A Defect in Booster or Ruptured Seam? : Why Shuttle Exploded Still a Major Mystery

A little more than a minute after the space shuttle Challenger lifted off pad 39B at the Kennedy Space Center Tuesday morning, a thin tongue of flame appeared between the left booster rocket and the shuttle’s main fuel tank. The flame was not seen by observers on the ground, but slow motion videotapes of the launch show it clearly.

Less than a second later, nearly 200,000 pounds of liquid hydrogen in the main fuel tank exploded in a massive fireball that enveloped the shuttle orbiter and sent both solid rocket boosters flying off in nearly opposite directions.

The explosion destroyed the orbiter, killed its passengers and damaged the future of the American space program.

Speculation on Defect

While there is little doubt about what happened, the problem of how it happened remained a major mystery. By the end of Tuesday, however, most of the speculation centered on two possibilities.

One is that a defect in the solid rocket booster caused exhaust flames to spew from the side of the rocket, igniting the main fuel tank.

The second is that a seam in the main fuel tank itself ruptured, releasing hydrogen that caught fire and ignited the explosion.

Investigators will probably not be able to determine precisely which of those scenarios occurred, however, until the wreckage of the boosters and the fuel tank are recovered, a process that could take several weeks.

Launching is the most hazardous time for the shuttle, as well as other rockets, both because they are carrying a full fuel load and because they are subjected to maximum stress during that period.

With all five rocket motors--three on the shuttle itself and one on each of the solid rocket boosters--firing, the vehicle has enough power to literally batter itself to pieces as it forces its way through the dense air near the ground.

For that reason, the three main engines on the orbiter are throttled back to about 65% of maximum thrust shortly after liftoff. They remain throttled back until the vehicle passes through the point of maximum wind resistance.

Maximum Stress of Flight

That point typically occurs about one minute and nine seconds into the mission, when the rocket is passing through an altitude of 4.9 miles at a speed of about 1,800 m.p.h. It is at that point that the shuttle is exposed to the maximum stress of the flight.

Above that altitude, the air thins rapidly and resistance to the rocket’s passage drops off sharply. The main motors are then throttled up to full power to complete the climb out of the atmosphere.

Shuttle commander Francis R. Scobee was apparently in the process of throttling up when the explosion occurred. The vehicle would thus have been exposed to two very strong stresses within a short period of time: the maximum stress from wind resistance and stress from the added acceleration caused by throttling up.

The fact that the explosion occurred during the period of maximum stresses strongly suggests structural failure.

No Indication of Problems

NASA Associate Administrator Jesse W. Moore also told a Tuesday afternoon press conference that there was no indication of problems on board on the part of the astronauts or the ground crew. This lack of prior warning also suggests catastrophic structural failure.

Because the videotapes of the explosion seemed to show a sheet of flame traveling between the solid rocket booster and the main fuel tank at right angles to the direction of flight, much speculation has centered on a failure of one of the solid rockets.

Computer-enhanced video images of the accident show that the tongue of flame was preceded by a small explosion at the base of the booster rocket.

The solid rocket boosters are, in effect, a hollow metal tube filled with propellant. The propellant itself consists of finely dispersed aluminum particles. The aluminum is mixed with ammonium perchlorate, which breaks down in the presence of heat to release oxygen that combines explosively with the aluminum.

These are mixed into an epoxy-like polymer that is solidified to hold the fuel in position until it is needed.

Fuel Burns Outward

A small channel runs lengthwise through the center of the solid rocket. When the rocket is ignited, the fuel burns outward from the channel, with the hot gases escaping through a nozzle at the tail.

If the solid fuel should be punctured or cracked, the flame would quickly open a hole through the crack and burn through the outer shell of the motor. That could be what happened Tuesday.

Initial speculation had been that icicles formed on the main fuel tank as moisture from the air condensed during the 10 hours of below-freezing weather that preceded--and delayed--the launching. One or more of the icicles might then have jarred loose during the flight and punctured a hole in the rocket motor.

That possibility seems unlikely, however. Leo Krupp, a former test pilot for Rockwell Corp., the shuttle’s manufacturer, noted that heat generated by the shuttle’s passage through the air would undoubtedly have melted any icicles long before the accident.

Exposed to Hot Exhaust

Furthermore, the fuel in the solid rocket motor is encased in both a thick layer of insulation and a metal casing; both the insulation and the metal casing are thickest at the bottom of the booster, where the explosion seems to have occurred, because that segment is exposed to the hot exhaust for the longest period of time.

It is unlikely that any piece of ice smaller than an office desk could have caused significant damage to the casing. Further testimony to the strength of the casing is the fact that one of the solid boosters appeared to emerge from the explosion intact.

If the booster rocket was the cause of the explosion, the source must have been an inherent defect in the rocket motor itself or damage to the motor sometime before launch.

An inherent defect is a possibility, but the motors had performed flawlessly on the previous 24 shuttle launches. The motors’ manufacturer, Thiokol Chemical Corp., forwarded all inquiries to NASA on Tuesday.

Fuel Possibly Froze

Another possibility is that the cold weather froze the solid fuel, causing it to crack. The epoxy binder should be relatively impervious to cold, however, and it is also unlikely that the fuel would freeze because of the heavy insulation between it and the outer casing.

A final possibility is physical damage. At his press conference, Moore acknowledged that a boom arm had hit the vehicle Sunday night. He pointed out, however, that the boom had hit a small box that protruded from the shuttle itself rather than the booster rocket, and that the damage was not serious.

In November, however, NASA had reported: “A section of a rocket motor intended to boost a teacher and six others into orbit . . . may have been damaged in an industrial accident.”

Workers at Cape Canaveral, the report said, “heard a ‘sharp, cracking sound’ when an overhead crane was being used to lift a handling ring attached to the rocket section.” That section was one of the eight sections that were combined to produce the two booster motors.

No NASA Response

NASA did not respond to inquiries about that incident on Tuesday.

If Tuesday’s explosion was not caused by the booster rocket, then it almost certainly was caused by a leak in the main fuel tank, which carried the liquid oxygen and liquid hydrogen that are fed to the orbiter’s three rocket engines.

Unofficial NASA sources speculated that a seam on the tank might have ruptured under stress, releasing hydrogen into the air. Because of all the heat generated by the rocket motors and the ship’s passage through the air, the hydrogen could have ignited on contact with the air or when it reached the rocket motor.

This scenario was supported by the presence of what appeared to be feathery flames around the tail of the left booster immediately before the large explosion.