Project

# DISCOVERER - VLEO satellites for EO

Goal: The vision of the DISCOVERER project is a radical redesign of Earth observation (EO) satellites for sustained operation at much lower altitudes than the current state of the art.

www.discoverer.space

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 737183. This reflects only the author’s view and the European Commission is not responsible for any use that may be made of the information it contains.

Date: 1 January 2017 - 31 March 2021

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## Project log

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Challenging space mission scenarios include those in very low Earth orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere through an intake and uses it as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow drag compensation for an unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing costs. Preliminary studies have shown that the collectible propellant flow for an ion thruster (in LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster's lifetime. In this paper we introduce the use of an inductive plasma thruster (IPT) as thruster for the ABEP system as well as the assessment of this technology against its major competitors in VLEO (electrical and chemical propulsion). IPT is based on a small scale inductively heated plasma generator IPG6-S. These devices have the advantage of being electrodeless, and have already shown high electric-to-thermal coupling efficiencies using O2 and CO2 as propellant. A water cooled nozzle has been developed and applied to IPG6-S. The system analysis is integrated with IPG6-S equipped with the nozzle for testing to assess mean mass-specific energies of the plasma plume and estimate exhaust velocities.
We present the validation of ADBSat, a novel implementation of the panel method including a fast pseudo-shading algorithm, that can quickly and accurately determine the forces and torques on satellites in free-molecular flow. Our main method of validation is comparing test cases between ADBSat, the current de facto standard of direct simulation Monte Carlo (DSMC), and published literature. ADBSat broadly performs well, except where deep concavities are present in the satellite models. The shading algorithm also experiences problems when a large proportion of the satellite surface area is oriented parallel to the flow, but this can be mitigated by examining the body at small angles to this configuration (${\pm}$ 0.1{\deg}). We determine the error interval on ADBSat outputs to be 1-3% whilst exhibiting a significantly shorter runtime than comparable methods. ADBSat can therefore be used as a viable alternative to DSMC for preliminary design studies involving a wide range of geometries and cases. It can also be used in a complementary manner to identify cases that warrant further investigation using methods such as DSMC. Thus, it is an ideal tool for determining the aerodynamic characteristics of future missions to VLEO.
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Challenging space missions include those at very low altitudes, where the atmosphere is the source of aerodynamic drag on the spacecraft, that finally defines the mission’s lifetime, unless a way to compensate for it is provided. This environment is named Very Low Earth Orbit (VLEO) and it is defined for h < 450km . In addition to the spacecraft’s aerodynamic design, to extend the lifetime of such missions, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP), in which the propulsion system collects the atmospheric particles to be used as propellant for an electric thruster. The system could remove the requirement of carrying propellant on-board, and could also be applied to any planetary body with atmosphere, enabling new missions at low altitude ranges for longer missions’ duration. One of the objectives of the H2020 DISCOVERER project, is the development of an intake and an electrode-less plasma thruster for an ABEP system. This article describes the characteristics of intake design and the respective final designs based on simulations, providing collection efficiencies up to 94%. Furthermore, the radio frequency (RF) Helicon-based plasma thruster (IPT) is hereby presented as well, while its performances are being evaluated, the IPT has been operated with single atmospheric species as propellant, and has highlighted very low input power requirement for operation at comparable mass flow rates P ∼ 60 W.
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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.
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We present the validation of ADBSat, a novel implementation of the panel method including a fast pseudo-shading algorithm, that can quickly and accurately determine the forces and torques on satellites in free-molecular flow. Our main method of validation is comparing test cases between ADBSat, the current de facto standard of direct simulation Monte Carlo (DSMC), and published literature. ADBSat exhibits a significantly shorter runtime than DSMC and performs well, except where deep concavities are present in the satellite models. The shading algorithm also experiences problems when a large proportion of the satellite surface area is oriented parallel to the flow, but this can be mitigated by examining the body at small angles to this configuration (± 0.1°). We recommend that an error interval on ADBSat outputs of up to 3% is adopted. Therefore, ADBSat is a suitable tool for quickly determining the aerodynamic characteristics of a wide range of satellite geometries in different environmental conditions in VLEO. It can also be used in a complementary manner to identify cases that warrant further investigation using other numerical-based methods.
This paper discusses the design and the performance achievable with active aerodynamic attitude control in very low Earth orbit, i.e. below 450 km in altitude. A novel real-time algorithm is proposed for selecting the angles of deflection of aerodynamic actuators providing the closest match to the control signal computed by a selected control law. The algorithm is based on a panel method for the computation of the aerodynamic coefficients and relies on approximate environmental parameters estimation and worst-case scenario assumptions for the re-emission properties of space materials. Discussion of results is performed by assuming two representative pointing manoeuvres, for which momentum wheels and aerodynamic actuators are used synergistically. A quaternion feedback PID controller implemented in discrete time is assumed to determine the control signal at a sampling frequency of 1 Hz. The outcome of a Monte Carlo analysis, performed for a wide range of orbital conditions, shows that the target attitude is successfully achieved for the vast majority of the cases, thus proving the robustness of the approach in the presence of environmental uncertainties and realistic attitude hardware limitations.
The Satellite for Orbital Aerodynamics Research (SOAR) is a 3U CubeSat mission that aims to investigate the gas–surface interactions (GSIs) of different materials in the very low Earth orbit environment (VLEO), i.e. below 450 km. Improving the understanding of these interactions is critical for the development of satellites that can operate sustainably at these lower orbital altitudes, with particular application to future Earth observation and communications missions. SOAR has been designed to perform the characterisation of the aerodynamic coefficients of four different materials at different angles of incidence with respect to the flow and at different altitudes in the VLEO altitude range. Two conventional and erosion-resistant materials (borosilicate glass and sputter-coated gold) have first been selected to support the validation of the ground-based Rarefied Orbital Aerodynamics Research (ROAR) facility. Two further, novel materials have been selected for their potential to reduce the drag experienced in orbit whilst also remaining resistant to the detrimental effects of atomic oxygen erosion in VLEO. In this paper, the uncertainty associated with the experimental method for determining the aerodynamic coefficients of satellite with different configurations of the test materials from on-orbit data is estimated for different assumed gas–surface interaction properties. The presented results indicate that for reducing surface accommodation coefficients the experimental uncertainty on the drag coefficient determination generally increases, a result of increased aerodynamic attitude perturbations. This effect is also exacerbated by the high atmospheric density at low orbital altitude (i.e. 200 km), resulting in high experimental uncertainty. Co-rotated steerable fin configurations are shown to provide generally lower experimental uncertainty than counter-rotated configurations, with the lowest uncertainties expected in the mid-VLEO altitudes ( $$\sim$$ ∼ 300 km). For drag coefficient experiments, configurations with two fins oriented at 90 $$^{\circ }$$ ∘ were found to allow the best differentiation between surfaces with different GSI performance. In comparison, the determination of the lift coefficient is found to be improve as the altitude is reduced from 400 to 200 km. These experiments were also found to show the best expected performance in determining the GSI properties of different materials. SOAR was deployed into an orbit of 421 km $$\times$$ × 415 km with 51.6 $$^{\circ }$$ ∘ inclination on 14 June 2021. This orbit will naturally decay allowing access to different altitudes over the lifetime of the mission. The results presented in this paper will be used to plan the experimental schedule for this mission and to maximise the scientific output.
