Bipropellant microthruster using nitrous oxide and dimethyl ether

Objectives

Today, microsatellites, weights of which are on an order of 100 kg or sometimes less than 1 kg, are designed and prototyped by some universities and startup companies. Some of them have been launched to the space and successfully sent some pictures of the Earth from the orbits. Some technological experiments were also conducted in some microsatellites. Microsatellites possibly provide universities, non-governmental organization, or private companies with opportunities to do something different in the space. On the other hand, almost all of the microsatellites have no thruster, which allows dynamic and precise maneuver: orbit transfer, deorbit, and formation flight.

Hence, we have proposed to use dimethyl ether (DME) and nitrogen oxide (N2O) as a bipropellant in order to provide an eco-friendly and high performance bipropellant thruster for spacecraft. DME and N2O, non-toxic liquefied gases, are neither toxic nor reactive to stainless steel. Hence, N2O/DME bipropellant is a sort of green propellant, an eco-friendly propellant.

Proposed bipropellant thruster

Among chemical propulsion device, bipropellant thrusters, in which oxidizer and fuel are separately stored in individual tanks, have relatively high specific impulse, which expresses how little propellant is necessary for yielding a certain thrust. Moreover, the thruster is readily throttleable simply by adjusting mass flow rate of propellant whereas solid propellant thrusters have difficulty in start, interrupt, and restart of combustion as well as variable thrust. Hence, most thrusters on satellites are bipropellant thrusters using Unsymmetrical Dimethyl Hydrazine (UDMH)/nitrogen tetra oxide (NTO) propellant.

On the other hand, conventional bipropellant thrusters have some unfavorable characteristics. The thruster necessitates tanks and valves and consequently has more complex structure than solid propellant thruster. Propellants such as UDMH and NTO are very toxic and reactive to materials for propellant feeding devices. Hence, in ground tests, a special system is necessary for exhaust gas treatment. For augmentation in performance, atomization is necessary when the propellant is injected into thrust chamber because evaporation of liquid propellant is a rate-determining process in combustion.

From these points of view, we proposed to use nitrous oxide (N2O) and dimethyl ether (DME) as a green bipropellant for thrusters. Neither N2O nor DME is toxic; N2O, i.e., the laughing gas, is used as anesthetics and cooking spray; DME is used as a propellant for gas dusters. Today, some research groups tries to apply DME to diesel engine.

N2O and DME are stored in a liquid form without cryogenic device; N2O DME have vapor pressures of 5 and 0.6 MPa at the room temperature, respectively. The propellant can be readily evaporated or liquefied by adjusting temperature and pressure. The vapor pressure allows the propellant to be fed in a gaseous form into thrust chambers, and resultantly requires no atomization. This will downsize and simplify thrust chambers and injectors.

Chemical equilibrium calculation showed that N2O/DME propellant yields a maximum theoretical specific impulse of 287 s at oxidizer to fuel ratio (O/F) of 3.5. The specific impulse is smaller than that of conventional liquid propellants; NTO/UDMH thruster yields a specific impulse of 320 s. Nevertheless, N2O/DME bipropellant thruster neither toxic nor reactive to materials.


Figure 1 N2O/DME bipropellant microthruster.

Prototyped thruster firing

A 0.3-N class prototyped thruster was tested at the atmospheric ambient pressure to show that the proposed N2O/DME thruster yields a thrust. The thruster is mounted on the pendulum of a vertical thrust stand, which is a torsional pendulum. Combustion is ignited with an spark plug, and stably sustained during propellant feeding without quenching nor flickering. The thruster successfully produced a stable thrust for 300 s.

Journal papers and conference proceedings

  1. Takamasa Asakura, Shouta Hayashi, Yasuyuki Yano, Akira Kakami, “Influence of Injector for Performance of N2O/DME Bipropellant Thruster,” Transactions of The Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan, in press.
  2. Tasuku Uraoka, Yoshikazu Iwao, Yasuyuki Yano, Akira Kakami, “Improvement of Combustion Stability of N2O/DME Bipropellant in Vacuum,” Transactions of The Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan, Vol. 14, ISTS30, pp. Pa_73-Pa_81.
  3. Tasuku Uraoka, Yoshikazu Iwao, Yasuyuki Yano, Akira Kakami, “Improvement of Combustion Stability of N2O/DME Bipropellant in Vacuum,” 30th International Symposium on Space Technology and Science, 2015-a-65, 2015/7/10, July 4-10 2015, Kobe, Japan.
  4. Tasuku Uraoka, Yoshikazu Iwao, Yasuyuki Yano, Akira Kakami, “Performance evaluation of N2O/DME bipropellant thruster in vacuum,” The 5th International Symposium on Energetic Materials and their Applications, PP-12, 2014/11/15, November 15-17 2014, Fukuoka, Japan.
  5. Keisuke Bando, Shinpei Watanabe, Akira Kakami, Takeshi Tachibana, “Characteristics of a micro thruster using N2O/DME as propellant,” The 5th International Symposium on Energetic Materials and their Applications, O13, 2014/11/15, November 15-17 2014, Fukuoka, Japan.
  6. Akira Kakami, Motoki Yamanaka, Tatsuya Matsushita and Takeshi Tachibana, “Performance of 1-N class liquefied gas propellant thruster,” 49th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, AIAA 2013-3986, June 15 2013, San Jose, California.