WASHINGTON — Internal space agency documents showed Wednesday that the agency had been concerned for years about the safety of the intricate components in the seal between segments of the shuttle's solid boosters but had enough confidence in one critical part to waive a requirement for a back-up system.
The possible failure of seals on the solid booster has been the most investigated and discussed of the potential causes of the explosion of the shuttle Challenger, largely because videotapes taken prior to the blast show a plume of flame emanating from the right booster, possibly coming from a seal.
'Loss of Mission'
The documents show that in December, 1982, National Aeronautics and Space Administration officials put synthetic rubber seals called O-rings in the most sensitive category of the space shuttle's long "critical items list," saying that their failure could cause "loss of mission, vehicle and crew due to metal erosion, burn-through, and probably case-burst resulting in fire and deflagration (intense, rapid burning)."
The heightened concern arose, NASA officials said Wednesday, when it was learned that one of the two O-rings sealing the seam between booster segments could become unseated in the fraction of a second that it takes the huge booster to ignite and build to a thrust of 3.3 million pounds.
During that instant, a "joint rotation" phenomenon can unseat the secondary O-ring, leaving the primary ring in the assembly as a "single failure point." This means that only one quarter-inch ring would be left to prevent flame from reaching the structure of the rocket case.
Still, space agency officials said they believed it safe to launch the rockets under such conditions because they had subjected the rings to tests at pressures more than double those expected during the 2.09 minutes that the two solid boosters burn during the shuttle's ascent to orbit.
Less than a month after the primary O-ring was put in Category 1 on the solid rocket booster's critical items list, a waiver was signed by Michael Weeks, the space agency's deputy associate administrator for manned space flight, allowing use of the boosters despite the potential problem.
Cause 'an Open Issue'
The shuttle investigation, said William Lucas, director of NASA's Marshall Space Flight Center, "has not concluded that the Challenger accident was caused by the solid rocket booster. Neither have we concluded that the SRB (solid rocket booster) did not cause it. The cause is still an open issue."
In addition to their excruciating analysis of thousands of pictures of Challenger's 73-second journey to disaster, investigators have reviewed films of earlier shuttle launchings. But Lucas said they have so far seen no flames indicating a leak in a solid rocket seam or a burn-through within one of the segments.
During those earlier launchings, officials continued to search for a solution that would alleviate their concerns about the seals. Meanwhile, documents showed, there were a dozen instances in which the primary O-ring showed erosion when the expended boosters were recovered and one case of damage to the secondary ring.
Among the possible fixes were a "capture feature" designed to reduce the "joint rotation" and the tendency of the secondary O-ring to become unseated, a wire or carbon mesh on the rings to make them seat more securely or the introduction of a new, larger O-ring.
However, NASA had been hesitant about making changes, officials said, for fear of introducing a new and unknown problem in the process of addressing their concerns about the O-rings.
The documents released by the space agency Wednesday had already been turned over to the presidential commission investigating the disaster.
Questions About Weather
On Tuesday, in its second open hearing, the commission had raised pointed questions about the possibility that launching of the shuttle in cold weather--the temperature had plunged to 27 the night before liftoff--had contributed to explosion in space.
Lawrence Mulloy, director of the booster program, said Wednesday that the launching proceeded with the approval of engineers from Morton Thiokol, manufacturer of the booster.
On the eve of the launching, Thiokol engineers looked at NASA data on the possible effect low temperatures might have on O-ring performance, Mulloy said.
"The initial recommendation of the Thiokol engineers was that we should launch within our experience base--which was that the O-ring temperatures should be 53 degrees."
Witnesses testifying Tuesday before the presidential commission investigating the accident said that, despite the freezing weather on the launching pad, they believed the internal temperature of the boosters' fuel to be in the 50s.
Then, Mulloy said, the Thiokol program manager "recommended proceeding with the launch under those temperature conditions based on the engineering analysis that had been done."