Hybrid Rocket Engine Control by the Electrostatic Field

Abstract

Design of the effective hybrid rocket engines requires development of adequate control methods for the condensed phase regression rates. There are two traditional approaches: prescription with catalytic additives and geometrical with oxidizer flux involution or burning channel profiling. Other methods are significantly less investigated, including using various fields for the combustion process control. The paper presents some experimental results on the influence of electrostatic field on the combustion rate for the PMMA - gaseous oxygen hybrid fuel engine. It was shown that it is possible to control engine thrust with parameters of the control field. Some analysis and model explanations are performed.

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Journal of Engineering Science and Technology Review 11 (1) (2018) 146 - 150
Note
Hybrid Rocket Engine Control by the Electrostatic Field
Reshetnikov S.M.1, Zyryanov I.A.1, Budin A.G.1 and Reshetnikov I.S2
1Vyatka State University, Kirov, 610000, Russia
2OGSS Research Center, Moscow, 117630, Russia
Received 3 September 2017; Accepted 15 January 2018
____________________________________________________________________________________________
Abstract
Design of the effective hybrid rocket engines requires development of adequate control methods for the condensed phase
regression rates. There are two traditional approaches: prescription with catalytic additives and geometrical with oxidizer
flux involution or burning channel profiling. Other methods are significantly less investigated, including using various
fields for the combustion process control. The paper presents some experimental results on the influence of electrostatic
field on the combustion rate for the PMMA - gaseous oxygen hybrid fuel engine. It was shown that it is possible to
control engine thrust with parameters of the control field. Some analysis and model explanations are performed.
Keywords: Hybrid rocket engine, Electrostatic field, Combustion rate, Engine thrust
____________________________________________________________________________________________
1. Introduction
A Hybrid Rocket Engine (HRE) is a kind of the chemical
rocket engines where one component is solid while second
one is either gas or liquid [1,2]. Hybrid rocket engines are
positioned between liquid propellant engines and solid
propellant engines. Main advantages of HRE are their
simplicity, cost efficiency and safety. Usage of natural and
organic components such as paraffin and polyethylene as
solid phase propellant and oxygen (hydrogen peroxide and
others) as second component allows creation of ecology-
friendly engines.
However HREs have a significant shortcoming - low
component combustion rate for solid phase which leads to
insufficient combustion efficiency and low propulsion
power. Known methods for increasing combustion rate are
chiefly based on injection of catalysts, optimization of
channel geometry and using turbulent oxidizer streams [1-3].
These methods however do not allow controlling rate of
burn of fuel during engine operation without changing of
either oxidant usage rate or injection method of oxidizing
agent.
It is possible to influence combustion process by
applying an electric field. A possibility to control
combustion of liquid hydrocarbons and polymers with static
electric field was demonstrated in [4-6]: flame temperature
dependence on presence of electrostatic fields [5], change in
phase transition parameters [6], deformation of flame edge
[4, 5]. Application of electrostatic fields to control
combustion rate appears particularly promising because
maintenance of electrostatic fields does not require
additional energy consumption while field’s shape, direction
and intensity can be easily controlled.
The paper investigate influence of electrostatic field on a
combustion rate of a solid fuel component for hybrid rocket
engine based on polymethylmethacrylate (PMMA) – oxygen
propellant pair. Impact of the field on propulsion power is
also studied.
2. The laboratory HRE model
Investigation of electrostatic field effect on combustion
processes in HRE have been conducted using specially
designed laboratory HRE. The overview of the installation is
shown in Fig. 1. The engine uses PMMA block as the solid
fuel component and the gas oxygen as the oxidizer. Engine
consists of (Fig. 2):
• oxidizing agent injection chamber
• ignition system
• electrode system
• combustion chamber made from the replaceable
PMMA cylinder block
• afterburning chamber
• nozzle.
Oxygen is supplied via regulator with mass flow rate up
to 80 kg/m2s. Ignition is provided by the nichrome spiral
located at the base of PMMA block. Dimensions of fuel
block are 200 mm length and 50 mm height/width. A
cylindrical channel of 20 mm in diameter is running along
the central axis of the fuel block. Exhaust is directed to
subsonic nozzle with critical dimension of 13.6 mm.
Propulsion power is measured by strain gauge with accuracy
of 1g.
Main feature of this engine setup is a possibility to apply
an electric field to the combustion area using various
configuration of electrodes. This paper presents data on the
case of “coaxial capacitor” where positive electrode (2 mm
in diameter) was located in the center along the channel in
the PMMA block and metal grid located on the outer side of
the block was used as negative electrode (50 mm in
diameter). To avoid electric contact with the flame the
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Reshetnikov S.M., Zyryanov I.A., Budin A.G. and Reshetnikov I.S/Journal of Engineering Science and Technology Review 11 (1) (2018) 146-150
147
positive electrode was placed inside the 5 mm diameter
quartz tube. In order to preserve quartz isolation properties
from destruction at high temperatures the tube was
constantly cooled by nitrogen flow supplied at 0.01 kg/m2s
rate. Voltage difference across electrodes in range of 35V-
35kV was maintained using high voltage source HCP35-
35000. Because leakage current between electrodes was
measured to be below 0.1uA electric field within the
chamber is static.
Fig. 1. The laboratory HRE installation
Fig. 2. HRE scheme
Measurement of the linear combustion rate was done
using two methods. The first approach was based on the
transparency of the PMMA block which allowed video
recording of the burning process. Linear combustion rate
was derived by checking markings along the fuel block.
Average linear rate is obtained after averaging over the
block’s length. In the second approach the average linear
combustion rate is derived from difference in weight of fuel
block before and after the experiment and duration of the
burning of the fuel [7].
3. Experimental results
Experimental investigations have been conducted at the
oxygen flow rates ranging between 15 and 80 kg/m2s.
Results for the time averaged combustion rates u, mm/s vs.
linear channel coordinate l, mm are shown in Fig. 3,
where subfigure a) corresponds to the absence of the
external field and b) corresponds to the applied external field
160 kV/m on the phase boundary. Oxygen flow rate was
equal to 23 kg/m2s. Horizontal lines show the average
combustion rates for the first (solid line) and second (dashed
line) methods.
a) b)
Fig 3. Average combustion rates vs. length of burnt channel of fuel block (explanation in the text)
Fig. 4 presents results on the dependence of the engine
thrust P, N vs. the time for various oxygen flows in the
absence (blue marks) and presence (red marks) of the
electric field. It can be seen that for all conditions
application of the field leads to the increment of the pulling
force up to 30%.

