EXTERNAL TANK

¶Issue

¶The External Tank was obviously involved in the accident. Was that involvement a cause or an effect? Findings

The Committee adopts the "Finding" of the Rogers Commis- sion that: "A review of the External Tank's construction records, acceptance testing, pre-launch and flight data and recovered hard- ware, does not support anything relating to the External Tank which caused or contributed to the cause of the accident."35
The External Tank ruptured under the forces of a failed Solid Rocket Booster motor. These forces were far outside of any possible design considerations that could have been applied to the External Tank.36 Discussion

¶The 154 foot long,²⁷²⁷ bid., p. 201. Rogers Commission Report, Volume 11, p. F-2. foot diameter external disposable fuel tank contains the liquid oxygen and liquid hydrogen used by the Space Shuttle's three main engines. Structurally, the External Tank (ET) serves as the keel, or backbone, of the Space Transportation System. The two Solid Rocket Boosters are attached to the Exter- nal Tank near the front and rear of the ET. The Space Shuttle Or- biter is also attached to the ET at one forward, and two aft attach- ment points.

¶The Committee was interested in the findings of the Rogers Com- mission concerning the External Tank because it was obvious to all that the External Tank was directly involved in the accident.

¶The Rogers Commission investigated five potential faults or fail- ures of the External Tank which could have contributed to the ac- cident.

¶They are:

Premature detonation of the External Tank Range Safety System; A structural flaw in the tank; Damage at lift-off; Structural overload; Overheating.

¶The Committee is satisfied that the Range Safety System on the External Tank did not cause the STS 51-L accident because:38

There is no flight data to support premature detonation of the ET range safety package;

¶ss Rogers Commission Report, Volume I, p. 42 36 Ibid., Volume 11, p. L-23. 3T Ibid., Volume I, p. 41 38 Ibid., Volume 11, p. L16.

The photographic evidence does not support premature deto- nation; Most of the explosive charges included in the ET Range Safety System were recovered with the ET wreckage, undeton- ated.

¶The Committee affirms the Rogers Commission finding that there is no data to support structural faults which might have propagated to a size which could have caused catastrophic failure of the External Tank.39

¶The evidence from the Ice Team examination, and photo analysis and television monitoring of the launch give no indication whatso- ever that there was any damage to the External Tank during launch and lift-off.

¶The examination of all flight data indicates that the maximum structural load on the External Tank on STS 51-L was less than 80 percent of the allowable design load from launch until the final ex- plosion. O

¶The Committee also affirms the Rogers Commission finding that the evidence does not support any theory of independent overheat- ing of the External Tank as a cause of the accident.41

¶The failed joint in the Solid Rocket Motor permitted the burning solid propellant gases to escape in the direction of the aft External Tank attachment strut. Temperatures and velocity of these gases caused a rapid erosion and deterioration of the aft strut to the point where it failed structurally under turbulence and maneuver- ing loads. There were no deficiencies in the design of the external strut. However, the strut was not designed to withstand the "blow- torch" effect of the propellant hot gas stream. During its investiga- tion, the Committee staff visited Martin Marietta's External Tank assembly plant in Louisiana. An issue raised during this visit was whether or not the strut could be relocated such that in the event of another #hint €ailure in that vicinity, the strut would not be damaged. It was learned that relocating the aft strut created more problems that it solved. Furthermore, it was also learned that the gas stream would have almost instantly cut through the insulation on the External Tank and destroyed it anyway. After the failure of the aft strut, the flame continued to bear on the bottom of the Ex- ternal Tank, breaching that tank at the joint of the aft dome. This caused the liquid hydrogen to escape from the tank. Once the flame had penetrated the tank at the weld of the aft dome the fail- ure of that weld spread rapidly and completely around the tank's diameter severing the dome from the rest of the tank. The burning hydrogen ignited by the flame then caused the External Tank to act much like a rocket and created an upward thrust. The right Solid Rocket Booster without the attachment strut to the External Tank, rotated around its long In SO doing, it may have

¶38 bid., Volume I, p. 42.⁴⁰⁴⁰ Ibid., Chart 4-2. Ibid.⁴¹⁴¹ Ibid., Chart "Conclusions." 4 2 Ibid.. Chart "Recommendations." 4 3 This'Chart was prepared by the Committee and is based on information obtained by Com. mittee staff during meetings at MSFC on June 30, 1986. Ibid.

¶*In It should be noted that the right Solid Rocket Booster did not awing outward at the bottom and cause the now of the booster to collide with the External Tank as had originally been thought. For this to have happened, the right Solid Rocket Booster would have extended out- ward at the bottom at a wide angle that is not supported by any of the photographic or telemet- ric evidence.

¶jammed the upper Solid Rocket Booster to the External Tank at- tachment structure and caused it to fail. That structure is located on a large cross beam in the intertank which in turn possibly dam- aged the cross beam. The other distinct possibility is that the burn- ing hydrogen forced the External Tank upward and caused the cross beam to be damaged and at the same time, caused a rupture of the oxygen tank. With the massive release of energy from the burning of the hydrogen/oxygen, the Shuttle system completely broke apart.

¶NASA's Accident Analysis Team determined that the Orbiter was destroyed by aerodynamic forces beyond design limits, not by the actual explosion of the External Tank. The report stated,

¶All fractures and material failures examined on the Orbiter, with the exception of the SSME's, were the result of overload forces and they exhibited no evidence of internal burn damage or exposure to explosive forces.41b