The Solar Electric Propulsion Stage program was started in the early 1970s with a goal to provide a primary ion propulsion system capable of operating at a fixed power for Earth orbital applications or over a wide power profile such as would be encountered in planetary missions. One of the potential planetary targets was an encounter with the comet Encke. The SEPS program included the development of 25 kW solar arrays, PPUs, thermal control systems, gimbals, throttleable / long-life 30 cm diameter ion thrusters, and mercury propellant storage and distributions systems. This multi-Center, multi-Contractor effort was ongoing for about 10 years with a NASA investment of approximately $30 million. Because of funding limitations, a planetary flight program was not carried out. Instead, a ground-based technology demonstration was pursued.
The thrust subsystem was a bi-module consisting of two thrusters, two PPUs, a propellant system, a gimbal system, thermal control, and supporting structure. This module would be a basic building-block of an electric stage with simple interfaces. The 30 cm thruster was designed for 2.6 kW input power with 128 mN thrust and a specific impulse of about 3000 s. The thruster/PPU was capable of throttling down to 1.1 kW. One of the early engineering model thrusters was tested for 10,000 hours over an input power range of 0.8 kW to 2.4 kW. Endurance tests of these 30 cm ion engines confirmed the need for spalling control of sputter-deposited discharge chamber coatings, and the need of low sputter-yield materials for the cladding of pole pieces and baffles. Other tests indicated that very small concentrations of nitrogen in the vacuum facility could significantly reduce wear on the upstream surface of the screen grid compared to that expected in space.
Subsequent to these EM thruster tests, a total of seven advanced engineering model thrusters were tested in segments including 3,940 hours and 5,070 hours and a total test time of 14,541 hours. Ninety-five percent of the test was implemented using either breadboard or brassboard PPUs which were of the series-resonant inverter design.
Electrical Input Power: 2.60 kW.
Thrust: 0.13 N (0.03 lbf).
Specific impulse: 3,000 s.
First Launch: 1972-1980.