As ships and robot submarines searched the ocean off Cape Canaveral for debris from the ill-fated Challenger shuttle, which might include all or part of the crew cabin, NASA`s interim board of inquiry was examining possible causes for the explosion that killed seven crew members.
The space agency was maintaining a strict silence about the direction of the inquiry by the six-member board, which met in Houston Friday, then returned to the Cape. But aerospace industry sources shed some light on the proceedings.
NASA investigators are checking reports that assembly equipment twice bumped into the sides of the liquid-fuel tank while it was being readied on the launch pad, the sources said.
The investigators are also exploring whether a nozzle in one of the shuttle`s two rocket boosters--attached on either side of the huge liquid-fuel tank--might have burned out, causing flaming gases to move up the side of the rocket, rather than earthward, the sources said.
According to the Associated Press, another possibility being studied by the board was that a tongue of flame from a leak midway in the right booster rocket triggered one of the explosive ``destruct`` packages in the aluminum liquid-fuel tank.
(The New York Times, quoting an unidentified source in Saturday editions, reported that a NASA study shows a sudden drop in power about 10 seconds before the Challenger exploded.
(The newspaper said officials believe the most plausible explanation for the sudden drop was flames burning through the side of the right-hand rocket. (NASA officials were said to believe that flames shooting out of the booster burned through the skin of the external fuel tank to which the booster rocket was strapped and through the liquid oxygen line inside, the newspaper said.
(Additional data supporting the theory were sent back from the Challenger, the source told the Times. )
One shuttle engineer, who asked not to be named, said he was questioned Wednesday by NASA investigators who said they were pursuing reports that a crane twice struck the sides of the 150-foot liquid-fuel tank.
According to the source, an employee of Lockheed Corp. who said he did not witness the alleged incident, investigators said workmen used portable X- ray machines to examine the struck area, but were unable to test the interior of the tank with the more sophisticated magnaflux technique, which detects stress and damage to metal parts, because the interior already was covered with insulation.
The interior consists of two separated chambers that contain liquid hydrogen and liquid oxygen, two of the most volatile substances known to man. The damage was alleged to have happened while crews from companies as diverse as Rockwell International Corp., Martin-Marietta Corp. and Lockheed worked to attach the fuel tank to the thinner, white solid rocket boosters and the orbiting vehicle itself.
A senior Air Force official at the Pentagon reached by telephone confirmed that he had received the same report this week.
Within hours of Tuesday`s explosion, NASA officials acknowledged at a press conference still another incident in which a crane struck the outer insulation of the liquid-fuel tank while the vehicle was standing upright on Pad 39B.
Walt Cooper, spokesman for the Denver-based Martin-Marietta Aerospace Corp., said the company had no reports about any other incidents in which damage occurred to the company`s tank.
He said Lockheed assembles the shuttle components once the tank arrives from Martin-Marietta`s Michoud plant in New Orleans, with help from crews employed by Rockwell and Martin-Marietta.
NASA has ordered its officials not to discuss the investigation as they seek to pin down the cause of the disaster.
But based on questions being asked of employees for the various contractors involved in the shuttle project, one of the possibilities being studied is whether efforts made to lighten the space vehicle to make it capable of carrying heavy military payloads increased the vulnerability of the fuel tanks to rupture.
Fuel-tank modifications allowed the company to increase the shuttle`s payload by 10,000 pounds in recent flights. Cooper emphasized that the changes in question have been in place during several shuttle flights and that engineers ``made certain that absolutely none of the changes decreased the structural strength of the craft.``
Concerns about possible nozzle burnout in one of the rocket boosters, which carry solid fuel, stem from televison pictures of the blast that appear to show flames from the booster moving upward--despite the fact that the vehicle was moving at 1,900 miles an hour.
According to the Lockheed engineer, in one test of the shuttle orbiter`s engines it was found that the nozzles that were almost 6 inches thick when manufactured had burned down to only three-eighths of an inch during flight.
According to Paul Hoven, a military hardware expert in Washington, flames from the rocket booster--because of either a leak or a malfunctioning nozzle
--could have triggered self-destruct devices on the liquid-fuel tank, which is attached to the underside of the shuttle for 8.5 minutes after launch.
Hoven said a number of experts have speculated in interviews with congressional staff investigators that once the flames reached that
``interface`` between the tank and shuttle they might have set off an explosive charge.