Calculator Inputs
Example Data Table
| Body | Mass | Engine Setup | Effective Thrust | Gravity | Approx TWR | Use Case |
|---|---|---|---|---|---|---|
| Kerbin | 42.5 t | 4 × 215 kN | 860 kN | 9.80665 m/s² | 2.06 | Strong launch vehicle |
| Mun | 12 t | 1 × 60 kN | 60 kN | 1.63 m/s² | 3.07 | Responsive lander |
| Duna | 18 t | 2 × 45 kN | 90 kN | 2.94 m/s² | 1.70 | Safe ascent stage |
| Eve | 55 t | 6 × 215 kN | 1290 kN | 16.7 m/s² | 1.41 | Difficult heavy ascent |
Formula Used
The main formula is:
TWR = Effective Thrust ÷ Weight
Weight = Craft Mass × Local Gravity
With KSP-style units, mass in metric tons multiplied by gravity gives weight in kilonewtons. Engine thrust is already entered in kilonewtons.
Effective Thrust = Thrust Per Engine × Engine Count × Throttle × Efficiency
Burn time uses:
Mass Flow = Effective Thrust ÷ (Isp × 9.80665)
Burn Time = Propellant Mass ÷ Mass Flow
Estimated delta-v uses:
Delta-v = Isp × 9.80665 × ln(Full Mass ÷ Dry Mass)
How To Use This Calculator
- Select a KSP body or enter custom gravity manually.
- Enter current craft mass in metric tons.
- Add thrust per engine and engine count.
- Adjust throttle and thrust efficiency for atmosphere or engine mode.
- Enter a target TWR for your mission phase.
- Add propellant mass and specific impulse for burn estimates.
- Press the calculate button to show results above the form.
- Use CSV or PDF export to save the mission setup.
Rocket Planning Guide
Why Thrust Ratio Matters
A thrust to weight ratio shows how strongly a craft can push against local gravity. In KSP, this value decides whether a launcher rises cleanly, hovers, or falls back. A ratio above one can lift vertically. A higher value gives better launch response, yet it may waste fuel if the craft fights air drag. Most Kerbin lifters work well near 1.3 to 1.7 at liftoff. Heavy upper stages can use lower values after leaving thick air.
Mass, Gravity, And Staging
The calculator treats mass in metric tons, thrust in kilonewtons, and gravity in meters per second squared. This matches common KSP planning because one ton under one meter per second squared equals one kilonewton of weight. Changing the selected body changes the weight side of the equation. The same engine stack can feel powerful on Minmus, moderate on Duna, and weak on Eve. Staging also matters. When tanks empty or boosters drop, mass falls and TWR rises. This tool estimates that change with dry mass and final ratio values.
Using Results For Design
Start by entering total craft mass, engine thrust, engine count, throttle, and thrust efficiency. Add propellant and specific impulse when you want burn time and delta-v estimates. Compare the calculated ratio with your target. If the margin is positive, the stage has extra lift capacity. If it is negative, reduce mass, add engines, or choose a lower gravity body. Net upward acceleration helps judge handling. Very high acceleration can make launches unstable, especially inside atmosphere.
Better Mission Decisions
Use the export buttons to save test cases before changing a design. Compare each stage after adding payload, landing legs, fairings, or science parts. For landers, check both full and nearly dry states. For launchers, check sea level thrust efficiency first, then test vacuum settings for upper stages. A balanced rocket is not always the one with the largest number. It is the one with enough control, enough fuel, and a ratio suited to its mission phase. Record atmospheric mode, planet choice, and payload mass with each run. Small notes make future revisions faster and help identify weak stages before launch attempts.
FAQs
What is thrust to weight ratio in KSP?
It compares engine thrust with craft weight under local gravity. A value above one means the craft can rise vertically, assuming control and atmosphere do not create major problems.
What TWR is good for Kerbin launch?
Many Kerbin lifters work well near 1.3 to 1.7 at liftoff. Lower values feel slow. Very high values can waste fuel or cause control issues.
Why does gravity change the result?
Weight depends on local gravity. The same rocket has lower weight on Mun or Minmus, but much higher weight on Eve. That changes the ratio directly.
Should I use sea level or vacuum thrust?
Use sea level thrust for launch checks inside thick atmosphere. Use vacuum thrust for upper stages, orbital burns, and airless body landers.
What does thrust efficiency mean?
It reduces rated thrust for atmosphere, engine mode, damaged settings, or conservative planning. A value of 90 percent means only 90 percent of listed thrust is used.
Why does final TWR rise?
Final TWR rises because propellant burns away. The craft becomes lighter while engine thrust stays similar, so the ratio increases near the end of the burn.
Can this estimate delta-v?
Yes. Enter propellant mass and specific impulse. The calculator uses the rocket equation to estimate delta-v from full mass and dry mass.
Can I export my result?
Yes. Use the CSV button for spreadsheet data. After calculating, use the PDF button to save a readable mission result summary.