Motivation

Fires in spacecraft? They can and have happened. On February 23, 1997 a faulty oxygen supply unit aboard the Mir space station caused a fire that damaged equipment and forced astronauts to put on emergency oxygen masks and officials to consider evacuation. Minor incidents of charred electrical components have even been reported in the Space Shuttle. Many cases of fires aboard aircraft have been reported. Could a fire happen aboard the International Space Station or in manned spacecraft missions, like the Mars mission? Considering the lengthy duration of these missions (the 10-20 years lifetime of the Space Station) and the amount of combustible materials in these facilities (circuit boards, electrical cables, packing materials, paper, trash), there is a high probability that a fire will occur sometime during the lifecycle of these facilities. Even one serious incident could be disastrous for the spacecraft and for NASA’s manned space program in general. Whether an incipient fire turns into a triviality or into a catastrophe will depend on our collective knowledge of fire in spacecraft.

The potential for injury to humans and damage to equipment worth billions of dollars has prompted NASA to investigate how fires behave in a spacecraft. Fire in an enclosed compartment, like in a spacecraft, can have serious consequences – you can’t open the doors, you can’t call the firefighters, the smoke and toxic gases produced by the fire cannot escape or be vented, and the fire may consume the cabin oxygen very quickly.

Also, detection of a fire in the absence of gravity is more difficult than on Earth because in normal gravity smoke and heat rise, making it easier to know where to put smoke detectors. In spacecraft, special measures must be taken to ensure that the smoke reaches the smoke detectors. NASA has an established fire prevention program for its space facilities that to date has been successful for the short duration missions of the Shuttle. Flammability of materials is primarily based on testing here on Earth. Unfortunately, materials do not burn the same way under zero-gravity conditions as they do in normal gravity. For example, when a candle burns on Earth, the hot gases from the flame rise, creating air currents that feed the flame and give it its familiar shape. However, without gravity, heat doesn’t rise and a candle flame becomes spherical. The controlling mechanisms of the combustion process change and diffusion of air to the fuel supply becomes the limiting factor.

Fig. 1. Left: Candle flame on Earth. Right: Candle flame in zero gravity.

One would hypothesize, then, that in spacecraft, because of the absence of gravity, materials are less flammable than on Earth, where buoyancy helps the fire. However, recent research has indicated that materials can ignite easier and burn under less stringent conditions in the spacecraft environments (reduced gravity, low velocity air circulation) than on Earth where buoyancy induces large air currents. These findings are the result of a thrilling and far-reaching research program into the behavior of fire in reduced gravity environments being conducted at the NASA Glenn Research Center, while working with other institutions under its sponsorship. This research encompasses studies of ignition, flame spread, smoldering, smoke evolution, and fire detection. The results from this work are bringing new light as to how materials burn in airflows with velocities lower than those achievable in normal gravity. This research is addressing many of the fire-safety concerns of the space program, and is providing insight into problems in fire-safety research.

Flammability Testing for Materials

Spacecraft designers need flammability data to select materials according to their potential fire exposure in the facility where they may be installed. In this project, researchers at the CPL are examining and characterizing the potential fire hazards of materials used aboard spacecraft under environmental conditions expected in those facilities (reduced gravity, small air currents). For this purpose, researchers at the UCB CPL are developing a new test apparatus, Forced Ignition and Spread Test (FIST), to characterize the flammability of materials. The FIST apparatus has the potential to provide NASA with a test methodology that will complement the existing protocol and to provide a more comprehensive assessment of material flammability for space applications. FIST is based on an ASTM test method (ASTM E 1321-93)), but reflects the conditions expected in space facilities. In particular, the effect of heat rising (or buoyancy-induced flows) is minimized so that the controlling mechanisms of the combustion process more closely approach those experienced in low-gravity conditions. The materials currently being tested include acrylic plastics (PMMA), composite materials such as blended polypropylene with glass fiber commonly used for paneling in the transportation industry, and laminated epoxy/glass often used in circuit boards.

Flammability Testing for Materials

The FIST apparatus consists of a small-scale wind tunnel in which samples of materials are exposed to an external radiant heat flux and varied airflow velocities. The external heat flux simulates a source of heat near the material and the flow of air simulates the circulation currents in the spacecraft. A hot wire placed near the surface, which simulates a hot spot adjacent to the material, ignites the volatile compounds that out-gas from the heated surface of the material. CPL currently has two functional FIST rigs. The rig shown in Figure 2 can be used to measure the time until ignition (ignition delay), the amount of mass a material must lose before it ignites (critical mass loss rate), the rate at which the flames spread over the surface (flame spread rate), and the heat released while burning from a sample of material (heat release rate). This rig can be operated with a large range of airflow velocities and external heat flux levels.

