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Title    : Spacecraft may fly on "empty"
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Date    : January 22, 1999


Spacecraft may fly on "empty" using propulsive tether concept NASA/Marshall project selected for development

It's not quite something-for-nothing, but the new International SpaceStation and other future spaceships may maintain their orbits withoutusing rockets if they follow the lead of an unusual concept selectedfor a test flight next year.

NASA recently announced that the Propulsive Small Expendable DeployerSystem - ProSEDS - was selected for development under the Future-Xspace technology development program.

"This will be a demonstration of a propellant-free propulsionsystem," said Les Johnson of NASA's Marshall Space Flight Center. "Weexpect that it will reduce the cost of space transportation."

Johnson leads a team of engineers and scientists at NASA's MarshallSpace Flight Center developing ProSEDS, a tether that will "plug in"to the same physics principle that powers electric motors. Forces canbe generated by sending a current through a wire loop - i.e. anelectrical circuit - while it lies in a magnetic field. In space, onepart of the electrical circuit is a long tether attached to anorbiting spacecraft. The return path of the circuit is supplied bythe electrically charged gas in the ionosphere. The magnetic field issupplied by Earth. When properly controlled, the forces generated bythis "electrodynamic" tether can be used to pull or push a spacecraftto act as a brake or a booster.

The ProSEDS concept builds on results from the second flight of theTethered Satellite System (TSS-1R) in 1996 which gave scientists asurprise. The system was designed so that the satellite would bebiased to a high positive voltage and collect electrons from theionosphere.

"The theoretical models we had for current collection from theionosphere before the mission were not accurate," said Dr. NobieStone, TSS project scientist at NASA/Marshall. "The tether-generatedcurrents were higher than we had expected, and this is good news forfuture applications."

Before the flight, the models predicted that the tether would produce0.5 amp (0.5 A) under ideal conditions. Instead, it produced morethan 1 amp under less than ideal conditions. About the same time, Dr.J. R. Sanmartin of the Polytechnic University of Madrid, Spain,predicted that a long bare wire tether can produce even more currentthat the insulated wire and biased end-body collector used by TSS.

"If this new bare wire tether works as advertised," Stone said. "itwould allow us to collect considerably more current for a givenlength of tether." As a result, shorter tethers could be used forpropulsion or to generate electrical power.

The ProSEDS flight will demonstrate braking. The payload will beattached to the second stage of a Delta II rocket launching a pair ofAir Force navigation satellites. Normally, these stages slowly spiralback to Earth over the next half year as atmospheric drag nibblesaway at their speed. By generating an electrical current, ProSEDSwill turn itself into an electromagnetic brake.

"We're going to show an orbital decay of at least 5 km (3 mi) a day,"Johnson explained. "It's not quick compared to a retrorocket, but itis much faster than natural decay. And it's being done without theuse of any propellant."

Johnson expects ProSEDS to operate for only one to three days -atomic oxygen erosion or other space hazards may limit its lifetime.But a more rugged operational unit could make an expended secondstage re-enter Earth's atmosphere in about 14 days.

Working with the Earth's magnetic field would benefit a number ofspacecraft, including the International Space Station. NASA plans toreboost the station several times a year with propellants brought upfrom Earth, an expensive proposition. The resupply cost could beeliminated by using a propulsive tether system weighing less than 200kg and having a tether 10 km long.

"With a relatively low development and operations cost of less than$50 million, a tether reboost system on the ISS could potentiallysave the program up to $2 billion over 10 years," wrote Johnson andMelody Hermann, another space engineer at NASA/Marshall in a July1998 report. The tether would also increase the time available formicrogravity experiments, a key justification for ISS, and cancelaerodynamic drag that would upset the more sensitive of thoseexperiments. Although using the same principles as the ProSEDSdemonstration, a propulsive tether on ISS would be powered byelectricity from the ISS's solar arrays: 5 kW of electricity wouldproduce a steady push of 0.5 Newton (about one-eighth of a pound).

Tether systems may also be useful in planetary exploration. With itsgreat gravity and magnetic field, Jupiter would seem a natural placefor a propulsive tether system to move a space probe into orbitaround the planet, then tour the moons, and even power thespacecraft. NASA/Marshall investigated just such an intriguingpossibility, but the answer was a surprising "maybe" rather than aresounding "yes."

While Jupiter has a strong magnetic field, the gravity gradient - ineffect, its "steepness" - is not strong enough to keep the tetherstraight as it pushes the probe.

"The use of tethers in the Jovian system presents entirely newchallenges and opportunities," wrote Johnson and Dr. DennisGallagher, a scientist at NASA/Marshall's Space Sciences Laboratory.If anything, the tether would produce more electricity than thespacecraft would need.





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