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SESK: Stuff Everyone Should Know · sesk.info/aero-specific-impulse-seconds

In what unit is a rocket engine's specific impulse usually measured?

Seconds.

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  1. Seconds ✔
  2. Pounds
  3. Miles per hour

Why: Specific impulse compares thrust to the weight of propellant used each second. When you divide those, the units cancel down to seconds.

Huh, didn't know that: Engineers shorten specific impulse to Isp.

The dad joke
Why did the engine brag about its specific impulse? Every second counted.
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References

Answer: Seconds. Specific impulse compares thrust to the weight of propellant used each second. When you divide those, the units cancel down to seconds.

  1. NASA Glenn, Specific Impulse www.grc.nasa.gov/www/k-12/airplane/specimp.html
    Isp = m/sec / m/sec^2 = sec
    Checked 2026-10-10.

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