What Fuel Do Rockets Use?

Rockets use fuels like refined kerosene (RP-1), liquid hydrogen (LH₂), and liquid methane (CH₄) for liquid-propellant engines, along with ammonium perchlorate composite propellant (APCP) for solid rocket boosters. These fuels pair with an oxidizer to combust, most commonly liquid oxygen (LOX). RP-1 with LOX is the most common combination, used on vehicles like Falcon 9 and historically the Saturn V.

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What Is Rocket Propellant Made Of?

Rocket propellant is made of two components: a fuel and an oxidizer. The fuel provides the chemical energy, and the oxidizer supplies the oxygen needed to burn it.

That second component is what separates rockets from jet engines and car engines. Those systems pull oxygen from the surrounding air, which doesn’t work above the atmosphere or in space. Rockets have to carry their own oxidizer to combust at any altitude.

The fuel and oxidizer are stored separately and only combine inside the combustion chamber. When they mix and ignite, the reaction produces high-pressure gas that expands through the nozzle and generates thrust.

What Are the Main Types of Rocket Propellant?

Rocket propellants fall into three categories: liquid, solid, and hybrid.

What Liquid Propellants Are Used in Modern Rockets?

Modern rockets rely on four main liquid propellant combinations: RP-1 with liquid oxygen, liquid methane with liquid oxygen, liquid hydrogen with liquid oxygen, and hypergolic propellants like hydrazine with nitrogen tetroxide.

Refined kerosene paired with liquid oxygen is the most common rocket propellant, having powered more orbital launches than any other combination. RP-1 is dense, stable at room temperature, and relatively inexpensive, which is why it anchors first stages on Falcon 9, Atlas V, Soyuz, and historically the Saturn V.

Methalox (liquid methane with LOX) has moved from niche to mainstream in recent years. SpaceX’s Raptor engines on Starship and Blue Origin’s BE-4 on New Glenn both run on methalox. The appeal: methane burns cleaner than kerosene, leaves less residue inside engine components, and can theoretically be produced on Mars from atmospheric CO₂, making it attractive for reusable and deep-space vehicles.

Liquid hydrogen with LOX delivers the highest performance of any chemical propellant combination, which is why it shows up on upper stages and deep-space missions where efficiency matters more than tank volume.

Hypergolic propellants ignite on contact without an ignition source, which makes them reliable for spacecraft maneuvering, upper stages, and in-space propulsion. Hydrazine and its derivatives paired with nitrogen tetroxide have powered everything from the Apollo lunar module to modern satellite thrusters. They’re toxic and corrosive, but their reliability in zero-gravity restarts keeps them in service.

What Propellant Powers Solid Rocket Boosters?

Solid rocket boosters run on ammonium perchlorate composite propellant (APCP), a mixture of oxidizer, metal fuel, and a rubber binder cast into a single solid grain. It’s the standard formulation for modern solid motors.

APCP typically contains three main ingredients:

• Ammonium perchlorate: the oxidizer, making up roughly 70% of the mixture.
• Aluminum powder: the metal fuel, adding significant energy to the reaction.
• Hydroxyl-terminated polybutadiene (HTPB): a synthetic rubber binder that holds everything together and also burns as fuel.

The Space Shuttle’s solid rocket boosters used APCP, as do the boosters on NASA’s Space Launch System and many military missiles. Smaller solid motors appear on launch vehicle upper stages, sounding rockets, and kick stages for satellite deployment.

Solid propellants offer high thrust, long shelf life, and mechanical simplicity, though once lit they burn until the grain is consumed.

How Do Hybrid Rocket Propellants Work?

Hybrid rocket propellants pair a solid fuel grain with a liquid or gaseous oxidizer that’s injected into the combustion chamber during operation. The fuel sits inert until the oxidizer is introduced and ignited.

The most common hybrid setup uses HTPB as the solid fuel with liquid oxygen or nitrous oxide (N₂O) as the oxidizer. Paraffin wax has also emerged as a high-performance fuel option in newer designs.

Hybrids can be throttled and shut down, unlike solid motors, because flow is controlled on the oxidizer side. They’re also safer to handle and transport since the fuel grain alone can’t combust without the oxidizer present.

They’re not commonly used, though, at least not yet. Virgin Galactic’s SpaceShipTwo uses a hybrid motor for suborbital flights, and hybrids appear in amateur rocketry, research vehicles, and some small launch concepts. Mainstream orbital launchers have mostly stuck with liquid or solid designs.

What Determines Which Propellant Gets Used?

Propellant selection comes down to matching performance characteristics to mission requirements. No single propellant wins on every metric, so engineers weigh tradeoffs based on what the vehicle actually needs to do.

The main factors include:

• Specific impulse: a measure of fuel efficiency, where higher values mean more thrust per pound of propellant consumed.
• Propellant density: denser fuels allow smaller tanks, reducing vehicle size and dry mass.
• Storability: whether the propellant can be held at ambient temperature or requires cryogenic cooling.
• Handling and safety: toxicity, corrosiveness, and ignition characteristics affect ground operations and infrastructure costs.
• Cost and availability: production volume, supply chain maturity, and raw material pricing.
• Mission profile: launch stages, in-space maneuvering, and deep-space propulsion each favor different propellants.

First stages often prioritize thrust and density, which favors kerosene or methane. Upper stages lean toward high specific impulse, which is where hydrogen earns its place. Spacecraft that need to restart engines years into a mission typically use hypergolic propellants for their reliability.

Fuel Is Only Half the Equation

What people call rocket fuel is really half of a propellant system. The other half is the oxidizer, which lets combustion happen where there’s no atmospheric oxygen to pull from.

A handful of proven combinations cover most of modern spaceflight: RP-1/LOX as the workhorse, methane and hydrogen for performance, hypergolics for reliability, and solid APCP for brute thrust.

Nick Vasco

Nick is an experienced B2B writer who brings his skill for crafting clear, easily digestible content to the industrial gas space.