Optimize your rocket designs for Kerbin, Mun, and beyond
Vessel Configuration
Total mass including fuel, payload, and engines.
Please enter a positive mass greater than 0.
Combined maximum thrust of all active engines in current stage (ASL or Vacuum).
Please enter a non-negative thrust value.
Kerbin (Surface) – 9.81 m/s²
Mun – 1.63 m/s²
Minmus – 0.49 m/s²
Duna – 2.94 m/s²
Eve – 16.70 m/s²
Tylo – 7.85 m/s²
Laythe – 7.85 m/s²
Space (Vacuum) – 0.00 m/s²
Custom Gravity…
Select the planet or moon where you intend to launch or land.
Enter the gravitational acceleration constant.
Thrust to Weight Ratio (TWR)
1.41
LIFTOFF POSSIBLE
Total Weight
152.06 kN
Net Acceleration
4.06 m/s²
Gravity Loss
9.81 m/s²
TWR Comparison Across Bodies
Celestial Body
Gravity (m/s²)
Calculated TWR
Status
Table 1: Performance of your vessel configuration across different celestial bodies.
Figure 1: Visual comparison of TWR on Kerbin vs. Moons and other Planets.
What is the thrust to weight ratio calculator ksp?
The thrust to weight ratio calculator ksp is a critical planning tool for players of Kerbal Space Program. It determines whether your rocket has enough engine power to overcome the gravitational pull of the planet or moon it is resting on. In simpler terms, it tells you if your rocket will fly upwards or sit on the launchpad burning fuel.
Thrust-to-Weight Ratio (TWR) is a dimensionless number. A TWR greater than 1.0 means your thrust exceeds your weight, allowing for liftoff. A TWR of exactly 1.0 means the rocket will hover, and anything less than 1.0 means gravity wins, and the vessel remains grounded.
This calculator helps seasoned engineers and new Kerbonauts alike avoid the embarrassing "revert to flight" scenario by simulating performance on various celestial bodies like Kerbin, Mun, and Minmus before you even leave the VAB (Vehicle Assembly Building).
Thrust to Weight Ratio Formula and Mathematical Explanation
The logic behind the thrust to weight ratio calculator ksp is based on Newtonian physics, exactly as simulated in the game. The formula is straightforward but requires consistent units.
TWR = Fthrust / (m × g)
Where:
Fthrust is the total force generated by the engines (in kiloNewtons, kN).
m is the total mass of the vessel (in tonnes, t).
g is the gravitational acceleration of the celestial body (in m/s²).
Note: In KSP, mass is displayed in tonnes. Since 1 tonne = 1000 kg and Thrust is in kN (1000 N), the units cancel out conveniently, so you do not need to convert tonnes to kilograms if Thrust is in kN.
Variables Table
Variable
Meaning
Unit (KSP Standard)
Typical Range
TWR
Thrust to Weight Ratio
Dimensionless
1.3 – 2.0 (Launch)
Mass (m)
Vessel Mass
Tonnes (t)
0.5t – 500t+
Thrust (F)
Engine Power
KiloNewtons (kN)
20kN – 5000kN+
Gravity (g)
Gravitational Pull
Meters per second²
1.63 (Mun) – 9.81 (Kerbin)
Practical Examples (Real-World Use Cases)
Example 1: The Early Career Hopper
Imagine you have built a simple sounding rocket early in your career mode.
Vessel Mass: 4.5 tonnes
Engine: LV-T30 "Reliant" (Thrust ~205 kN ASL)
Body: Kerbin (g = 9.81 m/s²)
Calculation:
Weight = 4.5 * 9.81 = 44.145 kN
TWR = 205 / 44.145 = 4.64
Result: With a TWR of 4.64, this rocket will accelerate extremely fast. While it will definitely fly, it might be too fast, encountering high aerodynamic drag in the lower atmosphere. It is efficient to throttle down.
Example 2: Mun Lander Descent Stage
You are designing a lander to touch down on the Mun.
Vessel Mass: 8.2 tonnes
Engine: LV-909 "Terrier" (Thrust 60 kN Vacuum)
Body: Mun (g = 1.63 m/s²)
Calculation:
Weight = 8.2 * 1.63 = 13.366 kN
TWR = 60 / 13.366 = 4.48
Result: A TWR of 4.48 is excellent for a lander. It provides plenty of authority to kill horizontal velocity and make a soft landing without burning fuel for too long (gravity losses).
