VII. CASING JOINT DESIGN Discussion (a) Introduction The fact that the aft field joint of the right-hand Solid Rocket Booster failed at the 300 degree location is overwhelmingly sup- ported by the evidence. Retrieval of two large pieces of the joint clearly show that they were destroyed by the heat and velocity of the gas flame emanating from the right-hand booster. Additional supporting evidence was found by reviewing the telemetry data and the photographs taken during launch and flight.' For the purpose of redesigning the joint it is important that the way in which the joint failed be determined as closely as possible. This determination, however, is difficult, if not impossible, to make with one hundred percent certainty. The evidence to support progress of the failure through the joint is incomplete. However, based on the recorded history of the joint problems encountered in flight and in test, based on the laws of physics, and based on behav- ior of the materials used in the joint, the following PROBABLE CAUSE is offered. (b) Probable Cause of Failure 1. Both the primary O-ring and the secondary O-ring were seated when the steel casings were mated. The pressure check verified this fact. However, from experience, the primary O-ring was seated in the upstream position as had been previously recognized by NASA and Thiokol engineers. (See Figure VII-1.) Rogers Cornmiasion Report, Volume I, pp. 22-23 and 78-79. (183) 184 . PRIMARY / -RING EEATED BUT INUPSTREAM WRONG) POSITION DOWNSTREAM- SECONDARY 0-R ING hEATED DOWNSTREAM (PROPER) POSIT ION )I; FIGUREVII-1 185 2. Upon ignition, the primary O-ring could not reseat at the 300 degree location in the downstream position, where it needed to be in time to prevent blowby. At this point there were too many defi- ciencies acting in unison which prevented the O-ring from reseat- ing in that location. First, proper spacing between the inner face of the tang and the opposing face of the inner leg of the clevis ap- proximately 0.020 inches, is critical. That spacing for Flight 51-L was too small, at the 300 degree location where the smoke was ob- served, to facilitate prompt reseating of the primary O-ring. Calcu- lations of segment diameters indicate the gap spacing was only 0.004 inches, near metal-to-metal contact. The ignition gases passed the O-ring at this location (See Figure VII-2). This condition did not exist elsewhere in the joint around the casings since the pri- mary O-ring was able to seat around the joint in other locations. Second, the low temperature throughout the night prior to launch left the fluorocarbon elastomer primary O-ring stiff and lacking in ability to spring into the downstream (seated) position at the 300 degree location in time, relative to the buildup of motor pressure, to provide a tight seal. The temperature of the aft field joint at time of launch was calculated by Thiokol after the accident to be 16 degree F. Part of the reason for this low temperature was the heat transfer away from the joint, by conduction through the aft attachment strut. The conduction was driven by liquid hydro- gen, which remained in the external tank overnight. The supercold fuel created a 430 degree temperature differential across the ship, drawing heat out of the joint and O-rings. At ignition, blowby occurred, either with erosion of the primary O-ring or without erosion. 64-420 0 - 86 - 7 186 -SPAC I NG "Too TIGHT" O-RING WILL NOT 1 SEAL 187 3. However, there could have been one or more than one blow- hole through the zinc chromate putty before ignition. One such blowhole could have been made at the 300 degree location either during the leak check at 200 psi prior to the seating of the primary O-ring or prior to the leak check when the casings were brought together. If so, there was a high probability that the primary 0- ring eroded at this point. The phenomena of blowing holes in the putty had been observed many times at post-flight dismantlement and had dramatically increased when the procedures were changed to increase the test pressure to 200 psi. Additionally, the Randolph putty had been found unsatisfactory on numerous occasions.2 4. Upon ignition of the Solid Rocket Motor, this blowhole would have facilitated and concentrated the hot propellant gas flame on the primary O-ring, and possibly the secondary O-ring as well. Alignment of O-ring erosion with the location of blowholes had been observed on numerous occasions.3 5. Between the time the casings were assembled and the launch, the secondary O-ring was unseated from its previously sealed posi- tion. The fact that it had been sealed has