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SOLAR ELECTRIC PROPULSION
SYSTEM TECHNOLOGY (SEPST)
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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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