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
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 contr