Fig.2 FIST apparatus

The other FIST rig that CPL uses is shown in Figure 3. This rig is unique because it is entirely contained within a pressure chamber so that experiments can be performed in varying atmospheric pressures. Because of its isolated nature, this rig can only be used to measure the time until ignition of a material.

Fig.3 Low pressure FIST apparatus

Current experimental work

NASA plans to retire the Space Shuttle in 2010 and is designing the next generation of space vehicle. This new vehicle, however, is being designed to operate with a different cabin environment than has been used in the past. Figure 4 below shows the cabin volume percent oxygen versus the total cabin pressure. The yellow diamonds represent the previously used cabin environments. For example, the Space Shuttle and the International Space Station operate with a cabin environment identical to that on Earth at sea level (1 atmosphere of pressure with 21% oxygen concentration). The Apollo missions used a much lower cabin pressure (5 psi or 34% of the pressure experienced at sea level) with pure oxygen.

Fig.4 Cabin environment for space vehicles.

Just like in SCUBA diving, astronauts can get decompression sickness (the “bends”) from the pressure difference between the vehicle and the suit. When the pressure suddenly reduces, the nitrogen in the blood stream bubbles out. Before an astronaut can suit up and go outside the space vehicle, he/she needs to pre-breathe pure oxygen to eliminate the nitrogen from the blood. The risk of decompression sickness increases when the change in pressure increases. A greater difference in pressure means that the astronauts must pre-breathe pure oxygen for a longer period of time. To decrease the risks and the pre-breathe time, the pressure difference must be reduced. However, if the suit pressure is increased, the suits will become too stiff and the astronauts wouldn’t be able to move. The only option is to decrease the pressure in the cabin of the vehicle. Unfortunately, if the only change made is to decrease the pressure, the astronauts will suffocate. In order to breathe normally when the pressure is reduced, it is necessary to increase the oxygen concentration of the air. The green (“normoxic”) line in Figure 4 indicates what the oxygen concentration would need to be at a given pressure so that a human feels like it is breathing sea level air. The blue line is the “hypoxic” curve and represents the oxygen concentration where breathing starts to become more labored, as if a human were at a high elevation, for example in Denver, Colorado. Ideally, the vehicle cabin would operate at or near the pressure used in the space suits but the oxygen concentration of the air required to breathe normally drastically increases the fire hazard onboard. For the next generation of vehicles, NASA has compromised by choosing the cabin environment shown in the figure by the yellow box. The goal of the current project is to understand just how the flammability of materials changes in this new environment.

Preliminary results

Experiments performed to date indicate that materials ignite faster in the cabin environment proposed for the next generation of space vehicles compared to normal atmospheric conditions on Earth. Figure 5 below can be used to explain why. Figure 5 shows the temperature of a material while it is being heated and subsequently ignited. The sharp spikes in temperature indicate when the material has ignited. There are two things we can infer from this graph. The first is that the material heats up faster when the pressure is reduced. In low pressure, the air is less dense. The reduced density means there is less of it that can be used to cool the material and cooling by convection is less effective. The second inference we can make from this graph is that the material ignites at a lower temperate, i.e., the temperature of the material right before ignition is lower when the pressure is reduced. Like in your car engine, if the mixture of air and fuel isn’t just right, combustion cannot occur. The fuel in this case is the vapor that off-gasses from the material as it is heated. Again, the air is less dense in low pressures. Because there is less air, less fuel is needed for combustion to occur so the material doesn’t have to be as hot to ignite.

Fig.5 Material temperature during ignition test

Previous experiments

Tests were conducted at UCB and on the NASA KC-135 aircraft, flying a parabolic trajectory (as shown in Figure 6 below). Using the KC-135 (or “Vomit Comet”), short periods of zero gravity can be achieved, simulating the conditions in spacecraft. A major finding from these experiments is that the time until a material ignites after being exposed to heat is significantly less in reduced gravity than here on Earth. Theoretical modeling also indicates that materials will ignite at lower heat fluxes than in normal gravity. This is primarily due to the reduction in heat losses from the material surface because of the small air currents in space facilities (about ten times smaller than in normal gravity). The results have very important implications since they indicate that materials will ignite more easily under the low gravity conditions expected in space facilities, and that consequently stricter design specifications or configuration control may be needed for fire safety. Figure 6

Fig.6 NASA’s KC-135 aircraft. “Vomit Comet.”

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