Encyclopedia Astronautica
1970.06.13 - Apollo 13 Review Board publishes result of investigation.

The explosion was found to have been caused by a bare-wire heating element within the fuel cell liquid oxygen tank. The element itself had burnt off its insulation through a combination of unimplemented specification changes early in the programme coupled with unauthorised procedures during ground testing.

The investigation terminated on June 15, when the Review Board accident report was released by NASA at a Headquarters press conference.

The Apollo 13 Review Board was established April 17 by George M. Low, NASA Deputy Administrator, and Thomas O. Paine, NASA Administrator, who appointed the Director of Langley Research Center, Edgar M. Cortright, as Review Board Chairman. On April 21 the members of the Board were named. In addition, by separate memos of April 20, the Aerospace Safety Advisory Panel was requested to review the procedures and findings of the Board and the Associate Administrator for Manned Space Flight was directed to provide records, data, and technical support as requested by the Board. The investigation indicated the accident was caused by a combination of mistakes and a somewhat deficient design. The following sequence of events led to the accident:

- After assembly and acceptance testing, the oxygen tank no. 2 that flew on Apollo 13 was shipped from Beech Aircraft Corp. to North American Rockwell (NR) in apparently satisfactory condition.

- However, the tank contained two inadequate protective thermostatic switches on the heater assembly, and they subsequently failed during ground test operations at Kennedy Space Center (KSC).

- In addition, the tank probably contained a loosely fitting fill tube assembly. This assembly was probably displaced during subsequent handling, which included an incident at the prime contractor's plant in which the tank was jarred.

- In itself, the displaced fill tube assembly was not particularly serious, but it led to improvised detanking procedures at KSC, which "almost certainly set the stage for the accident."

- Although Beech had not met any problem in detanking during acceptance tests, it was not possible to detank oxygen tank no. 2 using normal procedures at KSC. Tests and analyses indicate that the problem was gas leakage through the displaced fill tube assembly.

- The special detanking procedures at KSC subjected the tank to an extended period of heater operation and pressure cycling. "These procedures had not been used before, and the tank had not been qualified by test for the conditions experienced. However, the procedures did not violate the specifications which governed the operation of the heaters at KSC."

- In reviewing these procedures before the flight, officials of NASA, NR, and Beech did not recognize the possibility of damage from overheating. Many were not aware of the extended heater operation. In any event, adequate thermostatic switches might have been expected to protect the tank.

- A number of factors contributed to the presence of inadequate thermostatic switches in the heater assembly. The original 1962 specifications from NR to Beech Aircraft Corp. for the tank and heater assembly specified the use of 28-volt, direct-current power, which was used in the spacecraft. In 1965, NR issued a revised specification that stated the heaters should use a 65-volt dc power supply for tank pressurization; this was the power supply used at KSC to reduce pressurization time. Beech ordered switches for the Block II tanks but did not change the switch specifications to be compatible with 65-volt dc.

- The thermostatic switch discrepancy was not detected by NASA, NR, or Beech in their review of documentation, nor did tests identify the incompatibility of the switches with the ground support equipment (GSE) at KSC, "since neither qualification nor acceptance testing required switch cycling under load as should have been done. It was a serious oversight in which all parties shared."

- The thermostatic switches could accommodate the 65-volt dc during tank pressurization because they normally remained cool and closed. However, they could not open without damage with 65 volt dc power applied. They were not required to open until the special detanking. During this procedure, as the switches started to open when they reached their upper temperature limit, they were welded permanently closed by the resulting arc and were rendered inoperative as protective thermostats.

- Failure of the thermostatic switches to open could have been detected at KSC if switch operation had been checked by observing heater current readings on the oxygen tank heater control panel. Although not recognized at the time, the tank temperature readings indicated that the heaters had reached their temperature limit "and switch opening should have been expected."

- Subsequent tests showed that failure of the thermostatic switches probably permitted the temperature of the heater tube assembly to reach about 1,000 degrees F (810 K) in spots during the continuous eight-hour period of heater operation. Such heating had been shown by tests to damage severely the Teflon insulation on the fan motor wires near the heater assembly. "From that time on, including pad occupancy , the oxygen tank no. 2 was in a hazardous condition when filled with oxygen and electrically powered."

- Nearly 56 hours into the mission, the fan motor wiring, possibly moved by the fan stirring, short-circuited and ignited its insulation. Combustion in the oxygen tank "probably overheated and failed the wiring conduit where it entered the tank, and possibly a portion of the tank itself."

- The rapid expulsion of high-pressure oxygen which followed, "possibly augmented by combustion of insulation in the space surrounding the tank, blew off the outer panel to bay 4 of the SM, caused a leak in the high-pressure system of oxygen tank no. 1, damaged the high-gain antenna, caused other miscellaneous damage, and aborted the mission."

Based on the findings of the Board, a number of recommendations were made to preclude similar accidents in future space flights:

- The cryogenic oxygen storage system in the service module should be modified to:

- Remove from contact with the oxygen all wiring and unsealed motors that could potentially short-circuit and ignite adjacent materials; or otherwise ensure against an electrically induced fire in the tank.

- Minimize the use of Teflon, aluminum, and other relatively combustible materials in the presence of the oxygen and potential ignition sources.

- The modified cryogenic oxygen storage system should be subjected to a rigorous requalification program, including careful attention to potential operational problems.

- The warning systems on the Apollo spacecraft and in the Mission Control Center should be carefully reviewed and modified where appropriate, with specific attention to:

- Increasing the differential between master alarm trip levels and expected normal operating ranges to avoid unnecessary alarms.

- Changing the caution and warning system logic to prevent an out-of-limits alarm from blocking another alarm if a second quantity in the same subsystem went out of limits.

- Establishing a second level of limit sensing in Mission Control on critical quantities, with a visual or audible alarm that could not be easily overlooked.

- Providing independent talk-back indicators for each of the six fuel cell reactant valves plus a master alarm when any valve closed.

- Consumables and emergency equipment in the LM and the CM should be reviewed to determine whether steps should be taken to enhance their potential for use in a 'lifeboat' mode.

- MSC should complete the special tests and analyses under way to understand more completely the details of the Apollo 13 accident. In addition, the lunar module power system anomalies should receive careful attention. Other NASA Centers should continue support to MSC in the areas of analysis and test.

- Whenever significant anomalies occurred in critical subsystems during final preparation for launch, standard procedures should require a presentation of all prior anomalies on that particular piece of equipment, including those which have previously been corrected or explained. Critical decisions on flightworthiness should require the full participation of an expert "intimately familiar with the details of that subsystem."

- NASA should thoroughly reexamine all its spacecraft, launch vehicle, and ground systems containing high-density oxygen or other strong oxidizers, to identify and evaluate potential combustion hazards in the light of information developed in this investigation.

- NASA should conduct additional research on materials compatibility , ignition, and combustion in strong oxidizers at various gravity levels and on the characteristics of supercritical fluids. Where appropriate, new NASA design standards should be developed.

- MSC should reassess all Apollo spacecraft subsystems, and the engineering organizations responsible for them at MSC and at its prime contractors, to ensure adequate understanding and control of the engineering and manufacturing details at the subcontractor and vendor level. "Where necessary, organizational elements should be strengthened and in-depth reviews conducted on selected subsystems with emphasis on soundness of design, quality of manufacturing, adequacy of test, and operational experience."

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