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Operating satellites in Very Low Earth Orbit (VLEO) benefit the already expanding New Space industry in applications including Earth Observation and beyond. However, long-term operations at such low altitudes require propulsion systems to compensate for the large aerodynamic drag forces. When using conventional propulsion systems, the amount of storable propellant limits the maximum mission lifetime. The latter can be avoided by employing Atmosphere-Breathing Electric Propulsion (ABEP) system, which collects the residual atmospheric particles and uses them as propellant for an electric thruster. Thus, the requirement of on-board propellant storage can ideally be nullified. At the Institute of Space Systems (IRS) of the University of Stuttgart, an intake, and a RF Helicon-based Plasma Thruster (IPT) for ABEP system are developed within the Horizons 2020 funded DISCOVERER project. To assess possible future use cases, this paper proposes and analyzes several novel ABEP-based mission scenarios. Beginning with technology demonstration mission in VLEO, more complex mission scenarios are derived and discussed in detail. These include, amongst others, orbit maintenance around Mars as well as refuelling and space tug missions. The results show that the ABEP system is not only able to compensate drag for orbit maintenance but also capable of performing orbital maneuvers and collect propellant for applications such as Space Tug and Refuelling. Thus, showing a multitude of different future mission applications.
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This paper discusses the design and the performance achievable with active aerodynamic attitude control in very low Earth orbit, i.e., below 450 km in altitude. A novel real-time algorithm is proposed for selecting the angles of deflection of aerodynamic actuators providing the closest match to the control signal computed by a selected control law. The algorithm is based on a panel method for the computation of the aerodynamic coefficients and relies on approximate environmental parameters estimation and worst-case scenario assumptions for the re-emission properties of space materials. Discussion of results is performed by assuming two representative pointing maneuvers, for which momentum wheels and aerodynamic actuators are used synergistically. A quaternion feedback proportional-integral-derivative (PID) controller implemented in discrete time is assumed to determine the control signal at a sampling frequency of 1 Hz. The outcome of a Monte Carlo analysis, performed for a wide range of orbital conditions, shows that the target attitude is successfully achieved for the vast majority of the cases, thus proving the robustness of the approach in the presence of environmental uncertainties and realistic attitude hardware limitations.
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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.
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The Satellite for Orbital Aerodynamics Research (SOAR) is a 3U CubeSat that has been designed to investigate the aerodynamic performance of different materials at low orbital altitudes. The spacecraft has been developed within the scope of DISCOVERER, a Horizon 2020 project that aims to develop foundational technologies to enable sustainable operations of Earth observation spacecraft in very low Earth orbits (VLEO) i.e., those below 450 km. SOAR features two payloads: i) a set of steerable fins that can expose different materials to the oncoming atmospheric flow developed by The University of Manchester, and ii) a forward-facing ion and neutral mass spectrometer (INMS) that provides in-situ measurements of the atmospheric density, flow composition, and velocity from the Mullard Space Science Laboratory (MSSL) of University College London. These payloads enable characterisation of the aerodynamic performance of different materials at very low altitudes with the aim to advance understanding of the underlying gas-surface interactions in rarefied flow environments. The satellite will also be used to test novel aerodynamic attitude control methods and perform atmospheric characterisation in the VLEO altitude range. SOAR will perform the first in-orbit test of two novel materials that are expected to have atomic oxygen erosion resistance and drag-reducing properties, providing valuable in-orbit validation data for ongoing ground-based experimentation. Such materials hold the promise for extending operations at lower altitudes with benefits particularly for Earth observation and communications satellites that can correspondingly be reduced in size and cost. The platform for SOAR is largely based on GOMX-3 heritage and the spacecraft was assembled, integrated, and tested by GomSpace A/S. The satellite was launched on the SpX-22 commercial resupply service mission to the International Space Station in on 3 rd June 2021 was subsequently deployed into orbit on the 14 th June 2021. This paper presents the final preparations of SOAR prior to launch and provides an overview of the planned operations of the spacecraft following deployment into orbit.
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The operation of satellites in very low Earth orbit (VLEO) has been linked to a variety of benefits to both the spacecraft platform and mission design. Critically, for Earth observation (EO) missions a reduction in altitude can enable smaller and less powerful payloads to achieve the same performance as larger instruments or sensors at higher altitude, with significant benefits to the spacecraft design. As a result, renewed interest in the exploitation of these orbits has spurred the development of new technologies that have the potential to enable sustainable operations in this lower altitude range. In this paper, system models are developed for (i) novel materials that improve aerodynamic performance enabling reduced drag or increased lift production and resistance to atomic oxygen erosion and (ii) atmosphere-breathing electric propulsion (ABEP) for sustained drag compensation or mitigation in VLEO. Attitude and orbit control methods that can take advantage of the aerodynamic forces and torques in VLEO are also discussed. These system models are integrated into a framework for concept-level satellite design and this approach is used to explore the system-level trade-offs for future EO spacecraft enabled by these new technologies. A case-study presented for an optical very-high resolution spacecraft demonstrates the significant potential of reducing orbital altitude using these technologies and indicates possible savings of up to 75% in system mass and over 50% in development and manufacturing costs in comparison to current state-of-the-art missions. For a synthetic aperture radar (SAR) satellite, the reduction in mass and cost with altitude were shown to be smaller, though it was noted that currently available cost models do not capture recent commercial advancements in this segment...
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Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft. To extend the lifetime of such missions, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP) that collects atmospheric particles to be used as propellant for an electric thruster. The system would minimize the requirement of limited propellant availability and can also be applied to any planetary body with atmosphere, enabling new missions at low altitude ranges for longer times. IRS is developing, within the H2020 DISCOVERER project, an intake and a thruster for an ABEP system. The article describes the design and simulation of the intake, optimized to feed the radio frequency (RF) Helicon-based plasma thruster developed at IRS. The article deals in particular with the design of intakes based on diffuse and specular reflecting materials, which are analysed by the PICLas DSMC-PIC tool. Orbital altitudes $h=150-250$ km and the respective species based on the NRLMSISE-00 model (O, $N_2$, $O_2$, He, Ar, H, N) are investigated for several concepts based on fully diffuse and specular scattering, including hybrid designs. The major focus has been on the intake efficiency defined as $\eta_c=\dot{N}_{out}/\dot{N}_{in}$, with $\dot{N}_{in}$ the incoming particle flux, and $\dot{N}_{out}$ the one collected by the intake. Finally, two concepts are selected and presented providing the best expected performance for the operation with the selected thruster. The first one is based on fully diffuse accommodation yielding to $\eta_c<0.46$ and the second one based un fully specular accommodation yielding to $\eta_c<0.94$. Finally, also the influence of misalignment with the flow is analysed, highlighting a strong dependence of $\eta_c$ in the diffuse-based intake while, ...
