Development of repetitive type pellet injector with improved thermal insulation

A repetitive-type pellet injector was developed, and some improvements of thermal insulation were evaluated. The injector is an extruder type and its cylindrically shaped pellets have 2-mm diameter, 2-mm length, and a maximum velocity of 1.6 km/s. A single shot test showed that most of the excessive thermal energy for the cryogenic block comes from the propellant gas itself. A push-pull-type fast-opening/closing valve was developed for gas reduction. The opening and closing times are both within 0.8 ms, and the gas release is cut in half compared with the case in which the usual valve is used. A double-layered tube is used for connection between the valve and cryogenic block, and it can stop most of the thermal flow from the propellant gas. For repetitive operation, in addition to thermal conduction of the propellant gas, thermal conduction from the external field becomes responsible. Polyimide tubes were used for the insulation inside the block and, with a vacuum tank of 0.7 m/sup 3/, the steady-state condition of solid hydrogen is achieved for the repetition rate of 1 Hz and more than a 10-s operation time can be achieved for 5 Hz. This would be raised to 10 Hz with a higher pumping speed of about 5000 m/sup 3//s. >