SPACE shuttle mission 113 is about to come to a sudden end. Unknown to the crew, an invisible flame is eating through a bundle of cables behind a service hatch. The fire is consuming valuable oxygen. Smoke detectors are useless 鈥� there is too little smoke to trigger them. Without warning, the lights flicker and fail on a communication panel. Then comes the smell of burning.
The crew realise they are in extreme danger. Apart from the risk of burns and oxygen starvation, fire damage could leave them without the means to communicate with Earth or, worse, stranded in orbit. An emergency search uncovers the charred mass of cables, a faint yellow flash marks out the edges of the flames. As one crew member aims the extinguisher, a ball of flame bursts into the cabin, splits in two and dies out. With a quick blast from the extinguisher, the astronaut kills the blaze and averts a major disaster. The shuttle returns immediately to Earth.
Fiction? Possibly. But by no means fantasy. In November, NASA admitted for the first time that five 鈥渇ire incidents鈥� have occurred on board the shuttle since it started flying in 1981. Each time, smoke detectors failed to work and the crew only became aware of the danger after they smelt and saw smoke. NASA says the incidents were caused by short circuits or by electrical components overheating. 鈥淭he potential fires were immediately suppressed by switching off the circuits, without resorting to the use of fire extinguishers,鈥� says Howard Ross, a specialist in microgravity fires in NASA鈥檚 Lewis Research Center in Cleveland, Ohio. He insists, however, that the shuttle鈥檚 fire safety systems are adequate.
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The shuttle is not the only spacecraft to have suffered a fire. Ross says that a more severe incident occurred on the Soviet Salyut 7 space station which was launched in 1982. Details are scarce, but he believes that the fire was first controlled with an extinguisher and then by jettisoning the cabin atmosphere into space to remove any oxygen. By creating a vacuum, the fire would be extinguished instantly and any corrosive or noxious gases produced by the fire would be sucked out. The cabin pressure could only be restored after an emergency flight brought extra supplies of oxygen and nitrogen from Earth.
The possibility of fire is an increasing worry for designers of spacecraft. Experience in similarly isolated environments such as aircraft and submarines shows that unpredictable and unexpected fires do occur. In space, the fear is that these fires could burn undetected for some time and that, even when smoke is spotted, locating the source would be difficult.
Fires begin in much the same way wherever you are. To burn, a solid material must be heated to a temperature that drives volatile gases from its surface. If these gases are ignited, they react with oxygen giving out heat and light. It is this plume of oxidising gases that we call a flame. Much of the gas is oxidised directly to carbon dioxide and water, while some gas is converted into particles of carbon which rise from the flame as smoke. Other by-products can also form depending on the combustion materials. Fires that burn inefficiently, without converting all the carbon to carbon dioxide, produce a lot of soot which glows in the heat producing a yellow flame. More efficient flames produce less soot and are less bright.
The differences in the way fires burn on Earth and in space are caused by differences in gravity. On Earth, the hot, expanding gases in flames are forced to rise by cooler, denser gases in the surrounding air which displace them. Soot particles are swept along by these convection currents, creating a column of smoke above a fire. In orbit, however, a space vehicle and its contents experience almost no gravitational forces so convection currents cannot flow and the hot expanding gases can become spherical. Relying only on diffusion to supply oxygen and remove oxidation products, they often burn slowly but efficiently, producing few if any particles of soot. This means that the flames are almost colourless, generating only a faint, blue light created by chemical reactions in the flame. With no convection currents, heat can only be dissipated by radiation and conduction which is poor in air.
Detecting and locating fires in space is no easy task. Part of the problem is the lack of raw data on how fires burn in zero gravity. NASA astronauts have carried out only 10 experiments which involved deliberately lighting a fire in a space vehicle. The most recent was in September on space shuttle mission STS-64, where a sample of Plexiglas was burnt to find out exactly which combustion byproducts are produced in microgravity conditions.
Flame detector
The European Space Agency is also interested in minimising the chances of fires on board Columbus, the European section of the international space station now being designed. In 1993, the ESA commissioned Peter Bury, then head of the Fire Research Station, part of the Building Research Establishment in Hertfordshire, and Martin Shipp, a physicist at the station, to work out the best way to detect flames in space. 鈥淭he hope is that the results will be used to design a fire detection and suppression system for Columbus,鈥� explains Shipp.
The experiments were carried out in an aircraft, aptly named the vomit-comet by the people who work in it, which creates 20 to 25 seconds of reduced gravity by flying in a parabolic trajectory. Bury and Shipp burnt a range of substances, including propane, paper, cloth and balsa wood, inside a small combustion chamber containing detectors to measure the light, heat and smoke produced by the blaze. Different materials produce different combinations of light and heat as well as different types and amounts of soot. The plan is to build fire detection systems that can recognise such combinations.
The experiments, however, indicated that no single detector could recognise the presence of a fire in all the materials tested. Shipp believes that future detection systems will also need heat and light sensors and that more than one type of detector may be needed to measure smoke. Some detectors monitor changes in the optical transparency of the air, and smoke entering the detector causes the air to become more opaque. These are generally good at detecting the large smoke particles given off by smouldering fires. Small particles can be monitored more reliably with detectors which ionise the soot. As they pass through an electric field, the ionised particles allow a current to flow which can be measured. A computer will trigger the alarm when it spots the combination of signals from the detectors that indicate there is a fire.
Determining a suitable set of detectors is only half the problem 鈥� working out where to put them is just as important. On Earth this is easy: smoke detectors should be placed on ceilings because smoke rises. In the space shuttle and in the current design for Columbus, smoke detectors are built into the air-conditioning systems which draw air out of the living spaces, remove CO2, add oxygen and maintain the cabin temperature at between 18 掳C and 27 掳C. This system will work only if the air circulates rapidly within the entire volume of the vehicle 鈥� any stagnant areas could allow a fire to burn undetected.
