In-flight troubleshooting on Challenger

Rocketing toward its Saturday touchdown in California, space shuttle Challenger has developed its share of anomalies. But it's a remarkably tiny share for an experimental piece of equipment on its first flight.

When unexpected problems do crop up, Challenger's four-man crew and the ranks of flight controllers in the mission operations control room (MOCR) here at the Johnson Space Center carry out troubleshooting exercises calmly. The reason: The astronauts know from their years of intensive training that they are linked directly with a vast ground-based ''support tree'' of National Aeronautics and Space Administration (NASA) experts.

For the Space Transportation System's sixth mission (STS-6), the most glaring problem has been an apparent electrical-switch failure that developed in the US Air Force's 16-ton inertial upper stage rocket designed to lift a giant NASA communications satellite from the shuttle's 176-mile orbit into a 22,300-mile stationary Earth orbit.

A special team of NASA, Air Force, and private-industry experts is sending commands directly to the misplaced satellite, using its own maneuvering rockets to nudge it gently from an incorrect elliptical orbit into its intended higher, circular orbit.

Aboard Challenger itself, the crew has had to deal with a jammed OMS rocket-control device, a stalled fan, and other minor faults that are not expected to affect STS-6's mission objectives in any way.

Speaking for the enthusiastic crew as mission specialists Story Musgrave and Donald H. Peterson prepared Thursday for NASA's first spacewalk since 1974, STS- 6 commander Paul J. Weitz told Mission Control in Houston: ''I'm truly impressed with the way this vehicle's performed, and I think it's a real tribute to how much we've learned in the previous flights. . . .''

The preceding shuttle mission, STS-5, flown by NASA's first orbiter, Columbia , last November, ran into space suit problems that scrubbed a planned spacewalk in the vacuum of the ship's giant cargo bay. So NASA stretched Challenger's STS- 6 flight plan from two days to five, providing time for the postponed spacewalk.

Troubleshooter John D. Holt, chief of the guidance and propulsion systems branch here at Johnson Space Center, heads a ''support tree'' of space experts. When, as has happened with STS-6, a rocket fails to operate precisely as programmed, Mr. Holt's team of experts goes to work to identify the problem and find a solution. This procedure, he explains, may involve going all the way back to the person who assembled a suspect piece of equipment. Drawing on elaborately detailed contingency plans worked out on the ground months or years beforehand, Holt and his colleagues can provide quick access to the experts needed to correct specific faults.

Holt, who has worked with NASA space programs since 1966, explains that a piece of equipment as sophisticated as a $1 billion space shuttle is bound to encounter a variety of problems during each flight. He compares the shuttle with complex 1973 Skylab mission.

''We flew Skylab for a year,'' Holt says, ''and throughout that year we found we had some new anomaly showing up every day.''

He explains that the shuttle's own duplicate and often quadruplicate control systems give the shuttle crew the ability to solve many major mission-threatening or life-threatening problems on their own even if they lose all communication with STS ground controllers. But for lesser problems, ground personnel can use diagrams to locate faults, which could lie in the 300 miles of electric wiring at the heart of the STS avionics system.

The shuttle represents a new generation of complexity, Holt says, because ascent, orbit, reentry, and landing all place very specific and different demands on the vehicle. The multiple demands on the shuttle - to function as a reusable rocket, spacecraft, space plane, and glider - require the storage of far more information than can be carried even in the shuttle's array of five on-board computers.

Holt warns that information demands will increase tremendously when STS missions jump from the current five to more than 20 per year. Each mission will require specific computerized flight plans tailored to specific cargo weights and other requirements.

To handle far more flights per year, says Holt, astronauts and flight controllers must ''know what is important and not be confused by having too much data.'' But he is confident NASA will be able to handle future flights and their problems thanks to ''our accumulated knowledge gained from dealing with past flights.''

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