for compression ratio are observed. Then, the variation of flow incidence angle is simulated between 0 • and 15 • , yielding differences lower than 1.80%. Consecutively, the intake aspect ratio is varied between 10 and 40, for which a maximum discrepancy of 1.83% is measured and, finally, the drag coefficient of the intake is obtained in dsmcFoam+ to define the power density requirements.
... (Di Cara, et al., 2007) concluded that an air-breathing system was worthwhile at altitudes below 250 km by considering the propellant mass needed for conventional electric thrusters to provide a given operational lifetime in a dawn-dusk Sun synchronous orbit (SSO). (Romano, Binder, Herdrich, Fasoulas, & Schonherr, 2015) presented a method for finding drag in freemolecular flow, however a constant value for the drag coefficient was used in finding the thrust required for the purposes of reducing uncertainty. Unlike the previously noted studies, (Singh, 2014) and (Andreussi, et al., 2019) presented a more complete analysis of the spacecraft drag including the effect of lateral surfaces, such as solar arrays, which are assumed to be aligned parallel to the onset airflow. ...
... An range of 0.3-0.5 is used here as this is the typical range predicted by previous studies (Fujita, 2004) (Romano, Binder, Herdrich, Fasoulas, & Schonherr, 2015). The maximum possible stagnation compression ratio is derived from isentropic flow theory. ...
Conference Paper
Full-text available
Air-breathing electric propulsion (ABEP) enables long lifetime missions at very low orbital altitudes through the use of drag compensation. A model of the spacecraft is developed based on the interaction between thruster, intake and solar arrays. A quadratic solution is found in terms of specific impulse and evaluated at varying altitudes to identify the thruster performance required for drag-compensation. An upper altitude limit around 193km is based on a minimum thruster propellant density, resulting in approximate required values of Isp > 3200s and T/P > 7mN/kW for a realistic ABEP spacecraft. Simulations of the ABEP spacecraft orbit with time reveal an unavoidable orbit eccentricity due to non-spherical gravity and therefore increased atmospheric variability. A thruster control law is introduced which avoids a divergent altitude behaviour by preventing thruster firings around the orbit periapsis. Through the combination of an initial frozen orbit, thruster control and an augmented T/P, a stable long-term profile is demonstrated based on the performance data of a gridded-ion thruster tested with atmospheric propellants. An initial mean semi-major axis altitude of 200km, a spacecraft mass of 200kg, Isp = 5450s and T/P = 20mN/kW results in an altitude range of around 10-15km at altitudes of around 160-180km relative to the mean Earth radius for a constant, average solar activity.
... In 2015, the team conducted design research on the air intake of the atmosphere-breathing electric propulsion system [38]. Compared with the research of JAXA and BUSEK, it is found that, ideally, the inflow should be as high as possible and the backflow as low as possible in an air intake. ...
... Improve the Performance of Air Intake. Romano et al. [38] indicate that the flow rate of the atmospheric propellant needs to be greater than 3 mg/s to satisfy the thrust compensation at an altitude of 250 km. The configuration and material of the intake determine its ability to absorb atmospheric molecules. ...
