Scientists from Nasa and the National Institute of Standards and Technology (NIST) have found that current smoke detectors used on the International Space Station (ISS) are not able to detect all types of smoke.
The results were published in the Fire Safety Journal and recommend that ‘the next generation of spacecraft fire detectors must be improved and tested against smoke from relevant space materials.’
Detecting a fire in space requires a very different process than on Earth due to the lack of gravity. In microgravity, flames are spherical in shape, with smoke often aggregating into large particles or long chains that spread in all directions. This requires detectors to be placed in ventilation systems rather than on the wall.
Researchers also found that materials used on a spacecraft, which might have the potential to become fuel for a fire, are not the same as potential combustibles in a terrestrial environment. Smoke in microgravity may also have different properties depending on the source.
Since 2002 Nasa and NIST have been studying the behaviour of smoke in microgravity to help develop fast, sensitive and reliable methods for detecting it during spaceflight. More recently, they looked at the smoke particles produced by five materials commonly used aboard crewed spacecraft, defined their characteristics and evaluated how well they could be detected by two existing systems.
To accurately define these characteristics, Nasa and NIST researchers conducted the Smoke and Aerosol Measurement Experiment (SAME) aboard the ISS. In the experiment, an astronaut wrapped materials commonly used on the ISS—cellulose, Kapton, silicone rubber, Teflon and Pyrell—in wire filaments and loaded them into a rotatable carousel enclosed in one of the station’s glove boxes.
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By GlobalDataA software program then applied an electrical current to the wires to heat the materials and produce smoke. The smoke was then ‘aged’ in a chamber to simulate the time it would take to build up in a real fire scenario.
“By controlling and changing three factors: the rate of sample heating, the airflow passing around the heated material and the age of the smoke generated, we obtained valuable data about smoke from a variety of possible fire conditions,” said NIST engineer Tom Cleary, an author on the paper who calibrated the equipment used in SAME.
The results showed that the current ISS smoke detector was able to easily pick up the large smoke particles produced by overheating the cellulose, silicone and Pyrell samples.
However, the light-scattering photoelectric ISS detector frequently failed to detect the smaller smoke particles from Teflon and Kapton, which are both used extensively in electronics—the most likely fire and smoke source in space.
Nasa researcher and lead author Marit Meyer said: “Considering the wide array of materials and heating conditions possible in a spacecraft fire, as well as the complications from background aerosols in the cabin environment such as dust, we concluded that no single smoke detection method currently available is sensitive enough to detect all possible smoke particle sizes.
“More research is needed to better understand how fires behave in microgravity, and in turn, how best to detect them as early as possible by whatever type of smoke they create.”
Meyer added that the older ionisation detector from the space shuttle era fared only slightly better for Teflon smoke.
Nasa plans to continue conducting studies into space fires with the Spacecraft Fire Experiment, also known as Saffire. In 2016 and 2017 they conducted tests on unmanned cargo vessels, turning them into orbiting fire laboratories and monitoring the effects from the ground.
There are three more Saffire tests scheduled for 2019 and 2020, with NIST’s Tom Cleary again calibrating the smoke particle instruments.
