Liquid Oxygen (LOX) (O₂) is a cryogenic oxidizer propellant used as a oxidizer. Cryogenic — requires insulated tanks, boils off over time. Typical ISP: 270–460 s (depending on fuel) seconds.
Liquid oxygen, almost always shortened to LOX, is the cold liquid form of the same oxygen we breathe. In rocketry it is the most widely used oxidizer of all, supplying the oxygen a rocket needs to burn its fuel in the airless vacuum of space.
Quick facts
- What it is: molecular oxygen (O₂) in liquid form, written LOX or LO₂.
- Appearance: a clear, pale-cyan (light blue) liquid. It is strongly paramagnetic, meaning it can actually be held in place between the poles of a strong magnet.
- Boiling point: 90.19 K (−182.96 °C / −297.33 °F) at normal pressure — it must be kept this cold to stay liquid.
- Freezing point: 54.36 K (−218.79 °C / −361.82 °F).
- Density: about 1.141 kg per liter, slightly heavier than water and far heavier than oxygen gas.
- Role: oxidizer, not fuel — it provides oxygen so a separate fuel can burn.
- Where it comes from: pulled out of ordinary air by chilling and separating it (fractional distillation) in a cryogenic air-separation plant.
What it is and how it works
A rocket engine makes thrust by burning fuel, and burning needs oxygen. Here on the ground a fire pulls oxygen from the surrounding air, but in space there is no air. So a rocket has to bring its own oxygen along. That is the job of liquid oxygen: it is stored in an insulated tank, pumped into the combustion chamber, and mixed there with a fuel such as kerosene, hydrogen, or methane. The two react violently — this reaction is called combustion — producing hot, high-pressure gas that rushes out the nozzle and pushes the rocket forward.
Oxygen is liquefied (turned from gas to liquid) by chilling it below −183 °C because the liquid packs far more oxygen into the same space than the gas would. Think of how a tightly packed suitcase holds far more than a loosely filled one: the liquid lets a rocket carry plenty of oxidizer in a smaller, lighter tank. The trade-off is that LOX must stay extremely cold, so it is loaded shortly before launch and constantly “boils off” into gas. Tanks are vented and topped up right until liftoff — the reason countdowns include fueling holds. Some modern vehicles even sub-cool, or densify, the LOX below its boiling point to squeeze in still more propellant.
Why it matters
LOX is the workhorse oxidizer of orbital rocketry. It delivers high specific impulse — a measure of engine efficiency, essentially how much thrust you get per kilogram of propellant — better than storable oxidizers can. It is comparatively safe and non-toxic to handle, and it is cheap and abundant because it is simply made from air. Paired with different fuels, it covers nearly every need: LOX with hydrogen for maximum efficiency, LOX with kerosene for dense and simple high-thrust boosters, and the newer LOX with methane (“methalox”) as a balanced, reusable middle ground whose fuel could one day even be produced on Mars.
The downsides are real, though. Keeping LOX cold demands heavy insulation, constant boil-off venting, and last-minute fueling. And because it is such a powerful oxidizer, it makes ordinary materials dangerously flammable — oils, greases, and even LOX-soaked asphalt or coal can ignite or detonate from a spark, flame, or impact. LOX systems must therefore be kept scrupulously clean and built from oxygen-compatible materials. It remains the only cryogenic (extremely cold) oxidizer ever flown operationally; alternatives like nitrogen tetroxide can be stored at room temperature but are toxic and less powerful.
Notable examples
- Robert Goddard’s 1926 rocket — the first liquid-fueled rocket flight, burning LOX with gasoline. This was the founding use of LOX in rocketry.
- Saturn V (Apollo) — LOX with RP-1 kerosene in the giant F-1 first-stage engines, and LOX with liquid hydrogen in the upper-stage J-2 engines that sent astronauts to the Moon.
- Space Shuttle Main Engines (RS-25) — burned LOX with liquid hydrogen from the orange External Tank, reaching about 452 seconds of specific impulse. The same engines now fly on NASA’s Space Launch System (SLS).
- SpaceX Falcon 9 and Falcon Heavy — Merlin engines burn LOX with RP-1 kerosene (Merlin Vacuum about 348 s).
- SpaceX Starship and Super Heavy — Raptor engines burn LOX with liquid methane, about 380 s in vacuum. LOX also powers Atlas V, Soyuz, New Glenn, and many others.
First used in rocket engines by Robert Goddard in the 1920s. Has been the standard oxidizer since the dawn of the space age.
High performance, non-toxic, abundant, inexpensive, compatible with many fuels
Cryogenic boil-off, requires insulation, vigorous oxidizer requiring careful handling
