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NASA's official standard for testing spacecraft materials is called NASA-STD-6001B. This test applies a six-inch flame to the bottom of a vertically mounted material sample. The material fails the safety test if the flame burns upward more than six inches or if it drips burning debris. This method seems reasonable for Earth, but it contains a critical flaw: the test is conducted in Earth's gravity, and the environment of space is fundamentally different.
On Earth, air moves in predictable convection currents, and gravity creates a distinct "up" and "down." These conditions do not exist on the International Space Station (ISS) or other microgravity environments. In microgravity, fires do not burn upward with a pointed flame. Instead, flames form spherical blobs that spread slowly outward in all directions. These flames are largely sustained by the spacecraft's ventilation systems, which constantly circulate air.
Simply turning off the ventilation is not a viable solution to stop a fire. While a lack of air movement might slow a fire's spread, it could also cause materials to smolder. A smoldering material could wait for airflow to resume and then reignite into a full flame. The ideal way to understand this physics would be to conduct experiments directly on the ISS. However, lighting open flames inside a crewed, enclosed space station is too dangerous. Researchers have instead ignited roughly 1,500 very small, controlled flames to study basic combustion principles.
Because of these limitations, NASA developed a different method. They created the Spacecraft Fire Safety experiments, known as Saffire. These tests were performed inside uncrewed Cygnus cargo capsules. After the capsules completed their supply missions and departed from the ISS—but before they re-entered Earth's atmosphere to burn up—researchers remotely ignited large sheets of materials like cotton, fiberglass, and acrylic. They carefully observed how these materials burned in true, sustained microgravity.
The Saffire experiments revealed unexpected physics. Flames sometimes spread in the opposite direction of the airflow, contrary to what Earth-based models predicted. Researchers also found that flames could burn hotter on thinner materials in space. The data clearly showed significant discrepancies between the NASA standard test results and the actual behavior of fire in a spacecraft.