Development of Chemically Augmented Electrothermal Thrusters
By Shoko SHIBAGAKI,1) Kai WADA,1) Mitsutoshi TSUCHIYA, 2) Yuka ARAI, 2) Tomoyuki IKEDA, 3)
and and Hideyuki HORISAWA4)
1)

2)

Department of Mechanical Engineering, Graduate student, Tokai University, Kanagawa, Japan
Department of Aeronautics and Astronautics, Undergraduate student, Tokai University, Kanagawa, Japan3)
Department of Aeronautics and Astronautics, Associate Professor, Tokai University, Kanagawa, Japan
4)
Department of Aeronautics and Astronautics, Professor, Tokai University, Kanagawa, Japan(Received June 21st, 2017)
In this study, an investigation and development of non-hazardous, low-cost, and stable liquid propellants including theirmixtures under room temperature for arcjet thrusters were conducted. These materials should be energetic producing chemical
exothermic energy when given an input energy density above certain thresholds, or namely only with electrical inputs(discharges). Numerical investigations based on chemical equilibrium calculations were conducted to understand the detailsof combustion reactions and products of the electrically augmented reactions for various liquids. In addition, a preliminaryexperimental study was conducted to evaluate thermal characteristics of the arcjet thruster employing novel water-based
liquid propellants. To evaluate various liquids, variations of measured thermal output ratios of exhaust plumes from the arcjets
with flow rates (0.3 ~ 1.0 mg/s) of various liquid propellants were compared. As a experimental results, HFE showed thehighest outputs throughout various flow rates followed by acetic acid-ethanol mixture. Moreover, slight increases of output
ratios with the increase of flow rate could also be seen. Although the output ratios of a pure argon propellant case was about
5.3%, those of HFE showed about 9 ~ 10%, 7.5 ~ 8.8% for acetic acid – ethanol mixture, and 6 ~ 7% for water.
Key Words: Electrothermal Thruster, Liquid Propellant, Chemical Equilibrium Caliculation

1.

Introduction

storable as a liquid phase and can be shared with chemicalthrusters. However, due to its relatively high freezing point of274 K, temperature management is still necessary for thestorage in space environment. Since hydrazine is very reactiveand toxic, the materials for tanks and valves must be compatible
to the chemical properties.
In this study, an investigation and development of nonhazardous, low-cost, and stable liquid propellants including
their mixtures under room temperature for arcjet thrusters wereconducted. These materials should be energetic producing
chemical exothermic energy when given an input energy
density above certain thresholds, or namely only with electricalinputs (discharges). Since the objective of this study is aninvestigation of exothermic characteristics from the nonexothermic, stable, non-hazardous materials, these novel
propellants must be “exotic propellants” with abnormally
exothermic characteristics. Numerical investigations based on
chemical equilibrium calculations were conducted tounderstand the details of combustion reactions and products ofthe electrically augmented reactions for various liquids. In
addition, a preliminary experimental study was conducted toevaluate thermal characteristics of the arcjet thruster employing
novel water-based liquid propellants.
To discover and investigate exothermal properties of waterbased liquid propellants from chemically stable liquids instandard condition, some liquids and their combinations wereexamined. Moreover, calculation of hydrazine was conducted
as a widely used, standard liquid propellant for comparison.
Figs. 1 and 2 show the results for pure water, pure acetic acid,water solution of acetic acid (60mol%), HFE

The current trend towards miniaturized satellites, which is
not only mass limited but also power limited, has produced astrong interest in development of micropropulsion devices. 1-4)
Although in the past, all of those miniaturized satellites havelacked propulsion systems altogether, future micro-spacecraft
will require significant propulsion capability to provide a highdegree of maneuverability and capability. The benefit of using
electric propulsion for the reduction of spacecraft mass will
likely be even more significant for mass limited microspacecraft missions.
Various potential micropropulsion systems for microspacecraft applications, such as ion thrusters, field emission
thrusters, pulsed plasma thrusters (PPTs), resistojets, arcjets,
etc., have been proposed and are under significant development
for primary and attitude control applications.
Through the use of an arc for direct heating of the propellant
stream to temperatures much higher than the wall temperatures,
DC arcjet thrusters overcome the gas temperature and specificimpulse limitation of the resistojet thrusters and can yield
relatively high thrust power ratio among other electric
propulsion systems. Regarding propellants, various kinds ofgas have been applied to arcjet thrusters. Although relatively
low molecular-mass gaseous propellants such as hydrogengenerate a relatively high specific impulse, a need for a highpressure tank and leakage problems through seals and valves
are inevitable. These issues can be major disadvantages for theuse of the propellant of this type for long-term missions 5). At
present, hydrazine is the most common propellant because it is1

(Hydrofluoroether), water solution of HFE (60mol%), purehydrogen peroxide, acetic acid and ethanol solution, andhydrazine.

temperature characteristics among them (i.e., HFE: 1,000 ~3,900 K, HFE solution: 1500 ~ 3500 K) for a wide range of
specific powers (1 ~ 40 kJ/g). Subsequently, hydrogen peroxide
follows (1100 ~ 3100 K).
Variations of specific impulses with specific power (orspecific energy) for water, acetic acid, acetic acid solution,HFE, HFE solution, hydrogen peroxide, acetic acid and ethanol
solution and hydrazine are plotted in Fig.2. Similar to reaction
temperature, specific impulses increase monotonically withspecific power in all propellant cases. In a wide range of
specific powers (5 ~ 40 kJ/g) except 1 kJ/g, hydrazine shows
the highest values (330 ~ 610 s), followed by HFE solution (320~ 560 s). However, at 1kJ/g, HFE solution shows higher value(280 s) than hydrazine (230 s).