Reshetnikov S.M., Zyryanov I.A., Budin A.G. and Reshetnikov I.S/Journal of Engineering Science and Technology Review 11 (1) (2018) 146-150
148
Fig. 4. Propulsion power vs time for various oxidizer flows: a) 20 kg/m2s b) 23 kg/m2s c) 47 kg/m2s d) 51 kg/m2s
Numerical results are summarized in Tab. 1, where E
- intensity on the phase boundary, kV/m, 𝜌υ - oxidizer
mass flow rate, kg/m2s, α - coefficient of oxidizer excess,
Re - Reynolds number on the entrance of the PMMA block,
M - mass of burned PMMA, g, t - engine activity time, s.
Table 1. Experimental results
No
Е, kV/m
𝜌
υ, kg/m2s
α
Re
M, g
t, s
u, mm/s
P, N
1
0
15
1.97
17900
101
42
0.142
8.1
2
23
2.45
26600
121
42
0.171
15.1
3
31
2.61
35000
146
41
0.211
23.0
4
39
2.98
44900
144
36
0.237
30.1
5
47
3.27
53300
121
28
0.256
34.0
6
51
3.18
58900
123
25
0.291
39.8
7
61
3.66
69800
76
15
0.300
52.0
8
160
15
1.82
17900
117
45
0.154
9.5
9
23
1.90
26600
160
43
0.200
17.5
10
39
2.58
44900
120
26
0.273
31.5
11
47
2.84
53300
164
33
0.294
40.0
12
51
2.93
58900
112
21
0.315
43.3
13
266
20
1.78
23000
111
32
0.205
17.5
14
31
2.13
35000
96
22
0.258
28.1
15
47
2.60
53300
125
23
0.322
43.2
16
61
2.78
69800
193
29
0.394
60.0
On the base of the measured data it can be concluded that
application of the electrostatic field with intensity of 160 and
266 kV/m on the boundary between solid and gas
components (phase boundary) leads to increase in linear
combustion rates for the PMMA block of 14 and 31%
respectively. For HRE the correlation between the linear
combustion rate and the mass oxidizer flow is defined by the
combustion law [1,2]
u=A(
ρυ
)v
(1)
where A and v are constant parameters.
In examined cases combustion laws were u =
0.029(𝜌υ)0.57 at E=0 kV/m, u = 0.029(𝜌υ)0.57 at E=166 kV/m,
u = 0.038(𝜌υ)0.56 at 266 kV/m. In other words, increase in
the rate of combustion is reflected in the increase of the
coefficient “A” before the (𝜌υ) in formula (1), while the
constancy of exponent value v shows that the combustion
regime remains “mixed” and does not change for the
duration of experiment.
The increase of combustion rate in the presence of
electrostatic field increases engine thrust as well. Fig. 5
shows dependence of the averaged thrust vs. flow rate of
oxidizer and demonstrate expected increase due to influence

