Kerbal Space Program Delta-V Calculator
Your essential tool for planning successful missions in KSP. This kerbal space program delta v calculator uses the Tsiolkovsky rocket equation to determine your craft’s maneuvering capability.
Enter the vacuum specific impulse of your engine stage, found in the VAB/SPH. Measured in seconds (s).
The total mass of your craft or stage with all fuel.
The mass of your craft or stage after all fuel has been burned.
Select the unit for your full and dry mass inputs.
What is a kerbal space program delta v calculator?
A kerbal space program delta v calculator is a tool that computes the total change in velocity a rocket can achieve. In Kerbal Space Program (KSP), Delta-V (Δv) is the single most important metric for mission planning. It represents your spacecraft’s “fuel budget” for maneuvers. Every action—from achieving orbit, to transferring to another planet, to landing—costs a certain amount of Δv. If you don’t have enough Δv, your mission will fail. This calculator helps you determine if your rocket design has the capability to reach its destination and return. For more complex planning, you might also use a TWR calculator to ensure your engines have enough power.
The Delta-V Formula and Explanation
The calculation is based on the Tsiolkovsky Rocket Equation, a fundamental principle of rocketry. It relates the change in velocity to the engine’s efficiency and the ratio of the rocket’s initial (full) mass to its final (empty) mass.
The formula is: Δv = Isp * g0 * ln(mfull / mdry)
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Δv | Delta-V (Change in Velocity) | m/s | 0 – 10,000+ |
| Isp | Specific Impulse | seconds (s) | 80 – 4200 |
| g0 | Standard Gravity | m/s² | 9.80665 (constant) |
| ln | Natural Logarithm | Unitless | N/A |
| mfull | Full (Wet) Mass | tons (t) or kg | 0.5 – 5,000+ |
| mdry | Dry Mass | tons (t) or kg | 0.1 – 500+ |
Practical Examples
Example 1: Munar Injection Stage
Imagine you have a small probe in a stable Low Kerbin Orbit (LKO) and want to send it to the Mun. You need to calculate the Δv of your transfer stage.
- Inputs:
- Engine: “Terrier” Liquid Fuel Engine (Vacuum Isp ≈ 345s)
- Full Mass: 8.5 tons
- Dry Mass: 2.5 tons
- Results:
- Mass Ratio: 3.4
- Calculated Delta-V: ≈ 4,142 m/s
This is more than enough to get to the Mun and establish an orbit. For a complete mission plan, check out a delta-v map to see the requirements for each step.
Example 2: First Stage of a Heavy Lifter
You are designing the first stage to lift a heavy payload into orbit. This stage operates mostly in the atmosphere, so you use an average Isp.
- Inputs:
- Engine: “Mammoth” Liquid Fuel Engine (Atmospheric Isp ≈ 295s)
- Full Mass: 450 tons
- Dry Mass: 60 tons
- Results:
- Mass Ratio: 7.5
- Calculated Delta-V: ≈ 5,861 m/s
How to Use This kerbal space program delta v calculator
- Find Your Isp: In the Vehicle Assembly Building (VAB) or Space Plane Hangar (SPH), right-click on your engine to see its stats. Use the ‘vacuum’ Specific Impulse for stages that will operate only in space, or the ‘sea level’ Isp for launch stages. For stages that operate in both, you may need to use an average.
- Find Your Mass: In the bottom right corner of the VAB/SPH, click the “Engineer’s Report” icon. Here you can find the total (wet) mass of your craft. To find the dry mass, remove the fuel from all the tanks in your stage and check the mass again.
- Enter the Values: Input the Isp, Full Mass, and Dry Mass into the corresponding fields in the calculator.
- Select Units: Ensure you select the correct mass unit (tons or kg) that matches your input values.
- Interpret the Result: The calculator will instantly show you the total Δv for that stage. Compare this value to a community Delta-V map to see if you have enough “fuel budget” for your intended maneuver. Understanding orbital mechanics is key to using your Δv efficiently.
Key Factors That Affect Delta-V
- Specific Impulse (Isp): This is the most direct measure of an engine’s fuel efficiency. Higher Isp means more Δv from the same amount of fuel. Ion engines have very high Isp but low thrust, while solid rocket boosters have low Isp but high thrust.
- Mass Ratio (mfull / mdry): This is crucial. To maximize Δv, you want to maximize this ratio. This means making your rocket’s dry mass (tanks, engines, payload) as light as possible compared to the fuel it carries.
- Staging: Shedding empty tanks and heavy engines (dry mass) is the core concept of staging. By dropping a stage, the next stage has a much more favorable mass ratio, allowing it to provide a large amount of Δv. Perfecting your rocket staging is an art.
- Payload Mass: The heavier your payload (the part of the rocket that needs to get to the destination), the lower your overall mass ratio will be, thus reducing your total Δv.
- Engine Mass: A very efficient engine (high Isp) might also be very heavy. Sometimes, using a slightly less efficient but much lighter engine can result in a better overall mass ratio and more Δv. This is a key design trade-off.
- “Dry” Mass of Fuel Tanks: Even empty fuel tanks have mass. Using lighter tank structures can improve your dry mass and significantly increase your Δv.
Frequently Asked Questions
Q1: Why is my calculated Delta-V different from the in-game reading?
The in-game calculator automatically averages the Isp of multiple engines and accounts for atmospheric pressure changes. This calculator is best used for a single stage with a single engine type. For multi-engine stages, you would need to calculate an average Isp. A guide on advanced KSP calculations can explain this further.
Q2: What is a “good” mass ratio?
For chemical rockets, a mass ratio between 3 and 10 is typical for a single stage. Ratios above 9 are very difficult to achieve as they require extremely light structures. Liquid fuel tanks in KSP have a standard full-to-dry ratio of 9:1 (8 parts fuel, 1 part tank).
Q3: Does Thrust-to-Weight Ratio (TWR) affect Delta-V?
No, TWR does not directly affect the total Δv of a stage. However, TWR is critical for actually being able to use that Δv. If your TWR is less than 1 on Kerbin, you can’t even lift off the launchpad. A low TWR in space means your maneuver burns will take a very long time.
Q4: How do I find the mass of my craft in KSP?
The total mass is shown in the Engineer’s Report in the VAB/SPH. To get the dry mass, you can right-click on each fuel tank in your stage and drag the fuel slider to zero.
Q5: What’s the difference between vacuum and sea-level Isp?
Rocket engines are more efficient in a vacuum where there is no air pressure pushing against the exhaust. Engines designed for space (like the “Poodle” or “Terrier”) have a much higher vacuum Isp than sea-level Isp. Launch engines (like the “Mainsail”) are optimized to perform better in the atmosphere.
Q6: Why use tons instead of kilograms?
Kerbal Space Program’s UI displays mass in tons by default, so it’s often more convenient for players to use tons for their calculations. This calculator provides both options for flexibility.
Q7: Can I use this calculator for multiple stages?
You must use it for each stage individually. Calculate the Δv for your first stage. Then, for the second stage, its “full mass” is the total mass of the remaining rocket, and its “dry mass” is that same rocket but with the second stage’s fuel removed.
Q8: Does Delta-V account for atmospheric drag?
No, the pure Tsiolkovsky rocket equation does not. The Δv values on community maps for reaching orbit (typically ~3,400 m/s from Kerbin) include a buffer to account for overcoming gravity and atmospheric drag. Getting a deeper understanding of atmospheric flight can help you minimize these losses.