Calculator Inputs
Example Data Table
| Scenario | Body | Thrust | Mass | Gravity | Approx Ratio | Use Case |
|---|---|---|---|---|---|---|
| Medium launcher | Kerbin | 1500 kN | 100 t | 9.81 m/s² | 1.529 | Stable early ascent |
| Mun lander | Mun | 120 kN | 12 t | 1.63 m/s² | 6.135 | Landing and ascent |
| Duna ascent craft | Duna | 250 kN | 35 t | 2.94 m/s² | 2.430 | Thin atmosphere launch |
| Eve lifter | Eve | 8000 kN | 300 t | 16.70 m/s² | 1.597 | High gravity ascent |
Formula Used
The main formula is:
Thrust to Weight Ratio = Effective Thrust / Weight
For KSP style units, metric tons and kilonewtons work cleanly:
Weight in kN = Mass in tons × Local gravity
Effective thrust = Base thrust × Throttle factor × Atmosphere factor
Wet ratio = Effective thrust / (Wet mass × Gravity)
Dry ratio = Effective thrust / (Dry mass × Gravity)
Net acceleration = Effective thrust / Wet mass - Gravity
Maximum mass at target = Effective thrust / (Gravity × Target ratio × Safety factor)
How to Use This Calculator
- Enter your total stage thrust, or choose per-engine thrust.
- Add the engine count when using per-engine mode.
- Enter wet mass for launch checks.
- Enter dry mass to review late-stage performance.
- Select a KSP body, or use custom gravity.
- Adjust throttle and thrust factor for real mission conditions.
- Set a target ratio and safety factor.
- Press calculate and review the result above the form.
- Download the result as CSV or PDF when needed.
Understanding KSP Launch Balance
A good rocket needs more than a powerful engine. It needs enough thrust to beat gravity, keep control, and still leave fuel for staging. In Kerbal Space Program, the thrust to weight ratio shows this balance in one number. A value above one means the craft can rise from the selected body. A value below one means it cannot lift without help.
Why Ratio Matters
Kerbin launches often need a starting ratio near 1.2 to 1.8. A lower value may waste fuel during a slow climb. A very high value can create drag losses, steering issues, and structural stress. Small moons need less thrust because gravity is weak. Eve and other heavy worlds need far more thrust for the same mass.
Advanced Mission Checks
This calculator goes beyond a basic ratio. It converts units, applies throttle, checks local gravity, and estimates acceleration. It also compares your result with a target ratio. The payload margin helps you see how much extra mass the stage can carry. The excess thrust value shows how much force remains after holding the craft against gravity.
Dry and Wet Planning
Wet mass is useful for launch checks. Dry mass is useful near the end of a burn. Comparing both values can show how performance changes as fuel drains. A stage that starts gently may become very strong later. This is common with boosters, landers, and upper stages.
Using Results Wisely
Treat the output as a planning guide. Real flight also depends on aerodynamics, engine atmosphere curves, staging, fuel flow, pilot control, and ascent path. Sea level engines may lose thrust in thick air. Vacuum engines may perform poorly at launch. Always test important vehicles before a career mission.
Better Rocket Decisions
Use the example table to compare common scenarios. Change gravity for each destination. Adjust throttle to model limited engine power. Raise the target ratio for heavy launchers. Lower it for efficient landers. The best design is not always the strongest design. It is the craft that reaches the goal with stable control, enough margin, and fuel left for recovery.
Save each result and compare designs before final launch approval.
FAQs
What is thrust to weight ratio in KSP?
It compares engine thrust against vehicle weight under local gravity. A ratio above one means the craft can lift upward, assuming the thrust direction is useful and other flight conditions do not prevent movement.
What ratio is good for Kerbin launch?
A starting ratio around 1.2 to 1.8 is common. Heavy lifters may use different values. The best choice depends on drag, staging, engine type, control, and ascent style.
Why does gravity change the result?
Weight depends on local gravity. The same craft has a lower weight on small moons and a higher weight on strong gravity bodies. That changes the ratio without changing thrust or mass.
Should I use wet mass or dry mass?
Use wet mass for launch and early burn checks. Use dry mass to estimate late burn performance. Both values are useful because fuel drain can greatly increase the ratio.
Why is my craft marked marginal?
The craft can lift, but it is below your selected target after applying the safety factor. Add thrust, reduce mass, or accept a slower climb if the mission can handle it.
What does thrust factor percent mean?
It adjusts thrust for atmosphere or engine performance. Use lower values for poor sea-level performance. Use full value for rated thrust when no adjustment is needed.
Can this calculator model every flight issue?
No. It focuses on thrust, mass, gravity, and acceleration. Real flights also involve drag, steering, staging, fuel flow, engine curves, and pilot decisions.
Why download CSV or PDF results?
Downloads help compare several vehicle designs. CSV works well for spreadsheets. PDF is useful for saving, printing, or sharing a clean mission planning summary.