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Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft. To extend the lifetime of such missions, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP) that collects atmospheric particles to be used as propellant for an electric thruster. The system would minimize the requirement of limited propellant availability and can also be applied to any planetary body with atmosphere, enabling new missions at low altitude ranges for longer times. IRS is developing, within the H2020 DISCOVERER project, an intake and a thruster for an ABEP system. The article describes the design and simulation of the intake, optimized to feed the radio frequency (RF) Helicon-based plasma thruster developed at IRS. The article deals in particular with the design of intakes based on diffuse and specular reflecting materials, which are analysed by the PICLas DSMC-PIC tool. Orbital altitudes h=150−250km and the respective species based on the NRLMSISE-00 model (O, N2, O2, He, Ar, H, N) are investigated for several concepts based on fully diffuse and specular scattering, including hybrid designs. The major focus has been on the intake efficiency defined as ηc=Ṅout∕Ṅin, with Ṅin the incoming particle flux, and Ṅout the one collected by the intake. Finally, two concepts are selected and presented providing the best expected performance for the operation with the selected thruster. The first one is based on fully diffuse accommodation yielding to ηc<0.46 and the second one based on fully specular accommodation yielding to ηc<0.94. Finally, also the influence of misalignment with the flow is analysed, highlighting a strong dependence of ηc in the diffuse-based intake while, for the specular-based intake, this is much lower finally leading to a more resilient design while also relaxing requirements of pointing accuracy for the spacecraft.
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The Satellite for Orbital Aerodynamics Research (SOAR) is a CubeSat mission, due to be launched in 2021, to investigate the interaction between different materials and the atmospheric flow regime in very low Earth orbits (VLEO). Improving knowledge of the gas-surface interactions at these altitudes and identification of novel materials that can minimise drag or improve aerodynamic control are important for the design of future spacecraft that can operate in lower altitude orbits. Such satellites may be smaller and cheaper to develop or can provide improved Earth observation data or communications link-budgets and latency. Using precise orbit and attitude determination information and the measured atmospheric flow characteristics the forces and torques experienced by the satellite in orbit can be studied and estimates of the aerodynamic coefficients calculated. This paper presents the scientific concept and design of the SOAR mission. The methodology for recovery of the aerodynamic coefficients from the measured orbit, attitude, and in-situ atmospheric data using a least-squares orbit determination and free-parameter fitting process is described and the experimental uncertainty of the resolved aerodynamic coefficients is estimated. The presented results indicate that the combination of the satellite design and experimental methodology are capable of clearly illustrating the variation of drag and lift coefficient for differing surface incidence angle. The lowest uncertainties for the drag coefficient measurement are found at approximately 300 km, whilst the measurement of lift coefficient improves for reducing orbital altitude to 200 km.
The Satellite for Orbital Aerodynamics Research (SOAR) is a CubeSat mission, due to be launched in 2021, to investigate the interaction between different materials and the atmospheric flow regime in very low Earth orbits (VLEO). Improving knowledge of the gas-surface interactions at these altitudes and identification of novel materials that can minimise drag or improve aerodynamic control are important for the design of future spacecraft that can operate in lower altitude orbits. Such satellites may be smaller and cheaper to develop or can provide improved Earth observation data or communications link-budgets and latency. In order to achieve these objectives, SOAR features two payloads: (i) a set of steerable fins which provide the ability to expose different materials or surface finishes to the oncoming flow with varying angle of incidence whilst also providing variable geometry to investigate aerostability and aerodynamic control; and (ii) an ion and neutral mass spectrometer with time-of-flight capability which enables accurate measurement of the in-situ flow composition, density, velocity. Using precise orbit and attitude determination information and the measured atmospheric flow characteristics the forces and torques experienced by the satellite in orbit can be studied and estimates of the aerodynamic coefficients calculated. This paper presents the scientific concept and design of the SOAR mission. The methodology for recovery of the aerodynamic coefficients from the measured orbit, attitude, and in-situ atmospheric data using a least-squares orbit determination and free-parameter fitting process is described and the experimental uncertainty of the resolved aerodynamic coefficients is estimated. The presented results indicate that the combination of the satellite design and experimental methodology are capable of clearly illustrating the variation of drag and lift coefficient for differing surface incidence angle. The lowest uncertainties for the drag coefficient measurement are found at approximately 300 km, whilst the measurement of lift coefficient improves for reducing orbital altitude to 200 km.
\\ Updated preprint available at: https://arxiv.org/abs/2010.00489 \\ Renewed interest in Very Low Earth Orbits (VLEO) - i.e. altitudes below 450 km - has led to an increased demand for accurate environment characterisation and aerodynamic force prediction. While the former requires knowledge of the mechanisms that drive density variations in the thermosphere, the latter also depends on the interactions between the gas-particles in the residual atmosphere and the surfaces exposed to the flow. The determination of the aerodynamic coefficients is hindered by the numerous uncertainties that characterise the physical processes occurring at the exposed surfaces. Several models have been produced over the last 60 years with the intent of combining accuracy with relatively simple implementations. In this paper the most popular models have been selected and reviewed using as discriminating factors relevance with regards to orbital aerodynamics applications and theoretical agreement with gas-beam experimental data. More sophisticated models were neglected, since their increased accuracy is generally accompanied by a substantial increase in computation times which is likely to be unsuitable for most space engineering applications. For the sake of clarity, a distinction was introduced between physical and scattering kernel theory based gas-surface interaction models. The physical model category comprises the Hard Cube model, the Soft Cube model and the Washboard model, while the scattering kernel family consists of the Maxwell model, the Nocilla-Hurlbut-Sherman model and the Cercignani-Lampis-Lord model. Limits and assets of each model have been discussed with regards to the context of this paper. Wherever possible, comments have been provided to help the reader to identify possible future challenges for gas-surface interaction science with regards to orbital aerodynamic applications.
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To extend missions lifetime at very low orbits (VLO), an efficient propulsion system is required for drag compensation. Atmosphere-Breathing Electric Propulsion (ABEP) is a concept that collects atmospheric particles to be used as propellant for an electric thruster. The system could nullify the onboard propellant storage requirements. Moreover, it can be applied to any celestial body with atmosphere (Mars, Venus, Titan, etc.), enabling novel missions at VLO for long periods. Challenging is the operation with atmospheric propellant, especially atomic oxygen (AO), highly present in Earth orbit, that causes erosion of (not only) major propulsion system components. The RF helicon-based plasma thruster is designed, built, and set-into operation. It is contactless and features a novel antenna called the birdcage antenna, derived from heritage of magnetic resonance imaging (MRI) machines. A static magnetic field is applied as 1) required boundary condition for the helicon wave formation in the plasma, and 2) provides further electromagnetic acceleration of the plasma. The first tests on Ar, N2, and O2 show a low input power requirement Pf ∼ 60W, easy ignition, and minimized reflected power Pr. Finally, a B-dot magnetic inductive probe is designed and integrated to verify the presence of helicon waves in the plasma plume.
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Renewed interest in Very Low Earth Orbits (VLEO) - i.e. altitudes below 450 km - has led to an increased demand for accurate environment characterisation and aerodynamic force prediction. While the former requires knowledge of the mechanisms that drive density variations in the thermosphere, the latter also depends on the interactions between the gas-particles in the residual atmosphere and the surfaces exposed to the flow. The determination of the aerodynamic coefficients is hindered by the numerous uncertainties that characterise the physical processes occurring at the exposed surfaces. Several models have been produced over the last 60 years with the intent of combining accuracy with relatively simple implementations. In this paper the most popular models have been selected and reviewed using as discriminating factors relevance with regards to orbital aerodynamics applications and theoretical agreement with gas-beam experimental data. More sophisticated models were neglected, since their increased accuracy is generally accompanied by a substantial increase in computation times which is likely to be unsuitable for most space engineering applications. For the sake of clarity, a distinction was introduced between physical and scattering kernel theory based gas-surface interaction models. The physical model category comprises the Hard Cube model, the Soft Cube model and the Washboard model, while the scattering kernel family consists of the Maxwell model, the Nocilla-Hurlbut-Sherman model and the Cercignani-Lampis-Lord model. Limits and assets of each model have been discussed with regards to the context of this paper. Wherever possible, comments have been provided to help the reader to identify possible future challenges for gas-surface interaction science with regards to orbital aerodynamic applications.
