Specific Impulse

The engineering

Isp is engine gas mileage: seconds of thrust per unit weight of propellant, or equivalently the effective exhaust velocity divided by g₀. The league table to carry in your head — solids ~250 s, kerosene-LOX ~300–350, hydrogen-LOX ~450, and ion thrusters 2,000–4,000 s (at thrust levels measured in postage stamps).

The 'seconds' unit is a historical peace treaty: dividing by g₀ made the number identical in metric and imperial, so von Braun's team and everyone since could compare engines without unit fights. Physically the honest quantity is v_e — it's what the Tsiolkovsky card actually multiplies by the logarithm.

Where this math comes from

Goddard was measuring the efficiency of his powder and liquid motors at Clark University in the 1910s–20s — his 1919 monograph scandalized the New York Times with the idea of rockets working in vacuum — and exhaust velocity was his figure of merit from the start. The 1926 Auburn flight burned gasoline and LOX at an Isp a modern hobby motor would sneer at, and it changed the world anyway.

The g₀-normalized 'seconds' convention solidified in the 1940s–50s missile programs where German-metric and American-imperial teams collided. Chasing the number gave us staged combustion, the RL10 (first hydrogen engine, 1963, ~430 s) — and the recognition that chemistry caps near 460 s, which is exactly why electric propulsion exists.

1. 1903Konstantin TsiolkovskyExhaust velocity identified as rocketry's key metric.
2. 1919Robert Goddard'A Method of Reaching Extreme Altitudes' — measured nozzle efficiency.
3. 1926Robert GoddardFirst liquid-fuel flight — Isp leaves the test stand.
4. 1963Pratt & Whitney RL10Hydrogen-LOX flies — chemical Isp near its ceiling.

See the full timeline of the math behind every calculator →

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