Conference Paper
吸气式电推进(Atmosphere-Breathing Electric Propulsion,ABEP)系统在过去十年中对研究人员越来越具有吸引力。该系统可以摄取低轨道环境的稀薄大气分子作为推进工质来提供推力补偿,从而延长飞行器的工作寿命。该综述首先回顾了先前研究人员关于该系统开发作出的努力,然后总结了不同类型吸气式电推力器的主要性能,讨论了各类型推力器的优缺点,最后提出了将来进一步研究的可能途径。结果表明,利用低轨道稀薄大气分子作为推进工质进行推力补偿的潜力很大。但是,先前研究显示出的各种局限性,使得吸气式电推进系统目前还难以实际应用于空间任务。该系统的发展需要解决一些问题,例如进气效率,电离功率,电极腐蚀等。
... 轨道在轨运行的卫星数量众多且日趋饱和,而低于 250 公里高度范围内尚无任何卫星运行,超低空间轨道 资源成为卫星拓展运行范围、提升任务能力的新选择。超低轨道资源具有其独特的优势 [3] :离地面越近,意 味着更低的发射成本、更高的观测精度、更小的通讯损耗和更安全的运行环境。同时超低轨道飞行器的运行 也面临着诸多挑战 [4] :超低轨道空间由于稀薄大气的存在,使得卫星面临更为复杂的环境状况。轨道高度越 低,飞行时的大气阻力越大,对推进工质提出了更高要求。受到工质限制,卫星工作寿命也十分有限。为了 解决上述问题,吸气式电推进概念便应运而生。 如图 1 所示,吸气式电推进系统主要由进气装置和电推力器(包含工质电离与加速)两部分组成,其推 进工质为超低轨道残留的稀薄大气粒子。作为一种气体工质推力器,目前国内外开展的电推力器研究主要 有离子推进 [5,6] 、霍尔推进 [7,8] 、脉冲等离子体推进 [9][10][11] 、磁等离子体推进 [12] 和感应等离子体推进 [13][14][15] 等几种 类型。研究指出 [16] ,鉴于超低轨道具有大量含氧粒子,为避免电极腐蚀问题,无电极式的推力器是吸气式电 推进概念的优选发展对象。为了向电推力器提供充足的气体工质,进气装置成为了吸气式电推进系统的独 特组件。 图 1 吸气式电推进概念 [17] 最早提出了适用于吸气式离子发动机(ABIE)的进气装置,之后还经过了 JAXA [18][19][20] 的改进。该进气装置前端由环形的狭长网格状吸管组成,能够使平行于进气壁移动的中性粒子进 入通道而减少回流与泄露;后端为一个反射壁面,可以使来流分子反射至进气装置中心作为推进剂。该进气 装置的明显缺点是环形进气装置的中心部分会减少有效进气面积,反而增加大气阻力。 2007 年,ESA [21] 设计的进气装置同样具有网格状的前端导管,改进之处在于将进气装置和推力器装置 分开排布,从而使装置进气面积等于迎风面积,因此不需要克服额外的气动阻力,从而提高系统效率。 2012 年,BUSEK 公司 [22] 为火星大气收集设计了进气装置,该装置是一种前端为蜂窝状导管,后端为锥 体的长直管道。在漫反射条件下对二氧化碳的压缩倍率可达 250 以上。 2015 年,中国五一〇所 [23] 则提出了一种主动进气装置,该装置由多孔板、腔室和涡轮分子泵组成,能 够实现较高的收集效率和极高的压缩倍率。 2016 年至今,德国斯图加特大学 [24][25][26] 基于分子传输理论分析优化了进气装置,结果表明,六边形前端 导管和提出的增强漏斗形结构有利于大气工质的回收。为了提高六边形导管的填充率,进气道的截面也设 计成了六边形结构。 2017 年,美国科罗拉多大学 [27,28] 基于镜面反射模型分析了金字塔、锥形和抛物线型进气腔室对进气性 能的影响,结果表明抛物线的光学结构能使大气颗粒的收集效率达到 90%以上。 2018 年,俄国 TsAGI 和 RIAME 机构 [29][30][31] 为进气装置设计了一种独特的蓄能装置,其能够提高大气粒 子在进气装置内部的积蓄效果,从而提高收集效率和压缩性能。 2019 年,意大利 SITAEL 机构 [32] 对进气装置的前端网格状导管进行了改进,该结构呈多个同心的开口 环,每个环内被平均分割成了 3、9、15 和 21 个部分。比利时 VIK 机构 [33,34] 也做了类似的研究。 2020 年,国防科技大学则 [35] 总结了各种进气装置的主要构成部分,设计了一种由网格状导管、渐缩腔 体和长直管道组成的进气装置。 表1 ABEP系统进气装置简要总结 Tab.1 A brief review of intake device for ABEP system 机构 构型 收集效率 压缩倍率 JAXA [18][19][20] 网格状吸管、环形截面 <45% <180 ESA [21] 网格状吸管、圆柱截面 --BUSEK [22] 蜂窝状导管、长直管 20%~40% 250 五一〇所 [23] 多孔板、涡轮分子泵 41.67%~57.85% >10000 斯图加特大学 [24][25][26] 六边形导管、六边形截面 43.03% -科罗拉多大学 [27,28] 抛物线型腔体 >90% -TsAGI/RIAME [29][30][31] 蓄能装置 33%~34% 100~400 ESA/SITAEL [32][33][34] 开口环 28%~32% 95~140 国防科技大学 [35] 网格、渐缩腔、长直管 81. ...
