Drop tower experiment for performance evaluation of gas-liquid equilibrium thruster for small spacecraft
Abstract
JAXA/ISAS is developing the gas-liquid equilibrium thruster for a small spacecraft. In small spacecrafts, the thruster system must be simple and its weight must be light. This thruster system uses HFC-134a (1,1,1,2-tetrafluoroethane) , a kind of liquefied gas, as propellant because of its harmlessness and ease of handling. And this thruster stores propellant as liquid in the tank and ejects propellant as gas using the gas-liquid equilibrium pressure to produce thrust, so the propellant tank only needs to resist the vapor pressure of propellant. In this thruster system, the porous metal is also equipped in the tank for the following performance advantages: (1) liquid fuel retention: The porous metal reduces sloshing problems which cause bad effects on spacecraft attitude by retaining liquid propellant inside the porous metal: (2) vapor-liquid separation: The porous metal also helps propellant separate gas from liquid by advancing propellant vaporization on its large surface area and retaining liquid propellant using its surface tension. In last autumn, we carried out the experiment to evaluate these two advantages of porous metal under micro gravity condition using 50 meters drop tower in Hokkaido, Japan. The system of this experiment divides into two different systems. The first one evaluates liquid propellant retention performance by adding disturbance to liquid propellant absorbed in porous metal. The disturbance is centrifugal force and angular acceleration worked on the liquid propellant by rotating propellant tank controlled by motor. A high speed camera records the behavior of the liquid propellant. The other one evaluates the ability of gas-liquid separation on the case of propellant ejection. In this evaluation, the parameters are full filling porous metal or some ullage in the tank, nozzle diameters and the filling ratio of liquid propellant in the tank. As for (1) liquid fuel retention, in all conducted cases without propellant ejection, liquid propellant was retained in the porous metal. It is obvious that the porous metal is effective for decreasing of sloshing problems. We evaluated this effectiveness analytically based on head pressure worked on liquid propellant as centrifugal force and angular acceleration. As for (2)vapor-liquid separation, liquid propellant retained inside the porous metal started to boil right after propellant ejection occurred and propellant was ejected mixed with liquid. And after propellant ejection, it takes some time to recover tank pressure into saturated vapor pressure. To obtain stable thrust force, it is required to halt the propellant ejection just before the tank pressure reaches to the break point pressure and to take enough ejection intervals to recover the tank pressure. From these experiments, for practical use of this thruster system, that is, to avoid liquid propellant ejection, it is important to ensure adequate ullage volume of tank to achieve mission requirement.
- Publication:
-
38th COSPAR Scientific Assembly
- Pub Date:
- 2010
- Bibcode:
- 2010cosp...38.3628M