The white “smoke” one typically sees billowing from a rocket on a launchpad is primarily excess oxygen being vented from the vehicle. It raises the question as to why rockets carry oxygen in the first place.

Rockets must carry oxygen internally since all rocket fuel requires an oxidizer, typically in liquid oxygen form, to burn and produce the hot gases that propel the vehicle. Since they mainly operate in the vacuum of space, rockets cannot utilize oxygen in the atmosphere like conventional aircraft.

Like most forms of motorized transport on land (using fossil fuels) and all types of air transport (specifically jet aircraft), rockets use fuel that requires oxygen (used as an oxidizer) to burn and allow for engine combustion.

Commercial airliners typically fly at a cruising altitude of about 10 kilometers (33 000 feet). At this height, the air is thin enough to provide little resistance to the aircraft but still enough oxygen present to allow its engines to function.

Any higher, though, and jet engines will struggle to burn fuel due to a lack of oxygen. Rockets already spend a minimal amount of time in the atmosphere, and at altitudes above 10 kilometers, there is too little oxygen present to support engine combustion effectively.

Why Rockets Carry Oxygen In Addition To Fuel
A SpaceX Falcon 9 rocket vents oxygen during launch.

As a result, rockets need to carry all the oxygen they will require for fuel to combust internally. To better understand how this occurs, one needs a clear understanding of how rocket engines work.

How Rocket Engines Use Oxygen

The vast majority of modern rockets used for space travel primarily use a type of rocket engine called a liquid-propellant rocket engine. Different types of liquid propellants are used (more on that in the next section), but they all need to combine with an oxidizer to combust.

Illustration of the propulsion system of a typical liquid-propellant rocket, showing the separate fuel and oxidizer (liquid oxygen) tanks.

The liquid propellant and oxygen are kept in two large pressured tanks, the fuel & oxidizer tanks, respectively, within the rocket’s main structure.

To have enough oxygen available to last an entire mission, it has to be cooled down to ‑183° Celsius (‑297° Fahrenheit) to turn into a liquid that can be stored in a rocket’s oxidizer tank.

(The liquid oxidizer tank can also house the helium tanks that are often used to pressurize an orbital rocket’s propellant tanks. Learn more about why and how rocket propellant tanks are pressurized in this article.)

(This explains the “white smoke or steam” that escapes from a rocket’s structure on the launchpad, which is just excess gaseous oxygen evaporating as it is vented.)

During a rocket launch, the fuel and oxidizer are pumped from the tanks into a combustion chamber under high pressure using turbopumps. In the combustion chamber, the two compounds are mixed and ignited.

The hot gases that are produced as a result of this combustion are further pressurized and accelerated through the rocket engine’s nozzle, where it exists at supersonic speeds to provide the trust needed by the rocket to escape Earth’s atmosphere.

Since the oxygen is stored internally, it is always available to be used as an oxidizer to allow the fuel to combust, even in the vacuum of space where no atmospheric oxygen is available. This is what allows a rocket to use its thrusters in environments with or without any oxygen.

By controlling the flow and mixing of fuel & oxidizer, rockets are also able to control their thrust, throttle up and down, and even completely switch off their engines and restart them at a later stage when needed in space.

(Learn more about exactly how rocket engines work and the different components involved in this article.)

Although liquid-propellant rocket engines make up the vast majority of propulsion systems used in modern rockets, solid rocket boosters using solid rocket propellant are also still used for specific applications.

However, as the next section will illustrate, no matter what specific type of fuel is used in a rocket, some form of oxidizer is always required to allow for combustion, even in solid rocket propellants.

The Different Types Of Fuel Rockets Use

Image showing the three liquid-fueled main engines of the Space Shuttle Endeavour flanked by its two solid rocket boosters.

All modern rockets use a type of chemical compound as fuel that, combined with an oxidizer, is combusted to produce the hot gases that propel them forward. There are mainly two types of rocket fuel:

  • Liquid Rocket Propellant
  • Solid Rocket Propellant

Liquid Rocket Propellant

Liquid Rocket Propellant rockets all use a type of liquid fuel that is combined with an oxidizer (liquid oxygen or LOX) in a combustion chamber to produce the hot gases that provide thrust to the rocket. There are four primary liquid propellants:

  • RP-1 (refined kerosene called kerolox when combined with oxygen)
  • Hydrogen (called hydrolox when combined with oxygen)
  • Methane (called methalox when combined with oxygen)
  • Hypergolic Fuel (spontaneously combusts when compounds are combined)

Except for hypergolic fuel, all the other propellants have to be mixed with liquid oxygen (LOX) in a combustion chamber to ignite. Combined with oxygen, they are called kerolox, hydrolox, and methalox, respectively.

Hypergolic fuels are chemical compounds that spontaneously combust when coming into contact with each other without the need for an external ignition mechanism. This makes them ideal for use in space and other areas where quick, reliable combustion is needed.

But even hyperbolic fuels require an oxidizer to ignite and combust. They primarily consist of hydrazine (N2H4) or some derivative of the compound, while the oxidizer consists of nitrogen tetroxide (N2O4).

(Learn more about the different fuel types orbital rockets use, their characteristics, as well as the advantages and drawbacks of each fuel in this article.)

Solid Rocket Propellant

As the name suggests, solid rocket propellants consist of fuel in a solid state with an almost rubber-like texture. This characteristic gives them the advantage of being able to be stored and transported with relative ease and safety.

(Unlike liquid rocket propellants that need to be stored separately and only able to be transferred to the rocket’s main tanks minutes before liftoff on the launchpad.)

Just like all other rocket fuels, though, solid rocket propellants need an oxidizer to combust. It typically consists of aluminum powder (as the fuel) combined with ammonium perchlorate (as the oxidizer) and other binding agents to create the solid mix.

(Learn more about solid rocket propellant, how it is made, the different types of solid propellants, as well as its various advantages and drawbacks in the following article.)


As just illustrated, although their composition may differ, all types of rocket fuel (including liquid rocket propellant and solid rocket propellant) need a form of oxidizer to ignite. This may come in the form of liquid oxygen or another chemical compound acting as an oxidizer.

As a result, all modern rockets have to carry large quantities of oxygen onboard to combine with the fuel to provide the supersonic hot gases needed to generate the thrust required to escape Earth’s atmosphere and travel to Low Earth Orbit and beyond.

(On a side note, whenever a crewed rocket launch occurs, additional oxygen is also carried in the spacecraft/module that houses the astronauts/cosmonauts to provide a hospitable environment to breathe and work. This, however, is a subject for another article.)

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