Rocket fuel, often called rocket propellant, is the substance carried aboard a vehicle to produce thrust. Historically and in modern practice most rockets rely on chemical propellants that generate high-speed exhaust by controlled combustion or rapid gas expansion. The general concept of a propellant and its use in a vehicle are described in many technical summaries of rocketry: see propellant and the broader topic of rockets.

Types of propellant

Propellants appear in several forms. Solid propellants are homogeneous or composite mixes of fuel and oxidizer bound into a grain (solid). Liquid propellants store fuel and oxidizer separately and feed them into a combustion chamber (liquid). Pressurized gases or compressed working fluids are used in simple rockets and demonstrations (gas). There are also hybrid designs that combine a solid fuel with a liquid or gaseous oxidizer.

How they work and key components

Most chemical rockets operate by burning a fuel in the presence of an oxidizer to produce high-pressure hot gases; this process is commonly described as combustion. Typical systems separate the concepts of fuel and oxidizer: the fuel supplies chemical energy while the oxidizer supplies the oxygen needed for combustion in the absence of atmospheric air (oxidizer). Propulsion performance depends on the exhaust velocity, mass flow rate, and chamber conditions.

History and notable examples

Different propellants have been chosen for reasons of thrust, efficiency, storability and cost. The Space Shuttle combined both solids and cryogenic liquids: its two solid rocket boosters used a composite grain containing powdered aluminium and an oxidizer such as ammonium perchlorate, while the main engines burned liquid hydrogen with liquid oxygen. Simple educational devices, like a toy water rocket, use compressed air or other gases to expel water and produce thrust.

Applications, trade-offs and safety

Choice of rocket fuel balances specific impulse (a measure of efficiency), density, handling hazards, temperature requirements and ignition behavior. Cryogenic liquids offer high performance but require insulation and complex plumbing; solid motors are mechanically simple and store well but cannot be throttled or shut down once ignited. Toxicity, flammability and the potential for accidental detonation are important operational considerations for manufacturers and launch operators.

Further reading