Specific Impulse Calculator
Fuel Efficiency, Measured in Seconds
Specific impulse is a rocket engine's fuel efficiency rating, expressed in seconds — the higher the Isp, the more delta-v a given mass of propellant delivers. It can be derived two ways: from measured thrust and propellant mass flow rate on a test stand, or directly from the exhaust velocity of the propellant leaving the nozzle, since the two are proportional through standard gravity.
The Formulas
From exhaust velocity: Isp = Exhaust Velocity / g0
g0 is standard gravity, 9.80665 m/s², used here purely as a unit-conversion constant to express the result in seconds rather than m/s.
Where This Calculation Matters
- Comparing propulsion technologies — Isp is the standard basis for comparing chemical, electric, and nuclear thermal propulsion concepts on equal footing.
- Feeding the rocket equation — Isp is the key input to the Tsiolkovsky rocket equation, directly determining how much delta-v a given mass ratio can achieve.
- Engine test analysis — static-fire test data (measured thrust and propellant flow) is converted to Isp to validate an engine against its design specification.
- Mission architecture tradeoffs — a low-thrust, high-Isp electric propulsion system trades burn duration for dramatically better propellant efficiency than a high-thrust chemical engine.
Typical Specific Impulse by Propulsion Type
| Propulsion type | Typical Isp range |
|---|---|
| Solid rocket motor | 250–270 s |
| Kerosene / liquid oxygen (kerolox) | 300–350 s |
| Hydrogen / liquid oxygen (hydrolox) | 420–455 s |
| Ion / electric propulsion | 1,500–4,000+ s |
These are widely published approximate ranges for propellant classes, not guaranteed figures for any specific engine — actual Isp varies with nozzle design, chamber pressure, and expansion ratio.
How to Use This Calculator
- Choose the calculation mode: From Thrust & Mass Flow Rate, or From Exhaust Velocity.
- For the thrust mode, enter thrust in newtons and mass flow rate in kg/s.
- For the exhaust velocity mode, enter exhaust velocity in m/s.
- Select Calculate to get the specific impulse in seconds.
Related Calculations
Apply the resulting Isp to the Rocket Equation Calculator, or budget a mission profile with the Delta-V Calculator.