Freezing Temperatures Under Study for Role in Blast

Challenger’s solid rocket boosters, which are suspected of initiating the explosion that destroyed the spacecraft, were so near their minimum allowable temperature just before launch that investigators are now trying to determine if freezing temperatures played a role in the accident.

Specifications for launching the shuttle call for the internal temperature of the rocket structure and propellant to be between 40 and 90 degrees Fahrenheit, according to technical data provided by Morton Thiokol, manufacturer of the solid rocket.

Temperature Reached 24

The air temperature on the night before the launch at the Kennedy Space Center reached a low of 24 degrees. The temperature at launch was 32 degrees. Since the rockets are 12 feet thick and the rubbery rocket propellant is a good thermal insulator, the internal temperature changes very slowly.

Nevertheless, the boosters sat outside in the cold for so long that investigators are now examining whether key portions of the boosters may have fallen below the minimum specifications, according to a Morton Thiokol expert at Kennedy Space Center.

Officials at Morton Thiokol denied that the shuttle was launched when internal rocket temperatures were not within allowable limits. “There is no chance it could have been near 40 degrees,” said Gil Moore, director of external relations for Thiokol.

There have been persistent reports that Rockwell officials advised the National Aeronautics and Space Administration last Tuesday morning, just prior to the Challenger launch, to delay the mission because of weather conditions. Rockwell spokesmen said the company can neither confirm nor deny those reports.

Not Only Theory

This isn’t the only theory of what went wrong. So far, investigators have focused much of their efforts on determining whether faulty seams, flawed casings and poorly packed fuel might have caused the rocket motor malfunction.

Also, CBS reported Monday night that a former solid rocket test engineer, Richard White, said the reusable rockets have a history of cracks and corrosion problems in their aft skirts, the large structure on the rocket that rests on the launchpad.

The theory that cold weather played a role is being considered by teams at both Kennedy Space Center and Marshall Space Flight Center in Huntsville, Ala.

Temperatures below design limits may cause solid fuel to crack or separate from the case of the rocket, resulting in a possible burn through of the case, a Marshall expert told The Times.

Device Was Detonated

One of the solid rocket motors is thought to have experienced such a burn through of its outer wall, sending a white hot plume of gas against the nearby hydrogen fuel tank or detonating an explosive self-destruct device. The latest evidence suggests that the self-destruct device was detonated, according to Marshall investigators.

Every solid rocket motor is designed to operate in particular temperature ranges. Some military rockets are designed to operate at 65 degrees below zero. But the solid motor on the shuttle was not designed to these specifications, according to experts at Thiokol and independent rocket experts.

In addition, a disagreement has emerged among various contractors and NASA over the interpretation of those specifications.

“It’s not black and white,” said a Rockwell International engineer. “The world is not that simple.”

The specifications leave open to judgment exactly what is meant by a 40 degree average temperature of the structure, the Thiokol expert told The Times.

“I believe the bulk (average) temperature of the rocket was over 40,” the Thiokol expert said.

“Not all the pieces of the rocket will be the same temperature,” he said. “Some tiny dinky piece sticking out may have been below 40. That’s what they are looking at at Huntsville.”

The disagreement over interpretation is likely to be the focus of a “protracted legal battle,” the source speculated.

Reviewing Calculations

Investigators in Huntsville are reportedly reviewing computerized temperature calculations of the booster.

The solid rocket boosters do not contain sensors that will directly disclose their internal temperature.

“You calculate the internal temperature based on external temperature, wind chill factors and length of exposure,” said Moore, the Thiokol spokesman.

Moore said the internal temperature changes very slowly because the solid rocket motors contain 1 million pounds of propellant, a significant mass that would take “hours if not days” to drop below 40 degrees.

The temperature guidelines are “very, very conservative. There’s lots and lots and lots of room for safety built into that figure,” Moore said. “If it gets cold enough, it would eventually crack the propellant and it will blow up when that happens.”

The guidelines are meant to prevent that, Moore said, and he declined to say what, if anything, might have happened specifically to the Challenger’s boosters if they became too cold during last week’s cold snap in Florida.

A specialist in solid rocket motor technology in Los Angeles told The Times that a drop in temperature in critical localized areas can cause catastrophic problems even if the average internal temperature is far above specifications.

“It is possible if it got cold enough at the interface between the case and the propellant that you could have a problem,” he said. The case of the rocket, the area closest to the outside temperature, would be the coldest part of the entire rocket structure.

In January, 1985, NASA postponed the launch of a shuttle mission for three days when temperatures fell to lows of 19 degrees, about the same general area as last week’s conditions.

Request Declined

NASA declined a Times request to provide additional information regarding temperatures at the time of liftoff and before, saying all information on temperatures has been impounded by the investigation team.

“We do not lift off exceeding redline values,” said Jim Mizell, a NASA spokesman. “We are very cautious. That would have required a top management team decision.”

Ralph Vartabedian reported from Los Angeles and Maura Dolan from Kennedy Space Center, Fla. Staff Writers Peter H. King and Scott Kraft also contributed to this story.