... 轨道在轨运行的卫星数量众多且日趋饱和,而低于 250 公里高度范围内尚无任何卫星运行,超低空间轨道 资源成为卫星拓展运行范围、提升任务能力的新选择。超低轨道资源具有其独特的优势 [3] :离地面越近,意 味着更低的发射成本、更高的观测精度、更小的通讯损耗和更安全的运行环境。同时超低轨道飞行器的运行 也面临着诸多挑战 [4] :超低轨道空间由于稀薄大气的存在,使得卫星面临更为复杂的环境状况。轨道高度越 低,飞行时的大气阻力越大,对推进工质提出了更高要求。受到工质限制,卫星工作寿命也十分有限。为了 解决上述问题,吸气式电推进概念便应运而生。 如图 1 所示,吸气式电推进系统主要由进气装置和电推力器(包含工质电离与加速)两部分组成,其推 进工质为超低轨道残留的稀薄大气粒子。作为一种气体工质推力器,目前国内外开展的电推力器研究主要 有离子推进 [5,6] 、霍尔推进 [7,8] 、脉冲等离子体推进 [9][10][11] 、磁等离子体推进 [12] 和感应等离子体推进 [13][14][15] 等几种 类型。研究指出 [16] ,鉴于超低轨道具有大量含氧粒子,为避免电极腐蚀问题,无电极式的推力器是吸气式电 推进概念的优选发展对象。为了向电推力器提供充足的气体工质,进气装置成为了吸气式电推进系统的独 特组件。 图 1 吸气式电推进概念 [17] 最早提出了适用于吸气式离子发动机(ABIE)的进气装置,之后还经过了 JAXA [18][19][20] 的改进。该进气装置前端由环形的狭长网格状吸管组成,能够使平行于进气壁移动的中性粒子进 入通道而减少回流与泄露;后端为一个反射壁面,可以使来流分子反射至进气装置中心作为推进剂。该进气 装置的明显缺点是环形进气装置的中心部分会减少有效进气面积,反而增加大气阻力。 2007 年,ESA [21] 设计的进气装置同样具有网格状的前端导管,改进之处在于将进气装置和推力器装置 分开排布,从而使装置进气面积等于迎风面积,因此不需要克服额外的气动阻力,从而提高系统效率。 2012 年,BUSEK 公司 [22] 为火星大气收集设计了进气装置,该装置是一种前端为蜂窝状导管,后端为锥 体的长直管道。在漫反射条件下对二氧化碳的压缩倍率可达 250 以上。 2015 年,中国五一〇所 [23] 则提出了一种主动进气装置,该装置由多孔板、腔室和涡轮分子泵组成,能 够实现较高的收集效率和极高的压缩倍率。 2016 年至今,德国斯图加特大学 [24][25][26] 基于分子传输理论分析优化了进气装置,结果表明,六边形前端 导管和提出的增强漏斗形结构有利于大气工质的回收。为了提高六边形导管的填充率,进气道的截面也设 计成了六边形结构。 2017 年,美国科罗拉多大学 [27,28] 基于镜面反射模型分析了金字塔、锥形和抛物线型进气腔室对进气性 能的影响,结果表明抛物线的光学结构能使大气颗粒的收集效率达到 90%以上。 2018 年,俄国 TsAGI 和 RIAME 机构 [29][30][31] 为进气装置设计了一种独特的蓄能装置,其能够提高大气粒 子在进气装置内部的积蓄效果,从而提高收集效率和压缩性能。 2019 年,意大利 SITAEL 机构 [32] 对进气装置的前端网格状导管进行了改进,该结构呈多个同心的开口 环,每个环内被平均分割成了 3、9、15 和 21 个部分。比利时 VIK 机构 [33,34] 也做了类似的研究。 2020 年,国防科技大学则 [35] 总结了各种进气装置的主要构成部分,设计了一种由网格状导管、渐缩腔 体和长直管道组成的进气装置。 表1 ABEP系统进气装置简要总结 Tab.1 A brief review of intake device for ABEP system 机构 构型 收集效率 压缩倍率 JAXA [18][19][20] 网格状吸管、环形截面 <45% <180 ESA [21] 网格状吸管、圆柱截面 --BUSEK [22] 蜂窝状导管、长直管 20%~40% 250 五一〇所 [23] 多孔板、涡轮分子泵 41.67%~57.85% >10000 斯图加特大学 [24][25][26] 六边形导管、六边形截面 43.03% -科罗拉多大学 [27,28] 抛物线型腔体 >90% -TsAGI/RIAME [29][30][31] 蓄能装置 33%~34% 100~400 ESA/SITAEL [32][33][34] 开口环 28%~32% 95~140 国防科技大学 [35] 网格、渐缩腔、长直管 81. ...
Conference Paper
