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STRUCTURALLY INTEGRATED THRUSTER 5 (SIT-5)

A 5-cm-diameter mercury ion engine, called SIT-5, was developed circa 1970 for attitude control and NSSK of geosynchronous satellites. The thruster input power was 0.072 kW, and it provided a thrust of 2.1 mN at a specific impulse of 3000 s. Electrostatic thrust vectoring grids with a ±10 degree vectoring capability were baselined. The engine was successfully random vibration tested at 19.9g rms. The mass of the thruster and mercury storage and feed system was 2.2 kg. The propellant system could store 6.8 kg of mercury which could provide operation at full-power for approximately 30,000 hours. The envelope was about 31 cm long by 12 cm diameter. The SIT-5 development program focused on the thruster and feed system development; there was no PPU technology effort.

Hollow-cathode component tests demonstrated over 2800 simulated duty cycles. A separate test of the SIT-5 thruster was conducted for 9715 hours at a beam voltage of 1300 V, a thrust of 1.8 mN, and a specific impulse of 2500 s. During the initial 2023 hours, the thruster was operated with a translating screen grid thrust vector system. For the remainder of the test the thruster had an electrostatic thrust vector system. The electrostatic beam vector grids were operated at 5 degrees deflection for about 120 hours, at either 2 degrees or 4 degrees deflection for 1880 hours, and with no deflection for 5690 hours. There were a number of grid shorts that were successfully cleared by the application of 200 V to 400 V at currents from 6 mA to 70 mA. These tests were helpful in the later definition of grid-clear circuits for the IAPS, XIPS, and NSTAR thrusters.
The SIT-5 mercury propellant system was successfully tested for a period of 5400 hours in an independent test.

The preceding was an excerpt from:
Sovey, J. S., Rawlin, V. K., and Patterson, M. J.: "Ion Propulsion Development Projects in U. S.: Space Electric Rocket Test 1 to Deep Space 1." Journal of Propulsion and Power, Vol. 17, No. 3, May-June 2001, pp. 517-526.




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