⛽ Rocket Propellant Comparator — Isp, Density & Tank Size

Compare LOX/LH2, LOX/methane, kerosene, hypergolics, solids and hybrids on specific impulse and bulk density — then size the tanks and see the delta-v the same propellant mass buys.

Isp values are representative of flight engines at roughly 70–100 bar chamber pressure and expansion ratios of 40–100, not a guarantee for a specific engine.
Delta-v, vacuum Isp——
Bulk propellant density——
Tank volume——
Burn time at target thrust——

Delta-v from the same propellant mass and dry mass, ranked by vacuum Isp. The highlighted bar is the selected combination.

Selected combination

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    All combinations on the same propellant mass and dry mass — scroll sideways on a phone
    PropellantO/F massBulk kg/m³Isp vacIsp SLΔv vacTank m³Handling
    Why Isp alone is a bad way to choose a propellant — mixture ratio, density and the tank you have to carry

    Specific impulse is exhaust velocity divided by g₀, or thrust divided by weight flow: Isp = F/(ṁg₀). In vacuum, a hydrogen/oxygen engine can reach roughly 450 s; a kerosene/oxygen engine around 350 s; a solid composite around 260 s. But the figure is only comparable at similar chamber pressure, expansion ratio and nozzle area ratio. A 300 s engine with a huge vacuum nozzle and a 300 s engine optimised for sea level are different machines, and the same propellant can move by tens of seconds across that range.

    Bulk density is the mass of propellant divided by the volume of both tanks at the operating mixture ratio. It is why liquid hydrogen, despite the best Isp, needs a huge and heavily insulated tank: at an O/F of about 5.5 the bulk density is only around 345 kg/m³, against roughly 1,020 kg/m³ for LOX/RP-1. A dense propellant lets a smaller, lighter, lower-drag stage carry the same impulse; a high-Isp but bulky propellant can lose the mass advantage once tank and insulation mass are counted. This tool deliberately shows both columns and the tank volume so the trade is visible.

    Mixture ratio. The O/F values here are representative operating points, usually slightly fuel-rich of stoichiometric to control chamber temperature and improve Isp. Moving away from the optimum costs Isp and changes the density; real engines trim mixture ratio in flight for tank balance and performance. Solid motors and monopropellants have no O/F split: the propellant is a single formulated grain or a single liquid, so the volume is simply mass divided by density.

    Handling is part of the trade. Liquid hydrogen and methane are cryogenic; oxygen is a cryogenic oxidiser and a fire hazard; N₂O₄/MMH is hypergolic and toxic; hydrogen peroxide decomposes and attacks many materials; nitrous oxide can exothermically decompose; solids cannot be throttled or shut down; hydrazine is toxic and carcinogenic. The tool's labels are a prompt to read the safety data, not a handling procedure.

    What the delta-v means. Δv = Isp·g₀·ln(m₀/m_f) with m₀ = dry mass + propellant, ignoring gravity and drag losses. It is the ideal velocity change of a single stage, not a mission design. Real launch vehicles lose roughly 1.5–2 km/s to gravity and drag on the way to low Earth orbit, and staging changes the arithmetic.

    Representative values compiled from Sutton & Biblarz, Rocket Propulsion Elements; Huzel & Huang, Modern Engineering for Design of Liquid-Propellant Rocket Engines; NASA SP-125; and published engine data for the RL10, RS-25, Raptor, Merlin, RD-180 and typical APCP motors. Last reviewed: 2026-10-19. Not a design guarantee or a safety procedure.