Zeroing in on the O Rings

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Even the celebrities on the blue-ribbon commission charged with investigating the explosion of space shuttle Challenger were upstaged last week by a steady stream of disclosures. First, the New York Times revealed that NASA internal documents had long ago warned about problems with the crucial O rings, the two giant synthetic-rubber washers that seal each joint between the booster-rocket segments. Next, an article in Aviation Week & Space Technology spelled out in extraordinary detail how the starboard booster had caused Challenger's external liquid-fuel tank to explode. Then, NASA released pictures showing a mysterious puff of black smoke apparently emerging from the booster at lift- off. The 13-member panel, which includes former Secretary of State William Rogers, Nobel Laureate Physicist Richard Feynman and Astronauts Neil Armstrong and Sally Ride, seemed to have its hands full just keeping up with the new information.

The O rings, already suspect, were spotlighted early in the week when the Times printed details of memos leaked by an unnamed solid-fuel rocket expert. One document, written last July by Richard Cook, an agency budget analyst, noted that booster O rings had shown signs of charring on previous missions and could lead to a "catastrophic" situation.

Panel Chairman Rogers reacted defensively to the Times scoop. At a commission hearing, he asked Cook rhetorically: "Do you think your engineering experience based on the short time you've been at NASA improved your ability to pass judgment on what others had decided?" Cook, who has no engineering experience, seemed stunned and did not reply to the question but forcefully defended the facts in his memo. Two days later, he told the press that NASA engineers had "whispered" in his ear that because of the O-ring problems they "held their breaths" during every shuttle launch. In other testimony, one of NASA's booster experts, Lawrence Mulloy, conceded that damage to the rings had occurred previously. In the 171 joints from spent booster segments that NASA has examined, he said, six primary rings showed signs of erosion.

The week's most tantalizing story, in Aviation Week, reported that the jet of flame from the side of Challenger's right booster had either melted or wrenched loose the struts that held the booster to the lower end of the external tank. The booster then pivoted on its still intact upper-attachment fitting and crashed its nose into the tank wall. The escaping liquid oxygen and liquid hydrogen ignited, causing the fatal explosion. At week's end NASA had not commented on the report.

Nearly everyone, including the space agency, seemed to be zeroing in on a failure of the right booster rocket, probably at its bottom joint, as the event that initiated the tragedy. The puff of black smoke seen in the NASA photographs and videotape lends support to theories that an O ring was at fault. According to a flight "time-line" compiled by NASA and released at week's end the smoke first appeared .445 seconds after booster ignition. It swirled between the rocket and the external tank, near where the fatal burnthrough seems to have occurred. One solid-rocket specialist noted that because the puff was dark, it probably did not result from combustion of the booster's solid fuel, which produces light-colored smoke. More likely, it came from the burning O ring or the putty placed inside the rings to protect the seal.

What might have caused an O ring in the right booster to fail? Panelist Feynman demonstrated one possibility at the public hearing by conducting a simple experiment in front of the TV cameras. He placed a small section of a ring in a C-clamp and submerged it in a cup of ice water. Then, removing the section and releasing it from the clamp, he concluded, "The resilience is very much reduced when the temperature is reduced." That fact may be significant, because the booster joints that the O rings are supposed to seal shift under the enormous stresses of launch. If the rings are not resilient, ! they may not seat properly in their grooves, leaving gaps through which the hot gases can escape. Thus, Feynman asked, would the low temperature (38 degrees F) at Challenger's lift-off have increased the chance of failure?

NASA's Mulloy conceded that the rings start to lose their resiliency at a temperature of 50 degrees F. But despite some reservations expressed the day before the tragedy by booster manufacturer Morton Thiokol, Mulloy said, NASA technicians had concluded (and Thiokol experts concurred) that the seals would work. Mulloy later volunteered that even if the primary O ring failed, the backup ring "would seat as it has done in the past, even under those temperature conditions."

Mulloy's statement seemed at odds with a 1982 NASA report. The document concluded that because of shifting motions in the boosters at launch, the secondary O rings might not seat properly. But NASA decided that the shuttle could keep flying without an assured backup, knowing that the consequences of failure, in the agency's own words, could be "loss of mission, vehicle and crew."

At week's end, as NASA continued to maintain a stiff upper lip about both the rocket's defects and the shuttle's future, Rogers issued a terse but devastating statement: he had advised the President that after only one week of hearings, the commission "has found that the process (of decision making leading up to Challenger's launch) may have been flawed." As a result, NASA was being asked to exclude those of its personnel involved in the launch from any further role on the investigating teams.

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