Rocket Propellants Compared: Solid, Storable, Kerosene, Hydrogen and Methane

Propellant choice is the first big decision in rocket design. It sets how much push each kilogram delivers, how big the tanks must be, how hard the vehicle is to handle on the ground, and how many launches a team can afford. No single propellant wins on every measure, so each rocket family trades one strength for another.

This article compares the main propellant families using rounded figures from standard references. It explains why density matters as much as efficiency, and where each choice shows up in real vehicles.

Three families of propellant

Rocket propellants fall into three broad groups. Solid propellants mix fuel and oxidizer into one rubbery or cast block that burns from the surface inward. Storable liquids, mostly hypergolics, ignite on contact and can stay in tanks at room temperature for years. Cryogenic liquids, such as liquid oxygen with liquid hydrogen or methane, must be kept far below room temperature and boil off if left alone.

Each family has a different job. Solids give high thrust with simple hardware. Storable liquids suit missiles, satellites and long-duration spacecraft. Cryogenics give the highest efficiency for large launch vehicles and upper stages.

Solid propellants

Most solid rocket motors use ammonium perchlorate composite propellant, often called APCP. It mixes an oxidizer, powdered aluminum for energy, and a polymer binder such as HTPB that also holds the block together. The Space Shuttle's solid rocket boosters used this mix, with about 500 tonnes of propellant in each booster's case.

Solids have three strengths. They are simple, they can be stored for long periods, and they produce a large thrust from a compact package. Their weaknesses are equally clear. Specific impulse is modest, around 250 to 270 seconds in vacuum, and once a solid motor is lit it cannot be throttled or shut down. A flaw in the grain can cause catastrophic failure, which is why solid motor design focuses so heavily on casting quality and inspection.

Hypergolic storable liquids

Hypergolic propellants ignite as soon as they touch, so the engine needs no igniter. The classic pair is nitrogen tetroxide as the oxidizer with hydrazine or unsymmetrical dimethylhydrazine (UDMH) as the fuel. The Titan II launch vehicle used a storable hypergolic pair, and many spacecraft thrusters still rely on this chemistry.

Their specific impulse is good, around 310 to 320 seconds in vacuum. They are liquid at normal temperatures, so tanks can be lightweight and the vehicle can wait on the pad for long periods. The cost is toxicity. These chemicals are hazardous to people and to the environment, so they demand careful handling, strict protective equipment and expensive ground procedures.

Kerosene and liquid oxygen

RP-1, a refined kerosene, paired with liquid oxygen (LOX) is the workhorse of the modern launch industry. The Saturn V's first stage used it, and so does the SpaceX Falcon 9. RP-1 is dense, cheap, and easy to store in ordinary tanks. Liquid oxygen boils at about 90 kelvin, which is cold but manageable.

The pair delivers a specific impulse of roughly 300 to 350 seconds in vacuum, depending on the engine. It leaves carbon deposits, called coking, in cooling channels, which makes reuse harder. Engineers have worked around this with careful fuel cooling and by choosing engine designs that run clean.

Liquid hydrogen and liquid oxygen

Liquid hydrogen with liquid oxygen gives the highest specific impulse of any common chemical rocket. Engines such as the RS-25 and the J-2 reach roughly 420 to 450 seconds in vacuum. Hydrogen burns to water vapor, a light exhaust that moves quickly.

Hydrogen has a very low density. Liquid hydrogen weighs only about 71 kilograms per cubic metre and boils at about 20 kelvin, just above absolute zero. A tank full of it must be large and heavily insulated, and it still boils off over time. For a typical oxygen-to-hydrogen mixture ratio near 6 to 1, the combined propellant has a bulk density of only about 0.36 grams per cubic centimetre. The tanks, not the engine, often set the vehicle size.

Liquid methane and liquid oxygen

Liquid methane with liquid oxygen sits between kerosene and hydrogen. Methane boils at about 111 kelvin, which is easier to handle than hydrogen. Its specific impulse, roughly 350 to 370 seconds in vacuum, beats kerosene.

Methane also burns cleanly and leaves little carbon. This matters for reuse, because engines that run clean need less cleaning between flights. Methane can be made from carbon dioxide and water in principle, which has drawn interest for long missions, although production at launch scale is still a development task.

Density decides tank size

Specific impulse alone does not decide the best propellant. Density decides how big the vehicle must be to carry a given amount of propellant. A dense propellant packs more push into a smaller tank, which reduces structural mass and drag.

The table gives rounded values. Bulk density for the mixed propellants is calculated from the standard oxidizer-to-fuel mass ratios.

Propellant	Vacuum Isp (approx.)	Bulk density (approx.)	Example vehicle
APCP solid	268 s	1.8 g/cm3	Space Shuttle SRB
N2O4 / Aerozine 50	310 to 320 s	1.2 g/cm3	Titan II
LOX / RP-1	300 to 350 s	1.0 g/cm3	Falcon 9, Saturn V S-IC
LOX / LCH4	350 to 370 s	0.8 g/cm3	Starship (in development)
LOX / LH2	420 to 450 s	0.36 g/cm3	Saturn V S-IVB, SLS core

Read the table as a set of trades. Hydrogen wins on efficiency and loses on density. Kerosene is the reverse. Methane sits in the middle, which is one reason it has attracted attention for reusable designs.

Handling and ground systems

Propellant choice also drives the launch pad. Cryogenic propellants need loading systems that cool lines, manage boil-off, and vent gas safely. Hypergolics need sealed systems, emergency response and strict access rules. Solids need climate-controlled storage because temperature changes affect burn behavior.

Ground cost and turnaround time are often as important as the rocket itself. A propellant that is hard to load slows the launch rate, which matters for any program that aims to fly often.

Key takeaways

No propellant is best in every category. Solids offer simplicity and thrust, storables offer readiness, and cryogenics offer efficiency. Each family buys one advantage by accepting a cost somewhere else.

Density is the hidden factor. Hydrogen's high specific impulse is offset by enormous tanks, while kerosene and methane pack more push into a smaller space.

Future vehicles will keep balancing these trades. Methane has drawn attention because it sits between kerosene and hydrogen on both efficiency and density, and because it burns clean enough for repeated flights.