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Spacecraft may fly on "empty" using propulsive tether concept

NASA/Marshall project selected for development

Jan. 22, 1999: It's not quite something-for-nothing, but the new International Space Station and other future spaceships may maintain their orbits without using rockets if they follow the lead of an unusual concept selected for a test flight next year.

NASA recently announced that the Propulsive Small Expendable Deployer System - ProSEDS - was selected for development under the Future-X space technology development program.

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

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Right: Artist's concept depicts how a propulsive tether system works. Links to 963x1247-pixel, 262KB JPG. Credit: Dan Holland, NASA/Marshall Space Flight Center.

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

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

"The theoretical models we had for current collection from the ionosphere before the mission were not accurate," said Dr. Nobie Stone, TSS project scientist at NASA/Marshall. "The tether-generated currents were higher than we had expected, and this is good news for future applications."

initial deploy of tether and satelliteRight: Two views of deployment of the second flight of TSS-1R in 1996 showing the initial deployment of the tether and satellite, which acts as the "biased end-body voltage collector" (top), and the satellite at the end of the tether at approximately 1 mile (bottom).

Before the flight, the models predicted that the tether would produce 0.5 amp (0.5 A) under ideal conditions. Instead, it produced more than 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 current that the insulated wire and biased end-body collector used by TSS.

Left: The flight history of tethers in space starts with the Gemini program in 1966 and may extend to Europa and beyond in the 21st century. Links to 1007x735-pixel, 221KB JPG. Credit: Dan Holland, NASA/Marshall Space Flight Center.

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

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

The history of tethers - From an 1895 concept of an "elevator to space" to skyhooks to ProSEDS - learn how tethers have been affecting history and science through the years.

"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 it is much faster than natural decay. And it's being done without the use 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 second stage re-enter Earth's atmosphere in about 14 days.

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

Left: The cornerstone of the International Space Station - the combined Zarya (bottom) and Unity (top) modules - sails around the world after the crew of STS-88 completed assembly operations in December 1998. A propulsive tether system could replace most or all of the propellant refills that ISS will need for regular orbital reboosts. (NASA)

"With a relatively low development and operations cost of less than $50 million, a tether reboost system on the ISS could potentially save the program up to $2 billion over 10 years," wrote Johnson and Melody Hermann, another space engineer at NASA/Marshall in a July 1998 report. The tether would also increase the time available for microgravity experiments, a key justification for ISS, and cancel aerodynamic drag that would upset the more sensitive of those experiments. Although using the same principles as the ProSEDS demonstration, a propulsive tether on ISS would be powered by electricity from the ISS's solar arrays: 5 kW of electricity would produce a steady push of 0.5 Newton (about one-eighth of a pound).

Tether systems may also be useful in planetary exploration. With its great gravity and magnetic field, Jupiter would seem a natural place for a propulsive tether system to move a space probe into orbit around the planet, then tour the moons, and even power the spacecraft. NASA/Marshall investigated just such an intriguing possibility, but the answer was a surprising "maybe" rather than a resounding "yes."

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

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

Web LinksThe history of tethers - From an 1895 concept of an "elevator to space" to skyhooks to ProSEDS - learn how tethers have been affecting history and science through the years.
Plugged in to space. Proposed flight demonstration will show how to keep space clean - and boost satellites. (Oct. 15, 1998)
Up, up, and away (bit by bit)
Tethers in space describes the past, present, and promising future of tether applications in space, including technical details.
High wire act may be best way to explore Europa A March 13, 1998, story describing the use of propulsive tethers to explore Jupiter.
Tether science home page
Tethers Unlimited is a Clinton, Wash., company partnering with NASA/Marshall on ProSEDS and developing advanced tethers for other space applications.

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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack

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