combustion products. The pressure rise was sufficient to pop the rivets holding the aluminum exterior panel covering Sector4 and blow it out, exposing the sector to space and snuffing out the fire. The detached panel hit the nearby high-gain antenna, disabling the narrow-beam communication mode and interrupting communication with Earth for 1.8 seconds while the system automatically switched to the backup wide-beam mode. The sectors of the SM were not airtight from each other, and had there been time for the entire SM to become as pressurized as Sector4, the force on the CM's heat shield would have separated the two modules. The report questioned the use of Teflon and other materials shown to be flammable in supercritical oxygen, such as aluminum, within the tank. The board found no evidence pointing to any other theory of the accident. Mechanical shock forced the oxygen valves closed on the number1 and number3 fuel cells, putting them out of commission. The sudden failure of Oxygen Tank2 compromised Oxygen Tank1, causing its contents to leak out, possibly through a damaged line or valve, over the next 130 minutes, entirely depleting the SM's oxygen supply. With both SM oxygen tanks emptying, and with other damage to the SM, the mission had to be aborted. The board praised the response to the emergency: "The imperfection in 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." Oxygen Tank 2 was manufactured by the Beech Aircraft Company of Boulder, Colorado, as subcontractor to North American Rockwell (NAR) of Downey, California, prime contractor for the CSM. It contained two thermostatic switches, originally designed for the command module's 28-volt DC power, but which could fail if subjected to the 65 volts used during ground testing at KSC. Under the original 1962 specifications, the switches would be rated for 28 volts, but revised specifications issued in 1965 called for 65 volts to allow for quicker tank pressurization at KSC. Nonetheless, the switches Beech used were not rated for 65 volts. At NAR's facility, Oxygen Tank 2 had been originally installed in an oxygen shelf placed in the Apollo 10 service module, SM-106, but which was removed to fix a potential electromagnetic interference problem and another shelf substituted. During removal, the shelf was accidentally dropped at least , because a retaining bolt had not been removed.