Rocket Propulsion Calculator
Calculate rocket delta-V, exhaust velocity, and burn time using the Tsiolkovsky rocket equation.
About this calculator
The Tsiolkovsky rocket equation is the foundational relationship in rocket propulsion: it connects how much a rocket's velocity can change (delta-v) to how efficiently its engine burns propellant and how much of the vehicle's mass is propellant versus everything else. This calculator first converts specific impulse (Isp), a measure of engine efficiency given in seconds, into exhaust velocity by multiplying by standard gravity (9.80665 m/s²) -- a higher Isp engine expels propellant faster for the same mass flow, which is what makes it more efficient. It then takes the ratio of initial mass (wet mass, including propellant) to final mass (dry mass, after the burn) and applies the natural logarithm of that mass ratio, multiplying it by exhaust velocity to get delta-v.
This is why rocket designers care intensely about mass ratio: because delta-v depends on the LOG of the mass ratio, each additional unit of delta-v costs progressively more propellant mass, which is the core difficulty of spaceflight. Separately, the calculator estimates burn time by dividing the mass flow rate (thrust divided by exhaust velocity) into the total propellant mass consumed. Thrust force does not affect delta-v at all -- it only determines how quickly the same propellant mass is burned, trading burn time for thrust without changing the total velocity change achievable.
Inputs
Results
Delta-V (ΔV)
3,542.08 m/s
How to Use This Calculator
- Enter propellant specific impulse (Isp in seconds) from the engine's performance specification.
- Set initial (wet) mass (kg) and final (dry) mass (kg) after propellant burnout.
- Enter thrust force (N) produced by the engine.
- Review delta-V (m/s), exhaust velocity (m/s), mass ratio, burn time, and mass flow rate.
- Use the Tsiolkovsky rocket equation output to verify the mission delta-V budget is achievable.
How the result changes with Specific Impulse (Isp)
| Specific Impulse (Isp) | Delta-V (ΔV) |
|---|---|
| 150 | 1,771.04 m/s |
| 225 | 2,656.56 m/s |
| 450 | 5,313.12 m/s |
| 750 | 8,855.2 m/s |
What each input means
- Specific Impulse (Isp)
- Measure of engine efficiency in seconds. Higher values mean more efficient propulsion.
- Initial Mass (m₀)
- Total mass of the vehicle including propellant before the burn.
- Final Mass (mf)
- Mass of the vehicle after all propellant has been expended. Must be less than initial mass -- delta-V and burn time are floored at 0 if final mass reaches or exceeds initial mass (no propellant left to burn).
- Thrust Force
- Total thrust produced by the engine, used to calculate burn time.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSpecific Impulse (Isp) = 300, Initial Mass (m₀) = 10000, Final Mass (mf) = 3000, Thrust Force = 100000 = 4 input(s) provided
- Calculate Delta-VDelta-V3542.08 = 3542.08
- Calculate Exhaust VelocityExhaust Velocity2942 = 2942
- Calculate Mass RatioMass Ratio3.333 = 3.333
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does thrust force not change the delta-v this calculator reports?
Delta-v in the Tsiolkovsky rocket equation depends only on exhaust velocity (from specific impulse) and the ratio of initial to final mass -- thrust force does not appear in the equation at all. A higher-thrust engine burns the same propellant mass faster, which shortens burn time, but it does not change the total velocity change the vehicle can ultimately achieve from that propellant load.
How much more delta-v do I get from doubling specific impulse versus doubling propellant mass?
Doubling specific impulse exactly doubles delta-v, since exhaust velocity is a direct linear multiplier in the equation. Doubling the propellant mass (by lowering final mass relative to initial mass) increases delta-v only through the natural log of the new mass ratio, which grows much more slowly -- so for most realistic mass ratios, improving engine efficiency (Isp) is a more powerful lever than simply carrying more propellant.
What's the difference between initial mass and final mass in this calculator?
Initial mass (m0) is the vehicle's total "wet" mass at the start of the burn, including all propellant still on board. Final mass (mf) is the "dry" mass remaining after the propellant is fully consumed -- the structure, engine, and payload. The difference between the two is the propellant mass burned, and their RATIO (not their difference) is what drives delta-v through the equation's logarithm term.
Does a longer burn time mean the rocket achieves more delta-v?
No -- burn time and delta-v are calculated independently in this model. Burn time comes from dividing propellant mass by mass flow rate (which depends on thrust and exhaust velocity), while delta-v comes from exhaust velocity and the mass ratio alone. A slow, low-thrust burn and a fast, high-thrust burn of the same propellant mass and the same engine efficiency produce identical delta-v, just over different amounts of time.
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