WASHINGTON — Engineers concerned about the performance of solid rocket seals in cold weather suggested at one point prior to the launching of the space shuttle Challenger that liftoff not be attempted in temperatures below the previous low mark for shuttle departure, a National Aeronautics and Space Administration propulsion expert testified Tuesday.
Nevertheless, the Challenger and its crew of seven lifted off with the launch pad temperature at 38 degrees--13 degrees colder than the previous record low during a launching.
Under persistent questioning from the presidential commission convened to investigate Challenger's disastrous Jan. 28 explosion, the solid rocket project manager at NASA's Marshall Space Flight Center in Huntsville, Ala., provided significant elaboration about the level of preflight concern among engineers for Morton Thiokol, manufacturer of the rocket boosters.
"There were data presented," Lawrence B. Mulloy said, " . . . by Thiokol engineering that there was a suggestion that possibly the seal shouldn't be operated below any temperature that it had been operated under in previous flights."
Previously, NASA officials had admitted there had been discussions among NASA engineers and major shuttle contractors about the possible effect of cold weather on the launching, but said that there was no disagreement with the final decision to proceed. Mulloy's testimony provided the first details about the nature of the cold weather concerns and revealed that Morton Thiokol's engineers suggested no launch be attempted below 51 degrees.
Mulloy said any disagreements had been resolved prior to launching but he provided no elaboration.
A January launching in 1985 was conducted in 51-degree weather, the coldest ever in 24 shuttle flights before the Challenger mission. The next coldest liftoff temperature was 57 degrees, NASA officials said.
Photographs of the Challenger before the explosion have shown an unusual plume of hot propellant spewing from the right-hand solid rocket booster at or near one of four major seams in the multi-sectional missile. It has been widely speculated by experts inside and outside the space agency that the ever-widening plume triggered a catastrophic explosion in the shuttle's adjoining fuel tank.
The rockets are assembled into four sections at a Morton Thiokol plant in Utah and then hauled by rail to the Kennedy Space Center in Florida for final assembly. The joints that hold the sections together are secured with an elaborate system of pins, protected by belts.
Inside the rockets, two rings of synthetic rubber--the O-rings--help prevent the jellylike propellant from blasting through the seams after the rocket is lit. A carefully laid buffer of flame-proof putty prevents the propellant from eroding the O-rings, providing a third line of defense.
Commission member Richard P. Feynman, a Nobel Prize-winning physicist from Caltech in Pasadena, began questioning Mulloy about cold weather effects on the O-rings after conducting an impromptu experiment on a small piece of the synthetic rubber material.
While Mulloy testified, the physicist removed an O-ring from a cross-sectional sample of a joint that the NASA official had brought as a visual aid for his testimony. He asked a NASA aide to bring him a small vise and a glass of ice water.
He screwed the rubber into the vise and submerged it in the cold water. Then he pulled it out and removed the vise, watching to see how quickly the synthetic rubber sprang back to its original shape.
It bounced back less quickly than it did without being dunked--evidence, Feynman said, that the rings would be less resilient in cold weather.
The point was important because Mulloy had testified earlier that some previous cases of contamination in the seal system had occurred in the fractional seconds before the pressures of rocket ignition fully set the rings to form a perfect seal.
Thus, if the synthetic rubber responded more slowly in the cold, it might allow the hot propellant more time to seep under the seals and carve a path out of the joint.
Noting his "little experiment," the physicist asked Mulloy: "Does your data agree with this feature that the immediate resiliences in the first few seconds is very, very much reduced when the temperature is reduced?"
"Yes," Mulloy said. "In a qualitative sense. I just can't quantify at this time."
The NASA official testified that research data indicated the O-rings should operate properly in temperatures as low as minus-30 degrees. But Mulloy said the data "is refuted by some other test data, and that is why we're carefully running controlled tests under the specific conditions of 51-L (the mission designation) to understand the response of the O-ring seal . . . . "
Test Data Available