Enter spacecraft and mission values
Use the current active engines and the vessel's present wet mass.
Example mission data
| Mission | Thrust | Wet mass | Body | Throttle | Approximate TWR |
|---|---|---|---|---|---|
| Kerbin lifter | 2,000 kN | 160 t | Kerbin | 100% | 1.27 |
| Mun lander | 45 kN | 12 t | Mun | 90% | 2.07 |
| Eve ascent test | 3,200 kN | 150 t | Eve | 100% | 1.28 |
Formula used
TWR = [T × (Throttle ÷ 100) × (Modifier ÷ 100)] ÷ (m × g)
T is rated thrust in kilonewtons. m is wet mass in tonnes. g is selected local gravity in metres per second squared.
In these units, mass multiplied by gravity produces kilonewtons. The result has no unit. Net acceleration equals effective thrust divided by mass, minus local gravity.
How to use this calculator
- Enter thrust from all engines that will be active.
- Enter the craft's current wet mass in tonnes.
- Choose the body where the craft must lift or land.
- Set throttle and modifier for actual engine conditions.
- Choose a target TWR that suits the planned manoeuvre.
- Calculate, then inspect current TWR, margin, and net acceleration.
- Repeat after staging or large fuel and payload changes.
A custom gravity entry supports modded systems, different altitudes, and mission-specific planning.
Understanding KSP thrust-to-weight ratio
Why the ratio matters
Thrust-to-weight ratio shows whether a craft can overcome local gravity. It compares usable engine thrust with current vehicle weight. In KSP, the value changes during a mission. The same engines then create a larger ratio. A ratio above 1.00 gives upward thrust beyond gravitational weight. A ratio below 1.00 cannot hold a vertical hover. The best target depends on the mission. A slow lander needs a modest margin. A launch vehicle needs stronger early acceleration. Drag, steering losses, terrain, and engine response still matter. Pair it with delta-v, stability, and staging plans.
Local gravity changes every design
Local gravity is critical in every TWR calculation. Kerbin launch pads demand more thrust than Minmus flats. Eve needs an especially powerful ascent vehicle. Small moons allow gentle departures with limited thrust. Choose the body where the vehicle will operate. Then use that body’s surface gravity value. Gravity also changes slightly with altitude. This calculator uses the selected surface value for simple planning. Enter custom gravity for another altitude, modded world, or special scenario. Compare values carefully before moving designs between worlds. A dependable Duna lander may be weak on Kerbin.
Throttle and engine conditions
Engine thrust is not always equal to the editor’s listed value. Atmospheric pressure can change available output. Throttle settings also reduce current thrust. The thrust modifier field captures those effects. Enter 100 percent for full listed performance. Use a lower value for reduced output. The calculator multiplies rated thrust by throttle and modifier. This produces effective thrust. Effective thrust drives live TWR and net acceleration results. Check every engine group before liftoff. An accidental limiter can prevent launch. Full-throttle TWR shows the available ceiling before fuel mass changes.
Staging with a useful margin
Good staging makes a vehicle easier to fly. Calculate each stage with its own wet mass. Then repeat after boosters separate. A first stage benefits from a comfortable launch margin. Too little margin causes slow climbs and gravity losses. Too much margin can waste mass or create steering problems. Landers favor gentle control near the surface. Their target can be closer to 1.00. Add reserve for rough terrain, pilot response, and payload changes. The calculator estimates thrust needed for the target ratio. It also shows extra thrust or shortfall. Use those figures when choosing engines, fuel tanks, and throttle rules.
Checking results during a mission
Use the results before building and during flight checks. Enter the vehicle’s current mass, not only dry mass. Recalculate after major cargo, crew, or fuel changes. Use the target field for your operating margin. Minimum throttle helps with controlled landings and hovering. A value above 100 percent means the target is impossible now. Reduce mass, add engines, or select a lower target. Net acceleration describes upward acceleration after gravity. Positive values support ascent. Negative values reveal descent risk. Export results for design notes or mission reviews. TWR is simple, but regular checks prevent expensive surprises. Treat every stage, body, and throttle setting separately for each mission.
Frequently asked questions
What does TWR mean in KSP?
TWR means thrust-to-weight ratio. It compares usable engine thrust with the vessel’s weight under the selected body’s gravity. A higher number gives more upward acceleration, assuming the engines can maintain their stated output.
What TWR is useful for a Kerbin launch?
Many launch vehicles begin around 1.30 to 1.70 at Kerbin sea level. The best choice depends on drag, control authority, engine efficiency, and the planned ascent profile. Very high values can make steering harder.
Why should I use wet mass?
Wet mass includes propellant, payload, crew, and every attached part. It represents the heaviest launch condition. As propellant burns, mass drops and TWR rises, so an early-stage calculation should normally use wet mass.
Does the thrust modifier include atmospheric effects?
Yes. Enter a modifier below 100 percent when actual thrust is lower than the rating. This can represent atmospheric performance, engine limits, intake conditions, electrical limits, or any planned thrust reduction.
Does the selected body change the result?
Yes. Weight equals mass multiplied by local gravity. The same vessel has a much higher TWR on low-gravity bodies and a lower TWR on high-gravity bodies. Select the landing or launch body before calculating.
Is a TWR of 1.00 enough?
A TWR of 1.00 approximately balances local gravity. It can hover in an ideal case, but it has almost no upward acceleration. Use a margin above 1.00 for launches, terrain changes, and practical control.
What is the difference between current and full-throttle TWR?
Current TWR uses the selected throttle. Full-throttle TWR uses the same engine modifier at 100 percent throttle. Comparing both values shows whether a throttle adjustment can meet your target without changing the craft.
What does negative net acceleration mean?
Negative net acceleration means the effective thrust cannot overcome local gravity. The vessel will continue accelerating downward if no other force helps it. Increase throttle, reduce mass, add thrust, or choose a lower-gravity location.
Can I calculate a staged rocket?
Yes. Calculate each stage separately. Use the mass and active engine thrust after every separation event. This reveals weak transition stages, excessive acceleration, and the correct throttle limit for each stage.
Why is specific impulse not included directly?
Specific impulse measures propellant efficiency and helps estimate delta-v. TWR measures immediate force against weight. Both values matter, but an efficient engine can still produce inadequate liftoff thrust for a heavy vehicle.
Can I use custom gravity for modded worlds?
Yes. Select Custom gravity and enter the local value in metres per second squared. Use conservative margins while learning new craft designs. Plan every launch carefully for reliable, controlled orbital missions.