The RF Helicon-based plasma thruster (IPT) has been successfully operated operated on pure Argon, Nitrogen and Oxygen, showing very low power consumption and reflected power!

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The DISCOVERER project is developing technologies to enable commercially-viable sustained-operation of satellites in very low Earth orbits for communications and remote sensing applications. Operating closer to the surface of the Earth significantly reduces latency for communications applications and improves link budgets, whilst remote sensing also benefits from improved link budgets, the ability to have higher resolution or smaller instruments, all of which provide cost benefits. In addition, all applications benefit from increased launch mass to lower altitudes, whilst end-of-life removal is ensured due to the increased atmospheric drag. However, this drag must also be minimised and compensated for. DISCOVERER is developing several critical technologies to enable commercially-viable operations in at these lower altitudes including aerodynamic materials, aerodynamic attitude and orbit control methods, atmosphere breathing electric propulsion and an in-situ environment monitoring payload. The current status of these developments are summarised, along with the plans for the coming year.
Interest in operating spacecraft in very low Earth orbits (VLEO), those below approximately 450 km, is growing due to the numerous benefits offered by reducing altitude. For remote sensing and Earth observation applications, improvements in resolution can be achieved or smaller instruments used with associated benefits in cost or mission value. Similarly, for communications applications, link-budgets and data latency can be improved by reducing the operational altitude. However, a key challenge to sustained operations in lower altitude orbits is to minimise and compensate for the aerodynamic drag that is produced by the interaction with the residual atmosphere. A principal aim of the DISCOVERER project is to identify, develop, and characterise materials that can promote specular reflections of the residual atmosphere in VLEO whilst also remaining resistant to the erosive atomic oxygen that is predominant at these altitudes. In combination with geometric design, such materials would be able to reduce the aerodynamic drag experienced by satellites in orbit and would also be able to generate usable aerodynamic lift enabling novel aerodynamic attitude and orbit control. SOAR (Satellite for Orbital Aerodynamics Research) is a 3U CubeSat that has been designed to investigate the aerodynamic performance of different materials in the VLEO environment and provide validation data for further ground-based experiments. To achieve this, the spacecraft features a set of steerable fins that can expose different materials to the oncoming atmospheric flow. A forward-facing ion and neutral mass spectrometer (INMS) provides in-situ measurements of the atmospheric density and flow composition. SOAR is scheduled for launch to the ISS in March 2021. This paper will present the design of the spacecraft, the experimental method that will be used to investigate the aerodynamic properties of materials in orbit, and will provide an update on the status of the spacecraft as it prepares for launch.
In very low Earth orbits (VLEO), below 450 km altitude, the aerodynamic properties of satellites are primarily determined by the flow regime, free molecular flow, and the interaction of atomic oxygen with the surfaces of the spacecraft. The Rarefied Orbital Aerodynamics Research (ROAR) facility is a novel experimental facility designed to simulate these conditions in a controlled environment to characterise the aerodynamic properties of materials. It is built as part of DISCOVERER, a Horizon 2020 project developing the different technologies required to enable the sustainable operation of satellites in VLEO. Because ROAR isn't intended to perform erosion studies, it differs quite significantly from other atomic oxygen exposure experiments and its characteristics are discussed in this work. ROAR consists of an ultrahigh vacuum system, responsible for generating the free molecular flow conditions, a source of hyperthermal oxygen atoms at orbital velocities, and mass spectrometers; the latter used to characterise the gas-surface interactions, and therefore the material's aerodynamic performance. This paper includes a description of ROAR's main components, together with the experimental methodology for materials testing and early results. Among the main parameters to be considered are atomic oxygen flux, beam shape and energy spread, mass resolution, and signal-to-noise ratio.
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To extend missions lifetime at very low altitudes, an efficient propulsion system is required to compensate for aerodynamic drag. One solution is Atmosphere-Breathing Electric Propulsion (ABEP). It collects atmospheric particles to be used as propellant for an electric thruster. The system ideally nullifies the requirement of onboard propellant storage. An ABEP system can be applied to any celestial body with atmosphere (Mars, Venus, Titan, etc.), enabling new mission at low altitude ranges for longer times. Challenging is operation of the thruster on reactive chemical species, such as atomic oxygen (AO), that is highly present in low Earth orbit, as they cause erosion of (not only) propulsion system components, i.e. acceleration grids, electrodes, neutralizers, and discharge channels of conventional EP systems. For this reason, a contactless plasma thruster is developed: the RF helicon-based plasma thruster (IPT). The paper describes the thruster design, implementation, and first ignition tests. The thruster presents a novel antenna called the birdcage antenna that is implemented for decades in magnetic resonance imaging (MRI) machines. The design is supported by the simulation tool XFdtd ®. The IPT is aided by an externally applied static magnetic field that provides the boundary condition for the helicon wave formation within the plasma discharge. The antenna working principle allows to minimize losses in the electric circuit and provides, together with the applied magnetic field, acceleration of a quasi-neutral plasma plume.
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To extend lifetime of commercial and scientific satellites in LEO and below (100-250 km of altitude) the 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 exceeds the mass intake possible within reasonable limits, and that electrode erosion due to oxygen flow might limit the lifetime of eventual thruster systems. Pulsed plasma thruster can be successfully operated with smaller mass intake, and operate at relatively small power demands which makes them an interesting candidate for air-breathing application in LEO, and their feasibility is investigated within this study. Further, to avoid electrode erosion, inductive plasma generator technology is discussed to derive a possible propulsion system that can handle gaseous propellant with no harmful effects. Nomenclature E = discharge energy per pulse F D = drag force imposed on satellite f = discharge frequency h = orbital altitude m bit = mass shot per pulse n = number density t = orbital lifetime
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Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft. To extend such missions lifetime, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP). It collects atmospheric particles to be used as propellant for an electric thruster. The system would minimize the requirement of limited propellant availability and can also be applied to any planet with atmosphere, enabling new mission at low altitude ranges for longer times. Challenging is also the presence of reactive chemical species, such as atomic oxygen in Earth orbit. Such species cause erosion of (not only) propulsion system components, i.e. acceleration grids, electrodes, and discharge channels of conventional EP systems. IRS is developing within the DISCOVERER project, an intake and a thruster for an ABEP system. The paper describes the design and implementation of the RF helicon-based inductive plasma thruster (IPT). This paper deals in particular with the design and implementation of a novel antenna called the birdcage antenna, a device well known in magnetic resonance imaging (MRI), and also lately employed for helicon-wave based plasma sources in fusion research. The IPT is based on RF electrodeless operation aided by an externally applied static magnetic field. The IPT is composed by an antenna, a discharge channel, a movable injector, and a solenoid. By changing the operational parameters along with the novel antenna design, the aim is to minimize losses in the RF circuit, and accelerate a quasi-neutral plasma plume. This is also to be aided by the formation of helicon waves within the plasma that are to improve the overall efficiency and achieve higher exhaust velocities. Finally, the designed IPT with a particular focus on the birdcage antenna design procedure is presented
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Very low Earth orbits (VLEO), typically classified as orbits below approximately 450 km in altitude, have the potential to provide significant benefits to spacecraft over those that operate in higher altitude orbits. This paper provides a comprehensive review and analysis of these benefits to spacecraft operations in VLEO, with parametric investigation of those which apply specifically to Earth observation missions. The most significant benefit for optical imaging systems is that a reduction in orbital altitude improves spatial resolution for a similar payload specification. Alternatively mass and volume savings can be made whilst maintaining a given performance. Similarly, for radar and lidar systems, the signal-to-noise ratio can be improved. Additional benefits include improved geospatial position accuracy, improvements in communications link-budgets, and greater launch vehicle insertion capability. The collision risk with orbital debris and radiation environment can be shown to be improved in lower altitude orbits, whilst compliance with IADC guidelines for spacecraft post-mission lifetime and deorbit is also assisted. Finally, VLEO offers opportunities to exploit novel atmosphere-breathing electric propulsion systems and aerodynamic attitude and orbit control methods. However, key challenges associated with our understanding of the lower thermosphere, aerodynamic drag, the requirement to provide a meaningful orbital lifetime whilst minimising spacecraft mass and complexity, and atomic oxygen erosion still require further research. Given the scope for significant commercial, societal, and environmental impact which can be realised with higher performing Earth observation platforms, renewed research efforts to address the challenges associated with VLEO operations are required.
