Space Suit Air Supply Calculator

Stored gas is converted to breathable volume with the ideal gas law: a 1.2 liter bottle at 6,000 psi holds about 408 atmospheres of pressure, expanding to roughly 490 standard liters of oxygen. Consumption is set by metabolic rate. Because burning one liter of oxygen releases about 20.9 kJ, a moderate EVA workload of roughly 1,000 BTU per hour, or 293 watts, corresponds to about 0.84 liters per minute, while quiet rest is nearer 0.3 and strenuous tool work can exceed 1.3. Carbon dioxide output is scaled by the respiratory quotient, typically 0.8 to 0.85 for a mixed diet, which is why CO2 production runs slightly below oxygen uptake. NASA's Extravehicular Mobility Unit runs pure oxygen at 4.3 psi differential, low enough for the joints to flex yet giving the same oxygen partial pressure a person breathes at sea level, and it is certified for roughly 6.5 to 8 hours plus a 30 minute secondary pack. Several simplifications limit this estimate. Oxygen at 400 atmospheres is not an ideal gas; its compressibility factor is above one, so the true stored quantity is roughly 10 to 20 percent less than the ideal calculation suggests. The carbon dioxide danger time treats the suit as a fixed mixing volume, whereas real ventilation loops sweep gas past the scrubber continuously and are governed by inspired partial pressure, with flight rules keeping suit CO2 below about 7.6 mmHg. Oxygen is also rarely the true limiting consumable: sublimator feedwater for cooling, battery charge, and scrubber capacity usually bound the EVA first.

Pressure suits began in aviation rather than spaceflight. Russell Colley of B.F. Goodrich built the suit Wiley Post used for stratospheric flights in 1934, and that lineage led through the US Navy Mark IV suit to the Mercury program garment, which was pressure protection only and carried no independent life support. The first true extravehicular systems came in 1965: Alexei Leonov left Voskhod 2 on 18 March and found his suit had ballooned so stiffly that he had to bleed pressure to re-enter the airlock, and Ed White performed the first American EVA on 3 June from Gemini 4, breathing through an umbilical with a chest-mounted emergency pack. Self-contained life support arrived with Apollo, where ILC Dover built the A7L garment and Hamilton Standard supplied the Portable Life Support System, using lithium hydroxide for carbon dioxide removal and a water sublimator that rejects heat straight to vacuum. Apollo 17 crews worked outside for over seven hours at a stretch. The Shuttle-era Extravehicular Mobility Unit entered service in 1981 at 4.3 psi and was upgraded around 2001 with regenerable METOX canisters that bake out absorbed CO2 instead of being discarded. Russia's Orlan, flying since 1977, uses a rear-entry hatch and a higher 5.7 psi operating pressure that shortens pre-breathe. NASA's Exploration EMU work transitioned to industry in 2022, with Axiom Space contracted to deliver the Artemis lunar surface suit.