How to Use This Thrust to Weight Ratio Calculator KSP
Input Total Mass: Check the engineer's report in the VAB (bottom right corner) for your vessel's total mass in tonnes. Enter this into the "Total Vessel Mass" field.
Input Thrust: Right-click your engine in the part list or VAB to see its "Max Thrust". If you have multiple engines, add their values together. Note: Use "ASL" (At Sea Level) thrust for launch vehicles and "Vac" (Vacuum) thrust for space stages.
Select Celestial Body: Choose where you are calculating for. Default is Kerbin. If you are landing on Duna, select Duna to see if your engines are strong enough.
Analyze Results:
TWR < 1: You cannot lift off. Add engines or reduce weight.
TWR 1.0 – 1.2: Very slow liftoff. High gravity losses. Not recommended for launch.
TWR 1.3 – 1.7: Sweet spot for Kerbin atmospheric launch.
TWR > 2.0: Very high acceleration. Consider throttling down to reduce aerodynamic drag.
Key Factors That Affect Thrust to Weight Ratio Results
While the math is simple, several in-game factors can alter your effective TWR.
1. Atmosphere vs. Vacuum Thrust
Engines in KSP have two thrust ratings. The ISP (Specific Impulse) and thrust change based on atmospheric pressure. For a launch from Kerbin, always use the Atmospheric (ASL) thrust. Using Vacuum thrust numbers for a launch stage will give you a falsely high TWR result, leading to launch failures.
2. Fuel Consumption (Dynamic TWR)
As your rocket burns fuel, its mass decreases. Since TWR = Thrust / Mass, your TWR will increase continuously as you fly. A stage that starts with a modest 1.2 TWR might end with a bone-crushing 4.0 TWR before burnout. Check your delta-v calculator to manage staging effectively.
3. Staging Events
When you drop empty tanks or solid rocket boosters (staging), your mass drops instantly, causing a spike in TWR. Conversely, if you stage away a powerful engine to light a smaller upper-stage engine, your TWR might drop significantly. Always calculate TWR for each stage independently.
4. Gravitational Variation
Gravity decreases as you get further from the planet (Inverse Square Law). However, for initial launch calculations, using surface gravity is the safest baseline. If you are in high orbit, your local TWR will be higher than the surface TWR calculated here.
5. Throttle Limits
Solid Rocket Boosters (SRBs) in KSP cannot be throttled down in flight (though their thrust limiter can be tweaked in the VAB). Liquid fuel engines allow you to manage TWR actively. If your calculated TWR is too high, simply throttling down saves fuel by reducing drag.
6. Docked Payloads
If you are pushing a heavy payload or a docked vessel, ensure you include the entire combined mass in the calculator. Forgetting the mass of the docked rover is a common reason for transfer stage failures.
Frequently Asked Questions (FAQ)
What is the ideal TWR for launching from Kerbin?
Most experienced players aim for a TWR between 1.3 and 1.5 off the launchpad. This balances fighting gravity with not hitting terminal velocity (air resistance) too quickly.
Can I go to space with a TWR of less than 1?
No, not from the surface. You must generate more lift than your weight to leave the ground. However, once you are in orbit, a TWR of less than 1 is perfectly fine (e.g., using nuclear engines), though burn times will be longer.
Why does my TWR change during flight?
Your engines burn fuel, which has mass. As fuel is consumed, the vessel gets lighter. Since the thrust remains roughly constant (or increases in vacuum), the ratio of Thrust to Weight goes up.
Does TWR affect Delta-V?
Indirectly. TWR dictates how fast you can use your Delta-V. A high TWR minimizes "gravity losses" during ascent but might increase "drag losses" if you go too fast in the atmosphere.
How do I calculate TWR for the Mun?
Use the dropdown in our calculator to select "Mun". The gravity there is only 1.63 m/s², so a rocket that is too heavy to fly on Kerbin might fly easily on the Mun.
What units should I use for mass?
KSP uses metric tonnes. 1 unit of liquid fuel is not 1 tonne; check the total mass in the Engineer's Report inside the VAB.
Does the angle of ascent affect TWR?
TWR is a measure of potential acceleration. However, the effective vertical TWR decreases as you tilt over (gravity turn). You need a high initial TWR to get off the ground vertically before you can turn.
Is higher TWR always better?
No. Excessive TWR adds structural stress, increases aerodynamic heating/drag, and usually means you are carrying heavy, overpowered engines that reduce your overall Delta-V range.