added a research item
Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft. To extend such missions lifetime, an efficient propulsion system is required. One solution is Atmosphere-Breathing Electric Propulsion (ABEP). It collects atmospheric particles to be used as propellant for an electric thruster. The system would minimize the requirement of limited propellant availability and can also be applied to any planet with atmosphere, enabling new mission at low altitude ranges for longer times. Challenging is also the presence of reactive chemical species, such as atomic oxygen in Earth orbit. Such species cause erosion of (not only) propulsion system components, i.e. acceleration grids, electrodes, and discharge channels of conventional EP systems. IRS is developing within the DISCOVERER project, an intake and a thruster for an ABEP system. The paper describes the design and implementation of the RF helicon-based inductive plasma thruster (IPT). This paper deals in particular with the design and implementation of a novel antenna called the birdcage antenna, a device well known in magnetic resonance imaging (MRI), and also lately employed for helicon-wave based plasma sources in fusion research. This is aided by the numerical tool XFdtd®. The IPT is based on RF electrodeless operation aided by an externally applied static magnetic field. The IPT is composed by an antenna, a discharge channel, a movable injector, and a solenoid. By changing the operational parameters along with the novel antenna design, the aim is to minimize losses in the RF circuit, and accelerate a quasi-neutral plasma plume. This is also to be aided by the formation of helicon waves within the plasma that are to improve the overall efficiency and achieve higher exhaust velocities. Finally, the designed IPT with a particular focus on the birdcage antenna design procedure is presented.
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This paper analyses the use of aerodynamic control surfaces, whether passive or active, in order to carry out very low Earth orbit (VLEO) attitude maneuver operations. Flying a satellite in a very low Earth orbit with an altitude of less than 450 km, namely VLEO, is a technological challenge. It leads to several advantages, such as increasing the resolution of optical payloads or increase signal to noise ratio, among others. The atmospheric density in VLEO is much higher than in typical low earth orbit altitudes, but still free molecular flow. This has serious consequences for the maneuverability of a satellite because significant aerodynamic torques and forces are produced. In order to guarantee the controllability of the spacecraft they have to be analyzed in depth. Moreover, at VLEO the density of atomic oxygen increases, which enables the use of air-breathing electric propulsion (ABEP). Scientists are researching in this field to use ABEP as a drag compensation system, and consequently an attitude control based on aerodynamic control could make sense. This combination of technologies may represent an opportunity to open new markets. In this work, several satellite geometric configurations were considered to analyze aerodynamic control: 3-axis control with feather configuration and 2-axis control with shuttlecock configuration. The analysis was performed by simulating the attitude of the satellite as well as the disturbances affecting the spacecraft. The models implemented to simulate the disturbances were the following: Gravitational gradient torque disturbance, magnetic dipole torque disturbance (magnetic field model IGRF12), and aerodynamic torque disturbances (aerodynamic model DTM2013 and wind model HWM14). The maneuvers analyzed were the following: detumbling or attitude stabilization, pointing and demisability. Different VLEO parameters were analyzed for every geometric configuration and spacecraft maneuver. The results determined which of the analyzed geometric configurations suits better for every maneuver.
added a research item
Very low Earth orbits (VLEO), typically classified as orbits below 450 km to 500 km in altitude, have the potential to provide significant benefits to spacecraft over those that operate in higher altitude orbits. This paper provides a comprehensive review and analysis of these benefits to spacecraft operations in VLEO, with parametric investigation of those which apply specifically to Earth observation missions. The most significant benefit for optical imaging systems is that a reduction in orbital altitude improves spatial resolution for a similar payload specification. Alternatively mass and volume savings can be made whilst maintaining a given performance. Similarly, for radar and lidar systems, the signal-to-noise ratio can be improved. Additional benefits include improved geospatial position accuracy, improvements in communications link-budgets, and greater launch vehicle insertion capability. The collision risk with orbital debris and radiation environment can be shown to be improved in lower altitude orbits, whilst compliance with IADC guidelines for spacecraft post-mission lifetime and deorbit is also assisted. Finally, VLEO offers opportunities to exploit novel atmosphere-breathing electric propulsion systems and aerodynamic attitude and orbit control methods. However, key challenges associated with our understanding of the lower thermosphere, aerodynamic drag, the requirement to provide a meaningful orbital lifetime whilst minimising spacecraft mass and complexity, and atomic oxygen erosion still require further research. Given the scope for significant commercial, societal, and environmental impact which can be realised with higher performing Earth observation platforms, renewed research efforts to address the challenges associated with VLEO operations are required.
The Institute of Space Systems (IRS) of the University of Stuttgart has achieved within the H2020 EU DISCOVERER project the first ignition of the RF helicon-wave based Inductive Plasma Thruster (IPT)!
The thruster is the first of its kind: it is based on a RF-fed cylindrical birdcage antenna and is to produce helicon-waves.
The electromagnetic fields created by the antenna together with the helicon waves, provide both ionization and acceleration, removing the requirement of having accelerating grids and, most of all, the need of a neutralizer, as both ions and electrons accelerated together generating a quasi-neutral plume.
It can run on any propellant, the first tests run on Nitrogen (in the attached picture) and Argon.
The next step will be its optimisation and characterisation by means of advanced plasma diagnostics.
The IPT is the thruster for the Atmosphere-Breathing Electric Propulsion (ABEP) system that is developed within the DISCOVERER project. Propellant flexibility, contact less characteristics, and no neutralizer required are fundamental to cope with aggressive propellants such as atomic oxygen in the VLEO altitude range.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 737183. This reflects only the author’s view and the European Commission is not responsible for any use that may be made of the information it contains.

