Specific Impulse Calculator Formula

Understand the math behind the specific impulse calculator. Each variable explained with a worked example.

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Formulas Used

Specific Impulse

isp = exhaust_velocity / 9.80665

Thrust per Flow Rate

thrust_per_flow = exhaust_velocity

Variables

VariableDescriptionDefault
exhaust_velocityExhaust Velocity(m/s)3000

How It Works

Specific Impulse

Specific impulse (Isp) measures rocket engine efficiency: how much thrust per unit weight of propellant consumed per second.

Formula

Isp = ve / g0

  • *ve* = effective exhaust velocity (m/s)
  • *g0* = 9.80665 m/s2 (standard gravity)

    • Higher Isp means more efficient use of propellant. Chemical engines: 200-450 s. Ion engines: 1500-5000 s.

    Worked Example

    Engine with exhaust velocity 3000 m/s.

    exhaust_velocity = 3000
    1. 01Isp = ve / g0
    2. 02Isp = 3000 / 9.80665
    3. 03Isp = 305.9 s

    Frequently Asked Questions

    Why is Isp measured in seconds?

    It represents the time for which one kilogram of propellant can produce one kilogram-force of thrust. The seconds unit comes from dividing velocity by acceleration.

    Which rocket engine has the highest Isp?

    Ion engines (e.g., NASA's NEXT) achieve over 4000 s, but with very low thrust. Among chemical engines, the RL-10 hydrogen engine reaches about 465 s.

    Does high Isp always mean a better engine?

    Not necessarily. High Isp usually comes with low thrust. For launch vehicles, a balance of Isp and thrust is needed to overcome gravity.

    Ready to run the numbers?

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