Evaluation of Nontoxic Liquid Propellants for Chemically-augmented
Electrothermal Thrusters
Mitsutoshi Tsuchiya1), Yuka Arai1) , Nobuhito Ohnishi2), Ryuta Futada2), Hideyuki Horisawa3)
1)
Graduate student of Mechanical Engineering, Tokai University, Japan
Department of Aerospace and Astronautics Engineering Tokai University, Japan3)
Department of Aerospace and Astronautics Engineering , Professor Tokai University, Japan
2)

In this study, an investigation and development of non-hazardous, low-cost, and stable water-based liquid propellants including theirmixtures under room temperature were conducted for propellant of arcjet thrusters. From the results of numerical simulation, it wasshown that HFE and HFE solution showed high temperature characteristics (i.e., HFE: 1,000-3,900 K, HFE solution: 1500-3500 K) fora wide range of specific powers (1-40 kJ/g). Subsequently, hydrogen peroxide followed (1100-3100 K). In a wide range of specificpowers (1-5, 20-40 kJ/g) except 10 kJ/g, HFE solution showed the highest values (260-350, 500-710 s), followed by hydrazine (230-330,490-610 s). However, at 10 kJ/g, hydrazine showed higher value (410 s) than HFE solution (400 s). In addition, a preliminary
investigation of operational tests of arcjet thruster was conducted employing various liquid propellants.

Key Words : Low power arcjet thruster, Exothermic propellant, Liquid propellant, Non-hazardous propellant

1.

Introductionliquefied simply by adjusting pressure or temperature.
They demonstrated that the DME arcjet thruster wascomparable to the conventional thruster in discharge
voltage, power and plenum chamber pressure. Yanagidaet al. proposed a mixture of aluminum (either solid orpowder) and water7). Since the combustion reaction of themixture can release a very high energy, a thruster using
this propellant can either be a chemically augmented
arcjet thruster or an electrically augmented chemicalthruster, in which much higher thrust performances canbe expected than conventional chemical thrusters or eventhan hydrazine arcjets.
In this study, an investigation and development of
non-hazardous, low-cost, and stable liquid propellants
including their mixtures under room temperature forarcjet thrusters were conducted. These materials shouldbe energetic producing chemical exothermic energy
when given input energy density above certain thresholds,
or namely only with electrical inputs (discharges). Since
the objective of this study is an investigation of
exothermic characteristics from the non-exothermic,
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 andproducts of the electrically augmented reactions forvarious liquids. In addition, a preliminary experimental
study was conducted to evaluate thrust characteristics ofthe arcjet thruster employing novel water-based liquidpropellants.

Although propulsion systems on the majority ofspacecraft to date have consisted of chemical thrusters,
an increasing number of spacecraft launched recently use
arcjet thrusters as a high-performance alternative fornorth-south station keeping (NSSK) of geosynchronous
satellites1-4). With an arc for direct heating of thepropellant stream to temperatures much higher than thewall temperatures, DC arcjet thrusters overcome the gas
temperature and specific impulse limitation of theresistojet thrusters and can yield relatively high thrust
power ratio among other electric propulsion systems.
Regarding propellants, various kinds of gas have been
applied to arcjet thrusters. Although relatively low
molecular-mass gaseous propellants such as hydrogengenerate a relatively high specific impulse, a need for ahigh-pressure tank and leakage problems through seals
and valves are inevitable. These issues can be major
disadvantages for the use of the propellant of this type forlong-term missions 5). At present, hydrazine is the mostcommon propellant because it is storable as a liquid phaseand can be shared with chemical thrusters. However, due
to its relatively high freezing point of 274 K, temperature
management is still necessary for the storage in spaceenvironment. Since hydrazine is very reactive and toxic,
the materials for tanks and valves must be compatible tothe chemical properties.
For an alternative propellant, Kakami et al. proposed
Dimethyl ether (DME)6). Unlike hydrogen and hydrazine,
DME can be stored as a liquid phase under a relatively
low storage pressure without sophisticated temperature
management system owing to a freezing point of 130 K,
a boiling point of 119 K, and a vapor pressure of 6 atm atroom temperature. Catalyst is not necessary forpropellant gasification because DME is easily gasified or1