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After decades of traditional space businesses, the space paradigm is changing. New approaches to more efficient missions in terms of costs, design, and manufac-turing processes are fostered. For instance, placing big constellations of micro- and nano-satellites in Low Earth Orbit and Very Low Earth Orbit (LEO and VLEO) enables the space community to obtain a huge amount of data in near real-time with an unprecedented temporal resolution. Beyond technology innovations, other drivers promote innovation in the space sector like the increasing demand for Earth Observation (EO) data by the commercial sector. Perez et al. stated that the EO industry is the second market in terms of operative satellites (661 units), micro- and nano-satellites being the higher share of them (61%). Technological and market drivers encourage the emergence of new start-ups in the space environ-ment like Skybox, OneWeb, Telesat, Planet, and OpenCosmos, among others, with novel business models that change the accessibility, affordability, ownership, and commercialization of space products and services. This chapter shows some results of the H2020 DISCOVERER (DISruptive teChnOlogies for VERy low Earth oRbit platforms) Project and focuses on understanding how micro- and nano-satellites have been disrupting the EO market in front of traditional platforms .
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This paper analyses the use of aerodynamic control surfaces, whether passive or active, in order to carry out very low Earth orbit (VLEO) attitude maneuver operations. Flying a satellite in a very low Earth orbit with an altitude of less than 450 km, namely VLEO, is a technological challenge. It leads to several advantages, such as increasing the resolution of optical payloads or increase signal to noise ratio, among others. The atmospheric density in VLEO is much higher than in typical low earth orbit altitudes, but still free molecular flow. This has serious consequences for the maneuverability of a satellite because significant aerodynamic torques and forces are produced. In order to guarantee the controllability of the spacecraft they have to be analyzed in depth. Moreover, at VLEO the density of atomic oxygen increases, which enables the use of air-breathing propulsion (ABEP). Scientists are researching in this field to use ABEP it as a drag compensation system, and consequently an attitude control based on aerodynamic control could make sense. This combination of technologies may represent an opportunity to open new markets. In this work, several satellite geometric configurations were considered to analyze aerodynamic control:3 axis control with feather configuration and 2 axis control with shuttlecock configuration. The analysis was performed by simulating the attitude of the satellite as well as the disturbances affecting the spacecraft. The models implemented to simulate the disturbances were the following: Gravitational gradient torque disturbance, magnetic dipole torque disturbance (magnetic field model IGRF12), and aerodynamic torque disturbances (aerodynamic model DTM2013 and wind model HWM14). The maneuvers analyzed were the following: detumbling or attitude stabilization, pointing and demisability. Different VLEO parameters were analyzed for every geometric configuration and spacecraft maneuver. The results determined which of the analyzed geometric configurations suits better for every maneuver. This work is part of the H2020 DISCOVERER project. Project ID 737183.
Very Low Earth Orbits (VLEO) gathered interest due to the advantages of flying in lower altitudes, such as higher signal to noise ratio in the communications, possible reduction in the size, mass and cost of imaging payloads, less space debris in the orbits or lower propagation delay, among others. However, at these altitudes the aerodynamic forces and torques become the predominant disturbances and it must be considered in the design of the spacecraft. In this work atmospheric, magnetic and wind models were implemented in Xcos blocks to calculate the disturbances that affect the spacecraft and a panel method was implemented to study the aerodynamics with different geometries. The results of pointing maneuvers and attitude stabilization simulations comparing feather and shuttlecock geometries are presented. The models implemented in C and Scilab were used to create Xcos blocks that will be part of a toolbox.
IPT has been integrated at IRS facilities, the IPT laboratory model has passed vacuum testing! A lot of exciting work and results are ahead of us!
Happy new year!

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The operation of IPG6-S mitigates and promotes the following aspects: 1. Identification, development of needed test routines and procedures 2. Identification of relevant and needed plasma diagnostics and measurement techniques 3. Verification for the currently being ignited IRS-IPT. IPG6-S e.g. has been operated with a DC electromagnet that enhanced the power coupling by a factor of three. This hints to wave modes (Helicon) and/or power enhancement by inductance change, see S. Masillo et al.
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DISCOVERER is a European Commission funded project aiming to revolutionise satellite applications in Very Low Earth Orbits (VLEO). The project encompasses many different aspects of the requirements for sustainable operation, including developments on geometric designs, aerodynamic attitude and orbital control, improvement of intake designs for atmosphere breathing electric propulsion, commercial viability, and development of novel materials. This paper is focused solely on the description of the experimental facility designed and constructed to perform ground testing of materials, characterising their behaviour in conditions similar to those found in VLEO. ROAR, Rarefied Orbital Aerodynamics Research facility, is an experiment designed to provide a controlled environment with free molecular flow and atomic oxygen flux comparable to the real orbital environment. ROAR is a novel experiment, with the objective of providing better and deeper understanding of the gas-surface interactions between the material and the atmosphere, rather than other atomic oxygen exposure facilities which are mainly focused on erosion studies. The system is comprised of three major parts, (i) ultrahigh vacuum setup, (ii) hyperthermal oxygen atom generator (HOAG) and (iii) ion-neutral mass spectrometers (INMS). Each individual part will be considered, their performance analysed based on experimental data acquired during the characterisation and commissioning, thus leading to a complete description of ROAR's capabilities. Among the key parameters to be discussed are operational pressure, atomic oxygen flux, beam shape and energy spread, mass resolution, signal-to-noise ratio and experimental methodology.
Spacecraft operations below 450km, namely Very Low Earth Orbit (VLEO), can offer significant advantages over traditional low Earth orbits, for example enhanced ground resolution for Earth observation, improved communications latency and link budget, or improved signal-to-noise ratio. Recently, these lower orbits have begun to be exploited as a result of technology development, particularly component miniaturisation and cost-reduction, and concerns over the increasing debris population in commercially exploited orbits. However, the high cost of orbital launch and challenges associated with atmospheric drag, causing orbital decay and eventually re-entry are still a key barrier to their wider use for large commercial and civil spacecraft. Efforts to address the impact of aerodynamic drag are being sought through the development of novel drag-compensation propulsion systems and identification of materials which can reduce aerodynamic drag by specularly reflecting the incident gas. However, the presence of aerodynamic forces can also be utilised to augment or improve spacecraft operations at these very low altitudes by providing the capability to perform coarse pointing control and trim or internal momentum management for example. This paper presents concepts for the advantageous use of spacecraft aerodynamics developed as part of DISCOVERER, a Horizon 2020 funded project with the aim to revolutionise Earth observation satellite operations in VLEO. The combination of novel spacecraft geometries and use of aerodynamic control methods are explored, demonstrating the potential for a new generation of Earth observation satellites operating at lower altitudes.
Aerodynamic forces have often been proposed as a possible means to perform a variety of different attitude and orbit control manoeuvres in very low Earth orbits including pointing control, constellation and formation management, and re-entry interface targeting. However, despite interest and numerous studies conducted in this area there is has been lack of on-orbit demonstration of these manoeuvres beyond simple proof of aerostability and some operational use of differential drag for constellation maintenance. SOAR (Satellite for Orbital Aerodynamics Research) is a CubeSat mission and part of DISCOVERER, a Horizon 2020 funded project to develop technologies to enable sustained operation of Earth observation satellites in very Low Earth Orbits. SOAR is due to be launched in 2020 with the primary aim to investigate the interaction between different materials and the atmospheric flow regime in very low Earth orbits. This satellite, with its set of rotating aerodynamic fins, also offers the unique opportunity to demonstrate and test novel aerodynamic control methods in the very-low Earth orbit (VLEO) environment. This paper presents the approach to demonstrate novel aerodynamic control methods in-orbit that will be used on the experimental SOAR Cubesat. The aerodynamic manoeuvres and associated control methods selected for demonstration are first described. Simulations of the aerodynamic control manoeuvres and expected satellite dynamic behaviour are also presented, demonstrating potential advantages for spacecraft operations which can be achieved by utilising the natural aerodynamic forces present at these lower orbital altitudes.