吸气式电推进(ABEP)系统是用于极低轨道太空任务的电推进概念,该系统可以捕获稀薄大气作为电推力器的推进剂,以补偿低轨气动阻力。在最好的情况下,它可以使航天器长时间执行任务,而无需携带任何推进剂。作为ABEP系统的关键组件,进气装置可以收集稀薄大气作为推进工质,并将其驱动至电推力器。考虑到电推力器对推进剂质量和电离效率的要求,必须改善进气装置的性能,包括稀薄大气的捕获效率与压缩比。本文首先对以往研究的进气装置进行了总结分析,在此基础上设计了一种新的进气装置:该进气装置主要由渐缩腔体、前端导管和后端直管组成;然后根据地球大气模型,获得装置的进气边界条件,包括气体种类、中性温度和数密度等。最后采用DSMC方法研究了不同气壁相互作用条件下装置的进气性能,从而优化装置的进气性能。结果表明,在镜面反射和漫反射模型结合的情况下,抛物线型渐缩腔体、蜂窝状进气道,和特定长度短管的组合对进气装置更有利。装置的稀薄大气收集效率为65.79%,压缩倍率为210.2。
... The intake device developed by University of Stuttgart [61,62] are based on the transmission theory of rarefied flows [63,64]. The proposed intake design consists of honeycomb type ducts and a long slender cylindrical tube (called Enhanced Funnel Design, EFD) [65]. ...
Article
Increasing interest in development of very low Earth orbit (VLEO) has attracted more and more researchers to study atmosphere-breathing electric propulsion (ABEP) system in past several decades. This system can use rarefied atmospheric particles as the propellant of electric thrusters, and maintain a long lifetime mission without carrying any propellant from ground. As the key component of system, intake device can realize the collection and compression of atmospheric particles within limited frontal area, which determines the performance of whole ABEP system. This review summarizes the previous studies to develop intake devices, evaluates the corresponding performance and understands the model involved, including atmosphere model, flow physic model and so on. In addition, several continued researches for intake device are also presented, including ground experiment technologies, intake surface material development, space compressor and liquefaction technology. Wherever possible, comments have been provided to provide useful reference to researchers engaged in intake device for ABEP system.
... Different intake concepts have been proposed and studied for this purpose (see e.g. [3,7]), relying on different reflective properties of the intake material. In this paper, two intake types, based on diffuse and specular reflection of atmospheric particles, are considered. ...
