Throttleable solid propellant microthruster using laser-assisted combustion

December 16, 2016

Objectives

Solid propellant thrusters have attractive characteristics for space propulsion device on satellites: simple structure and resultant reliability. Nevertheless, throttling including start, interruption and restart of combustion is difficult because solid propellants can autonomously maintain combustion. Hence, liquid propellant thrusters, which are throttleable by adjusting valves, are usually applied to satellite propulsion systems for attitude control, station keeping, and orbit transfer. On the other hand, liquid propellant thrusters require tanks and tubes, which complicate satellite-systems design. Today, microsatellites, weights of which are on an order of 100 kg or sometimes less than 1 kg, are designed and prototyped by universities. The microsatellites require compact and light-weight components including thrusters.

From this point of view, we proposed a throttleable solid propellant microthruster using laser heating, where thrust is variable by adjusting laser power. The proposed thruster would have not only the preferable characteristics of the conventional solid propellant thruster: simplicity, robustness, and light-weight, but those of liquid propellant thrusters: variable thrust including interruption and restart of thrust production. Up to now, we have investigated the mixture ratio and ingredients for combustion control by use of laser heating, and designed prototypes to show thrust is throttleable in the proposed microthruster.

Throttleable solid propellant microthruster using laser heating

Figure 1 Schematic diagram of solid propellant microthruster using laser assisted combustion.

By adjusting mixture ratio and adding extra ingredients, combustion of solid propellant is controllable by adjusting external heat source such as laser and arc plasma. Combustion is sustained only while the solid propellant is irradiated with laser. The propellant is combustion controllable solid propellant (non-self combustible solid propellant), which we have developed. Ingredients and mixture ratio were investigated such that combustion is controllable with laser. Figure 1 shows a schematic diagram of the proposed thruster.

In the proposed thruster, thrust production is started by starting laser heating on solid propellant surface, and interrupted by switching off laser. Moreover, thrust is variable by adjusting laser-driving current or laser-heated area of solid propellant. Hence, the proposed thruster requires no rapid depressurizing device for throttling.

Burning rate measurement and combustion controllability

Figure 2 Schematic of strand burner.

Burning rate, the velocity of burning surface regression, i.e. flame-spreading velocity, was measured in a strand burner with laser-introducing window shown in Fig. 2. Laser beam of 808 nm in wavelength was guided from 45-W laser diode through an optical fiber to a collimation lens. Inside the strand burner, a solid propellant sample is heated with laser in nitrogen atmosphere. The back pressure was variable by adjusting nitrogen flow rate, ranging from 0 to 1 MPa (abs). For evaluating burning rate, the solid propellant combustion was recorded with a video camera.

Figure 3 Dependence of burning rate on back pressure at laser power density ranging from 0.3 to 0.8 W/mm².

Figure 3 shows the dependence of burning rate r on back pressure P. As you see, burning rate was increased with back pressure. Fitting with Vieille's law below 0.18 MPa yields a pressure exponent n ranging from 0.3 to 0.6. Pressure exponent, which is less than 1, indicates that laser-assisted combustion of combustion-controllable solid propellant was applicable to thrusters. (If n exceeds 1, thrust chamber pressure would diverge or oscillate.) Evaluation of burning rate allows us to design thrusters; determines nozzle throat area and laser heated area on solid propellant for producing target thrust.

The video shows a combustion test in a strand burner. Solid propellant sample of 2×2×20 mm³ in size was placed inside the stand burner, and was irradiated with 808-nm laser beam from its right side. Laser beam is invisible but recorded with CCD cameras; in the movie, the beam was seen as purple light.

Laser irradiation was started at approximately 4 s after the start of recording. Combustion is initiated immediately after the start of laser heating. During laser heating, combustion is stably sustained without flickering. By interrupting laser heating, combustion is also interrupted. From the result, combustion was controllable with laser heating.

Thrust measurement using prototyped thruster

A prototype microthruster was tested in a vacuum chamber for the purpose of evaluating performance and showing the throttleability using laser heating. The vacuum chamber was evacuated with a rotary pump to reduce a back pressure below 0.03 atm (3 kPa). The prototype has a transparent acrylic thrust chamber that can transmit laser beam with comparatively low laser-beam power loss. As shown in the movie, thrust production is successfully started and interrupted with adjustment of laser heating.

Journal papers and proceedings

Akira Kakami, Daichi Haraguchi, Yusuke Matsuura and Yasuki Yano, "Effect of carbon black on performance of laser-controlled solid propellant microthruster," Joint Propulsion Conference, Cincinnati Ohio, USA, AIAA 2018-4484, July 9-12, 2018.
Shota Isakari, Takamasa Asakura, Daichi Haraguchi, Yasuyuki Yano, Akira Kakami, "Performance evaluation and thermography of solid-propellant microthrusters with laser-based throttling,” Aerospace Science and Technology, DOI: 10.1016/j.ast.2017.08.027, pp. 99-108, December 2017.
Shota Isakari, Shingo Onizuka, Yasuyuki Yano, and Akira Kakami, “Performance Evaluation of a Throttleable Solid Propellant Thruster using Laser Heating,” Transactions of The Japan Society for Aeronautical and Space Sciences, Aerospace Technology Japan, Vol. 14, ISTS30, pp. Pa_61-Pa_66.
Akira Kakami, Shota Terashita and Takeshi Tachibana, “A Laser Heating Method for Estimating Thermal Balance of Burning Solid Propellants,” Science and Technology of Energetic Materials, Vol. 70, No. 6 , pp.145-151, December 2009.
Akira Kakami, Ryoma Hiyamizu, Kiyotaka Shuzenji and Takeshi Tachibana, “Laser-Assisted Combustion of Solid Propellant at Low Pressures,”Journal of Propulsion and Power, Vol. 24, No. 6, pp. 1355-1360, November 2008.
Masayuki Shimoda, Taketoshi Hamada, Akira Kakami, and Takeshi Tachibana, "Prototype of 0.1 N-class Solid Propellant Thruster with Laser-Controlled Combustion," 29th International Symposium on Space Technology and Science, 2013-a-06, June 5 2013, Nagoya Japan.
Akira Kakami, Hamada Taketoshi, Shigeki Ishihara and Takeshi Tachibana, “Laser assisted combustion of solid propellant for a 100 mN class variable thrust rocket motor,” 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, AIAA-2012-4042, July 31 2012.
Shinichiro Masaki, Ryoma Hiyamizu, Akira Kakami, Hideyuki Horisawa and Takeshi Tachibana, “Preliminary Experiment on Solid Propellant Microthruster Using Laser-Controlled Combustion,” 2nd Asian Joint Conference of Propulsion and Power, Kitakyushu, Japan, January 28 2005.

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