CONTACT: Stanford University News Service (650) 723-2558
1 1 8/11/92 CONTACT: Joel Shurkin, News Service (415) 725-1944 Moisture may have caused tethered spaceflight failure, expert says
STANFORD -- Moisture in the long, delicate tether that failed to unwind in space may have caused the shuttle Atlantis to fail in its attempt to generate electricity and study the ionosphere of earth last week, a Stanford engineer said.
Had it succeeded, the entire structure - the Atlantis, the tether and an Italian satellite at the tether's end - would have been easily the largest structure ever flown in space. Despite the failure, NASA officials said they learned a lot about how a tethered object behaves in space, but very little about generating electricity.
The shuttle's daring attempt to fly with a tethered satellite above it - an experiment designed largely at Stanford - experienced mechanical, rather than scientific or theoretical, failure last week when the tether and its satellite cap refused to deploy more than 257 meters (845 feet). It was supposed to go 20 kilometers (12.4 miles) above the spacecraft.
The seven Atlantis astronauts finally reeled back the tether after several tries and landed safely on earth Saturday, Aug. 8.
"In some ways, it was quite successful," said Stanford's Roger Williamson, a senior research scientist on the project and one of the originators of the idea. The shuttle flight "determined that the concept was valid and that a tethered structure was stable, at least as far out as it went" - the part that worried the scientists most.
Williamson said the problems reeling the tether out might have been caused by moisture gathering in the coiled, braided tether. The whole system was tested and packed in air-conditioned, dehumidified rooms in Houston and then sent to Florida, where moisture could have condensed from the hot, humid air.
In space, the moisture would have frozen, stiffening the tether.
"Everything that happened could have been explained by moisture," he said.
Much of the preliminary design work was done at Stanford and directed by electrical engineering Professor Peter Banks, who is now on administrative leave to serve as dean of engineering at the University of Michigan. Researchers from Utah State University also were involved. Many people from Stanford's Space Telecommunications and Radioscience Laboratory were at the Johnson Manned Spaceflight Center in Houston during the mission.
Some design was based on a concept called SURFER, set out by a group of 40 Stanford students (including three undergraduates) in 1986 as a Banks-led project.
Under the original plans, the Atlantis crew was to deploy the satellite at the end of the long copper and plastic tether, about the width of a strand of spaghetti, from the cargo bay of the shuttle using the space-age version of a fishing reel, a four-story boom.
Moving a copper wire through any electric field generates power. If completely deployed, this wire could produce as much as 5,000 volts as it traveled through the electrons and ions in the ionosphere, the engineers believed.
Besides pointing the way to a useful source of power for future spacecraft, the experiment had other possible ramifications, including artificial gravity in space (the acceleration of the satellite at the end of the tether produced an artificial gravity), and "sky hooks" to scoop payloads from the surface and bring them into orbit. Those were among the reasons NASA was willing to invest $379 million in the experiment. The satellite itself at the end of the tether cost $191 million.
The system was to drag through the ionosphere for 30 hours before the astronauts reeled it back in.
NASA officials admitted from the beginning that this was among the riskiest missions ever flown on a shuttle, but most of their concerns were centered on the flight characteristics of an ungainly structure, and the complicated and potentially hazardous procedure for reeling it back in. When the satellite was drawn close to the shuttle, NASA was worried it might be difficult to control; collisions in space are particularly dangerous.
They apparently worried about the wrong problem.
The tether was wound 26,059 times in 56 layers coiled in the cargo bay of the shuttle. The astronauts tried three times to unwind Tuesday but failed to get it to deploy very far.
Jeffrey A. Hoffman, the astronaut running the project on Atlantis, tried again Wednesday, slowing reeling almost 30 meters (90 feet) of the tether back in to the shuttle. He was then to reverse the direction of the reel and try to send the tether out faster than before.
But the electric motor driving the reel failed and the whole mechanism appeared jammed. The shuttle was threatened with having to cut itself loose from the tether and satellite.
The astronauts finally freed the tether by jiggling the boom back and forth, but by this time NASA decided it would not try to deploy the device again, for now giving up on the experiment.
The next problem was getting the satellite and tether back into the shuttle bay, which NASA assumed would be both difficult and dangerous. Pilot Col. Loren J. Shriver nudged the Atlantis under the tether and satellite, prepared at any moment to blast the spacecraft away if a collision seemed imminent. Two astronauts also were standing by to go outside the spacecraft if necessary. The retrieval, however, proved effortless, saving the satellite for another day.
The satellite was back in the shuttle 24 hours and four minutes after it was first lifted out of the cargo bay. It had produced at most 60 volts of electricity, usually only 40 or 50.
The mission was not a complete technological disaster, however. Atlantis was able to orbit a $213 million European satellite, although it took some trouble-shooting to get it in its proper orbit.
And NASA officials said they did learn that a tethered structure is not as difficult to control as they had feared.
"From a flight dynamics standpoint, we learned an awful lot," flight controller Charles Shaw said. "We didn't get out quite as far as we wanted to," but the whole experience proved "very reassuring."
Williamson agreed. The further the tether went out, the more stable the structure would be, so Atlantis flew in its most perilous configuration with no trouble.
Although NASA has not put in for funding for a repeat mission, the Italian government apparently has requested another attempt, especially since it has the equipment back. Williamson said there was significant support from the scientific community for another attempt.
lid, pw, ban, bab, lan, eng 1-3, sci 1-7 10 tetherfails
This is an archived release.
This release is not available in any other form.
Images mentioned in this release are not available online.
© Stanford University. All Rights Reserved. Stanford, CA 94305. (650) 723-2300.