In what might sound like a rather frightening first, NASA plans to set fire in a spacecraft in orbit. Scheduled for May, the Spacecraft Fire Experiment-I (Saffire-I) will take place aboard an unmanned Cygnus cargo ship after it completes its resupply mission to the International Space Station (ISS). The purpose of this controlled pyrotechnic experiment is a better understanding of large-scale fires in zero gravity and improving spacecraft design safety.

Since the fatal launch pad fire during a rehearsal for the Apollo 1 mission in 1967 that cost the lives of three US astronauts, NASA became so safety conscious about anything that could be set alight that the crews of the Skylab missions in 1973 complained there was so much fire retardant in the towels and washcloths on the spacelab they were useless.

Yet, despite decades of work, large-scale fires in space are still poorly understood. On Earth, safety engineers routinely set fire to houses, airplanes, and ships to better understand how they burn and how to make them. Spacecraft, on the other hand, have been largely neglected due the impracticality of giving them a thorough testing. Though a lot of work has gone into fire safety, all the large-scale tests have been conducted on the ground.

The problem is, fire acts very differently in space. On Earth, fire rises, which is why candle flames have their characteristic tapering. In zero gravity, things aren't the same. With no gravity to pull down denser, colder air, there's no air convection, so the fire and its combustion by-products hover about in a sphere rather than flow away. The result is that a candle in a spacecraft would soon extinguish itself and a large fire would behave in ways that are difficult to predict.

Until now, standards have been based on Earthside fire safety standards, which introduces a large level of uncertainty, and current practices are mostly theoretical and based on computer models. Meanwhile, what experiments have been conducted in space have been limited to samples no larger than 10 cm (4 in) in length and width because anything larger is considered too dangerous to conduct on the space station.

Being run by NASA's Glenn Research Center along with input from a number of international partners, the Saffire experiment's purpose is to assess current models of how fires grow and spread in weightlessness as a way to improve spacecraft safety. It's designed to simulate the amount of flammable material that's likely to be present in a spacecraft fire and to examine the flammability limits of materials by using a sample material about 1 m (39 in) long – which is an order of magnitude larger than any previous space fire experiment.

Fortunately, Saffire doesn't involve setting fire to an oily rag, tossing it into the spacecraft, and slamming the door. Instead, three Cygnus cargo ships, beginning with CRS OA-6 scheduled to lift off on March 22, will have a special experimental module installed for a controlled burn. According to NASA, aside from the safety factor, the Cygnus spacecraft was chosen because it has the required volume, power, communication systems, and availability.

After CRS OA-6 leaves the ISS in May following its resupply mission, it will not return to Earth, but will instead be filled with station rubbish and burn up in the Earth's atmosphere somewhere over the South Pacific Ocean. Before this happens the Saffire module will automatically carry out the fire experiment while sensors and cameras transmit data and images of the event to mission control.

Saffire consists of a 40 x 100 cm (16 x 39 in) panel of flammable material in a box with vents at one end and fans at the other to facilitate airflow, but to prevent any sparks or burning fragments from escaping into the cargo bay. According to preprogrammed instructions, hot wires will ignite the material and a suite of avionic sensors will record temperature, oxygen, carbon dioxide, and other factors while LEDs and cameras measure the length of the sample as it burns. In addition, thermocouples and a calibrated radiometer will track the spread of the flame.

The three Saffire experiments will be conducted on successive Cygnus missions. The space agency says that the findings will not only be useful for spacecraft safety, but also for mines, submarines, and aircraft, which are susceptible to similar fire hazards.

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