Space Shuttle Challenger
The flight of Space Shuttle Challenger began at 11:38 am EST on January 28, 1986. It ended 73 seconds later in an explosive burn of hydrogen and oxygen propellants that destroyed the External Tank and exposed the Orbiter to severe aerodynamic loads that caused complete structural breakup. After all was said and done, seven people were killed, and many more left in grief along with a shuttle in pieces with no apparent reason as to why.
To understand everything completely, a brief timeline is needed to show just what happened exactly during this flight. At .678 seconds into flight, a strong puff of gray smoke was spurting from the vicinity of the aft field joint on the Solid Rocket Booster. The two pad cameras that could have pinpointed the exact location of the smoke were inoperative. This area of the SRB was facing the External Tank.
The black color and dense composition suggested that the grease, joint insulation and the rubber O-rings in the joint seal were being burned and eroded by the hot propellant gases. From 37 seconds to 64 seconds, Challenger encountered several high-altitude wind shear conditions, which the guidance, navigation and control system sensed and countered. When the main engines were throttled up to 104 percent throttle, a flickering flame appeared on the right Solid Rocker Booster in the area of the aft field joint at 58.788 seconds into flight. At 59.262 seconds, the flame was visible without image enhancement. About the same time, telemetry showed a pressure difference between the chamber pressures in the left and right boosters. As the flame increased in size, it was deflected rearward, which directed the flame onto the surface of the External Tank.
A visual indication that the flame breached the External Tank was at 64.660 seconds, when there was an immediate change in shape in color. This indicated that the flame was mixing with the leaking hydrogen from the External Tank. Within 45 milliseconds of the breach of the External Tank, a bright, sustained glow developed on the underside of the Challenger between it and the External Tank. At 72 seconds, a series of events occurred in extremely rapidly succession that terminated the flight. At 72.2 seconds, the lower strut linking the SRB and the External Tank was severed and pulled away from the weakened hydrogen tank permitting the right SRB to rotate around the upper strut. At 73.124 seconds, a circumferential white vapor pattern was observed blooming from the side of the External Tank bottom dome. This was the beginning of the structural failure of the hydrogen that culminated in the entire aft dome dropping away. At the same time, the rotating right SRB impacted with the intertank structure and the lower part of the liquid oxygen tank. These structures failed at 73.137 seconds, as shown by the white vapors appearing in the intertank region. Within milliseconds, there was a massive burning of the hydrogen streaming from the failed tank bottom and the liquid oxygen breach in the area of the intertank. Challenger blew up into thousands of pieces while traveling at Mach 1.92 at an altitude of 46,000 feet.
Main Reasons for Accident
The Presidential Commission Report on the Space Shuttle Challenger accident discovered that the failure of the pressure seal in the aft field joint of the right Solid Rocket Booster. The specific failure was the destruction of the seals that are intended to prevent hot gases from leaking through the joint during the propellant burn of the rocket motor. To arrive at this conclusion, the Commission reviewed all available data and then developed specific scenarios and the range of most probable factors. First, a combustion leak through the right Solid Rocket Motor aft field joint initiated at or shortly after ignition weakened the External Tank initiating the structure breakup and the loss of the Space Shuttle Challenger. Second, the Commission determined that this was indeed an accident and not an act of sabotage. Third, significant out-of-round conditions existed between the two segments joined at the right Solid Rocket Motor aft field joint. Such as, the diameters of the two Solid Rocket Motor segments had grown due to prior use. The growth resulted in a condition at launch where the maximum gap between the regions of the joint's O-rings was no more than .008 inches, whereas the average gap would have been .004 inches. The lack of roundness of the segments occurred at the initiation of the assembly operation. Fourth, the air temperature at the time of launch was 36 degrees Fahrenheit; 15 degrees lower than any other launch. This was so cold that ice had formed on the pad, the rocket, and the instruments around the pad.
Fifth, O-ring resiliency is directed to its temperature. A compressed O-ring at 75 degrees Fahrenheit is five times more responsive in returning to its uncompressed shape that a cold O-ring at 30 degrees Fahrenheit. This means that a warm O-ring will follow the opening to the gap, whereas a cold O-ring may not. Sixth, the primary mechanism that actuates O-ring sealing is the application of gas pressure to the upstream side of the O-ring as the ring sits in its groove. For this to work, a space between the O-ring and its upstream channel wall should exist during pressurization. Seventh, the O-rings showed signs of thermal distress. A leak path accompanied each instance of thermal O-ring distress in the insulating putty. Eighth, there was a possibility that water was in the joint and if this was the case the water would have froze which would inhibit proper secondary seal performance. In view of the findings, the Commission concluded that the cause of the Challenger accident was the failure of the pressure seal in the aft field joint of the right Solid Rocket Booster. This failure was due to a faulty design (unacceptably sensitive to a number of factors.) These factors included the effects of temperature, physical dimensions, the character of materials, the effects of reusability, processing and the reaction of the joint to dynamic loading.
A modified joint and seal, essentially an "inside-out" version of the old joint and seal, was improved by this faulty joint. Joint rotation pushes the tanks harder against the O-rings, thereby making an even tighter seal. The development of more resilient materials to coat and manufacture with can only aid in making the design better.
The decision to launch Challenger was flawed. Those who made that decision were unaware of the recent history of problems concerning the O-rings, the written recommendation from the contractor advising against the launch below 53 degrees Fahrenheit, and the continuing opposition of the engineers at Thiokol after the management reversed it position. They did not have a clear understanding of Rockwell's concern that it was not safe to launch because of ice on the pad.
It took seven months, 31 days, 52 aircraft and 6,000 workers to search for the shuttle debris. Some of the things found were McAuliffe's lesson plans for space, a wing from the Orbiter, the forward fuselage, and the crew compartment. When the compartment was brought up, it became evident that some of the astronauts had been alive during the three-to-four minute fall to sea.
Movie of the Space Shuttle Challenger- shows the launch of shuttle and the explosion of the shuttle Challenger Links
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