Conference Paper
Very low Earth orbits (VLEOs) with altitudes in the range of 150-250 km promise considerable benefits for Earth observation instruments or communication devices compared to traditional low Earth orbits. The operation of spacecraft in this altitude regime requires, however, that the drag caused by the residual atmosphere is compensated in order to avoid fast orbital decay. A solution to achieve this could be the application of atmosphere breathing electric propulsion (ABEP). This paper discusses aspects of system design-particularly of configuration design-of a satellite platform for VLEO employing an ABEP system with a cathode-less thruster. The focus is thereby on a comparison between "slender body" spacecraft configurations, similar to GOCE's design and "flat body" spacecraft configurations. For a demonstrator spacecraft with both an Earth observation and a telecommunications payload, drag coefficients as well as performance requirements on the ABEP system are calculated and compared for both configuration options.
... The thermal mass flux Γ is defined as in Eq. 10.2 [43]. ...
Book
This dissertation deals with the development of Atmosphere-Breathing Electric Propulsion (ABEP) technology, that can enable propellant-less continuous orbiting in very low Earth orbits (VLEO). It uses an intake in front of the spacecraft to collect the residual atmosphere and deliver it to an electric thruster as propellant, finally utilizing the cause of aerodynamic drag as source of thrust. A literature review is presented to give the ABEP state-of-the-art of the technology and the most relevant performance parameters are highlighted. The application of ABEP in VLEO is investigated by applying analytical equations based on atmospheric models and intake efficiencies based on the outcome of this work, and available state-of-the-art thruster efficiencies. Such analysis derives the collectible propellant flow, the aerodynamic drag, and the power required to fully compensate the drag. The case of GOCE using an ABEP system is presented, as well as its application in very low Mars orbit (VLMO). The intake and the thruster are investigated and designed within this dissertation. Three ABEP intakes designs are hereby presented, based on gas-surface-interaction prop- erties. Two are based on fully diffuse reflections, delivering collection efficiencies ηc < 0.5 and one based on fully specular reflections of ηc < 0.95. Their sensitivity to misalignment with the flow is analysed as well highlighting the specular design of being more robust compared to the diffuse one by maintaining relatively high ηc even for large angles. The ABEP thruster is based on contactless technology: there is no component in direct contact with the plasma, and a quasi-neutral plasma jet is produced. This enables operation with multiple propellant species (also aggressive such as atomic oxygen in VLEO) and densities, and does not require a neutraliser. The thruster is based helicon plasma discharges to provide higher efficiency compared to inductive ones.
Conference Paper
Full-text available
This paper summarizes the results of the RAM-EP system concept study. The study involved the investigation of the feasibility of using electric propulsion together with gas collected from the atmosphere to provide thrust to counteract the S/C altitude decay caused by drag. This is in order to allow orbit altitude control with a defined thrust profile and within the typical budgets of an Earth Observation type of mission. The final objective was to enable low altitude missions (below at least 250 km) and / or long lifetime missions above 250 km. Moreover the study aimed to apply the concept to a reference technology demonstration mission that could be of interest for Earth Observation.
Conference Paper
Full-text available
Challenging types of mission scenarios include those in Earth orbit (i.e., LEO, GEO), where the residual atmosphere, especially at low altitudes, creates significant drag to the S/Cs and forces their orbit to decay. For drag compensation propulsion systems can be used requiring on-board propellant and electric power. Enhancing lifetime of Earth-orbiting satellites without any substantial increase in costs is an important objective for governmental as well as commercial operators. An air-breathing electric propulsion system (RAM-EP) ingests the air of the residual atmosphere through a mechanical intake and uses it as propellant for an electric thruster. This system theoretically allows a S/C to orbit for an unlimited time without carrying propellant on board. Moreover a new range of altitudes (120-250 km) can be accessed, filling the gap between ramjet atmospheric propulsion and LEO space propulsion, thereby enabling many new scientific missions. Preliminary studies according to [2] have shown that the propellant flow necessary for electrostatic propulsion exceeds the available mass intake with reasonable limits, and that electrode erosion due to aggressive gases, such as oxygen, highly present in LEO, might limit the thrusters lifetime. The electrode-less design of inductive plasma generators - IPG - solves this issue. Characterisation of such plasma generators using pure O2 and CO2 gases exists and shows significant electric-to-thermal coupling efficiencies [10]. A system analysis is shown within this work to derive main design drivers for a RAM-EP mission application. Atmospheric modelling, orbit considerations, heat fluxes, drag force, air intake, and available mass flow for a wide altitude range have been investigated. Preliminary results have shown that full drag compensation is possible. The small-scale inductive plasma generator IPG6-S of the University of Stuttgart is continually improved and used as test bed for RAM-EP using IPG source. A set of mass flows has been defined, depending on altitude, inlet area, and intake efficiency to simulate relevant mission conditions. IPG6-S has been tested for mass flow rates between 120 mg/s down to 0:25 mg/s with air and O2. Mean mass-specific energies of the plasma ave been assessed and used to estimate exhaust velocities for the system analysis.
