Why Rockets Don’t Need Oxygen

As many articles and videos throughout the internet explain, a rocket moves in accordance with Newton’s third law of motion by ejecting matter, in one direction so that the reaction force produces motion in the opposite.

The object of a rocket motor is to provide thrust, it does this by ejecting a stream of (usually hot) gas at high velocity. Liquid bipropellant engines combine a fuel with an oxidiser in either a solid or liquid form, then combust the mixture to produce gases. This is exhausted and voila! You have your stream of high velocity gas.

Some sources explain this using the analogy of an air-breathing jet engine. They describe the ability of rockets to operate in the absence of air by stating that unlike jet engines, rockets “must take the oxygen with them”. While this is accurate of operational rockets (at the time of writing) this isn’t technically correct and is disproven by chlorine trifluoride. In 1948, Bert Abramson of Bell Aircraft fired chlorine trifluoride with a hydrazine fuel. In following years, Rocketdyne produced two engines which similarly used a chlorine trifluoride oxidiser. To understand how these engines were possible, and how we can fire a rocket engine without oxygen we must understand how rockets propellants produce energy.

Propellants store their energy in the bonds between their atoms. To release this energy, we require a reaction which breaks bonds, but whose products release more energy than those bonds took to break. In the case of (liquid) hydrogen and (liquid) oxygen, the energy released when forming water is greater than that required to break the hydrogen-hydrogen bonds and the oxygen-oxygen bonds, giving us a viable bipropellant.

Because of our meddling in bond breaking, we are dealing with electrons and their movements. Thus, we enter the realm of redox reactions.

One of the major players within redox chemistry is the oxidising agent, or oxidiser. This is where our rocket oxidiser gets its name. Oxidisers are any chemical species that lose their electron(s) in a reaction. No limitations regarding oxygen are made. Two examples of oxidisers are oxygen, and the chlorine trifluoride we mentioned earlier.

If any molecule which loses its electron(s) can be our oxidiser, what properties make for a good oxidiser? In reality, the decision is a series of compromises where we consider a host of factors depending on the intended use. These factors include: energy released by the reaction, reaction time, freezing point, safety, production, cost, and more. In theory, we seek an oxidising agent that releases as much energy as possible. So in the realm of idealism and hypotheticals, oxygen need not be present.

In reality, rocket chemists must make a series of compromises, and liquid oxygen is consistently above average, leading to its preferential use over other oxidisers, like chlorine trifluoride.

To learn more about rocket propellant chemistry, I recommend reading Ignition! By John D. Clark.