Cheat sheet
From Kerbal Space Program Wiki
Kerbal Space Program rocket scientist's cheat sheet: delta-V maps, equations and more for your reference so you can get from here to there and back again.
Contents
Delta-V (dV)
Basic calculation
- For atmospheric dV value, use atmospheric thrust values.
- For vacuum dV value, use vacuum thrust values.
- Use this equation to figure out the dV per stage:
- Equation:
- dV = ln ( Mstart / Mend ) * Isp * g
- Explained:
- dV = ln ( Starting Mass / Ending Mass ) X Isp X 9.81
- Example:
- Single Stage Rocket that weighs 23 tons when full, 15 tons when fuel is emptied, and engine that outputs 120 Isp.
- dV = ln ( 23 Tons / 15 Tons ) X 120 Specific Impulse X 9.81m/s = Total dV of 1803.2 m/s2
Transitional dV (a.k.a. true dV when launching from Kerbin)
- How to calculate the dV of a rocket stage that transitions from Kerbin atmosphere to vacuum.
- Assumption: It takes approximately 1000 m/s2 of dV to escape Kerbin's atmosphere before vacuum dV values take over for the stage powering the transition.
- Note: This equation is an guess, approximation, and is not 100% accurate. Per Chris: "The results will vary a bit depending on your TWR and such, but it should usually be pretty darn accurate."
- Equation:
- TdV = ( ( dVatm - 1000 ) / dVatm ) * dVvac + 1000
- Explained:
- Transitional dV = ( ( Total dV in atmosphere - 1000 m/s2) / Total dV in atmosphere ) X Total dV in vacuum + 1000
- Example:
- Single Stage with total atmospheric dV of 5000 m/s2, and rated 6000 dV in vacuum.
- Transitional dV = ( ( 5000 dVatm - 1000 dV Required to escape Kerbin atmosphere ) / 5000 dVatm ) X 6000 dVvac + 1000 dV Required to escape Kerbin atmosphere = Total dV of 5800 m/s2
dV maps
Various maps developed by KSP fans.
- dV Total Values
- http://wiki.kerbalspaceprogram.com/w/images/7/73/KerbinDeltaVMap.png
- http://www.skyrender.net/lp/ksp/system_map.png
- dV Change Values
- dV KSP Nomogram
Thrust to weight ratio (TWR)
- This is Newton's Second Law.
- If ratio is less than 1, you will not lift off the ground.
- Equation:
- TWR = F / (m * g) > 1
- Explained:
- TWR = Force of Thrust / ( Total Mass X 9.81 ) > 1
- Example:
- 200 kiloNewton rocket engine on a 15 ton rocket launching from Kerbin Space Center.
- TWR = 200 kN / ( 15 Tons total Mass X 9.81 m/s2 ) = 1.36 which is > 1 which means liftoff!
Combined specific impulse (Isp)
- When Isp is the same for all engines in a stage, then the Isp is equal to a single engine. So six 200 Isp engines still yields only 200 Isp.
- When Isp is different for engines in a single stage, then use the following equation:
- Equation:
- Isp = ( F1 + F2 + ... ) / ( ( F1 / Isp1 ) + ( F2 / Isp2 ) + ... )
- Explained:
- Isp = ( Force of Thrust of 1st Engine + Force of Thrust of 2nd Engine...and so on... ) / ( ( Force of Thrust of 1st Engine / Isp of 1st Engine ) + ( Force of Thrust of 2nd Engine / Isp of 2nd Engine ) + ...and so on... )
- Example:
- Two engines, one rated 200 Newtons and 120 Specific Impulse; another engine rated 50 Newtons and 200 Specific Impulse.
- Isp = (200 Netwons + 50 Newtons) / ( ( 200 Newtons / 120 ) + ( 50 Newtons / 200 ) = 130.89 Specific Impulse
See also
Links to collections of reference materials.
