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SOLAR ELECTRIC PROPULSION SYSTEM TECHNOLOGY (SEPST)

The objective of the Solar Electric Propulsion System Technology program at JPL was to demonstrate a complete breadboard IPS that would be applicable to an interplanetary spacecraft. The focus of this program was directed toward thruster performance improvements, PPU and control technology, and power matching and switching. Most of the program efforts were conducted in the late 1960s and early 1970s. The 20 cm diameter mercury ion engine first employed a thermally heated oxide cathode and later-on used a hollow cathode. Maximum thruster power was 2.5 kW which enabled thrusting at 88 mN and a specific impulse of about 3600 s. Three basic servo-loops were demonstrated, and they were similar in concept to the two loops used in the SERT II technology. Servo-loops included an ion beam current to main vaporizer loop, a discharge voltage to cathode vaporizer loop, and a neutralizer keeper voltage to neutralizer vaporizer loop. The closed-loops, to first order, maintained the thrust level, the propellant efficiency, and the floating potential from neutralizer common to facility or spacecraft ground.

PPU development centered around the beam power supply. The beam power supply had 8 inverters and had an efficiency of 89% to 90% over a bus voltage range from about 53 V to 80 V.48 The PPU was integrated with the thruster, 2:1 power throttling with closed-loop control was demonstrated, and HV recycle algorithms were developed. Initial BBPPU efficiencies were about 84%-86%, and subsequent experimental BBPPUs had efficiencies of 88%-90%. The experimental BBPPUs, which provided 2.5 kW, had a specific mass of 5.4 kg/kW. Later work at NASA LeRC in the 1970s focused on the development of 30 cm diameter ion engine which operated at derated power levels compared to the SEPST engine. The 30 cm diameter thruster system, using mercury propellant, was brought to engineering model status under the SEPS program.

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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