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Flying a satellite at very low earth orbit is a technological challenge. It presents advantages, such as increase resolution in optical payloads, reduce costs of launch and enhance the use of air breathing propulsion and specular materials. The density of the atmosphere at these altitudes is much higher, behaving as a free molecular flow. This has severe implications in the increase of drag torques and forces that has to be analyzed in depth. We analyze the effects and the perturbations to small satellites, affecting their dynamics, performance and lifetime by implementing and analyzing realistic models of the environment at VLEO.
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Challenging space missions include those at very low altitudes, where the atmosphere is source of aerodynamic drag on the spacecraft, therefore an efficient propulsion system is required to extend the mission lifetime. One solution is Atmosphere-Breathing Electric Propulsion (ABEP). It collects atmospheric particles to use as propellant for an electric thruster. This would minimize the requirement of limited propellant availability. The system could be applied to any planet with atmosphere, enabling new mission at these altitude ranges for continuous orbiting. Challenging is also the presence of reactive chemical species, such as atomic oxygen in Earth orbit. Such components are erosion source of (not only) propulsion system components, i.e. acceleration grids, electrodes, and discharge channels of conventional EP systems (RIT and HET). IRS is developing within the DISCOVERER project an intake and a thruster for an ABEP system. This paper deals with the design and first operation of the inductive plasma thruster (IPT) developed at IRS. The paper describes its design aided by numerical tools such as HELIC and ADAMANT. Such a device is based on RF electrodeless discharge aided by externally applied static magnetic field. The IPT is composed by a movable injector, to variate the discharge channel length, and a movable electromagnet to variate position and intensity of the magnetic field. By changing these parameters along with a novel antenna design for electric propulsion, the aim is to achieve the highest efficiency for the ionization stage by enabling the formation of helicon-based discharge. Finally, the designed IPT is presented and the feature of the birdcage antenna highlighted.
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Challenging space missions include those at very low orbits, where the atmosphere is source of significant drag on a satellite. Therefore, an efficient drag-compensation propulsion system is required to extend the mission lifetime. One solution is Atmosphere-Breathing Electric Propulsion (ABEP), a system that collects atmospheric particles and directly uses them as propellant for an electric thruster, therefore minimizing the requirement of limited propellant availability. The system is theoretically applicable to any celestial body with atmosphere. This would enable new mission types due to the new altitude ranges available for continuous orbiting. Challenging is also the presence of reactive chemical species, such as atomic oxygen in Earth orbit, erosion source of (not only) the propulsion system components, i.e. acceleration grids, electrodes and discharge channels of conventional EP systems such as RIT and HET. IRS is developing within the DISCOVERER project an intake and a thruster for an ABEP system. This paper, deals with the design of novel contact-less RF thruster, the inductive plasma thruster (IPT) based on a novel antenna design.
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DISCOVERER is a €5.7M, 4 1/4 year Horizon 2020 funded project which aims to radically redesign Earth observation satellites for sustained operation at significantly lower altitudes. The satellite based Earth observation/remote sensing market is one of the success stories of the space industry, having seen significant growth in size and applications in recent times. According to Euroconsult, the EO data market from commercial and government operators, such as from data distributors, is expected to double to $3 billion in 2025 from an estimate of$1.7 billion in 2015. Yet key design parameters for the satellites which provide the data for this market have remained largely unchanged, most noticeably the orbit altitude. Operating satellites at lower altitudes allows them to be smaller, less massive, and less expensive whilst achieving the same or even better resolution and data products than current platforms. However, at reduced orbital altitude the residual atmosphere produces drag which decreases the orbital lifetime. Aerodynamic perturbations also challenge the ability of the platform to remain stable, affecting image quality. DISCOVERER intends to overcome these challenges by carrying out foundational research in the aerodynamic characterisation of materials, in atmosphere-breathing electric propulsion for drag-compensation, and in active aerodynamic control methods. A subset of the technologies developed will also be tested on an in-orbit demonstration CubeSat. In order to put these foundational developments in context, DISCOVERER will also develop advanced engineering, commercial, and economic models of Earth observation systems which include these newly identified technologies. This will allow the optimum satellite designs for return on investment to be identified. DISCOVERER will also develop roadmaps defining the on-going activities needed to commercialise these new technologies and make Earth observation platforms in these very low Earth orbits a reality.
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Challenging space mission scenarios include those in low altitude orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere particles through an intake and uses them as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow to orbit for unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing costs. Preliminary studies have shown that the collectible propellant flow for an ion thruster (in LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster lifetime. In this paper an inductive plasma thruster (IPT) is considered for the ABEP system. The starting point is a small scale inductively heated plasma generator IPG6-S. These devices are electrodeless and have already shown high electric-to-thermal coupling efficiencies using O2 and CO2. The system analysis is integrated with IPG6-S tests to assess mean mass-specific energies of the plasma plume and estimate exhaust velocities.
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For a satellite formation to maintain its intended design despite present perturbations (formation keeping), to change the formation design (reconfiguration) or to perform a rendezvous maneuver, control forces need to be generated. To do so, chemical and/or electric thrusters are currently the methods of choice. However, their utilization has detrimental effects on small satellites’ limited mass, volume and power budgets. Since the mid-80s, the potential of using differential drag as a means of propellant-less source of control for satellite formation flight is actively researched. This method consists of varying the aerodynamic drag experienced by different spacecraft, thus generating differential accelerations between them. Its main disadvantage, that its controllability is mainly limited to the in-plain relative motion, can be overcome using differential lift as a means to control the out-of-plane motion. Due to its promising benefits, a variety of studies from researchers around the world have enhanced the state-of-the-art over the past decades which results in a multitude of available literature. In this paper, an extensive literature review of the efforts which led to the current state-of-the-art of different lift and drag-based satellite formation control is presented. Based on the insights gained during the review process, key knowledge gaps that need to be addressed in the field of differential lift to enhance the current state-of-the-art are revealed and discussed. In closer detail, the interdependence between the feasibility domain/the maneuver time and increased differential lift forces achieved using advanced satellite surface materials promoting quasi-specular or specular reflection, as currently being developed in the course of the DISCOVERER project, is discussed.
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Orbiting at lower orbital altitudes, where the residual atmosphere is source of aerodynamic drag requires an efficient drag-compensation system for satellite lifetime extension. One solution is proposed by using Atmosphere-Breathing Electric Propulsion (ABEP), a system that collects atmospheric particles and directly uses them as propellant for an electric thruster. Challenging is also the presence of reactive chemical species at low altitudes, such as atomic oxygen. This is an erosion source of (not only) the propulsion system components, i.e. acceleration grids, electrodes and discharge channels of conventional EP systems such as Radio frequency ion thrusters (RIT) and Hall-effect thrusters (HET). The thruster for an ABEP is proposed to be an Inductive Plasma Thruster (IPT) based on an electrodeless design. Hereby the first step is an efficient plasma source working on atmospheric propellant. Starting from IPG6-S as test-bed, a small scale inductively heated plasma generator at IRS [1]-[3], the mechanisms of RF power absorption by plasma in the low-pressure inductive discharges are analysed numerically and experimentally. The application of a relatively low external magnetic field is reported to enhance plasma density and power transfer efficiency [4]. Performances of both magnetized and unmagnetized plasma source, in terms of plasma resistance and density, are evaluated for different frequencies, input power, magnetic field intensity, pressure, temperature, plasma density profile, discharge channel and antenna dimensions. Investigations on plasma parameters such as its resistance RP and the absorbed power, are based on numerical simulations and supported by theoretical and experimental results. In particular, the application of a magnetic field is foreseen to improve the coupling by increasing both RP and absorbed power. A preliminary design of the plasma source for the IPT, currently under development, is also presented.
