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Liquid oxygen, sometimes abbreviated asLOX orLOXygen, is a clear, palecyanliquid form ofdioxygenO2. It was used as theoxidizer in the first liquid-fueled rocket invented in 1926 byRobert H. Goddard,[1] an application which is ongoing.
Liquid oxygen has a clear, palecyan color and is stronglyparamagnetic: it can be suspended between the poles of a powerfulhorseshoe magnet.[2] Liquid oxygen has a density of 1.141 kg/L (1.141 g/ml), slightly denser than liquid water, and iscryogenic with a freezing point of 54.36 K (−218.79 °C; −361.82 °F) and a boiling point of 90.19 K (−182.96 °C; −297.33 °F) at 1 bar (14.5 psi). Liquid oxygen has anexpansion ratio of 1:861[3][4] and because of this, it is used in some commercial and military aircraft as a transportable source of breathing oxygen.[citation needed]
Liquid oxygen is also a very powerful oxidizing agent: organic materials will burn rapidly and energetically in liquid oxygen. Further, ifsoaked in liquid oxygen, some materials such ascoal briquettes,carbon black, etc., candetonate unpredictably from sources of ignition such as flames, sparks or impact from light blows.Petrochemicals, includingasphalt, often exhibit this behavior.[5]
Thetetraoxygen molecule (O4) was predicted in 1924 byGilbert N. Lewis, who proposed it to explain why liquid oxygen defiedCurie's law.[6] Modern computer simulations indicate that, although there are no stable O4 molecules in liquid oxygen, O2 molecules do tend to associate in pairs with antiparallelspins, forming transient O4 units.[7]
Liquid nitrogen has a lower boiling point at −196 °C (77 K) than oxygen's −183 °C (90 K), and vessels containing liquid nitrogen can condense oxygen from air: when most of the nitrogen has evaporated from such a vessel, there is a risk that liquid oxygen remaining can react violently with organic material. Conversely, liquid nitrogen orliquid air can be oxygen-enriched by letting it stand in open air; atmospheric oxygen dissolves in it, while nitrogen evaporates preferentially.[citation needed]
Thesurface tension of liquid oxygen at its normal pressure boiling point is 13.2 dyn/cm (13.2 mN/m).[8]
In commerce, liquid oxygen is classified as anindustrial gas and is widely used for industrial and medical purposes. Liquid oxygen is obtained from theoxygen found naturally inair byfractional distillation in acryogenic air separation plant.[citation needed]
Air forces have long recognized the strategic importance of liquid oxygen, both as an oxidizer and as a supply of gaseous oxygen for breathing in hospitals and high-altitude aircraft flights. In 1985, the USAF started a program of building its own oxygen-generation facilities at all major consumption bases.[9][10]
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Liquid oxygen is the most commoncryogenic liquidoxidizer propellant forspacecraft propulsion applications, usually in combination withliquid hydrogen,kerosene ormethane.[11][12]
Liquid oxygen was used in thefirst liquid fueled rocket. TheWorld War IIV-2 missile also used liquid oxygen under the nameA-Stoff andSauerstoff. In the 1950s, during theCold War both the United States'Redstone andAtlas rockets, and theSovietR-7 Semyorka used liquid oxygen. Later, in the 1960s and 1970s, the ascent stages of theApollo Saturn rockets, and theSpace Shuttle main engines used liquid oxygen.[citation needed]
As of 2025, many active rockets use liquid oxygen:
'We are going to do methane,' Musk announced as he described his future plans for reusable launch vehicles including those designed to take astronauts to Mars within 15 years, 'The energy cost of methane is the lowest and it has a slight Isp (Specific Impulse) advantage over Kerosene' said Musk adding, 'and it does not have the pain in the ass factor that hydrogen has.' ... SpaceX's initial plan will be to build a lox/methane rocket for a future upper stage codenamed Raptor. ... The new Raptor upper stage engine is likely to be only the first engine in a series of lox/methane engines.
