Rockets and human power

The two extremes of transport energy: chemical rockets and muscle-powered bicycles.

Conversion and use · Transport systems · No. 116

• Conversion and use

A Falcon 9 burns about 28 kg of propellant per kilogram it puts in orbit; a cyclist needs 21 kcal per km.

Chart: Illustration: Rockets and human power

Where it comes from

Chart: Map of where it comes from

A rocket carries both its fuel and its oxidiser, burns them and throws the hot gas backwards through a nozzle. Propellant makes up about 89% of a Falcon 9 at lift-off.

Three things to know

1. The hydrogen-burning RS-25 already reaches about 87% of the chemical ceiling on specific impulse, so reuse, not better engines, cut launch costs.
2. One Falcon 9 booster had flown 36 times by August 2026, spreading the cost of the first stage.
3. Per kilometre, a cyclist uses about one-thirteenth of the energy of a jet passenger.

The journey

1. 1 PropellantFuel plus oxidiser carried on board
2. 2 TurbopumpsForce propellants into the combustion chamber
3. 3 CombustionBurns at about 206 bar in RS-25
4. 4 NozzleExpands hot gas into a very fast jet
5. 5 OrbitPayload reaches about 7.6 km per second

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