kerbal calculator

What this Kerbal calculator helps you do

Building reliable rockets in KSP usually comes down to three numbers: delta-v, TWR (thrust-to-weight ratio), and burn time. If any one of these is off, missions fail in ways every Kerbal pilot knows well: wobbly ascents, failed captures, or landers that can’t get home.

This tool gives you fast estimates for a single stage so you can make better design decisions before launch. It is useful for:

• Checking whether a stage has enough delta-v for a transfer, landing, or return.
• Verifying your launch TWR on different celestial bodies.
• Estimating burn duration for transfer windows and maneuver node execution.

The math behind the numbers

1) Delta-v (Tsiolkovsky rocket equation)

Delta-v is computed as:

Δv = Isp × g₀ × ln(m_wet/m_dry)

Where Isp is specific impulse in seconds, g₀ is 9.80665 m/s², and masses are your wet and dry stage masses. This is the core measure of how much velocity change your stage can produce.

2) Thrust-to-weight ratio (TWR)

TWR is:

TWR = Thrust / (Mass × local gravity)

A TWR above 1.0 means you can accelerate upward from the surface of that body. For launch stages on Kerbin, players commonly target roughly 1.3 to 1.7 at liftoff. Landers can often run lower if terrain is forgiving and piloting is precise.

3) Burn time

The calculator estimates fuel flow from thrust and Isp:

ṁ = Thrust / (Isp × g₀)

Then burn time is fuel mass divided by mass flow. This gives a practical estimate for how long your engine can burn at full throttle.

How to use this in mission planning

Kerbin launch stage

For your first stage, pay attention to wet TWR on Kerbin. If it is below 1.0, you won’t leave the pad. If it is too high, drag and steering losses can increase. For many medium rockets, starting around 1.4 is comfortable.

Mun or Minmus lander

For landers, check TWR on the target body. A craft that feels weak on Kerbin can be perfectly capable on Minmus. At the same time, don’t ignore delta-v margins: include enough for descent corrections, ascent, rendezvous, and pilot error.

Duna transfer/capture stage

Transfer stages usually care most about total delta-v and burn time. Very long burns can reduce maneuver efficiency, especially with low-thrust engines and heavy payloads. If burn time grows too large, splitting the maneuver or redesigning staging may help.

Rule-of-thumb targets

• Kerbin liftoff TWR: about 1.3 to 1.7
• Vacuum transfer stage: prioritize high Isp and adequate burn duration
• Landing stage: aim for comfortable TWR margin (often 1.5+ on target body)
• Mission safety margin: carry extra delta-v for course corrections and mistakes

Common design mistakes this calculator can catch

• Dry mass accidentally larger than wet mass (invalid stage setup).
• Using sea-level Isp expectations for vacuum-only stages.
• Ignoring local gravity when validating landing or ascent capability.
• Underestimating burn time for large transfer maneuvers.

Final thoughts

A good Kerbal calculator doesn’t replace piloting skill, but it drastically improves mission reliability. Use it early in the VAB: test stage values, compare engine options, and tune fuel loads before your first launch attempt. A few minutes of planning can save hours of rescue missions.

Fly safe, overbuild only when necessary, and remember: if the numbers look questionable, send probes first.