Speculation Rife but Hard Evidence Is Coming Slowly

Speculation about the cause of the Challenger explosion accumulated rapidly Wednesday, but hard evidence was being gathered much more slowly.

New theories varied from an internal collapse of the main fuel tank to an inadvertent detonation of the craft’s main self-destruct device. But no one knew for sure.

Many experts were particularly disheartened by the news that the shuttle’s two solid rocket boosters were destroyed just after the explosion. An Air Force range officer activated a self-destruct mechanism aboard the boosters when one of them threatened to crash in a populated area.

Investigators had hoped that recovery of the boosters intact would yield solid evidence about the cause of the tragedy.

NASA officials hope to reassemble the burnt and broken pieces of the main fuel tank and the orbiter to attempt to find clues about where and how the explosion occurred. Until all of the pieces are recovered, however, their primary hope--albeit a slim one--is to find a hint in the data telemetered to the ground immediately before the explosion.

NASA Associate Administrator Jesse W. Moore indicated in a Wednesday press conference that none of the controllers monitoring computer displays in Houston had noted anything unusual. The possibility exists that a malfunction occurred so quickly that it did not register on the computer displays, which are updated only once per second.

The investigating team will go back over tapes of the telemetry at very slow speeds to search for any unusual readings that might have presaged what would happen. In particular, they will probably look for such things as a sudden drop in thrust of the booster rockets or the main rocket motors, a sudden decrease in fuel flow to the main motors, or a loss of pressure in the main fuel tank.

In the absence of any firm evidence, speculation has centered on the two main components of the propulsion system, the solid booster rockets that assist the orbiter’s main engines in propelling the vehicle to high altitude, and the fuel tank that supplies liquid fuel to those main engines.

Computer-enhanced, ultra-slow-motion video images of the explosion seem to show at least three separate events prior to the explosion that consumed the orbiter.

The first hint of danger was the appearance of feathery orange flames around the tail of the left solid booster rocket. Those flames were followed by brighter flames, perhaps a small explosion, in the same general area. The third event was the appearance of what seemed to be a sheet of flame arrowing from a point about three or four feet above the tail of the booster to the main fuel tank.

Engulfed in Flame

The entire vehicle was then engulfed in a ball of flame that seemed to originate at the nose of the fuel tank. The entire sequence of events occurred in less than a second.

On Wednesday, Moore refused to speculate about the cause of the explosion “on the basis of the video images.”

A leading theory, however, is that the main fuel tank ruptured, allowing hydrogen to flow out. That hydrogen could have been ignited by heat generated by the vehicle’s passage through the air or by contact with the rocket exhaust.

The rupture itself might have been caused by an icicle dislodged during the flight, by a collapse of the bulkhead in the main fuel tank separating the liquid hydrogen and liquid oxygen, or by the failure of a turbine on the pump that forces liquid fuel into the orbiter’s motors.

Below-Freezing Weather

Icicles were formed on the shuttle from condensed water vapor during the 10 hours of below-freezing weather that preceded the launching. Officials from NASA and Rockwell International--the shuttle’s manufacturer--inspected the shuttle immediately before launching.

At the press conference Wednesday, Moore was asked if Rockwell had asked that the launching be scrubbed because of the ice problems. Moore said that after the inspection, NASA and its major contractors were satisfied that ice presented no significant hazard. As one NASA source noted later, “The worst we expected was that the ice might chip off a tile or two.”

The theory that a failure of the internal bulkhead caused the explosion is flawed in part because even if the liquid oxygen and liquid hydrogen mixed inside the tank, a heat source would still be needed to ignite them. An internal failure of the fuel tank would also not account for the flames and explosions around the booster.

Some engineers thus favor the possibility that a high-speed turbine on the fuel pump of the shuttle failed, sending parts of the turbine through the walls of the fuel tank and thereby triggering the explosion. If that is what happened, a study of the telemetry might show a pressure drop in the feed to the motors.

Speculation on Fuel Tank

In any case, the fuel tank attracts speculation because it is the only part of the shuttle that is designed to be disposable. When its fuel load is expended, the tank separates from the shuttle and re-enters the atmosphere, normally disintegrating somewhere over India.

Because engineers want it to break up on re-entry, and because excess weight is always a major problem on shuttle components, the tank is designed with walls as thin as safety permits. NASA has even left off the customary coat of white paint on the tank on recent missions because the paint weighed 600 pounds.

This relative fragility of the tank compared to the rest of the shuttle’s components could have contributed to mechanical failure of some sort during the period of maximum liftoff stress immediately preceding the explosion.

Tube Filled With Fuel

The second area of speculation involves the solid boosters because that is where the first explosions seem to have been centered. In essence, the booster is a hollow tube filled with fuel. Combustion occurs in a channel lengthwise through the center of the tube. If a crack or fault developed in the fuel, flame would quickly open a hole through the fuel and burn a hole through the outer casing.

There had been speculation that the booster might have been damaged before launch. NASA had reported in November that one of the segments of the boosters scheduled for this launch had been involved in an industrial accident.

A NASA spokesman reported Wednesday, however, that the damaged segment had been placed in storage and replaced with a new one.

Perhaps the most intriguing explanation of the explosion was offered Wednesday by Raymond Wiech, president of Maya Electronics in San Diego. Wiech is a physicist with extensive experience working on solid-fuel rockets.

Self-Destruct Device

He thinks a malfunction of the booster detonated a self-destruct device that runs the length of the shuttle and main fuel tank. Unlike the self-destruct devices on the boosters themselves, this device is designed to destroy the orbiter itself if it goes out of control and heads for a populated area.

Wiech and Bud Weisenberg--a Los Angeles-based space consultant who has also worked on solid fuel rockets--at a press conference Wednesday cited several pieces of evidence to support their theory.

The most important point, they said, was that the video images showed an orange-yellow flame before the explosion. Hydrogen burns without color, which is why exhaust from the orbiter’s engines is practically invisible.

The presence of color in the flame suggests that it was from the solid rocket motor, they argued. Wiech argued that a spectrometric analysis of the TV image should show conclusively that the flame was from burning rocket fuel.

They also noted that the flame that appeared immediately before the explosion was in an area adjacent to the detonation package for the self-destruction device.

Kinetics of Combustion

And finally, they argued that the kinetics of liquid fuel combustion would not have permitted the type of explosion observed if the cause had been a hydrogen leak. Wiech contended that it would take several seconds for the hydrogen and oxygen to mix sufficiently for an explosion of such force.

For comparison, he noted that when the old Atlas rockets used to blow up on the launching pad, the explosion did not resemble the massive, instantaneous one seen Tuesday, but rather took several seconds to develop into large explosions.

The detonation device, in contrast, is designed to cause a much more rapid mixing of the hydrogen and oxygen so that the explosion completely demolishes the vehicle.

Wiech and Weisenberg postulated that the extremely cold weather in Florida affected the solid propellant and led to the leak. Their calculations, they said, indicated that the fuel could have pulled away from its insulation just about the time of the explosion.

In 1983, several solid rocket boosters were recalled by the manufacturer, Thiokol Chemical Corp., because a protective lining on the exhaust nozzle was defective. During a test firing of the rocket, NASA said at the time, the protective lining came within eight seconds of burning through. The problem was thought to have been corrected.

David Smollar in San Diego contributed to this story.