TOP > Report & Column > The Forefront of Space Science > 2008 > Develop a Low Environmental Impact Propellant
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Development of thruster using HAN-based solution Having discussed rocket motors above, we will now move on to small engines called thrusters. Although small compared to rocket motors, thrusters are a very important propulsion system. They are used as side jets (SJ) on rockets or for satellite attitude control (RCS). Two kinds of fuels are most commonly used: hydrazine (N2H4) and hydrazine-based two-component type. Both, however, are carcinogenic and therefore hard to handle. In the search for materials with less toxicity and the potential for improved performance, a number of compounds have been tested until now. Hydroxyl Ammonium Nitrate (HAN: NH3OHNO3) is one of them. This material is very promising apart from the drawback that it burns like a wild horse. It burns so fiercely that researchers in the world have had much difficulty in handling it. Our group has for many years pursued an idea to tame it. A few years ago, we managed to develop a HAN-based solution that has a gentle combustion property within the range of thruster operation without sacrificing performance. A thruster using this solution is now undergoing tests. At present, we are testing a thruster with 20N class thrust (Fig. 3). Compared to the aforementioned hybrid rocket using GAP with 200N class thrust, it is tiny. Nevertheless, we were pleased because we were able to carry out a “full-size testEof the thruster from scratch at our laboratory. 
In Table 1, we compare the performance of HAN-based solution (SGP163) and hydrazine. HAN-based solution exceeds in density specific impulse by 70% compared to hydrazine and also is substantially superior in melting point. We would like to adopt it immediately, but its weak point is, ironically, its high-performance (i.e., high combustion temperature). The difficulty is the “heat resistanceEof the catalyst and the catalyst’s retainer material. We expect to overcome it through appropriate control of the temperature field in thruster after a full examination of the design parameters. Last year, our research was backed by specially allocated funding at the discretion of JAXA’s President. With this help, we managed to set the conditions and find a raw material that would enable combustion lasting the 100 sec. necessary for the rocket phase (i.e., for side jet). We successfully accomplished a specific impulse of 240 sec. and improved its combustion efficiency to nearly 90%. By the way, Mr. Toshiyuki Katsumi, a doctoral student of the Graduate University for Advanced Studies who is actively engaged in the experiment, presented a paper summarizing the experiment results and combustion mechanism of the HAN-based solution at the 6th International Seminar on Flame Structure held in Brussels, Belgium, September 15 to 17, 2008. As a result, he was awarded the paper prize given to young researchers. However, we have not yet reached “the ideal goal of 10,000 secEin total injection time, a reference to the durability required for satellite attitude control. In Brussels, a leading figure in this field from the U.S. applied pressure when he said, “I have high expectations of your project.EOur staff is now working earnestly because we are in a critical point where we can show our mettle. I plan to attend an international symposium to be held at Pune, India, in late-October to deliver a guest lecture about combustion mechanism of GAP and its application to hybrid rockets. Although the presentation I can make is, of course, limited, I intend to bluff a little. 
Keiichi HORI |
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