Air

Air (78 % nitrogen, 21% oxygen, etc.) used as an inert cold gas, can be held under pressure and released by valves to create thrust. Ambient air can be scooped up by air intakes and used in turbojet, turbofan, ramjet, scramjet, or other airbreathing engines and burned with fuel as an oxidizer. It also can be mixed with the rocket exhaust to augment thrust.

Pressurized air was used in the Vostok and Zenit spacecraft for attitude control jets. It was also proposed in some manned spacecraft to ensure maneuvering near space stations without releasing corrosive by-products.

In LACE (Liquid Air Combustion Engines), air is collected during ascent, chilled, liquefied, and stored for use as rocket engine oxidizer once ambient air at high altitude is no longer sufficient to produce thrust. In single-stage-to-orbit variants the air may be liquefied prior to use, and later the motor converts to pure rocket propulsion, using on-board liquid oxygen for the final push to orbit. In the air augmented rocket, air is collected by an intake surrounding the rocket body, and used to augment the rocket exhaust.