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The Forefront of Space Science

Develop a Low Environmental Impact Propellant
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We are now conducting basic research on a hybrid rocket using GAP. Experiments on 200N-class thrust are now being performed using a small “motorE(i.e., a rocket-propulsion system using solid fuel). Fig. 2 shows a combustion test of the hybrid rocket using GAP and gas oxygen. The motor is small but very powerful. Its high performance had been verified before as this size of motor had attained 98% combustion efficiency. On the other hand, since we can significantly lower its burning rate by blending a small amount of a certain polymer into the GAP, it is possible to change the thrust by creating layers of GAP which are different in burning rate. GAP also has the property that its specific impulse changes sensitively in response to slight differences in the oxidizer-fuel ratio. We can therefore control the thrust by changing the amount of oxidizer injected. A combination of these two features would allow us to change thrust in a wide range, and eventually provide great freedom of rocket-motor design.

GAP loses self-combustibility when the above-mentioned additive polymer amount is increased. Using this composition in this marginal region, we can create a variety of options. For example, after feeding oxidizer and burning by igniter, we can halt combustion by cutting off the oxidizer. Then, we can re-ignite by adding oxidizer. We expect that our research will proceed to a flight model through repetition of small experiments to create a larger model gradually incorporating these features.

The hybrid-rocket concept is not new. It has been considered for the keywords “high-performanceEand “safety,Eas well as “low environmental impact.ESince hybrid rockets did not perform much better than solid rockets, however, there was little talk of them replacing solid rockets in the past. Because of the current importance given to “safety,Ehowever, the hybrid is now under re-evaluation worldwide. In Japan, Prof. Toru Shimada of ISAS launched a research group comprising members mainly from JAXA and universities to research the hybrid rocket from various aspects. It is expected to research a variety of topics, not only hybrid rockets using GAP. The group will probably lead the world in hybrid-rocket research.


Figure 2
Figure 2. Combustion test of GAP/oxygen hybrid rocket


Solid fuel that can realize both hydrogen-chloride free and fine-particle free

Next is option (3). At present, it is difficult to produce sold fuel to satisfy both requirements at once, i.e., hydrogen-chloride free and fine-particle free. This is because the new, hydrogen-chloride-free oxidizer is not yet completed. With GAP, however, we have a prospect for a fine-particle-free fuel to cope with the “upperEspace (outer space) environment. One possible candidate is a combination of the existing oxidizer, ammonium perchlorate, and GAP. The performance of this combination is almost the same as that of traditional fuel containing aluminum. In addition, although slightly technical, there is a possibility that the fuel can burn normally without “oscillationEwhen it is used in spherical motors for the upper stage, which hardly oscillate at burning. This problem with this idea is that the fuel is “limited to spherical motorsEand there are few spherical motors. Nevertheless, we should pay attention to this as a fallback idea to cope with the debris problem.



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