Reshetnikov S.M., Zyryanov I.A., Budin A.G. and Reshetnikov I.S/Journal of Engineering Science and Technology Review 11 (1) (2018) 146-150
149
of electrostatic field. Error in measurement of engine thrust
is less than 5%.
Fig. 5. Thrust vs oxidizer mass flow rate
To summarize, in presence of electrostatic field engine
thrust was increased on average by 12% when field’s
intensity on the phase boundary was 160 kV/m and by 21%
when exposed to the field with intensity of 266 kV/m.
4. Discussion
Main mechanism of the electric field influence on the flame
is effect of mass forces, which may be similar to the “ionic
wind”, i.e. formation of charged particles flowing in the
direction to oppositely charged electrode which in turn pulls
neutral particles into the flow [8-10]. It is also known that
the PMMA flame contains a surplus of positive particles [4].
The configuration of electrodes used in this work is such that
charged particles must move closer to the combustion
surface thus increasing the heat flux to the condensed phase
and, as a consequence, combustion rate. Previously it has
been shown [5] that the electric field in the flame zone
increases heat flux up to 30%. But at the same time rising of
the combustion rate leads to the change of flow from the
surface of products of pyrolysis reaction. This flow is
usually taken into account via the blow-in parameter B [11].
Calculation based on the standard method [11] predicts that
the flame front will depart from the surface. This, in turn,
decreases the heat flux to the surface. Thus increase of the
heat flux due to mass forces effect is compensated by the
change in blow-n parameter. Mass forces mechanism cannot
be used solely to explain observed effects.
It was shown [12] that electric field impacts both
kinetics and mechanism of thermal degradation of polymers.
Visual examination of the reaction layer of PMAA surface
during combustion showed that the surface is covered with
the cylindrical cavities 30-50 um deep and 10-15 um in
diameter. The total amount, allocation and distribution of
these cavities remain constant during the experiment. The
presence of the electric field initiates process of cavity
formation due to the decrease in amount of work required
for its formation [13]. It was observed experimentally: the
field with the intensity E = 160 kV/m results in the increase
of cavity concentration to 11%, with E = 266 kV/m to 32%.
Because cavity formation is the result of the bulk nature of
the PMMA pyrolysis, the electric field intensifies this
process.
Fig. 6. Normalized dependence of cavities concentration and
combustion rate vs field tense
There is a direct relationship between the concentration
of cavities N, m-2 and the linear combustion rate (Fig. 6). It
can be seen from the figure, the dependence in first
approximation can be considered as linear and can be
described as 1st order equations:
! ΔN/N0=
α
E
(2)
! Δu/u0=
β
E
(3)
where correlation parameters are α = 1,02·10-6 m/V and β =
0,99·10-6 m/V. Taking into account experimental error (5%)
these parameters can be considered to be equal. Thus
increase of the linear combustion rate is determined by the
cavity concentration in the surface layer, which, in turn,
increases proportionally to the intensity of the applied
electrostatic field.
5. Conclusion
This paper shows that an external electric field applied
between electrodes can be used to control combustion rate
and propulsion power of HREs. Our experiments
demonstrated an increase of propulsion power of up to 30%
using E = 266 kV/m field. The electrostatic field mainly
intensifies thermal degradation of condensed component of
PMMA – oxygen fuel system in HRE.
This is an Open Access article distributed under the terms of the
Creative Commons Attribution Licence
______________________________
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