Risk-based spacecraft fire safety experiments

Abstract

This paper utilizes the scenario approach of risk assessment to identify modeling needs and, in turn, experiments that would aid in the development of models that would meet these needs. Due to the closed environment of a spacecraft and the lack of egress, fire on-board may pose a severe problem. There are many differences between a fire on-board the spacecraft and one in a terrestrial facility and they must be accounted for in the assessment of risk. Both the risk assessment methodology and the phenomena-based models must be modified. This paper discusses some of the methodology modifications, as well as special experimental results. Multiple experiments have been conducted in terrestrial and microgravity environments in order to construct and validate models required for the assessment and management of risk on-board spacecraft. A logic diagram analyzing the ways in which the crew may be injured and/or the spacecraft may be damaged, as well as operating experience, have identified wire overheating events as being potentially significant accident initiators. As a result, the experiments have concentrated on quantifying the pyrolysis event of a wire being overheated with excessive current. A preliminary set of experiments at the 2·2-second NASA Lewis Drop Tower has led to several observations. The event is violent due to the high heating rates. At these high heating rates, a jet of hot gases and smoke was observed. Frequently the conductor would melt down, sometimes ejecting molten pieces of the copper conductor. The event poses a threat to targets in the near vicinity and further away. Also, the smoke particle size distribution is shifted towards larger sizes in a microgravity environment. This may prove very important in designing a smoke detector. While significant results were obtained from these tests, longer durations of microgravity are required for further quantification to be possible.