Week 23: Efficiency: Isp

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Not every rocket uses chemical propellant burned quickly — some use much smaller amounts of propellant moved much faster, trading thrust for incredible efficiency.

History

Ion and electric propulsion systems generate only a tiny amount of thrust — far too little to lift off a launch pad — but they use their propellant so efficiently that they're ideal for long, slow maneuvers once a spacecraft is already in space.

Forces

Specific impulse (Isp) measures how efficiently a motor uses its propellant — essentially, how much thrust it gets per unit of propellant consumed over time.

Math

Isp = ve / g0

Worked example (metric): an ion engine with an exhaust velocity of 30,000 m/s has an Isp of 30,000 / 9.81 ≈ 3,058 seconds — compared to just a few hundred seconds for a typical chemical motor.