In 1991, computer simulations of the air circulation in Colombus identified just this problem. Edwin Galea, a mathematician at the University of Greenwich in London, simulated the airflow in space and on Earth. 鈥淯nder normal gravity, the system would detect fires perfectly, but there were distinct differences in microgravity which prevented detection.鈥� The main problem was that air was flowing straight from air-conditioning inlets to the outlets without circulating round the cabin. The air-conditioning system has subsequently been redesigned to improve the circulation.
But this raises another problem. Increasing the airflow in microgravity brings more oxygen to the fire. Early experiments, in which the air was still, showed that flames burn more slowly in the zero gravity. More recent work on board the shuttle which used fans to create an air flow, has shown that materials burn more quickly in microgravity than on Earth. Even the movement of astronauts could create draughts strong enough to fan the flames.
In any case, improved air circulation may not increase the chances of detecting a fire early on. NASA鈥檚 scientists believe that the shuttle鈥檚 smoke alarms did not work because the small amount of soot produced picked up static charge and stuck to the walls of the craft before reaching the detectors. The sheer amount of soot produced in fires on Earth prevents this effect from being significant. In space it remains a problem, but one solution could be to use more heat detectors to detect the first signs of a blaze. A large number of these detectors could also help to pinpoint a fire. At the moment, if a fire alarm is triggered, crews must inspect the cabin to locate the source of any smoke.
The shuttle incidents have all involved electrical equipment but there is another potential danger. Some experiments carried out in space involve flammable gases: any leak could produce an explosive mixture inside the vehicle. Earlier this year, David Lozinski, a mathematician at the University of Illinois at Urbana-Champaign began developing mathematical models of such situations to determine how such gases mixed with oxygen burnt in zero gravity.
鈥淭he models predict that there is an absolute flammability limit, a concentration of gases below which it is safe to light a match,鈥� says Lozinski. He also discovered that at slightly higher concentrations, the mixture burns as balls of fire about one or two centimetres across. At still higher concentrations, any spark could cause an explosion. Lozinski鈥檚 models could lead to a fire prevention system that monitors the concentration of gases on board and warns the crew if they approach flammability limits.
Gravitational jitter
One problem, however, is that Lozinski鈥檚 models work only in zero gravity. In orbit, however, the force and direction of the gravity is constantly changing 鈥� albeit by small amounts. This phenomenon known as G-jitter, is affected by people moving about in the vehicle, liquids sloshing around in tanks, and even changes in the Earth鈥檚 gravitional field as the vehicle orbits. Such an effect could create small convection currents which the model does not take into account.
It is clear that daling with fires in microgravity is more complicated than scientists first thought. The current generation of spacecraft was designed after early experimentns suggested that fires in space were less likely and less potent than on Earth. 杏吧原创s now know this to be untrue and yet the designers of future space vehicles have not taken this into account. Of more immediate concern is the shuttle itself. Bury says the five incidents so far throw doubt on the effectiveness of the shuttle鈥檚 fire detection system. NASA, however, has no plans to ground the shuttle. The next flight is planned for early March. (see Diagram).
Fighting fires in space
HOW do you extinguish a fire in space? The problem is more complicated than it sounds since the chosen method must work in a confined environment, leave a nonpoisonous atmosphere and create manageable amounts of waste.
NASA currently uses aircraft-style extinguishers which rely on halon gases such as monobromotrifluoromethane or halon 1301. The shuttle is equipped with six halon extinguishers. The advantage of this gas is that it extinguishes fires in atmospheric concentrations below 5 per cent while seriously affecting people only at concentrations above 10 per cent. It operates by reacting with the hydrogen and oxygen free radicals in a flame, thereby preventing them from taking part in further combustion. The disadvantage, however, is that by-products of the reactions at any concentration include acidic fumes of hydrogen fluoride and hydrogen bromide which are harmful to breathe and highly corrosive.
On Earth, these gases can quickly be vented as soon as the fire is extinguished, but space vehicles do not carry enough spare gas to refill the cabin. NASA believes the crew could operate well enough in such conditions to carry out an emergency return to Earth. Once on the ground, the fumes would be flushed out to prevent damage to equipment.
The production of halons has been banned since the beginning of this year because of the damage they inflict on the ozone layer. At the moment, NASA is allowed to use up its reserves of halon, but even that is under review. It has not decided what to use instead.
Carbon dioxide extinguishers are another possibility, but the drawbacks are even more severe. CO2 extinguishes fires in concentrations above 25 per cent by preventing oxygen reaching the flame and by conducting heat. But an atmosphere of more than 5 per cent is harmful to humans so it is not possible to use it in a cabin which cannot readily be ventilated. Also, because more of it is needed and because CO2 boils at a lower temperature than halon-1301, it must be stored in liquid form at higher pressures. This requires thick metal containers which are very heavy 鈥� an enormous penalty in space vehicles.
Fine water mists are another possibility. The water would cool the burning material and extinguish any flames. However, the lack of gravity in space complicates the design of such an extinguisher. On Earth, water falls to the bottom of such an extinguisher. On Earth, water falls to the bottom of such an extinguisher and so compressed gas at the top can force it out. In microgravity, the water and gas would mix and so a way of keeping them separate must be developed. A further problem is that after extinguishing the fire, water would float around and could cause further damage to electrical equipment. A way of mopping it up would have to be developed.
A last resort is to move all the crew into another area of the vehicle, and then jettison the atmosphere to remove any oxygen. This might sound comparatively simple, but it is fraught with complications. Apart from problems with refilling the cabin, any items that contain even the smallest pocket of air could explode due to the sudden reduction in pressure.