WASHINGTON — Behind the expected conclusion that the immediate cause of the Challenger disaster was the failure of a rocket seal, the presidential commission investigating the Jan. 28 explosion reported a far more complex and disturbing account of the multibillion-dollar accident that claimed seven lives and NASA's once-spotless reputation.
Using computer reconstructions of Challenger's flight and small-scale tests of booster components, the panel concluded that the O-rings and the rocket joints they were supposed to protect were in fact wrongly designed 13 years before the doomed Challenger flight and never changed despite persistent doubts about their integrity.
For that, the panel indicted a National Aeronautics and Space Administration bureaucracy that ignored a crucial safety problem and a rocket maker, Morton Thiokol Inc., with an apparently equal distaste for bad news.
The panel stated that several previous shuttle missions skirted the same catastrophe that felled the Challenger flight. What may have doomed Challenger and spared the others, it implied, was an ominous conjunction of man-made and natural events.
The circumstances included soaking rains that may have formed puddles within the suspect rocket joint, frigid weather that reduced the O-rings' resiliency, an asbestos joint putty that failed to perform as expected, a steel rocket section that was not perfectly round at a crucial location and launching stresses and gusty high-altitude winds that flexed and shook the booster at the worst possible points and times.
In purely physical terms, the panel stated, the destruction of the Challenger and its crew appears to have begun at 11:38.00.688 a.m. last Jan. 28--barely two-thirds of a second after the shuttle's two solid rocket motors were ignited--as the spacecraft struggled to rise from Kennedy Space Center's Pad 39B into its first inches of flight.
At that instant, a single jet of inky smoke spurted skyward from near a joint that connects two sections of the aft portion of the spacecraft's right-hand booster rocket. It was followed 0.158 seconds later by a second jet of smoke; then, in the next 1.664 seconds, by seven more, escaping in rough syncopation with liftoff vibrations that were then flexing the left and right rocket boosters some three times a second.
In the two minutes before they burn off, the two rockets are supposed to propel the shuttle off the ground to a speed of about 1,900 m.p.h. The plumes emerged from that part of the right-hand rocket that abutted the shuttle's external fuel tank. The huge red tank was actually two sheathed tanks, one filled with liquid hydrogen and the other with liquid oxygen, that fed the Challenger's main engines and were meant to keep the shuttle on its course to orbit after the booster rockets had burned off.
First Plume Noted
No other abnormalities in the booster rocket were visible until some 58.8 seconds into the flight, when film of the mission revealed a first faint plume of rocket thrust bursting from the same location.
Less than a half-second later, the plume had become "continuous and well-defined," the commission stated. At 60 seconds into the flight, thrust pressure in the right-hand rocket began to decline compared with that of the undamaged left-side booster. Then, according to the commission, the following sequence occurred in rapid-fire succession:
60.988 seconds: The plume continuously deflected off the external tank.
62 seconds: The shuttle's left-hand booster began changing the direction of its thrust to offset the sideways push of escaping gas from the right booster leak.
64.660 seconds: Hydrogen began burning from a hole seared in the external tank.
72.2 seconds: The plume bored through the lower strut holding the booster to the external tank. The bottom of the rocket flailed outward, forcing the top into the oxygen-filled part of the external fuel tank.
73.124 seconds: The hydrogen tank began to disintegrate and the exploding gas "created a sudden forward thrust of about 2.8 million pounds," pushing into and bursting the bottom of the oxygen tank.
Within milliseconds, a massive explosion of mixing hydrogen and oxygen destroyed the spacecraft, rupturing the Challenger orbiter's own rocket fuel tanks and enveloping the orbiter in a reddish brown fireball.
The shuttle was at 46,000 feet, moving upward at 1.92 times the speed of sound. Its computers functioned to the last millisecond, shutting down the orbiter's three main engines one by one until the last radio transmission was recorded, 74.13 seconds into the mission.
The recovered right-hand booster sections show a burned-out hole, 33 inches wide. That is larger than the estimated 6- to 8-inch gap that triggered the explosion because the booster continued to burn for 37 seconds after the fireball. It was finally destroyed on radio command from the ground.