Rocket Fuel Explained: From Kerosene to Methane

Quick answer: RP-1 is a highly refined kerosene burned with liquid oxygen; it is dense and cheap but leaves soot (Falcon 9, Electron). Liquid hydrogen gives the best efficiency but needs huge, very cold tanks (RS-25, RL-10). Liquid methane sits between them, burns cleanly and can in principle be made on Mars, which is why Starship, New Glenn, Vulcan and Neutron use it.

Every rocket is fundamentally a controlled explosion pointed downward. The choice of what to explode, the propellant, is one of the most consequential engineering decisions in rocket design, influencing everything from engine complexity and vehicle size to reusability and operational cost. The space industry is in the midst of a major propellant transition, with methane emerging as the fuel of choice for next-generation launch vehicles. Here's why.

Rocket Propulsion Basics

Chemical rockets work by combusting a fuel with an oxidizer to produce hot gas, which is expelled through a nozzle to generate thrust. The key performance metric is specific impulse (Isp): a measure of how efficiently a propellant generates thrust, expressed in seconds. Higher Isp means more thrust per kilogram of propellant consumed.

But Isp isn't everything. Propellant density matters (denser propellants need smaller, lighter tanks), storability matters (some propellants boil off rapidly), handling difficulty matters (some are toxic or cryogenic), and cost matters (especially when you're buying propellants by the hundreds of tons).

RP-1 Kerosene: The Workhorse

RP-1 (Rocket Propellant-1) is a highly refined form of kerosene, paired with liquid oxygen (LOX) as the oxidizer. It's the most widely used rocket fuel in history.

Advantages:

• Dense: RP-1 has a density of ~0.81 g/cm³, enabling compact tank designs and smaller vehicle diameter
• Storable at room temperature: Only the LOX side requires cryogenic handling
• Well-understood: Decades of engine development have optimized kerosene/LOX combustion
• Relatively cheap: RP-1 costs approximately $2-5 per kilogram

Disadvantages:

• Moderate Isp: ~311 seconds at sea level (Merlin engine), ~340s in vacuum, good but not optimal
• Coking: Kerosene leaves carbon deposits (soot) in engine components, complicating reuse. SpaceX has invested heavily in solving this for Falcon 9 reusability, but the Merlin engines still require post-flight inspection and cleaning
• Not producible in space: RP-1 cannot be manufactured from extraterrestrial resources

Notable users: SpaceX Falcon 9/Heavy (Merlin engines), Rocket Lab Electron (Rutherford engines), Soyuz (RD-107/108), Saturn V first stage (F-1 engines), Atlas V first stage (RD-180).

Liquid Hydrogen: Maximum Performance

Liquid hydrogen (LH2) paired with LOX delivers the highest specific impulse of any chemical propellant combination: approximately 450 seconds in vacuum for engines like the RL-10 and RS-25.

Advantages:

• Highest Isp: More thrust per kilogram of propellant than any other chemical option
• Clean combustion: The exhaust is essentially water vapor, with no soot, no toxins
• Proven for upper stages: Ideal where Isp matters most, in vacuum, where every second of specific impulse translates to hundreds of kg of additional payload

Disadvantages:

• Extremely low density: LH2 density is just 0.07 g/cm³, 11 times less dense than RP-1. This requires enormous tanks, increasing vehicle size and structural mass
• Ultra-cryogenic: LH2 must be stored at -253°C, just 20 degrees above absolute zero. It boils off continuously, requiring constant replenishment until launch
• Hydrogen embrittlement: Hydrogen atoms penetrate metal crystal structures, weakening them over time. This requires specialized (expensive) materials and careful inspection protocols
• Explosive: Hydrogen is extremely flammable with a wide flammability range in air (4-75%), making handling hazardous
• Expensive infrastructure: Hydrogen production, liquefaction, transport, and storage infrastructure costs significantly more than kerosene or methane

Notable users: Space Shuttle main engines (RS-25), SLS core stage (RS-25), Ariane 5/6 (Vulcain), Delta IV (RS-68), Centaur upper stage (RL-10).

Liquid Methane: The Future

Liquid methane (LCH4) paired with LOX is the propellant combination of choice for almost every next-generation launch vehicle. The transition from kerosene and hydrogen to methane is the most significant propellant shift since the dawn of liquid rocketry.

Advantages:

• Clean-burning: Methane combustion produces minimal soot compared to kerosene, dramatically improving engine reusability. SpaceX's Raptor engines can be re-fired with minimal refurbishment, a critical enabler for rapid reuse
• Good Isp: ~330s sea level, ~363s vacuum (Raptor), significantly better than kerosene, approaching hydrogen performance with much higher density
• Reasonable density: 0.42 g/cm³, less dense than kerosene but far denser than hydrogen, enabling practical vehicle designs
• Moderately cryogenic: Stored at -162°C, much warmer than hydrogen's -253°C, simplifying tank design and reducing boiloff
• Cheap and abundant: Methane (natural gas) is widely available and costs approximately $1-3 per kilogram
• ISRU potential: Methane can be produced on Mars from atmospheric CO₂ and subsurface water ice via the Sabatier reaction, the original reason Elon Musk chose methane for Starship. A Mars-refuelable rocket is essential for the return trip

Disadvantages:

• Less flight heritage: Methane engines are newer and less proven than kerosene or hydrogen alternatives, though Raptor is rapidly accumulating flight data
• Lower density than kerosene: Methane tanks must be ~50% larger than equivalent RP-1 tanks

Notable users: SpaceX Starship (Raptor), Blue Origin New Glenn (BE-4), ULA Vulcan first stage (BE-4), Rocket Lab Neutron (Archimedes), Relativity Terran R (Aeon R), Stoke Space Nova (full-flow engines), LandSpace Zhuque-2 (the first methane-fuelled rocket to reach orbit, in July 2023), ESA Prometheus (development).

Solid Propellants: Simple but Limited

Solid rocket motors use a pre-mixed fuel and oxidizer (typically aluminum powder in an ammonium perchlorate rubber binder) cast into a solid grain. Once ignited, they burn until the propellant is exhausted; they cannot be throttled or shut down.

Advantages: Simple, reliable, storable for years, high thrust density, no plumbing or turbopumps. Ideal for strap-on boosters (SLS, Ariane 5) and military missiles (where storability is critical).

Disadvantages: Lower Isp (~260s), cannot be throttled or restarted, toxic exhaust (hydrochloric acid), and not reusable.

Why Methane Won

The industry's convergence on methane reflects a fundamental shift in priorities. In the expendable rocket era, maximum Isp (hydrogen) or maximum density (kerosene) mattered most because every kilogram of propellant was used once. In the reusable rocket era, engine reusability and operational simplicity matter most because the vehicle will fly hundreds of times. Methane's clean-burning characteristics, combined with its good-enough Isp, reasonable density, low cost, and Mars ISRU potential, make it the optimal all-around propellant for the reusable future.

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