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SOLAR ELECTRIC PROPULSION
STAGE (SEPS)
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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 Enke.
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
dollars. Because of funding limitations, a
planetary flight program was not carried
out; rather, 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 a
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 for 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 engineering model
thruster tests, a total of seven advanced
engineering model thrusters were tested in
segments, including two at 3,940 hours and
5,070 hours long, with a total test time of
14,541 hours. Ninety five percent of the
tests were implemented using either
breadboard or brassboard PPUs which were of
the series-resonant inverter design.
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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