been verified by the pres- sure check made 28 days before when the casings were joined. Either the secondary O-ring was unseated by joint rotation coupled with O-ring stiffness or by the formation of ice in the joint. During the intervening period, as the Shuttle stood on Pad 39B waiting for the launch, 7 inches of rain had fallen and some could have easily penetrated the joints. The access of rain water into the joints was proved when STS-9 was disassembled and water poured out of the assembly pin holes. In tests conducted after the accident, it was confirmed that the water in the aft field joint would have turned to ice, and that the ice could have dislodged the secondary O-ring, pushing it upstream into a non-sealed position. In this posi- tion, it is doubtful that the secondary O-ring could have sealed at ignition. 6. One of the three Solid Rocket Booster to External Tank aft at- tachment struts is also connected at the 300 degree location, just a few inches below the aft field joint. As the Space Shuttle system stood on the launch platform at Pad B on January 28, the large External Tank was gradually filled with liquid hydrogen and liquid oxygen. Liquid hydrogen, at a temperature of 423 deg F below zero, and liquid oxygen, at a temperature of 297 deg F below zero, caused the tank to contract as it was filled. Since the Solid Rocket Boosters are firmly bolted to the launch platform, a lateral force of approximately 190,000 pounds pulled sideways on the aft attach- ment strut and the Solid Rocket Motor casing, including the joint that failed.4 Refueling of the tank was accomplished early on the morning of January 28. At ignition, the 190,000-pound force was instantly released when the SRB hold-down bolts were blown loose. For the next two and a half seconds the right Solid Rocket Motor field joints experienced a 3 cycle per second vibratory load caused by the sudden release of * NASA, MSFC Memo, Miller to Horton, April 12, 1984. Thiokol, “Erosion of SRM Preasure Seals,” TWR 15160, Chart A-9, August 19, 1985 “Seal damage alwa s has associated putt blowhole.” 4 NASA, &FC, “51-L Analysis &erview,” April 25,1986, p. H-203. 188 the lateral force.6 The ignition pressure increased the joint spacing. Also, the flow of motor gases through the blowhole at the 300 degree location could have resulted in damage to the primary 0- ring. The evidence of smoke at the 300 degree location is unlikely without O-ring damage. 7. Smoke at launch, clearly visible in the photographs, stopped at 2.7 seconds when the vibratory load damped out and the joint sealed. The sealing of the breach at the 300 degree location was made possible by blockage from burned material, probably consist- ing of a mixture of insulation and aluminum oxide. Post-accident tests performed by Morton Thiokol proved that aluminum oxide could have successfully plugged the joint at 2.7 seconds. While the smoke at ignition appeared to be intermittent, that appearance was probably a result of air and main engine exhaust currents. 8. At T+37 seconds into the flight, the Shuttle encountered wind gust loads in conjunction with planned maneuvers. Components of these gust and maneuvering loads were transmitted to the Solid Rocket Booster through the External Tank attachment strut. Based on the prescence of smoke at liftoff, these forces were transmitted to a joint already weakened by erosion and heat damage. 9. At 43 seconds into the flight, the main engines throttled back as the Shuttle reached “Max q” (maximum dynamic pressure). Four seconds later, the main engines had throttled up to 104% power and the geometry of the Solid Rocket Motor propellants had increased thrust. At this point, the motor pressure increased to 609 psi. Additional structural loads resulted from turbulence. Flight 51-L experienced the most severe turbulence of any Shuttle flight and, although the loads were within the allowable design limits, those design limits did not consider a joint that had already failed.s It is unknown how much the combined effect of wind gust loads, ma- neuvering loads and an increase in thrust contributed to the acci- dent. But the combined effects of these forces could have dislodged the burned material at the previously breached section of the joint. 10. Shortly after the vehicle was loaded by these turbulent forces, at T+58 seconds, a flame appeared from the same general region where the puffs of smoke had been seen. But, this time the joint was continuously breached by the burning propellant gases. In a little over two seconds, the flame had grown and acted as a blowtorch to burn through the hydrogen tank.