Decisions on flammability problems related to coax cables in Apollo CMs - .
Nation: USA. Program: Apollo. Spacecraft: Apollo CSM; CSM Electrical. Top NASA and North American Rockwell management personnel discussed flammability problems associated with coax cables installed in CMs. It was determined that approximately 23 meters of flammable coax cable was in CM 101 and, when ignited with a nichrome wire, the cable would burn in oxygen at both 4.3 and 11.4 newtons per square centimeter (6.2 and 16.5 pounds per square inch). Burning rates varied from 30 to 305 centimeters per minute, depending upon the oxygen pressure and the direction of the flame front propagation. The cable was behind master display panels, along the top of the right-hand side of the cabin, vertically in the rear right-hand corner of the cabin, in the cabin feed-through area, and in the lower equipment bay. The group reviewed the detailed location of the cable, viewed movies of flammability tests, examined movies of the results of testing with fire breaks, discussed possible alternatives, and inspected cable installations in CMs 101 and 104.The following alternatives were considered:
- Replace all coax cable.
- Wrap all coax cable with aluminum tape.
- Partially wrap the cable to provide fire breaks. Tests at North American indicated that a 102-millimeter segment of wrapped cable with four layers of aluminum foil would provide a fire break. MSC tests indicated such a fire break was not adequate for multiple cables.
- Leave the installation as it was.
The following factors were considered in reaching a decision for spacecraft 101:
- The wiring in that spacecraft had been completed for several months. All subsystems had been installed and protective covers had been installed. Complete replacement or complete wrapping of all coax cables would be time consuming; it might take as long as three months, when taking retest into consideration. Additionally, in spite of extreme care, complete replacement or wrapping might do considerable damage to the installed wiring, and even partial wrapping might cause damage in many areas.
- The coax cable could not self-ignite under any conditions.
- In most installations, the coax cable was a separate bundle and not part of other wire bundles. An exception was the feed-through area in the lower right-hand corner of the cabin, where the coax cable was intertwined with other wires. Although power cables existed in this area, these were not high-current-carrying cables.
- A minimum number of possible ignition sources existed in the vicinity of the coax cables, and a complex series of events would be required to ignite the cable.
In view of these factors, decisions for spacecraft 101 were:
- The cable would be flown essentially as installed. The only exception was that the vertical cable bundle in the right-hand corner of the spacecraft would be wrapped with layers of aluminum tape. Each cable in this bundle would be individually wrapped.
- An analysis by North American would document all other wiring near the coax cable, including the wire size, functions, maximum currents carried, and degree of circuit-breaker protection.
- All possible ignition sources near the coax cable would be documented.
- Tests would be made in boilerplate (BP) 1250 to determine the effects of fire breaks inherent in the installation.
In making these decisions, NASA and North American recognized that they were contrary to existing criteria and guidelines. Those present agreed that the decisions were an exception and in no way should be construed as a change or relaxation of the criteria and guidelines. The basic reason for the exception was summarized as follows: "As a result of the clean installation of the coax cables, the lack of external ignition sources, and the complete job done in cleaning up the spacecraft from the flammability viewpoint, the risk of igniting the coax cables is exceedingly small. This risk is believed to be less than would likely be incurred through possible damage to existing installations had a decision been made to replace or wrap the cables."The installation in spacecraft 2TV-1 would not be changed. This decision was made fully recognizing that more flammable material remained in 2TV-1 than in 101. However, the burning rate of coax cable had been demonstrated as very slow, and it was reasoned that the crew would have sufficient time to make an emergency exit in the vacuum chamber from 2TV-1 long before any dangerous situations would be encountered.
Officials also agreed that coax cable in boilerplate 1224 would not be ignited until after the results of the BP 1250 tests had been reviewed.
