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Developing international standards for material flammability in spacecrafts<\/h3>\n\n\n\n
Laboratory of Space Utilization at Hokkaido University is a leading hub for microgravity combustion research in Japan.
Professor Fujita Osamu, a foremost expert in the field and former head of the laboratory, served as the founding principal investigator of the JAXA FLARE project (Fundamental Research on International Standard of Fire Safety in Space) launched in 2012.
Under the project the laboratory has been advancing the development of international standards for evaluating material flammability in microgravity, in collaboration with space agencies from Japan (JAXA), the U.S. (NASA), European Union (ESA), France (CNES), and Germany (DLR), as well as university researchers in Japan, the United States, France, and Germany.<\/p>\n\n\n\n
Fire is a deadly hazard in crewed spacecraft. The materials to be brought inside the spacecraft are tasted for safety following the flammability evaluation standard (NASA-STD-6001).<\/p>\n\n\n\n
\u201cThe results of a test are evaluated as pass or fail and behind such practical criteria is experience rather than detailed understanding of underling mechanisms,\u201d points out Assistant Professor Yusuke Konno. \u201cMoreover, the conditions under which materials perform in spacecraft cannot be easily reproduced on Earth. While oxygen concentration and pressure can be matched in special chambers used for testing, research shows that microgravity can alter the flammability of materials in space.\u201d<\/p>\n\n\n\n
To achieve \u201cweightlessness\u201d on earth for an experiment one may launch a sounding rocket, ride a parabolic flight or drop the test container from an experimental drop-tower, but large-scale preparations, availability and costs limit the number of possible experiments. Numerical simulations offer an accessible low-cost way to obtain data on effects of microgravity.
\u201cAnother key factor to be understood is the relationship between oxygen concentration, atmospheric pressure, and flammability under microgravity,\u201d explains Dr. Konno. \u201cFor example, astronauts currently spend around four hours preparing to leave the spacecraft, breathing pure oxygen and exercising to reduce nitrogen in the body and prevent decompression sickness. NASA\u2019s Artemis program for manned moon exploration seeks to shorten this time by increasing oxygen levels in the spacecraft atmosphere, but higher oxygen concentrations also increase flammability.
To better understand these risks, Dr. Konno uses numerical combustion analysis based on detailed descriptions of chemical reaction mechanisms available in the literature. His team believes that defining flammability limits in relation to oxygen concentration can enable development of more reliable fire safety standards.<\/p>\n\n\n\n
Focusing on the refrigerant R32 to unravel the chemical reactions during ETFE combustion<\/h3>\n\n\n\n
Previous research by Dr. Konno, Professor Fujita, and their collaborators showed that ethylene-tetrafluoroethylene (ETFE), a fluoropolymer recognized for its excellent flame-retardant properties and widely used as wire-insulation material in space crafts has expanded flammability range in micro-gravity, and that the flame behavior is affected by air flow velocity. Their findings suggested that increased flammability is connected to the residence time of the released gases within the reaction zone in micro-gravity. However, detailed mechanisms remained unclear.
In fluoropolymers such as ETFE fluorine atoms act as halogenated flame retardants, which are known to suppress combustion primarily by inhibiting gas-phase chain reactions through a radical-trapping mechanism that captures reactive intermediates. Dr. Konno and his colleagues selected difluoromethane (CH\u2082F\u2082), also known as the refrigerant R32, which belongs to the same broader hydrofluorocarbon (HFC) family as ETFE, as a representative organofluorine compound to study gas-phase decomposition products of fluoropolymers. R32 has gained attention as a lower\u2013global warming potential alternative to traditional carbon-based refrigerants, such as CFCs; its mild flammability has also driven extensive combustion research, and its behavior has been well documented in recent years.
The numerical simulations revealed a clear relationship between oxygen concentration, airflow velocity, and flammability. In particular, the researchers found that R32 can sustain combustion even at low oxygen concentrations if the flow residence time (the average time chemical species spend in the reaction zone) is long. Through reaction pathway analysis, they provided, for the first time, a physical and chemical explanation for why ETFE exhibits increased flammability in microgravity.
They also compared the one-dimensional counterflow diffusion flame model with a zero-dimensional model that excludes the effects of flow and physical structure on flame and closely resembles micro-gravity conditions. This comparison highlighted that organofluorine compounds such as ETFE exhibit unique combustion behavior, with clear differences between zero-dimensional conditions and structured flames, where chemical kinetics and transport phenomena are strongly coupled. Their findings have implications for fire-safety in space.<\/p>\n\n\n\n
\u201cOur choice of R32 as a model gas has drawn interest of refrigerant combustion researchers at other universities, as well as international researchers specializing in chemical reaction analysis, and has already led to collaborative projects,\u201d Dr. Konno adds. \u201cLooking ahead, I hope to expand this work beyond ETFE to investigate the combustion characteristics of various halogenated polymers and to elucidate the mechanisms of organic flame retardants.\u201d<\/p>\n\n\n\n Microgravity research is becoming increasingly competitive worldwide with the United States and China rapidly expanding its space capabilities leading the field. In the past, Kamisunagawa, a former coal-mining town north-east of Sapporo, was home to one of the world\u2019s largest drop towers, repurposed from a decommissioned mine shaft in 1991. With a total length of about 710 meters, it allowed experiments under high-quality microgravity conditions for up to 10 seconds, the longest in the world. \u201cResearchers from around the world used to gather there, often preparing at Hokkaido University before departing to conduct their experiments,\u201d says Dr. Konno. Although the facility unfortunately closed in 2003, a 50-meter drop tower \u201cKosmotorre\u201d jointly owned by a local company and Hokkaido University is now operating in Akabira town. It offers approximately 3 seconds of microgravity and is open to anyone.<\/p>\n\n\n\n The network formed by researchers who once conducted experiments at the Kamisunagawa facility, users of the Akabira drop tower, and students mentored by Professor Fujita continues to thrive. Dr. Konno has been part of this network since his undergraduate years, when he joined the FLARE project.![\"\"](\"https:\/\/pr.eng.hokudai.ac.jp\/rc\/en\/wp-content\/uploads\/sites\/2\/2026\/03\/featured_02_04-1600x1066.jpg\")
Building on Hokkaido\u2019s microgravity legacy to advance fundamental combustion research<\/h3>\n\n\n\n
Yet. Dr. Konno notes that Hokkaido is widely known among researchers as a place with a long and well-established history in microgravity research.<\/p>\n\n\n\n
\u201cAmong the many phenomena studied in engineering, combustion and fire are particularly complex. They involve the interplay of heat and mass transport with rapid chemical reactions, and many aspects remain not yet fully understood. While empirical knowledge is important, fundamental research is essential to grasp the underlying mechanisms,\u201d concludes Dr. Konno. Building on the knowledge cultivated in combustion engineering at Hokkaido University and supported by an expanding research network, the Laboratory of Space Utilization continues to pursue a deeper understanding of the fundamental nature of combustion.<\/p>\n\n\n\n![\"\"](\"https:\/\/pr.eng.hokudai.ac.jp\/rc\/en\/wp-content\/uploads\/sites\/2\/2026\/03\/featured_02_05-1600x1066.jpg\")
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