Burns a candle in a spaceship

Practical space travel: How does a flame burn in weightlessness?

Fire is one of the greatest dangers in a spaceship or on a space station. Because from there you cannot simply escape outside or open a window. The astronauts on board the Russian Mir space station had to react quickly when a fire suddenly broke out there in February 1997. Fortunately, they succeeded in locating the source of the heavy smoke development and helping to extinguish the fire.

Blue and round

But despite this literally burning danger, there are some crucial differences between a fire on earth or in a weightless spacecraft. In recent years, scientists have repeatedly investigated what these differences look like through experiments on the International Space Station or in one of its supply transporters.

It shows that a flame looks completely different in weightlessness and behaves differently. Instead of long drawn out shining and flickering like an earthly candle flame, a space fire glows silently and forms a spherical ball of fire. This is also not yellowish, like the candle flame, but predominantly blue - like the flame center directly on the wick of an earthly candle.

When there is no buoyancy

But how does this come about? The main reason is the lack of gravity. On earth, it ensures that gases rise or fall according to their density. When air is heated, as is the case in a candle flame, the hot combustion gases rise and with them soot particles.

Because the soot is also flammable, these particles ignite a little above the wick and then form the yellowish part of the candle flame. The decisive factor, however, is that the upflow of the hot gases draws fresh air into the flame from below. The oxygen it contains provides new nourishment for the combustion process.

It is different in weightlessness: Here the hot combustion gases have no buoyancy and therefore there is no suction that brings in new oxygen. Instead, the flame can only draw oxygen from the surrounding air. As a result, she is constantly on the verge of suffocation and only slowly glows. At the same time, due to the lack of air flow, it takes on a spherical shape. Because no soot particles rise, there is also no yellow flame component - the weightless flame appears blue.

Towards the draft

But there is another special feature of a fire in space, as recently shown by an experiment in the empty Cygnus supply module of the ISS. For this purpose, a ribbed piece of Plexiglas was blown on one side with a steady stream of air. Then a flame was lit at the other end. Something surprising happened: instead of going out or spreading with the air flow, the flames migrated towards the air flow. Under normal gravity this would be impossible.

Here, too, the reason is weightlessness: Because heated air does not rise, the gases in the flame remain stratified and do not mix. The flame therefore only gets fresh oxygen where it is hit by the air flow. Accordingly, the flame is most active here and spreads surprisingly quickly against the air flow. So trying to blow out a flame in a spaceship would be a really bad idea.

Increased risk of deflagration

And something else is different: the flame shields the area under it which is heated and degassed against the ingress of air and combustion. This means that the gas lying behind it cannot ignite. In the experiment you can therefore see that the flame front moving forward against the oncoming flow does not leave a burning surface behind. Instead, a lot of flammable but unburned flue gas is produced under it. Even the smallest disturbance of the air currents can supply this hot, unburned flue gas with oxygen and lead to a dramatic deflagration.

Something similar happens if, in an earthly apartment fire, a layer of hot, flammable gases collects under the ceiling and this suddenly receives fresh oxygen by opening a window - a dangerous cloud of flames occurs. In fact, the team of the SAFFIRE experiment has already experienced such a deflagration in space: In the previous experiment, the flame in the Cygnus transporter seemed to have already gone out and the airflow was switched on again. This caused a sudden and strong flare-up of the fire.

These are important findings for emergencies - for extinguishing a fire on the ISS space station or in a space capsule. Because if a crew member moves quickly near the fire or tries to extinguish the fire with a jet, this can create the air flow that really heats up the flames. In the worst case, the person even comes across the fire directly.