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The new NRLMSISE-00 empirical atmospheric model extends from the ground to the exobase and is a major upgrade of the MSISE-90 model in the thermosphere. The new model and the associated NRLMSIS database now include the following data: (1) total mass density from satellite accelerometers and from orbit determination (including the Jacchia and Barlier data sets), (2) temperature from incoherent scatter radar covering 1981-1997, and (3) molecular oxygen number density, [O2], from solar ultraviolet occultation aboard the Solar Maximum Mission. A new component, ``anomalous oxygen,'' allows for appreciable O+ and hot atomic oxygen contributions to the total mass density at high altitudes and applies primarily to drag estimation above 500 km. Extensive tables compare our entire database to the NRLMSISE-00, MSISE-90, and Jacchia-70 models for different altitude bands and levels of geomagnetic activity. We also explore scientific issues related to the new data sets in the NRLMSIS database. Especially noteworthy is the solar activity dependence of the Jacchia data, with which we study a large O+ contribution to the total mass density under the combination of summer, low solar activity, high latitude, and high altitude. Under these conditions, except at very low solar activity, the Jacchia data and the Jacchia-70 model indeed show a significantly higher total mass density than does MSISE-90. However, under the corresponding winter conditions, the MSIS-class models represent a noticeable improvement relative to Jacchia-70 over a wide range of F10.7. Considering the two regimes together, NRLMSISE-00 achieves an improvement over both MSISE-90 and Jacchia-70 by incorporating advantages of each.
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Very low earth orbit satellites enable researchers to find out about aeronomy, accurate gravity and magnetic field mapping, and high-resolution earth surveillance. They orbit the earth at an altitude of lower than 250 km, where the effect of atmospheric drag cannot be discounted. In order to use this orbit, some kind of propulsion for drag make-up is required and propellant mass increase proportionally to the mission time. The Air Breathing Ion Engine (ABIE) is a new type of electric propulsion system which can be used to compensate the drag of a satellite. In the ABIE propulsion system, the low density atmosphere surrounding the satellite is taken in and used as the propellant for the Electron Cyclotron Resonance (ECR) ion engines to reduce the required propellant mass. Therefore ABIE is a promising propulsion system for aerodynamic drag free missions longer than two years. Feasibility and performance of the ABIE depend on the compression ratio and an air intake efficiency. Generally, pressure of a discharge camber is lower than a propellant tank pressure in propulsion system, and the propellant flows to the reaction chambcr from the tank. In the case of ABIE, a static pressure of atmosphere which corresponds to tank pressure is lower than the discharge chamber pressure. The air intake is the most important component to realize the ABIE. The temperature of the atmosphere is from 700K to HOOK at 200km, which is sufficiently low compared with the orbital velocity of 8km/s. Therefore, it can be said it is a uniform and well collimated supersonic flow parallel to the orbital direction. Moreover, the density is thin enough and it is a free molecular flow. The air intake consists of a collimator section and a reflector section. The collimator section will be composed of gaps between concentric cylinders. This part does not intercept the entering neutral particles, and they impact the reflection part on the downstream side directly. However, the backflow from the discharge chamber to the upstream side through the collimator section cannot easily leak out, because it is thermalized to the same level of temperature as the chamber walls and it has a velocity in a random direction. We simulate the relation between the ABIE and the rarefied atmosphere on such a super low earth orbit in a vacuum chamber. We verified the pressure rise inside the air intake.
