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The following findings are the product of about 7 weeks of concentrated review of the Apollo 13 accident by the Apollo 13 Review Board. They are based on that review, on the accident investigation by the Manned Spacecraft Center (MSC) and its contractors, and on an extensive series of special tests and analyses performed by or for the Board and its Panels.
Apollo XIII Related Links
 Apollo Mission Summary
Apollo Air-to-Ground Transcript
(the REAL DEAL)
Apollo 13 Cryo Data Screen Captures
Apollo 13 Review Board Findings
Grumman Aerospace's Towing Bill
Apollo XIII: The Movie
Additional Apollo 13 References
This FAQ is maintained by Anthony W. Haukap
The current version of this FAQ document can always be found on my website at:
While every attempt is made to present accurate data it should be noted that the author makes no guarantee as to the accuracy of any information provided in this document, and is not responsible for any consequences of its use.
After assembly and acceptance testing, the oxygen tank no. 2 (S/N XTA0008) which was originally installed on Apollo 10 was shipped from Beech Aircraft Corporation to North American Rockwell (NR) in apparently satisfactory condition.
The oxygen tank no. 2 heater assembly contained two thermostatic switches designed to protect the heaters from overheating. The thermostatic switches were designed to open and interrupt the heater current at 80 degrees + 10 degrees F. The heaters are operated on 28 V dc in flight and at North American Rockwell. The heaters are operated on 65 V ac at Beech Aircraft Corporation and 65 V dc at the Kennedy Space Center. These higher voltages are used to accelerate tank pressurization. The thermostatic switches were rated at 7 amps at 30 V dc. While they would carry this current at 65 V dc in a closed position, they would fail if they started to open to interrupt this load. Neither qualification nor acceptance testing of the heater assemblies or the tanks required thermostatic switch opening to be checked at 65 V dc. The only test of switch opening was a continuity check at Beech Aircraft in which the switch was cycled open and closed in an oven. The thermostatic switches had never operated in flight because this would only happen if the oxygen supply in a tank were depleted to nearly zero. The thermostatic switches had never operated on the ground under load because the heaters had only been used with a relatively full tank which kept the switches cool and closed.
A number of factors contributed to the presence of inadequate thermostatic switches in the heater assembly. The original 1962 specifications from North American Rockwell to Beech Aircraft Corporation for the tank and heater assembly specified the use of 28 V dc power, which is used in the spacecraft. In 1965, NR issued a revised specification which stated that the heaters should use a 65 V 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 V dc. After receipt of the Block II oxygen tank specifications from North American Rockwell, which required the tank heater assembly to operate with 65 V dc GSE power only during tank pressurization, Beech Aircraft did not require their Block I thermostatic switch supplier to make a change in the switch to operate at the higher voltage. NR did not review the tank or heater to assure compatibility between the switch and the GSE. MSC did not review the tank or heater to assure compatibility between the switch and the GSE. No tests were specified by MSC, North American Rockwell, or Beech Aircraft to check this switch under load.
At North American Rockwell, Downey, California, in the attempt to remove the oxygen shelf assembly from SM 106 (Apollo 10), a bolt restraining the inner edge of the shelf was not removed. Attempts to lift the shelf with the bolt in place broke the lifting fixture, there by jarring the oxygen tanks and valves. The oxygen shelf assembly incorporating S/N XTA0008 in the tank no. 2 position, which had been shaken during removal from SM 106, was installed in SM 109 (Apollo 13) one month later. An analysis, shelf inspection, and a partial retest emphasizing electrical continuity of internal wiring were accomplished before reinstallation.
It is probable that the tank contained a loosely fitting fill tube assembly. Dimensioning of the short Teflon and Inconel tube segments of the cryogenic oxygen storage tank fill line was such that looseness to the point of incomplete connection was possible in the event of worst case tolerance buildup. The insertion of these segments into the top of the tank quantity probe assembly at the point of its final closure and welding was difficult to achieve. Probing with a hand tool was used in manufacturing to compensate for limited visibility of the tube segment positions. This assembly was probably displaced during subsequent handling, which included the incident at the prime contractor's plant in which the tank was jarred.
The attempt to detank the cryogenic oxygen tanks at KSC after the CDDT by the standard procedures on March 23, 1970, was unsuccessful with regard to tank no. 2. A special detanking procedure was used to empty oxygen tank no. 2 after CDDT. This procedure involved about 8 hours of continuous heating with repeated cycles of pressurization to about 300 psi with warm gas followed by venting. It was employed both after CDDT and after a special test on March 30, 1970 to verify that the tank could be filled. There is no indication from the heater voltage recording that the thermostatic switches functioned and cycled the heaters off and on during these special detanking procedures. At the completion of detanking following CDDT, the switches are only checked to see that they remain closed at -75 degrees F as the tank is warmed up. They are not checked to verify that they will open at +80 degrees F. Tests subsequent to the flight showed that the current associated with the KSC 65 V dc ground powering of the heaters would cause the thermostatic switch contacts to weld closed if they attempted to interrupt this current.
Although Beech did not encounter any problem in detanking during acceptance tests, it was not possible to detank oxygen tank no. 2 using normal procedures at KSC after the CDDT. Tests and analyses indicate that this was due to gas leakage through the displaced fill tube assembly. In itself, the displaced fill tube assembly was not particularly serious, but it led to the use of improvised detanking procedures at KSC which almost certainly set the stage for the accident. In reviewing these procedures before the flight, officials of NASA, North American Rockwell, and Beech Aircraft did not recognize the possibility of damage due to overheating. Many of these officials were not aware of the extended heater operation. In any event, adequate thermostatic switches might have been expected to protect the tank.
As shown by subsequent tests, failure of the thermostatic switches probably permitted the temperature of the heater tube assembly to reach about 1000 degrees F in spots during the continuous 8-hour period of heater operation. Such heating has been shown by tests to severely damage the Teflon insulation on the fan motor wires and melt solder in the vicinity of 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. The cryogenic oxygen storage tanks contained materials that could be ignited and which will burn under the conditions prevailing within the tank, including Teflon, aluminum, solder, and Drilube 822.
It was not until nearly 56 hours into the mission, that the fan motor wiring, possibly moved by the fan stirring, short circuited and ignited its insulation by means of an electric arc. The resulting combustion in the oxygen tank probably overheated and failed the wiring conduit where it enters 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.
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