What is the cabin pressure for spacecraft?

Question

At what altitude on earth do astronauts breath air in space?

On pressurised aircraft this is compared to breathing air at 8000ft.

Answer 1

Present space vehicles operate at standard sea-level atmospheric pressure, with the Shuttle Orbiter reducing the cabin pressure to 70.3 kilopascals (kPa) or 10.2 pounds per square inch (psi) prior to EVA operations. The ISS also operates at a nominal sea-level pressure to maintain compatibility with the Shuttle Orbiter and the Russian Soyuz vehicles.

Answer 2

Earlier spacecraft such as Apollo and Gemini used oxygen-only cabin atmospheres, with a total gas pressure of about 5 psi (roughly the partial pressure of oxygen in Earth atmosphere).  This reduced the need for strength in the cabin pressure vessel.  However, the absence of a diluent gas (e.g., nitrogen) actually increased the in-flight fire hazard aboard the spacecraft by allowing more energy from a spark to head toward ignition rather than toward heating the diluent.

Further, cabins were pressurized at launch to a pressure 3-5 psi above that of the outside air (to activate and validate the outflow valve operations).  Since those spacecraft were provided only with an oxygen plumbing and tanking apparatus, the launch cabin pressure was around 17 psia of pure oxygen, presenting a very hazardous condition.  This was the cause of the tragic Apollo 1 fire which claimed the lives of Gus Grissom, Ed White, and Roger Chaffee in a launch-pad test.

As a result, the Apollo spacecraft was modified to allow the high-pressure launch atmosphere to be a composition of oxygen and nitrogen, to be replaced gradually with pure oxygen at low pressure as the spacecraft climbed to orbit.  Tests about the Skylab space station in the 1970s confirmed that the diluent gas played a significant role in in-flight fire hazard mitigation, so subsequent manned spacecraft have been designed to support Earth normal air.