| The Relativity Gyroscope Satellite, Gravity Probe B (GP-B) Program, uses helium thrusters for attitude control and drag compensation. These thrusters use helium gas which is generated in cooling the experiment as their propellant. The current differential proportional thruster design for helium flow near the nominal flow rate has been developed by Bull (1973) and Chen (1983). Using a differential restrictor to modulate the flow, its maximum flow rate is twice the nominal flow rate.; There is a possibility that GP-B may need a thruster having a much wider flow dynamic range than the current design. A dual mode wide dynamic range helium thruster has been developed with a flow dynamic range one to two orders of magnitude wider than the previous design. It has a restrictor and an annular nozzle formed by a plug inside a circular opening. At low flow rates, fine modulation can be achieved by changing the restricted gap (valve opening) with the nozzle throat area kept constant. At higher flow rates, the thruster gives rapidly increasing throat area as the valve opens with almost no restrictor effect. At a nominal helium flow rate, the Reynolds number is as low as 1 with this design.; Tests have been performed to verify the wide dynamic range performance and to understand the properties of the annular nozzle. A mathematical model of the thruster has been developed to match the experimental thruster performance and to provide a basis for future design. By choosing the geometry of the annular nozzle and restrictor, one can design the thruster with desired performance characteristics. |
