Specific impulse converter
Convert rocket and jet-engine efficiency between seconds, exhaust velocity and fuel consumption.
That's 1.06× as efficient as a Merlin engine at sea level, while a Merlin vacuum engine is 1.16× more efficient. Its exhaust leaves at about 2.942 km/s.
What it means
How efficient is that engine?
Rocket figures are typical published values, at sea level unless marked vacuum. The jet-engine figure is for an airliner at cruise.
Every unit
Your specific impulse in every unit
Learn
Three things worth knowing
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Seconds × 9.81 = m/s
A 300 s engine throws its exhaust out at about 2,940 m/s. Double the specific impulse and each kilogram of propellant gives twice the push.
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Why seconds
It is the number of seconds one kilogram of propellant could hold up its own weight in thrust. Because gravity cancels out of that definition, US and European engineers get the same number.
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High Isp is not high thrust
An ion engine at 3,000 s would not lift a sheet of paper, and a solid booster at 250 s lifts a space shuttle. Specific impulse is about economy; thrust is about strength.
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Background
About specific impulse
Specific impulse is how much push an engine gets out of each kilogram of propellant. Quoted in seconds, it reads the same in metric and imperial, which is why rocketry kept it.
Multiply it by standard gravity and you have the effective exhaust velocity: the faster the gas leaves, the less of it you need to carry.