Rockets and human power: Engineering methods

Kerosene, hydrogen, methane or solid: picking a propellant

What this tab covers The engineering methods that can do this conversion, compared on efficiency, scale, economics, limitations and reliability.

Chemical rockets differ by propellant pair; human power is the low-energy extreme of transport.

Comparison

Method	Efficiency	Scale	Economics	Limitations	Reliability
Kerosene and liquid oxygen (kerolox), e.g. Merlin 1D³⁷	Isp 283 s sea level, 312 s vacuum	854 kN per engine; nine on a Falcon 9 first stage	cheap, storable fuel; enables booster reuse	lower Isp; soot can foul reusable engines	no Falcon 9 mission failures in 165 launches in 2025
Hydrogen and liquid oxygen (hydrolox), e.g. RS-25²	Isp 452.3 s vacuum	512,300 lb (about 2.3 MN) vacuum thrust; 7,774 lb dry mass	expensive engines and cryogenic handling	hydrogen is very bulky (large tanks) and very cold	flight-proven on the Space Shuttle and SLS
Methane and liquid oxygen (methalox), e.g. Raptor, BE-4¹¹⁷	Isp between kerolox and hydrolox (no verifiable official figures published for Raptor or BE-4)	New Glenn (seven BE-4 engines) flew its second mission in November 2025; Starship (Raptor) made five suborbital test flights in 2025	designed for full or partial reuse	young technology	limited flight history
Solid rocket motors	lower Isp than liquid engines	strap-on boosters, small launchers and missiles	simple and storable	no throttling or restart	high, few moving parts
Bicycle and human power⁵⁶	rider about 18-22%; chain up to 98.6%	tens to hundreds of watts	lowest energy per km of any vehicle	low power, weather, distance	simple; chain wear and dirt are the main losses

Hydrogen gives the best specific impulse but needs huge tanks; kerosene gives compact, cheap stages; methane sits between and suits reuse; solids give simple, high thrust. For launch cost, reuse matters more than engine efficiency. Per kilometre, a cyclist uses about one-thirteenth of the energy per km of a jet passenger (calculated), and a rocket uses vastly more than either because it must reach 7.6 km/s.

How each method works

Kerosene and liquid oxygen (kerolox), e.g. Merlin 1D

Refined kerosene (RP-1) and liquid oxygen are pumped into a combustion chamber; dense fuel keeps tanks small.

Hydrogen and liquid oxygen (hydrolox), e.g. RS-25

Liquid hydrogen and oxygen burn at high chamber pressure; very light exhaust gives the highest chemical Isp.

Methane and liquid oxygen (methalox), e.g. Raptor, BE-4

Liquid methane is cleaner-burning than kerosene and denser than hydrogen, suited to reuse and to making propellant on Mars.

Solid rocket motors

Fuel and oxidiser are pre-mixed in a rubbery solid cast into the casing; once lit it cannot be turned off.

Bicycle and human power

Leg muscles turn food energy into pedal force; a chain and sprockets transmit it to the wheel.

Sources

RS-25 engine specification sheet L3Harris (Aerojet Rocketdyne), 2024
Falcon 9 Wikipedia (compiling SpaceX data; figures as reported), 2026
Bicycle efficiency study (Spicer et al., Johns Hopkins University news release) ScienceDaily, 1999
Gross efficiency and the relationship with maximum oxygen uptake in young elite cyclists during the competitive season (Artetxe-Gezuraga et al.), Journal of Human Kinetics 67, 123-131 Journal of Human Kinetics, 2019
SpaceX shatters its rocket launch record yet again: 165 orbital flights in 2025 Space.com, 2025
New Glenn launches NASA ESCAPADE, lands fully reusable booster Blue Origin, 2025