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Aviation History
1970
1970 - 1198.PDF
30 fUGHT Fire down below BOTH INDUSTRY AND NASA have come in for criticism from the accident review board which was set up to investigate the near-catastrophic accident to Apollo 13 last April. The admirable self-criticism which distinguished the earlier accident to Apollo 204 in January 1967 is evident again here. The report is prefaced by the comment: "The Apollo 13 accident, which aborted man's third mission to explore the Moon, is a harsh reminder of the immense difficulty of this undertaking. Perfection is not only difficult to achieve, but difficult to maintain. The imperfection of Apollo 13 constituted a near-disaster, averted only by outstanding performance on the part of the crew and the ground control team which supported them." The cause of the explosion in the service module, which occurred 55hr 52min after launch, was, according to the board, an electrically initiated fire in the No 2 cryogenic oxygen tank, which (together with No 1 tank) supplies oxygen to the three fuel cells and to the environmental control system. These tanks, made by Beech, are spherical, and each contains 3201b of liquid oxygen, a quantity-measuring capacitance-probe and a heater tube. The last contains two heating coils controlled by thermostat switches, and two electrically driven fans. The purpose of the fans is to promote circulation within the liquid oxygen and maintain a uniform temperature within it, while periodic operation of the heaters evaporates some of the oxygen so as to maintain a constant supply pressure of about 9001b/sq in. The report draws attention to the difficulty of ensuring that the gauge and heater assemblies could be in stalled without damage to the insulation of the wiring. Extended operation of the heaters is a part of the pre-flight test procedures. The board noted that officials of Nasa, North American Rockwell and Beech did not recognise the possibility of damage due to heating. In any event, the thermostatic switches might have been expected to provide adequate pro tection for the tank. Unfortunately they did not. The 1962 NAR specification to Beech, calling for 28V d.c. heater assem blies, was changed in 19,65 to call up 65V d.c. heaters in order to bring the tank up to pressure more rapidly. The switch specification was not changed, and the discrepancy was not picked up by Nasa, Beech or NAR in their review of docu mentation. Because the ground test schedule did not call for switch cycling under load, the incompatibility continued un detected. "It was", said the board, "a serious oversight in which all parties shared." The switches could operate at 65V during tank pressurisa- tion because they normally remained cool and closed. But they could not open without damage at this voltage, and, during a continuous 8hr operation at Cape Kennedy, to drain the tank after tests, it appeared that they welded closed. The effect of this was that electrical power to the heaters would not be cut off when the temperature had reached 27 °C. As a result, the temperature of the heater tube must have reached about 550°C, causing damage to the Teflon insulation of cables adjacent to the heater. The effect of these tests, says the board, is that a potentially hazardous situation existed undetected from the time that this "de-tanking" procedure was carried out on No 2 tank. During the flight, the No 2 tank fans were turned on at 55hr 53min 20sec GET (ground elapsed time). The heaters remained off at this time, though they had operated normally prior to the accident. Two seconds later a power supply anomaly occurred, consistent with a short-circuit initiated by local arcing. A pressure rise in the tank was noted 13sec Arrangement of the three fuel cells, the two liquid hydrogen tanks and the two oxygen tanks in Bay 4 of the hexagonally structured service module. The panel covering this bay was blown clear after the rupture of liquid oxygen tank No 2 later, again compatible with a short circuit in a fan motor or its leads (the electrical energy in the tank would normally have been insufficient to cause the observed rise); the pressure rose to a maximum of l,0081b/in2 (the relief valve pressure is l,0001b/in2). The pressure then began to fall, and the tank then failed in a way not fully determined, but at the same time as the crew recorded hearing a sharp report. The escape of oxygen would have pressurised the particular bay in which the tank was situated, and this would have been adequate to cause the cover panel to blow off. At the same time a slow loss of pressure in No 1 oxygen tank occurred, possibly (says the board) as the rupture of a feed line caused by the loss of integrity of the first tank. With both oxygen tanks failed, the fuel cells were rendered com pletely inoperative, and the only electrical supplies (apart from the limited-capacity batteries in the command module, used only for re-entry after the service module has been jettisoned) were those in the lunar module. Under nine headings the board makes recommendations for
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