WASHINGTON — In recommending that the space shuttle be launched, senior officials of the manufacturer of Challenger's solid rocket boosters acknowledged that cold temperatures could hamper performance of crucial rocket seals at liftoff but asserted that backup seals should prevent a dangerous leak of propellant, NASA documents showed Thursday.
The consent form signed by Morton Thiokol Inc. Vice President Joe C. Kilminster, released by the National Aeronautics and Space Administration, provides a significant clue in the escalating effort to determine why senior company officials overruled their engineers on the eve of the Jan. 28 flight and recommended that the launch proceed.
Pressure to Launch
According to a Morton Thiokol official, 15 mid-level engineers had remained opposed to the decision to launch even after four top management officials overruled them. These engineers had expressed concern that overnight subfreezing temperatures at Kennedy Space Center, Fla., would adversely affect the performance of the rockets' seals, which increasingly has become the focus of the investigation.
Also on Thursday, Sen. Ernest F. Hollings (D-S.C.) told reporters he had talked with Allan McDonald, the Morton Thiokol engineer who had been most vigorously opposed to the launch, and was told that the company's recommendation to consent to launch was produced by "tremendous pressure from NASA."
At the same time, three members of the presidential commission investigating the Challenger disaster visited Morton Thiokol's Utah facility to interview engineers and company officials involved in the internal debate over the launch.
At issue were rocket seals called O-rings, which are designed to prevent 6,000-degree propellant from escaping out of joints between sections of the shuttle's twin 149-foot rockets. The seals have secondary O-rings that are intended to provide a backstop should the primary O-rings fail.
Kilminster's consent form noted that temperatures forecast for the launch would make the O-rings 20 degrees colder than in any previous mission. Although temperature data was "not conclusive," he said, the cold could cause the primary rings to move more slowly as they "seated," or settled themselves in the first fraction of a second of ignition--thus preventing the formation of a perfect seal.
Rings Would Form Seal
But, Kilminster wrote, any propellant that passed by the first rings would only force the secondary rings to quickly seat themselves and form a viable seal.
"If the primary seal does not seat," Kilminster wrote, "the secondary seal will seat."
The contractor's confidence in the backup set of rubber seals appeared to conflict with NASA's acknowledgment three years ago that the crucial system could no longer be counted on to be redundant.
NASA had determined that extraordinary pressures on the rocket during the first few seconds of liftoff sometimes unseated the secondary seals, rendering them useless should propellant burn past the primary rings.
Furthermore, the space agency had conceded that this scenario could cause a chain of events that would end with an explosion similar to the one that claimed the Challenger and its crew of seven.
The concession that the system was no longer strictly redundant required NASA to place the O-rings on its list of so-called critical items--those whose failure could destroy the spacecraft and crew; it also required NASA to sign a form indicating that it was confident the primary O-rings would not fail.
Margin of Safety
This was done in 1983, and since then the space agency and Morton Thiokol have been working on ways to increase the margin of safety in the seals. None had failed completely in the previous 24 missions.
Had Morton Thiokol refused to recommend the flight proceed as scheduled, NASA policy would have dictated that it be postponed. Instead, Challenger lifted off in 38-degree temperatures and exploded 73 seconds later.
Company officials said three senior officials in addition to Kilminster made the decision to recommend a launch, despite a lengthy discussion in which engineers argued that it would not be safe to launch in temperatures far colder than any experienced before.
"The engineers were very displeased," said one Morton Thiokol source. "They thought a serious mistake had been made."
Most of the pre-launch debate, which was conducted over several hours, also included NASA and Morton Thiokol officials at Kennedy Space Center and the space agency's officials at the propulsion center in Huntsville, Ala.
A Morton Thiokol official, who asked not to be identified, confirmed that at one point Lawrence Mulloy, the NASA propulsion expert most involved in the discussion, objected to the engineers' concerns about cold weather by asking, "When would you have us launch, in April?"
NASA officials at Huntsville said Mulloy was not available for comment Thursday.