ADDITIONAL OF INVESTIGATION AVENUES

¶Issue

¶Could the accident have been caused by some failure other than failure of the joint between the casings?

¶44 This memo was part of a package release, NASA, 86-100, draft, July 21, 1986.

¶Finding

¶As of September 15, 1986, the Committee has not found any credible evidence to support any cause of the Challenger accident, other than the failure of the aft casings joint in the right-hand Solid Rocket Booster. Nor has there been any substantial evidence of a secondary or parallel failure on Flight 51-L. Discussion

¶After the accident, the Committee waited until the Rogers Commission completed its work in order not to interfere with the progress being made by the Commission appointed by the President. By the time that work was completed the preponderance of evidence clearly pointed to a failure in the field joint of the right Solid Rocket Booster. However, the Committee was obligated to explore other possibilities which could have led to the same type of failure. Among these possibilities were the following: a failure of the propellant, a structural flaw in the steel casing, and separation of the NBR insulation from the casing. In addition, the Committee was contacted by and sought additional testimony from private cit- zens who offered their concerns and hypotheses pertaining to the cause of the Challenger accident. These included the following: either a main engine fire or a fire in the main engine compartment of the Orbiter, inadvertent firing of an OMS engine, inadvertent firing of one or more thrusters on the Orbiter, overloading of the aft field joint due to excessive "moment' developed in transit of the Shuttle from the VAB to the launch pad and the use of four separate propellant casings instead of one large casing without field joints.

Main Engine Fire

¶The photographs in Volume I of the Rogers Commission report on pages 26 and 27 indicate a bright spot in the vicinity of the main engine compartment. Photographic evidence is customarily taken to be accurate. In this case, however, it must be realized that the photographs were taken from roughly three miles away and that they were enhanced by computer methods. Computer enhancement has the ability to highlight bright objects and subdue dull ones. In this way, the photographs become distorted, that is, the difference between light and dark becomes unrealistically pro- nounced. The bright spot in the photographs does, in fact, look like a flame. The second consideration is that the orientation between the Orbiter and the ground where the cameras were is difficult to visualize and leads to erroneous conclusions. During flight the main engines are monitored continuously for changes in pump speed, temperature and pressure. There was no indication whatso- ever of a malfunction with the main engines. A fire in the main engine compartment outside of the engines is not credible because of the lack of combustible material to support a fire of any appreciable magnitude. The exception would be a hydrogen leak. But, that was not supported by telemetry data. In addition, NASA has submitted photographs to the Committee from four past successful launches which show the same bright spots. The Committee, therefore, has rejected this as a cause of the accident or as an independent problem.

Independent Firing of the OMS Engine or Orbiter Thrusters

¶The theory that either the OMS engines or the Orbiter thrusters were inadvertently activated and fired is also based on the same photographs stated previously. Those photographs show a bright spot in the same general area where these engines and thrusters are. The Committee has received photographs from Flights 41-G, 61-A, 61-13, and 61-C, all of which show similar "bright spots') in the same location as those seen on Flight 51-L. The Committee is still evaluating the possibility of a second failure in this regard and has requested additional telemetry data from Flight 51-L. Had the thrusters been firing, however, it would have had little impact on the launch of the Challenger. The thrust from these tiny engines is insignificant compared to the thrust from the two Solid Rocket Boosters and the main engines. The inadvertent activation of the OMS engine has been ruled out on the basis of telemetry data received from NASA. NASA has stated that the bright spot seen in the photographs is a reflection from the plume of the Solid Rocket Booster motors. Neither of these possibilities contributed to the Challenger accident.

Overloading of the Joint

¶It is true that in transit from the VAB to the launch pad, the Shuttle system, standing erect on the launch platform and being carried by a crawler, does experience a left-hand turn. At that time, because of the configuration of the Shuttle system, an additional moment, that is force times a distance, is transferred to the field joints including the aft joint on the right-hand Solid Rocket Booster that failed. However, this moment exerts a force which is only 10 percent of the force that the joint receives during other phases of the launch operations. The Committee concluded that this had no impact on the Challenger accident.

Insulation Debonding

¶The Committee investigated the possibility of separation of the insulation from the inside of the motor casings as a potential cause of the Flight 51-L accident. Had the insulation broken lose from the casing, there would have to have been a condition which would have permitted the burning propellant gases to get between the insulation and the casing. Furthermore, there would have to be a continuous gas flow at that point for the propellant gas to transfer a sufficient amount of heat to the casing to cause a failure. This would require an extremely large debonding of the insulation which has never been seen on any Shuttle flight when the Solid Rocket Motors were returned and disassembled for use later. When the Shuttle motors were inspected after usage, what remained of the insulation has always been in place with little damage. A debonding accident would have had to provide tremendous amounts of heat and again would have required a very high flow of the gas into the area where the debonding occurred. That flow of gas would have to be continuous and there is no rationale for envisioning how that could happen. In the case of the Shuttle Solid Rocket Motor design, the pressure acts to maintain the bond between the insulation and the casing, not to remove it. In the absence of these requirements the Committee found that debonding of the insulation was not a cause of the accident.

Crack in the Propellant

¶The Committee investigated whether or not a crack in the propellant could have contributed to loss of the Challenger. A crack in the propellant would have increased the burning surface of the propellant after ignition. This increase in the surface would have resulted in a n increase in t h thrust from the right Solid Rocket Motor. There was no evidence during the flight of 51-L of a greater thrust in the right Solid Rocket Motor. In additional, a propellant failure would have been more explosive in nature and would not have been observed as one continuous gas flame in a localized area. Consequently, it was concluded that a propellant failure did not contribute to the cause of the accident.

Crack in Motor Casing

¶The Committee was concerned that a crack in the rocket motor casings might have caused the accident if it was located in the same general area where the smoke and flame was observed during launch. All of the casings used on Flight 51-L were hydroproofed at 1.1. maximum expected operating pressure. Had there been a signficant crack in the casing it would have failed the hydroproof test. However, it could be argued that a crack developed between the test and the time the Solid Rocket Motor segments were assembled at the Kennedy Space Center. The failure of cracks under the pressures, such as those contained within the Solid Rocket Motors, would have been a catastrophic failure. The casings would have failed instantly at ignition because cracks in high carbon steel would propagate at a rate near the speed of sound. This is inconsistent with the smoke seen during the early part of the launch, and the lack of smoke or flame up until 58 seconds into the launch. It is also inconsistent with the pieces of the rocket motor casings which were recovered from the ocean which clearly show the abra- sion of the hot rocket propellant gases. Consequently, a crack in the casing was ruled out as a contributing cause of the accident.

Joint putty temporarily holds and then releases full motor pressure

¶During the post-accident tests conducted by NASA and Thiokol, it was learned that the performance of the putty used in the joint can be quite variable. In some instances, including temperatures as warm as 75"F, the joint putty can hold back the full operating pressure inside the motor without transferring any of this pressure to the O-rings.45 In this circumstance, the O-rings will not "seat" and, as the joint "rotates" due to the pressure build-up within the motor, contact can be lost between the O-rings and the metal surfaces they are meant to If the putty were then to release high pressure gases into the joint, these gases could "blow-by" the O-rings, thus causing the joint to fail. This scenario is not a likely failure mode for STS 51-L, because it would produce a leak across a broad area of the joint rather than a small localized leak as observed in the Challenger accident.

¶45RogersCommission Report, Volume I, p. 64 4 6 NASA, briefings from staff.

ADDITIONAL OF INVESTIGATION AVENUES