Rocket Engine Performance Calculator
Compute thrust, specific impulse, mass flow rate, exhaust velocity, total impulse, burn time, propellant mass and ideal Δv for liquid or solid rocket engines.
1. Select input mode
Choose whether you want to start from thrust and specific impulse, or from propellant mass and burn time. You can also mix inputs: the calculator will use whatever is provided and solve the rest consistently.
Results
Engine performance
- Thrust F
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- Specific impulse Isp
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- Exhaust velocity ve
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- Mass flow rate ṁ
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- Burn time tburn
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- Propellant mass mprop
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- Total impulse Itot
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Vehicle performance
- Initial mass m0
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- Final mass mf
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- Mass ratio m0/mf
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- Ideal Δv (Tsiolkovsky)
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- Average thrust-to-weight (at liftoff)
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Notes: All calculations assume constant thrust and Isp during the burn and ignore gravity and drag losses unless you account for them separately in your Δv budget.
How this rocket engine performance calculator works
This tool implements the standard rocket performance relationships used in introductory propulsion and mission design. It is suitable for quick trade studies, model rocketry, and educational use.
Key formulas
Specific impulse and exhaust velocity
Specific impulse \(I_{sp}\) is defined as thrust per unit weight flow of propellant:
\[ I_{sp} = \frac{F}{\dot{m} \, g_0} \]
where:
- \(F\) = thrust (N)
- \(\dot{m}\) = propellant mass flow rate (kg/s)
- \(g_0 = 9.80665\ \text{m/s}^2\) = standard gravity
The effective exhaust velocity \(v_e\) is:
\[ v_e = I_{sp} \, g_0 \]
Mass flow rate and propellant mass
From thrust and Isp:
\[ \dot{m} = \frac{F}{I_{sp} \, g_0} \]
For a burn of duration \(t_{burn}\):
\[ m_{prop} = \dot{m} \, t_{burn} \]
Total impulse
Total impulse is the time integral of thrust. For constant thrust:
\[ I_{tot} = F \, t_{burn} \]
Tsiolkovsky rocket equation (ideal Δv)
The ideal velocity change achievable by a burn is:
\[ \Delta v = v_e \ln \left(\frac{m_0}{m_f}\right) \]
where:
- \(m_0\) = initial mass (wet)
- \(m_f\) = final mass (dry)
- \(v_e\) = effective exhaust velocity
Typical values for rocket engines
- Small model rocket motors: \(I_{sp} \approx 70\text{–}120\ \text{s}\)
- Solid boosters (launch vehicles): \(I_{sp} \approx 240\text{–}290\ \text{s}\)
- Kerolox (RP-1/LOX) engines: \(I_{sp} \approx 280\text{–}330\ \text{s}\)
- Hydrolox (LH₂/LOX) engines: \(I_{sp} \approx 430\text{–}460\ \text{s}\) in vacuum
Limitations
- Assumes constant thrust and Isp during the burn.
- Ignores gravity and aerodynamic drag losses.
- Does not model multi-stage vehicles; treat each stage separately.
- Pressure thrust and nozzle expansion effects are captured only via the effective Isp you provide.
FAQ
What does specific impulse (Isp) tell you about a rocket engine?
Specific impulse measures how efficiently an engine converts propellant into thrust. Higher Isp means you get more total impulse from the same propellant mass, or equivalently, you need less propellant for the same Δv.
Should I use vacuum or sea-level Isp?
Use vacuum Isp for in-space maneuvers and upper stages. Use sea-level Isp for liftoff performance estimates. If you only have one value, note which condition it corresponds to and be consistent when comparing engines.
How accurate are the Δv results?
The Δv value is the ideal (lossless) result from the Tsiolkovsky equation. Real missions experience gravity losses, drag, steering losses, and throttling, so the actual achieved Δv will be lower. Mission designers typically add a margin of 5–20% depending on the profile.
Can I use this for hybrid or electric propulsion?
Yes, as long as you know thrust, Isp (or exhaust velocity), and mass flow rate, the same relationships apply. For low-thrust electric propulsion, remember that burns can be very long and gravity losses become more complex to model.