Most of us have heard of the dangers of space flight such as solar storms, cosmic rays, leaks to the space crafts, and so on, but there are some ordinary problems too. TV programs like to have space ships in battles, whereupon “shields fail” (there is no such thing as a shield, except the skin of the craft, but let’s leave that pass) and we then have fire. When you stop and think about it, fire of a space ship would be a nasty problem. It burns material that presumably had some use, it overheats things like electronics, which will stop them working, then we come to the real problem: if you don’t have spares, you cannot fix it. You often see scenes where engineers have been running around “beating the clock” but what do they use for parts? If they are going to make parts, out of what? If you say, recycle, then at the very least they should be assiduously collecting “smashed stuff”.
Accordingly, it would make sense for astronauts to prevent fires from starting in the first place. You may recall Apollo 1. The three astronauts were inside the command module practising a countdown. The module used pressurised oxygen, and somehow a fire broke out. Pure oxygen and flammable material is a bad mix, and the astronauts died from carbon monoxide poisoning. The hatch opened inwards, and the rapid increase of pressure from the fire meant that it was impossible to open the hatch. The fire was presumed to have started by some loose wiring arcing, and igniting something. Now, we know better than to have pure oxygen, but the problem remains. Fire in space would not be good.
One obvious defence is to reduce the amount of combustible materials. If there is nothing to burn, there will not be a fire, but that is not entirely practical, so the next question is, how do fires burn in space? At first sight that is obvious: the organic matter gets hot and oxygen reacts with it to make a flame. However, there is more to it.
First, how does a fire burn on Earth? For a simple look, light a candle. What you see is the heat melts the wax, the wax runs up the wick and vaporises. The combustion involves breaking the wax down into a number of smaller molecules (which can be seen as smoke if combustion is incomplete) and free radial fragments, which react with oxygen. Some of the fragments combine to form carbon (soot, if it doesn’t burn further). The carbon is important because it glows, giving off the orange colour, but it also radiates a lot of heat, and that heat that radiates downwards melts the wax. What you will notice is that the flame moves upwards. That is because the gas is hot, hence it expands and occupies less volume than a similar number of moles of air. Going up is simply a consequence of Archimedes’ Principle. As it goes up, it sucks in air from below, so there is a flow of gas entering the flame from below, and exiting above. If you can get hold of some methanol, you could light that. Its formula is CH3OH, which means there are no carbon-carbon bonds, which means it cannot form soot. Therefore, it will burn with a light blue coloured flame and it does not radiate much heat. Methanol burning on your skin will not burn you as long as the skin is below the flame.
Which brings us to space. Since fire is possible on a space ship, NASA has done some experiments, partly to learn more about fire, but also to learn how to put them out on a space ship. The first difference is that in the absence of gravity, flames do not go up, after all there is no “up”. Instead, they form little spheres. Further, since there is no gravity, Archimedes’ Principle is no longer important, so there is nothing to suck fresh air in. Oxygen has to enter by diffusion, and oxygen and fuel combine in a narrow zone on the surface of the sphere. The “fire” continues with no significant flame, and further, while a normal fire burns at about 1500 – 2000 degrees K, these fires using droplets of heptane eventually form cool fire, reaching temperatures of 500 – 800 degrees K. Also, the chemistry is different. On Earth, flames usually produce water, carbon dioxide and soot. In microgravity they were producing water, carbon monoxide and formaldehyde. In short, a rather poisonous mix. Cool fires in space are no less dangerous; just different dangerous. Dealing with them may be different too. Extinguishers that squirt gas may simply move the fire, or supply extra air to it. So far, I doubt we have worked out our final methods for fighting fires in space, but I am sure the general principle is to have the fewest possible combustible materials in the space ship.