safety issues in spacecraft. Indeed, our scientific understanding of fire on Earth is still emerging, and specialized studies in microgravity have been invaluable to that effort. In this context, the ISS is unique in providing sufficient time in a microgravity environment to achieve definitive evaluation of spacecraft fire phenomena—unachievable by short-duration microgravity facilities such as drop towers and unrealizable by computation that lacks sufficient spatial resolution. Thus, should fire safety gaps be identified that must be filled in order to reduce the fire risks on exploration missions to acceptable levels, the ISS is the only facility that could be used to conduct the studies. Such risk-reducing studies might include the effects of fire on humans and equipment and the design implications for suppressing, escaping, or correcting the damage of fire.

The presentations by NASA to the panel were ambiguous in describing the projects in the proposed continuation of the ISS program that are deemed essential for human space exploration (Mars).d Fire safety prioritization appears to have focused on fire detection and fire suppression without evidence that these are the most critical fire safety areas for future missions.e In addition, the two areas that appear to have been retained in ISS research planningdetection and suppression—were not presented in sufficient detail to allow the panel to assess whether NASA’s plans to address them are adequate. NASA needs to set design performance objectives for fire detection, suppression, and prevention and then demonstrate the certainty of achieving them. Fire poses an uncertain risk for the success of long-term spaceflight. Some of the areas of importance that were reduced or eliminated from the ISS program include these:

  • Tests for flammability and material screening. NASA currently has several tests for flammability.f However, these fire tests may not provide the proper levels of risk reduction for the new exploration program. For example, it has been noted that NASA-STD-6001 Test 1 results do not map quantitatively to results in the low- or partial-gravity environments of an exploration vehicle or habitat.5 Although NASA has performed extensive research to establish a replacement for the upward flammability test in a 1-g environment, it is not clear to the panel that this has been accomplished. NASA appears to be canceling the FEANICS program that is designed to verify the new test with ISS experiments.

    Previous reviews have stated the need for achieving prediction of surface flame spread in microgravity and fractional gravity.6 In order to rationally choose construction materials for spacecraft interiors that minimize the risk of, and danger from, fire and combustion, risk programs that include a better understanding of flame spread in microgravity are a primary consideration.7

  • Oxygen system safety. Combustion involving pure oxygen sources is a particular hazard in spacecraft and can result in temperatures capable of turning most materials into fuels. It was noted in a previous NRC report that the aspects of ignition, flame spread, and extinguishment that are unique to oxygen fires are critical research areas for human exploration.8 Thus, the use of oxygen generators and high-O2 atmospheres are matters of special concern.

  • Smoldering and pyrolysis. Smoldering and transition to flaming combustion are significantly different processes in microgravity than on Earth.9 The mechanisms that could enhance smoldering

d  

The projects indicated for continued funding appear to be smoke detection (DAFT and SAME) and smoke suppression (FLEX), the latter using the combustion integrated rack. The project related to material flammability and development of a new NASA test method (FEANICS) appears to have been dropped from the ISS but might be carried out on a limited basis on the ground.

e  

Assessment of Directions in Microgravity and Physical Sciences Research at NASA (NRC, 2003, p. 87) lists a number of fire-related issues that are at the level of critical impact on technology needed for human space exploration.

f  

For example, NASA-STD-6001 Test 1 looks at upward flame propagation. Other tests include the cone calorimeter (ASTM E 1254) for materials that fail the upward flame propagation test, ASTM D 93 for the flashpoint of liquids, and a special test for electrical wire flammability, containers, and metal flammability, which uses a version of the upward flame propagation test in pure oxygen.



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