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The air-breathing ion engine (ABIE) is a new type of electric propulsion system to be used to compensate the aerodynamic drag of the satellite orbiting at extremely low altitudes. To save the propellant mass for a long operation lifetime, it inhales the low-density atmosphere surrounding the satellite and use it as the propellant of ion engines. Since feasibility and performance of the ABIE depend strongly on the compression ratio and the air-intake efficiency, numerical analysis has been performed by means of the direct-simulation Monte-Carlo method to clarify the characteristics of the air-intake performance in highly rarefied flows. Influences of the flight altitude, the aspect-ratio of the air-intake duct, and the angle of attack are investigated.
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To extend the lifetime of commercial and scientific satellites in low Earth orbit (LEO) and below (100–250 km of altitude) recent years showed an increased activity in the field of air-breathing electric propulsion as well as beamed-energy propulsion systems. However, preliminary studies showed that the propellant flow necessary for electrostatic propulsion at these altitudes exceeds the mass intake possible within reasonable limits, and that electrode erosion due to oxygen flow might limit the lifetime of eventual thruster systems. The pulsed plasma thruster (PPT), however, can be successfully operated with smaller mass intake and at relatively low power. This makes it an interesting candidate for air-breathing application in LEO and its feasibility is investigated within this paper. An analysis of such an air-breathing PPT system shows that for altitudes between 150 and 250 km, drag compensation is at least partially feasible assuming a thrust-to-power ratio of 30 mN/kW and a specific impulse of 5000 s. Further, to avoid electrode erosion, inductively heated electrothermal plasma generator technology is discussed to derive a possible propulsion system that can handle gaseous propellant without unfavorable side effects. Current technology can be used to create an estimated 4.4 mN of thrust per 1 mg/s of mass flow rate, which is sufficient to compensate the drag for small satellites in altitudes between 150 and 250 km.
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Plasma flows with high Knudsen numbers cannot be treated with classic continuum methods, as represented for example by the Navier–Stokes or the magnetohydrodynamic equations. Instead, the more fundamental Boltzmann equation has to be solved, which is done here approximately by particle based methods that also allow for thermal and chemical non-equilibrium. The Particle-In-Cell method is used to treat the collisionless Vlasov–Maxwell system, while neutral reactive flows are treated by the Direct Simulation Monte Carlo method. In this article, a combined approach is presented that allows the simulation of reactive, partially or fully ionized plasma flows. Both particle methods are briefly outlined and the coupling and parallelization strategies are described. As an example, the results of a streamer discharge simulation are presented and discussed in order to demonstrate the capabilities of the coupled method.
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A collisionless gas flows through the interior of a tube of circular cross-section, having both an entrance and an exit and which reflects molecules diffusely. Suitable complementary variational principles are used to obtain upper and lower bounds for the transmission probability of the tube. The numerical results, believed to be the most accurate to date, are compared with those of other authors.
Molecular Gas Dynamics and the Direct Simulation of Gas Flows. Clarendon, Oxford, 1994. 2 P. Clausing. ¨ Uber die strömung sehr verdünnter gase durch röhren von beliebiger länge Complementary variational principles for Knudsen flow rates
  • A Bird
  • R J Cole
A. Bird. Molecular Gas Dynamics and the Direct Simulation of Gas Flows. Clarendon, Oxford, 1994. 2 P. Clausing. ¨ Uber die strömung sehr verdünnter gase durch röhren von beliebiger länge. Annalen der Physik, 404(8):961– 989, 1932. 3 R. J. Cole. Complementary variational principles for Knudsen flow rates. IMA Journal of Applied Mathematics, pages 107–115, 1977.