Shuttle Probe Sifts Subtle Clues : At Least 21 Unusual Events Detected Before Explosion

Computers monitoring the space shuttle Challenger’s flight provided some slight clues to ground controllers on what was to come as early as 13 seconds before the fatal explosion, according to a launch time-line released Friday by space agency officials.

The chronology shows a small drop in pressure in the right solid rocket booster 60.16 seconds into the launch, a reading most likely visible to controllers but which they probably regarded as normal, NASA spokesman Jim Mizell said.

In fact, the new computer data shows there were as many as 21 unusual events in a 73-second flight which ground controllers, until the end, had regarded as normal.

The pressure drop, which sources have said was so slight that it failed to raise caution flags, “would be the first opportunity you’d have to see something was wrong,” Mizell said.

‘Started at Ignition’

But the shuttle sent a continuous stream of subtle computer messages that show much more was happening.

“What this shows is that it was a continuous cause and effect. The cause probably started at ignition, and the effect was the plume and the explosion,” Mizell said. “It was a gradual process of deterioration till they got out there.”

While the pressure drop would have been the first anomaly apparent in the control room, the first unusual event actually occurred 0.44 seconds after ignition, with a puff of black smoke near a joint on the right solid rocket booster.

A failure in the seals that join the segments of the right booster has been widely viewed as the most likely cause of the accident, and none of the subsequent clues contradict that scenario, Mizell said.

The smoke lasted a little more than two seconds into the flight. Then, at 58.77 seconds, a camera picked up another puff of smoke on the right rocket booster, followed less then a second later by a well-defined plume in the same area.

Pressure Drop Detected

It was shortly after that when the pressure drop in the rocket booster was detected.

Then, the shuttle began a whole series of subtle maneuverings--movements of its control surfaces--which might be interpreted as normal but which probably meant the craft was compensating for a loss of thrust in the right booster, Mizell said.

At 66.17 seconds, bright spots began appearing in the plume near the booster.

At 66.62 seconds, cameras picked up a “bright sustained glow” on the side of the right booster and, a second later, there was a drop in the rate of liquid oxygen fuel pumped into the shuttle’s main engines.

At 72.14 seconds into the flight, the computers showed movement of one of the rocket boosters, at first outward and then a pitch forward.

Shuttle Moved to Left

The entire shuttle began moving rapidly to the left a fraction of a second later, at about the same time that fuel pressure from both the hydrogen and oxygen tanks began to drop.

At 73 seconds, the right rocket booster was measuring 24 full pounds per square inch less than the left--a significant decrease.

Cameras detected a “sudden cloud” along the side of the external fuel tank, and at 73.2 seconds, Challenger exploded.

NASA officials said the computer and camera data will allow them to simulate various launch conditions and equipment failures, then compare them with the data to see if they match Challenger’s readings.

The time-line was reviewed by the 12-member investigating commission appointed by President Reagan, which concluded two days of closed meetings and tours at Kennedy Space Center Friday afternoon.

‘Most Likely Cause’

During one of Friday’s tours, commission member Richard P. Feynman, a Nobel prize-winning physicist from Caltech, said a failed seal in the shuttle’s rocket booster appears to be “the most likely cause” of the explosion at this stage of the inquiry.

“It’s certainly true that that looks like the most likely cause,” he said. “If you made me say right now what is the most likely cause, then I might say that.”

But former Secretary of State William P. Rogers, chairman of the panel, said “no possibility has been excluded” in the probe to determine what caused the explosion that killed all seven Challenger crew members.

“We had a good deal of information provided, we’ve asked hundreds of questions, and I think the commission is able to say we’re making some progress,” Rogers said.

“No possibility has been excluded. The only portion of the shuttle that appears to be exonerated is the orbiter itself.”

Temperature an Issue

While Rogers said the 38-degree temperature on launch will be a major issue in the commission’s next open hearing later this month, he refused to confirm a report from Aviation Week and Space Technology that much lower temperatures had been recorded on the right solid rocket booster.

The magazine, citing unidentified sources, said the commission is looking at reports that an ice inspection team 90 minutes before liftoff recorded temperatures of 7 degrees and 9 degrees Fahrenheit at two points on the right booster--below flight safety limits.

The phenomenon could have been caused by a leak of super-cold liquid hydrogen fuel from the external tank, or any other of a number of possibilities, the magazine said.

Mizell said he was not aware of the details of the investigation. But he said it was not likely that a hydrogen leak from the main tank could go undetected for 90 minutes.