5,000 kg to 2,000 kg at 300 s
Mass ratio is 2.5. Effective exhaust velocity is 300 × 9.80665 = 2,941.995 m/s. Multiplying by ln(2.5) gives 2,695.72 m/s.
Rocket Equation Calculator
Use the Tsiolkovsky ideal rocket equation to connect wet mass, dry mass, specific impulse, mass ratio, and ideal delta-v.
What this calculator does
Enter the vehicle mass before the modeled burn, the mass after propellant is expended, and engine specific impulse. The result is an ideal velocity change for one stage, not a trajectory or mission-performance result.
Calculate ideal delta-v
Interactive calculator
Enter your values
Wet mass must be greater than dry mass. Select kilograms or pounds and use that unit for both mass inputs.
Primary result
Ideal delta-v2,695.72 m/s
Effective exhaust velocity2,942 m/s
Mass ratio2.5
Propellant mass3,000 kg
Propellant fraction60%
Dry-mass fraction40%
A 5,000 kg wet mass, 2,000 kg dry mass, and 300 s specific impulse produce about 2,695.72 m/s of ideal delta-v.
Formula and methodology
The ideal rocket equation is Δv = Isp × g₀ × ln(m₀ ÷ mf).
- Δv is ideal velocity change in meters per second.
- Isp is specific impulse in seconds.
- g₀ is standard gravity, 9.80665 m/s².
- m₀ is wet mass and mf is dry mass after the modeled propellant is used.
Mass ratio is m₀ ÷ mf. Propellant fraction is (m₀ − mf) ÷ m₀.
Worked examples
Why mass ratio is nonlinear
Doubling propellant does not double delta-v because the mass-ratio term uses a natural logarithm. Structural mass and staging therefore matter to the ideal model.
How to use the result
Sensitivity hint: Specific impulse moves delta-v in direct proportion, while wet-to-dry mass ratio moves it logarithmically. A small dry-mass change can matter more when the mass ratio is already high.
Common mistakes
- Using propellant mass as dry mass instead of subtracting it from wet mass.
- Mixing kilograms and pounds between the two mass fields; the ratio only works when the units match.
- Treating ideal delta-v as delivered spacecraft velocity without gravity loss, drag, steering loss, reserves, or staging.
What to do next
Compare the ideal result with a mission budget that separately accounts for losses, reserves, and each stage. Use engineering tools for any real propulsion design.
Sources and assumptions
- NASA Glenn: Ideal Rocket Equation — equation, variables, and ideal-model boundary.
- NIST CODATA: standard acceleration of gravity — g₀ constant used in the conversion from seconds of Isp to m/s.
Limitations: This page models a single ideal burn with constant effective exhaust velocity. It does not model thrust, burn time, atmospheric drag, gravity loss, staging, guidance, structural limits, or safety margins.
Rocket Equation Calculator FAQs
What is wet mass in the rocket equation?
Wet mass is the vehicle mass before the modeled propellant is expended. It includes dry hardware, payload, and the propellant used in that burn.
Can I enter pounds instead of kilograms?
Yes, if wet and dry mass use the same mass unit. Their ratio is unitless. Output remains meters per second because Isp is converted with standard gravity.
Why is calculated delta-v higher than a real velocity gain?
The ideal equation omits gravity, drag, steering, reserve, and operational losses. A mission budget subtracts those separately.
Does this calculator handle multiple stages?
No. Calculate each stage with the mass it carries during that stage, then use a proper multistage vehicle model before combining results.