2.

Experimental

In this study, to evaluate thermal characteristics of plasma
plumes from arcjet, a water-cooled arcjet with a convergentnozzle (without divergent nozzle) was employed and operated
in atmosphere, as shown in Fig.3.
Fig. 1. Variation of reaction temperatures vs specific powers
for liquids.

Fig. 3. Cross section of water-cooled arcjet.

Fig. 2. Variation of specific impulses vs specific powers forliquids.
Among them, hydrogen peroxide, acetic acid and ethanol
solution, HFE solution and HFE (Hydrofluoroether) showedinteresting characteristics.
Variations of reaction temperatures with specific power (orspecific energy) for water, acetic acid, acetic acid solution,HFE, HFE solution, hydrogen peroxide, acetic acid and ethanol
solution and hydrazine are plotted in Fig.4. In all propellants,
reaction temperatures increase monotonically with specificpower. Especially, HFE and HFE solution show highFig. 4. Schematic of DC power source.
As for the power source of the arcjet, a DC power source2

(maximum voltage 500 V, maximum current 40 A) and a highfrequency igniter, shown in Fig.4, were employed.
A schematic of experimental setup is shown in Fig.5. To
evaluate thermal characteristics of the exhaust plume of thearcjet, a calorimeter was developed and used in this experiment.

3.

Results and Discussion

Temporal variations of thermal outputs measured fromexhaust plumes of the arcjet for various water-based liquid
propellants are shown in Fig.7.
A typical output signal of temporal variation of input electrical
power of the arcjet in operation for argon propellant is shownin Fig.8.
In Fig.7, the flow rate of the liquid propellant added to theprimary propellant (argon gas) was controlled to a constantvalue of 0.3 mg/s for water, HFE, and a mixture of acetic acidand ethanol.
As shown in this figure, HFE showed the highest averagevalue of about 40 W, followed by a mixture (8:2) of acetic acidand ethanol (37 W at 250 s), and water (36 W at 250 s).

Fig. 5. Schematic of experimental setup.

Fig. 7. Results of thermal outputs from arcjet exhaust plumes
for various propellants.

Fig. 6. Schematic of calorimeter to measure evaluate thermal
characteristics of the plasma plume from the arcjet.

A schematic of the calorimeter is illustrated in Fig.6. As
shown in this figure, the conical calorimeter consists of ahelical tube made of aluminum. Differences of watertemperatures of outlet and inlet of the tube in the calorimeter,
which captures a plasma plume exhausted from the arcjet,
namely receiving a heat flux from the arcjet, were measured
with thermocouples and recorded with a data logger.
The heat received with the calorimeter was estimated as a
product of the difference of water temperature, specific heat ofwater, and a mass flow rate of water (0.3 g/s). The mass flowrate of the water was controlled with a mass flow controller.
For the propellant of the arcjet, argon gas of a flow rate of 8
slm was used as a standard, main propellant.
With the main propellant, liquid propellants, such as water,HFE, and acetic acid solution, were added with the flow rate of
0.3 to 1.0 mg/s.

Fig. 8. Temporal variation of input power of the arcjet inoperation.

Figure 9 shows variations of measured thermal output ratios
of exhaust plumes from the arcjets with flow rates (0.3 ~ 1.0mg/s) of various liquid propellants. The thermal output ratio isdefined as a ratio of an average thermal output to an electricalinput power of the arcjet. Plots in Fig.9 are average values of 33

runs of operation.
From the figure, it can be seen that results of HFE show thehighest outputs throughout various flow rates followed byacetic acid-ethanol mixture. Moreover, slight increases ofoutput ratios with the increase of flow rate can also be seen.
Although the output ratios of a pure argon propellant case isabout 5.3%, those of HFE show about 9 ~ 10%, 7.5 ~ 8.8% foracetic acid – ethanol mixture, and 6 ~ 7% for water.

1)

From the simulation, it was found that HFE and HFEsolution show high temperature characteristics among
them (i.e., HFE: 1,000 ~ 3,900 K, HFE solution: 1500 ~3500 K) for a wide range of specific powers (1 ~ 40 kJ/g).
Subsequently, hydrogen peroxide followed (1100 ~ 3100
K).
At 1kJ/g, HFE solution showed higher specific impulse
value (280 s) than hydrazine (230 s).
From the experiment, HFE showed the highest averagevalue of about 40 W, followed by a mixture (8:2) of acetic
acid and ethanol (37 W at 250 s), and water (36 W at 250s).
HFE showed the highest outputs throughout various flow
rates followed by acetic acid-ethanol mixture. Moreover,slight increases of output ratios with the increase of flowrate was also shown. Although the output ratios of a pureargon propellant case was about 5.3%, those of HFE
showed about 9 ~ 10%, 7.5 ~ 8.8% for acetic acid – ethanolmixture, and 6 ~ 7% for water.

2)
3)

4)

References1)

2)

Fig. 9. Variations of measured thermal output ratios of exhaust
plumes from the arcjets with flow rates of various liquid
propellants.

3)

4.

4)

Conslusion

In this study, numerical investigations based on chemical
equilibrium calculations were conducted to understand thedetails of combustion reactions and products of the electricallyaugmented reactions for various liquids. In addition, apreliminary experimental study was conducted to evaluate
thermal characteristics of the arcjet thruster employing novel
water-based liquid propellants. To evaluate thermal
characteristics of plasma plumes from arcjet, a water-cooled
arcjet with a convergent nozzle (without divergent nozzle) wasemployed and operated in atmosphere.

5)

6)

7)

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