Irving Pinkel

The NASA Spacecraft Fire Hazards Steering Committee report.

J. David Reid

Bromine containing derivates of phosphonitrillic chloride were investigated as possible durable flame retardants for cotton fabrics. These, including brominated allyl phosphontrilate, 2-3 dibromopropyl phosphonitrilate, and the bromoform adduct polymer of allyl phosphonitrilate were applied to cotton fabric as polymertic products from organic solvent or from aqueous emulsions. Evaluation tests on tear strength and the durability of the flame resistance to laundering were made.

D.E. Cagliostro, H. Goldstein, J.A. Parker

Reactions of the silica reinforcement fiber and ablation char of the Apollo heat shield have been investigated by laboratory tests in an arc image furnace at temperature levels up to 5000 degree R, pressure up to 0.7 atm, and heat flux similar to reentry, 400 Btu/ft sec, and by actual reentry test. Micro chemical analyses and X-ray diffraction studies have been made to determine the presence of SIC formation in the char. Experimental data and analytical predictions of thermal and density profiles have been compared for ablation of virgin heat-shield and precharred materials to determine the effects of SIC formation on ablation performance. In all analyses, general agreement was found between chemical composition and he thermal predictions for laboratory tests and reentry materials. In all ablated materials SiC was formed in the front surface of the char. The highest SiC content found was 58% by weight and found in a high-pressure environment. The SiC formed was found to act as a heat sink in the ablation process and can lower the front surface temperature by 300 degrees R.

Richard G. Hill

Tests under simulated flight conditions were conducted on a Self-Generating Overheat Detection System installed in a C-140 engine nacelle. They were run with the system in its normal configuration and also with a section of the detection cable pinched, opened and shorted. The system was monitored for fire response time as well as for false alarms.

The system performed well in its normal, pinched and opened, configuration, but the alarm time was increased by over 100 percent when the cable was shorted. No false alarms were noted during testing.

The ability to detect a fire in a spacecraft in its earliest stages is difficult to predict. The spacecraft is an extremely complex vehicle in which most of he system and/or equipment likely to act as initiators and propagators of fire are built into hidden areas or installed in semi-sealed cabinets. It is possible that over-heated; incipient fire conditions could exist for a long time before becoming visible. The purpose of this presentation is to discuss one class of devices capable of sensing these conditions and thus providing an early warning. These are so-called “smoke detectors’, which can detect the presence of many of the invisible and visible products of pyrolysis which are given off by most potentially combustible materials when over-heated or in early stages of combustion.

In principle smoke detection is one of the simplest and least ambiguous of the available techniques, which might be used for identifying the presence of an active or incipient fire. Normally, combustible materials such as hydrogen for fuel cells and propellants for altitude stabilization and propulsion will be stored exterior to the pressurized living area. Many of the materials likely to be brought on board a spacecraft will be relatively fire resistant. Much of the materials, synthetic polymers of many kinds, do not burn easily, and if involved in pyrolysis through overheating or arcing, break down into complex compounds which should be relatively easy to detect. There will undoubtedly be some relatively flammable materials on board, such as ethylene glycol in heat exchangers or even Kleenex for the crew. Insofar as is known, the products of pyrolysis of all of the materials which are inside the spacecraft can be detected by the smoke detectors. The problem is in space as it is in ground installation, primarily one of proper sampling. This will involve considerable study and research to determine the best location for the sensing units or sampling tubes. Serious consideration should be given to special flight experiments to develop thorough understanding of the factors involved at both zero “g” and partial conditions.

Most smoke particles range from 0.01 micron to 1.0 micron in size, which are too small to be seen by the human eye. Tobacco smoke for instance, is composed of particles in the 0.01 to 0.1 micron size, and is visible as a haze only when sufficiently concentrated. In the extreme cases, particles of combustion can range up to 10 microns or larger, but these are generally such substances as fly ash or dusts of various kinds and are not considered here.

Several kinds of smoke/ionization detectors are presently commercially available, although none are suitable for application to space flight without repackaging or re-engineering. All of the systems may be intentionally deactivated for certain normal spacecraft functions and all systems can be built to automatically reset to the detection mode when the alarm condition is corrected. The success of any of these systems is critically dependent on the careful matching of requirements with the available hardware. They fall into basically three categories.