Difference between revisions of "Cheat sheet"

Revision as of 13:05, 25 June 2013

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.

Delta-V (dV)

Basic calculation

1. For atmospheric dV value, use atmospheric thrust values.
2. For vacuum dV value, use vacuum thrust values.
3. 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)

1. How to calculate the dV of a rocket stage that transitions from Kerbin atmosphere to vacuum.
2. 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.
3. 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

1. http://wiki.kerbalspaceprogram.com/w/images/7/73/KerbinDeltaVMap.png
2. http://www.skyrender.net/lp/ksp/system_map.png

• dV Change Values

1. http://i.imgur.com/duY2S.png

• dV KSP Nomogram

1. http://ubuntuone.com/1kD39BCoV38WP1QeG6MtO6

Thrust to weight ratio (TWR)

1. This is Newton's Second Law.
2. 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)

1. 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.
2. 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.

• Tutorials
• The Drawing Board: A library of tutorials and other useful information