2. Prediction
Thrustersof

Chemically-augmented

monotonically with specific power. Especially, HFE andHFE solution show high temperature characteristics
among them (i.e., HFE: 1,000-3,900 K, HFE solution:800-3300 K) for a wide range of specific powers (1-40kJ/g). Subsequently, hydrogen peroxide follows (11003100 K).
Variations of specific impulses with specificpower (or specific energy) for water, acetic acid, acetic
acid solution, HFE, HFE solution, hydrogen peroxide,
acetic acid and ethanol solution and hydrazine are plottedin Fig. 2. Similar to reaction temperature, specificimpulses increase monotonically with specific power inall propellant cases. In a wide range of specific powers(1-5, 20-40 kJ/g) except 10 kJ/g, HFE solution shows thehighest values (260-350, 500-710 s), followed byhydrazine (230-330, 490-610 s). However, at 10 kJ/g,
although values are very close, hydrazine shows higher
value (410 s) than HFE solution (400 s).

ArcjetAs for the numerical simulation, equilibrium
calculations of chemical reactions of water-based liquids
and mixtures to estimate thrust performances for a given
nozzle condition of an arcjet thruster were conductedusing a simulation code, NASA Chemical Equilibriumwith Applications (CEA) developed by S. Gordon and B.
J. McBride13). To include additional electrical heating
through arc discharges occurring in an arcjet thruster, anelectrical input power was assumed and added to anenthalpy, or power, of formation of reactants, orpropellant components.
To discover and investigate exothermal properties ofvarious liquid propellants from chemically stable liquids
in standard condition, some liquids and theircombinations were examined. Moreover, calculation of
hydrazine was conducted as a widely used, standard
liquid propellant for comparison.
Figures 1 and 2 show the results for pure water, pureacetic acid, water solution of acetic acid (60 mol%), HFE(Hydrofluoroether), water solution of HFE (20 mol%),
pure hydrogen peroxide, acetic acid and ethanol solution,and hydrazine. Among them, hydrogen peroxide, acetic
acid and ethanol solution, HFE solution and HFE(Hydrofluoroether) showed interesting characteristics.
Variations of reaction temperatures with specific power(or specific energy) for water, acetic acid, acetic acidsolution, HFE, HFE solution, hydrogen peroxide, acetic
acid and ethanol solution and hydrazine are plotted in Fig.
1. In all propellants, reaction temperatures increase

3. Preliminary Experiment
Although our final goal is to utilize water-based
liquid propellants of an arcjet thruster, an objective of thispart is to examine a newly developed arcjet thruster andits evaluation system.
Schematic illustrations of the experimental setup,thrust stand, and a photo of the arcjet thruster installed on
the thrust stand are shown in Figs. 3 to 5. The thruster
consists of a copper-tungsten anode and a thoriatedtungsten cathode. A boron-nitride (BN) insulator (body)was employed for insulation of the cathodes and anode.

Figure 2. Variation of specific impulses vs specific powers
for liquids.

Figure 1. Variation of reaction temperatures vs specificpowers for liquids.

2

propellant operated with an average discharge voltage of
50 V and an average discharge current of 11 A.

Gas

Figure 3. Experimental setup.
Figure 6. Photo of plasma plume exhausted froma Ar- arcjet thruster.

A typical output signal of the thrust stand at thepreliminary test is shown in Fig. 7. Following a thrust of
0.08 N with a cold gas jet of nitrogen, a thrust of 0.16 N
was obtained during discharge operation.

Figure 4. Schematic of Calibration method.

