Conference 7.286::space

717.0. "Arcjets Yield Better Performance, Longer Satellite Life" by PRAGMA::GRIFFIN (Dave Griffin) Tue Mar 26 1991 19:39

3/14/91: ARCJETS YIELD BETTER PERFORMANCE, LONGER SATELLITE LIFE    

RELEASE: 91-40


	NASA's Lewis Research Center, Cleveland, has the lead role in
developing the arcjet thruster technology recently selected for
stationkeeping use on AT&T's Telstar 4 communications satellites.

	Arcjet systems offer a significant improvement in propellant
use over chemical and other electrically-augmented thrusters.  The
savings realized can be used to increase a satellite's on-orbit
lifetime or payload mass.  Alternatively, launch mass can be
decreased so that a smaller rocket booster can be used.

	The arcjet research and technology program at Lewis Research
Center began in 1983.  A major objective was to bring advanced
electric propulsion to operational status.  This included component
research necessary to demonstrate the required performance, life and
integration issues associated with the arcjet system.

	In an arcjet, a direct current electrical arc is used to heat
the decomposition products of hydrazine propellant to very high
temperatures.  Although the arc core temperature can reach 31,123
degrees F, the nozzle walls are protected by a cool gas boundary
layer.

	The hot, slightly ionized gas exits the rocket nozzle at an
average velocity 1.5 to 2 times that attained in conventional
thrusters.  For example, the 1.8 kW arcjet systems developed by
Rocket Research Co. (RRC), Redmond, Wash., for the Telstar 4 program,
provide a specific impulse (thrust divided by the propellant
consumption rate) of about 500 seconds.

	This compares to a state-of-the-art resistojet system
(another type of electrical thruster) that would provide 300 seconds
of specific impulse for the same task.  The 1.8 kW design is modeled
closely after a flight-type 1.4 kW system developed by RRC under a
Lewis-sponsored program.

	Switching to arcjet systems for north-south stationkeeping on
a geosynchronous communications satellite can reduce propellant
requirements by several hundred pounds.  This savings can extend
satellite lifetime by more than 50 percent or allow the satellite to
shift to a less powerful, less expensive launch vehicle.

	The arcjet technology developed by NASA's Lewis Research
Center and U. S. industry is the most advanced in the world.  The
Telstar 4 thrusters are the only arcjets accepted for operational use
on a spacecraft.

	Lewis and industry continue to study arcjet thruster systems
and their interactions with host spacecraft systems.  Results to date
suggest that electromagnetic interference with satellite systems
should be minimal and that there will be no problem sending radio
signals through the thruster exhaust plume.

	Lewis researchers also are investigating a range of power
options to enhance the versatility of hydrazine arcjet technology.
Examples include low power (1 kW) systems for power-limited
satellites and high specific impulse systems for advanced
communications satellites.  The use of high specific impulse hydrogen
arcjet systems also is being explored for primary propulsion.

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