SAWAI SHUJIRO

This paper proposes control system for a new micro gravity experimental system called BOV (Balloon-based Operation Vehicle). BOV uses a free-fall capsule with double-shell structure to prevent influence of aerodynamic disturbance. Additionally, BOV is raised to 40km by a high altitude balloon to extend micro gravity duration to 30 (or possibly 60) seconds. Thus we realize a medium duration micro gravity experimental system with good micro gravity environment. In this system, the most characteristic point is double-shell structure to realize drag-free system. The inner shell can fall freely since the outer shell is controlled not to collide with the inner shell. In the experiments, we realize good-quality micro gravity and realize to continue moderate micro gravity duration. This system is now improving and near future we can practically utilize BOV's system for moderate micro gravity duration with low-cost easily.

A novel way to conduct microgravity experiments using high altitude balloon is now under development at ISAS/JAXA. By dropping the rocket shaped vehicle from the high altitude balloon, high quality microgravity environment can be served during its free fall of 30 to 60 sec. This paper summarizes the outline of free fall vehicle, which is called as BOV (Balloon based Operation Vehicle). The preliminary results of its second flight test are also presented to show its feasibility.

To realize micro-gravity environment of long duration and good quality with moderate cost, an experimental system which is released from a high altitude balloon has developed. The system has a double-shell drag-free structure and the outer shell, namely flight capsule, is controlled not to collide with the inner shell to realize micro-gravity environment of good quality. This paper shows the design philosophy and the configuration of the system. The system was successfully tested twice, in May, 2006 and in May, 2007. Outline of the test results and future development plan are also described.

Hydrazine is used for most conventional monopropellant thrusters for spacecraft propulsion systems. Due to its toxicity, considering environmental pollution and safe handling, it has been suggested to apply "green propellants" to spacecraft propulsion systems. Among green propellants, HAN (Hydroxyl Ammonium Nitrate)-based liquid propellant has promising characteristics of lower toxicity, higher performance (higher density and specific impulse) and longer storability. Therefore, this attractive propellant is considered as a promising candidate of monopropellant for replacing hydrazine propellant. Though it has some advantages shown above, it is very difficult to control its combustion because of sometimes unpredictable reactivity under high pressure conditions. In recent research, it was found that the reactivity of HAN-based propellants can be somewhat reduced by the addition of methanol as a fuel.In our previous study, reactivity of HAN-based propellant with methanol and some catalysts was investigated. In the measurement, oxidizer-rich propellant shows higher reactivity with S-405, which is used commonly for hydrazine monopropellant thrusters. Then, firing tests with a thruster were conducted to understand the effects of methanol additive amount. Although stable combustion was observed, thruster performance was much lower than predicted.In this study, more firing tests were conducted with changes of methanol additive ratio and thruster configuration (combustion chamber length and catalyst bed layer length) to obtain fundamental combustion characteristics of HAN-based thruster for higher thruster performance. As a result, some valuable data as well as development problems for higher-performance thruster design

A new composition of the hydroxyl ammonium nitrate (HAN)-based solution containing ammonium nitrate, methanol, and water had been developed for monopropellant in a reaction control system (RCS) as an alternative to conventional hydrazine. Comparing this solution with hydrazine, Isp is 20% higher, density is 1.4 times, the freezing point is much lower, and the toxicity is low, which makes this solution promising as a RCS propellant. The linear burning rate of the solution is moderate at the operating pressures of an RCS thruster. However, it was found that the linear burning rate had some characteristics whose mechanisms had not been understood. The combustion mechanism of this solution was investigated, the burning behavior was observed using a medium speed camera, and a temperature profile for the combustion wave was measured with a fine 2.5 mu m-diameter thermocouple. From these results, the instability of the liquid interface may trigger a sudden increase in the burning rate to a violently high region, and methanol was found to be effective in reducing the bubble growth rate in the solution. For RCS thruster use, reactivity with several catalysts was evaluated in an open-cup test. Consequently, the S405 catalyst for hydrazine showed the best performance among Pt-based, Pd-based, Ru-based, Ir-based, and S405 catalysts. Thruster tests were conducted successfully using S405 in both the pulsing and continuous modes. As a result, it was found that S405 has little effect in activation of the reaction of methanol contained in the propellant. High-concentration Pd catalyst was found to improve the decomposition characteristic of the solution.

The second flight of microgravity experiment system using a free fall capsule from a high altitude balloon was conducted in May 2007. Using a drag free control, around 10(-4) G gravity conditions were obtained for 30 seconds. Results of a combustion experiment with Japanese sparker conducted inside the microgranity experimental unit were also reported

The first microgravity experiment using a new free-fall capsule released from 40km altitude was performed on May, 2006 based on a drag-free technique. The fundamental data for analyzing the drag-free control,the flight sequence, and the wireless communication between the capsule and a control room were successfully obtained in the first test flight.