Figure 7. Results of thrust (At water 8 mg/s).
Output signals of normalized thrusts for HFE, acetic
acid, and water propellants, mixed to Ar gas, in which
values of thrust are normalized with that of pure Ar case,are shown in Fig.8. As shown in the figure, comparing to
the thrust of pure Ar operation, the addition of thoseliquids to Ar propellant is effective for augmentation ofthrust.
Table 1 shows average increased values of thrusts
compared to the thrusts of pure Ar operation for various
propellant mass flow conditions. From the Table, it isshown that the highest thrust can be obtained with a flow

Figure 5. Photo of An Arcjet with Thrust Stand.
As an example of the operational experiment of anarcjet thruster with Ar gas propellant, a photo of theplasma plume exhausted from the arcjet is shown in Fig.
6. In this experiment, Ar gas was employed for the3rate of 6 mg of acetic acid, in which the value of thrust
was about twice as much as pure Ar conditions.
2)

3)

4)
Figure 8. Results of normalization thrust forvarious propellants.

5)

K, HFE solution: 800-3300 K) for a wide range of
specific powers (1-40 kJ/g). Subsequently, hydrogen
peroxide followed (1100-3100 K).
In a wide range of specific powers (1-5, 20-40 kJ/g)
except 10 kJ/g, HFE solution showed the highest
specific impulses (260-350, 500-710 s), followed byhydrazine (230-330, 490-610 s). However, at 10 kJ/g,hydrazine showed higher value (410 s) than HFE
solution (400 s).
A preliminary investigation of operational tests ofarcjet thruster was conducted employing various
liquid propellants. Examinations of thrust
performances of various potential liquid propellants
are currently being conducted.
As shown in the figure, comparing to the thrust ofpure Ar operation, the addition of those liquids to Ar
propellant is effective for augmentation of thrust.
From the Table, it is shown that the highest thrust
can be obtained with a flow rate of 6 mg of acetic
acid, in which the value of thrust was about twice asmuch as pure Ar conditions.

Table. 1 Results of normalization thrust

performance for various propellants.

2.

3.

Flow rate[mg/s]

H2O

HFE

Acetic Acid2

1.57

1.08

1.30

4

1.131.23

1.13

6

1.14

1.53

1.93

8

1.28

1.10

1.34

10

1.20

1.04

1.20

References
1 Birkan, M.A., “Arcjet and Arc Heaters: An Overviewof Research Status and Needs,” Journal of Propulsion andPower, vol. 12 1996, pp.1011-1017.

Butler, G.W., and Cassady, R.J., “Directions for Arcjet
Technology Development, “Journal of Propulsion andpower, vol. 12 1996, pp.1026-1034.
23 Auwerter-Kurtz, M., Glocker, B., Golz, T., Kurtz, H.L.,

Masserschmid, E.W., Riehle, M., and Zube, D.M., “ArcjetThruster Development,” Journal of Propulsion and Power.
Vol. 12 1996, pp.1077-1083.
4 Martinez-Sanchez, M., and Pollard, J.E., “Spacecraft
Electric propulsion-An Overview,” Journal of Propulsionand power., vol. 14 1998, pp.688-699.
5 Kakami, A., Yokote, J., Ebara, I., Tachibana, T.:“Application of dimethyl ether to arcjet thruster aspropellant,” Vacuum vol. 83 (1) (2008), pp.77-81.
6 Mueller, J., “Thruster Options for Microspacecraft: AReview and Evaluation of Existing Hardware and Emerging
Technologies,” AIAA Parer 1997, 97-3058.
7 Yanagida, N., Horisawa, H.: “Al-Water FedChemically Augmented DC Arcjet Characteristics,” AIAAPaper 2014, 2014-3

Conclusion
In this study, an investigation and development of
non-hazardous, low-cost, and stable water-based liquid
propellants including their mixtures under room
temperature were conducted for propellant of arcjet
thrusters. Following results were obtained.
1) From the results of numerical simulation, it wasshown that HFE and HFE solution showed hightemperature characteristics (i.e., HFE: 1,000-3,900

4