posted 04-28-2006 03:54 PM              

Source: MISSILES AND ROCKETS (March 14, 1966)

San Diego, Calif.—Use of electric power to propel U.S. spacecraft started in September 1965, when an electro-thermal thruster adjusted the velocity of a Vela nuclear detection satellite. Further use of such thrusters is planned, and mission studies and reliability tests are speeding the application of ion propulsion to station-keeping of manned and unmanned satellites, and to unmanned deep-space probes.

Details of the advances electrical propulsion is making toward useful application were released here recently at the Fifth Electric Propulsion Conference of the American Institute of Aeronautics and Astronautics. The meeting provided a notably practical assembly of data on what had previously tended to be a largely speculative subject.

Dr. Ernst Stuhlinger, who has been associated with chemical propulsion development since Germany’s World War II V-2 rocket, told M/R that electric propulsion has now been proven much more feasible then chemical rocket programs were at their inception.

Heated gas—In the Vela type of thruster, or resistojet, cold gas is passed through an electric heating coil. TRW Systems Group used a 90-watt heater to raise by 50% the specific impulse of a nitrogen thruster on Velas 5 and 6, launched by an Atlas-Agena in July, 1965. TRW engineers report that the result has been a major gain in maneuvering capability, desirable because of the increase in operating life shown by this series of satellites.

A resistojet with a power of 3kw has been tested for 25 hours by Marquardt Corp. for the Air Force, and plans are currently being made for a 1,000-hour test.

Resistojets have also been selected by Douglas Aircraft Co. in a preliminary evaluation of station-keeping and stability control for NASA’s Manned Orbiting Research Laboratory.

MORL is conceived as a6-9 man vehicle for launching by a Saturn IB early in 1972. In orbit the 260-in.-dia vehicle would be 44.2 ft. long and weigh 30,000 lbs. This weight would increase to 100,000 lbs. when a full complement of Apollo resupply vehicles was attached.

Use of an 11-kw radio-isotope-fueled Brayton cycle is anticipated, and both resistojets and radioisotope thrusters appear feasible for long-term missions. Resistojets were selected as most promising for this application because they have been in development and testing for several years.

Low-thrust—Development of low-thrust ion engines was described by Hughes Aircraft Co., NASA’s Lewis Research Center and Electro-Optical Systems, Inc. These would be used for station-keeping in synchronous communications satellites, weather satellites, and decoy satellites, First NASA flight of this type of ion engine is expected in 1968 on an Applications Technology Satellite, where it will be used for east-west station-keeping. The interaction of the thruster with communications and attitude control systems will be watched closely, for future satellite and interplanetary probe design.

One of the most comprehensive studies of the feasibility of applying ion engines to spacecraft was described at the meeting by Jerry P. Mullin of NASA’s office of Advanced Research and Technology. This was a joint NASA /Air Force study involving Jet Propulsion Laboratory, NASA-Lewis, Hughes Aircraft Co., and EOS. JPL/Hughes studies of spacecraft were carried into the detail of a Mars mission Phase 1 study. Lightweight, large-area solar panels were projected to power the engines.

Reviewing the studies, Mullin concluded that “we now have an existing body of technology, and solar electric propulsion looks extremely good for certain classes of interplanetary applications. There should be interest in these systems for missions that are currently under study.”

Studies ended—The joint studies have now been concluded. The NASA and AF missions call for divergent lines of development. OART is continuing to fund JPL studies for development of a methodology for mission analysis of Solar Electric-Propelled Spacecraft (SEPS). NASA-Lewis is also carrying out further studies in this area. The space agency’s Marshall Space Flight Center and Office of Space Sciences and Applications are funding still other investigations.

A preliminary look has been given to the characteristics of a Jupiter mission carried out with solar-electric or nuclear-electric propulsion. This study at JPL for an 800-day mission in 1971 shows that where the nuclear-powered spacecraft can arrive at Jupiter with a very low approach velocity, the solar-electric-powered spacecraft arrives at a speed comparable to that of a chemical-powered mission.

Other potential missions for an SEP spacecraft are a Mercury trip, a solar probe, and an out-of-the-ecliptic flight. The latter is of interest not only scientifically but also as a good engineering test of the solar-electric propulsion concept, as the probe would stay above the Earth but move up out of the plane of the ecliptic. This would make engineering tests of the interaction of the ion engine with out systems simpler.

New Studies—The next series of studies are likely to push forward into greater detail in spacecraft design, moving away from feasibility studies and towards optimization.

Among the questions to be resolved are whether large ion beams and solar arrays result in a spacecraft which builds up a static charge on itself, what sort of field develops, and whether the plasma beam can be used to expel excess charge.

Another area of importance is the possibility of beam divergence and impingement on different areas of the spacecraft, such as optical attitude sensors.

The high currents in the engine system may produce radio-frequency interference. There may be a problem with up-link communications, so the requirements for antenna arrays and on-board receiver amplification will be studied.

New concepts in power conditioning that are reaching the hardware stage will be examined. The interplanetary solar-powered spacecraft produces some unique conditioning requirements—although the solar power input drops as the spacecraft moves away from the Sun, the solar cells become more efficient due to the drop in temperature, and the optimum load varies.

Advantages—The advantages offered by solar-electric propulsion, however, make these investigations well worth-while. Hughes studies carried out for JPL compared an SEP spacecraft with an all-chemical spacecraft, both to be launched on a Mars mission in 1971.

Results showed that the electrical space-craft can put almost twice as much useful weight into Mars orbit.

The Hughes study predicts total program cost for the 1971 solar-electric-propelled spacecraft as $375.9 million, compared with $194.5 million for all-chemical spacecraft, not including the costs of the scientific payload and lander. The costs of the scientific payloads for the two spacecraft are projected as $626 million and $163 million , respectively. However, although total program cost of the SEP spacecraft is about twice that of an all-chemical spacecraft, the cost per pound of scientific payload in Mars orbit in about one-half.

The Hughes study considered a spacecraft with a solar power of 48 kw 21-Earth and 10 mercury-bombardment engines, eight operating and two in stand-by. Specific impulse was 4,000 sec. and propellant weight 1,600 lbs. The thrusters, their controls, feed system, and propellant tanks were integrated into one unit located externally on the prime spacecraft structure. The unit is attached to the spacecraft bus by means of a translation mechanism that allows alignment of the thrust vector with the spacecraft’s center of gravity, minimizing disturbances torques.

Jettisoned—To minimize the retro system for entering into Mars orbit, the electric propulsion unit and many of the solar panels are jettisoned on final approach to the planet.

A similar design was also used for a spacecraft of 10 kw power launched by an Atlas-Centaur. This would use six ion engines, for both spacecraft the lightweight large-area solar-cell arrays were designed by Boeing Co. Hughes, working under an Air Force contract, has been development a flexible solar array expected to provide 20 kw of power for a spacecraft launched by Atlas-Centaur.

------------------
Scott Schneeweis