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Challenging space mission scenarios include those in very low Earth orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere through an intake and uses it as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow drag compensation for an unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing the required effort for the launcher by achieving these low orbits. Preliminary studies have shown that the collectible propellant flow for an ion thruster (in LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster's lifetime. In this paper we present the advances on the design of an inductive plasma thruster (IPT) for the ABEP. The IPT is based on a small-scale inductively heated plasma generator IPG6-S. IPG have the advantage of being electrodeless, and have already shown high electric-to-thermal coupling efficiencies using O2 and CO2 as propellant. IPG6-S requires a scaling of the discharge channel to meet with power requirement and expected collected mass flows, as well as optimisation of the accelerating stage, to provide the required thrust to the spacecraft. Tests have been performed to verify some of the parameters and are as well presented within this paper.
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Using several small, unconnected satellites flying in formation rather than a single monolithic satellite has many advantages. As an example, separate optical systems can be combined to function as a single larger (synthetic) aperture. When the aperture is synthesized, the independent optical systems are phased to form a common image field with its resolution determined by the maximum dimension of the array. Hence, a formation is capable of much finer resolution than it could be accomplished by any single element. In order for the formation to maintain its intended design despite present perturbations (formation keeping), to perform rendezvous maneuvers or to change the formation design (reconfiguration) control forces need to be generated. To this day, using chemical and/or electric thrusters are the methods of choice. However, their utilization has detrimental effects on small satellites’ limited mass, volume and power budgets. In the mid-eighties, Caroline Lee Leonard published her pioneering work [1] proving the potential of using differential drag as a means of propellant-less source of control for satellite formation flight. This method consists of varying the aerodynamic drag experienced by different spacecraft, thus generating differential accelerations between them. Since its control authority is limited to the in-plane motion, Horsley [2] proposed to use differential lift as a means to control the out-of-plane motion. Due to its promising benefits, a variety of studies from researches around the world have enhanced Leonard’s work over past decades which results in a multitude of available literature. Besides giving an introduction into the method the major contributions of this paper is twofold: first, an extensive literature review of the major contributions which led to the current state-of-the-art of different lift and drag based satellite formation control is presented. Second, based on these insights key knowledge gaps that need to be addressed in order to enhance the current state-of-the-art are revealed and discussed. In closer detail, the interdependence between the feasibility domain and advanced satellite surface materials as well as the necessity of robust control methods able to cope with the occurring uncertainties is assessed.
We want to revolutionize Earth observation by operating satellites at much lower altitudes than usual. Our satellites will be smaller, lighter and cheaper to launch while achieving a better resolution than today’s devices.
Our main research questions:
1) How can we make use of the residual atmosphere for aerodynamic control of the platform?
2) Can we make use of the residual atmosphere as propellant for drag compensation and mission lifetime extension?
3) Are there aerospace materials that may reduce induced drag on spacecraft surfaces?
4) Can we develop new-disruptive system and business modelling for new stakeholders in the Earth Observation market?
European Union’s Horizon 2020 research and innovation programme under grant agreement No 737183

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An atmosphere-breathing electric propulsion system (ABEP) [1] captures the residual atmosphere of a planet and uses it as propellant for an electric thruster to counteract the drag. The system would theoretically allow orbiting for unlimited time without on-board propellant storage. A new range of altitudes, e.g. 120-250 km in Earth orbit, the Very-Low Earth orbit (VLEO), for permanent orbiting can be accessed, thereby enabling new scientific missions. ABEP can be conceptually applied to any planet with atmosphere. IRS has several decades of heritage on the development of inductively heated plasma generators (IPG) [2] [3]. Such devices are electrodeless, removing the issue of electrode erosion that reduces performance over time (see RIT, HET). Aggressive gases such as O as propellant, highly present in VLEO, will cause even faster erosion. IRS is currently developing an inductive plasma thruster (IPT) for ABEP application within the H2020 DISCOVERER project [4].
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Challenging space mission scenarios include those in low altitude orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere through an intake and uses it as propellant for an electric thruster that compensates the drag. Theoretically applicable to any planet with atmosphere, the system might allow to orbit for an unlimited period without carrying propellant on-board. IRS has several decades of heritage on the development of inductively heated plasma generators (IPG). Such devices are electrodeless, therefore issues of potential electrode erosion are eliminated. This paper deals with the complete refurbishment of a facility that was previously used for RIT testing, for the use of IPG6-S, a small scale IPG with an input power up to 3.5 kW. This facility allows more reliable test conditions. First operational and performance tests of IPG6-S have been performed. IPG6-S serves as test bed for the development of an inductive plasma thruster (IPT) for ABEP application. A newly designed water-cooled de Laval nozzle has been built and applied to IPG6-S. The nozzle is modular, it has the possibility of having various configurations so to assess its performance in terms of plasma acceleration and thrust production. Within this paper plasma plume energy has been measured by means of a cavity calorimeter and correlated to current, power, and pressure in the injector head.
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Challenging space mission scenarios include those in very low Earth orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere through an intake and uses it as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow drag compensation for an unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing costs. Preliminary studies have shown that the collectible propellant flow for an ion thruster (in LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster's lifetime. In this paper we introduce the use of an inductive plasma thruster (IPT) as thruster for the ABEP system as well as the assessment of this technology against its major competitors in VLEO (electrical and chemical propulsion). IPT is based on a small scale inductively heated plasma generator IPG6-S. These devices have the advantage of being electrodeless, and have already shown high electric-to-thermal coupling efficiencies using O2 and CO2 as propellant. A water cooled nozzle has been developed and applied to IPG6-S. The system analysis is integrated with IPG6-S equipped with the nozzle for testing to assess mean mass-specific energies of the plasma plume and estimate exhaust velocities.
added a research item
This paper describes the approach of the authors in creating a new space systems design course for Aeronautics students in Catalonia. Exploration roadmaps provide interest and relevance, and so the course development principle was to associate material with actual projects and real ideas, rather than textbooks. In particular, the NASA and ESA roadmaps proved to be an excellent source for examining a number of disciplines, and joining them together in a consistent way. The course concluded with a team project, which with the help of agency professionals, generated a small incremental step in mission definition for Venus exploration. - directly linked to the Solar System Exploration Roadmap and the VEXAG white paper. This paper presents the course, the links to exploration, a description of the process, a summary of the team project conclusions, and additional examples of the ‘strategy of teaching strategies’. Preprint
added a project goal
The vision of the DISCOVERER project is a radical redesign of Earth observation (EO) satellites for sustained operation at much lower altitudes than the current state of the art.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 737183. This reflects only the author’s view and the European Commission is not responsible for any use that may be made of the information it contains.