How do i calculate deltaV for a ship ?

[Supernir]

I have seen many deltaV calculators that tell you how much deltaV you need to get from a certain point to another but none of them tell you how much deltaV your ship has.

So how do you calculate deltaV for your spacecraft ?

Thanks

[GoSlash27]

delta V = 9.81* Isp * ln (Wf/Wd) where

Isp = your engine's Isp

ln = natural log function

Wf= weight of your rocket including fuel

Wd= weight of your rocket without fuel.

I also worked out an equation to calculate how much rocket you need to generate a known amount of delta v with a known payload. I'll post a link to it...

[Tortoise]

The most popular and easiest mod to use to calculate DV would be Kerbal Engineer Redux (KER), you should download it.

[GoSlash27]

http://forum.kerbalspaceprogram.com/threads/80803-Helpful-rewording-of-the-rocket-equation

[Vanamonde]

This question about how to play the game has been moved to the Gameplay Questions sub-forum.

[cantab]

Mods like Engineer Redux, Mechjeb, or VOID will calculate delta-V for you quite easily. Except when they get it wrong.

To do it yourself, the rocket equation has been mentioned,

ÃŽâ€v = 9.8 × I_sp × ln(m₀/m₁)

It needs to be applied one stage at a time. Serial staging is easy enough to handle, as is crossfeed/asparagus staging though it's more tedious, but for non-crossfeed core and booster setups you'll need to separately calculate how much fuel the core has used when the boosters separate. It also assumes constant specific impulse, which won't be the case when ascending through atmosphere, but launches tend to not be perfect anyway.

To get the values from the game, the simplest methods are as follows:

I_sp, the specific impulse, can be read off the engine info in the VAB/SPH, or when the engine is running by right-clicking it.

If you have different engines, the specific impulse used needs to be a weighted average. I'm not sure if it's weighted by thrust or by fuel flow.

m₀, the initial mass, can be found in the map view during flight or in the tracking station. With the vessel focussed (used Backspace in the map view), open the information box on the right of the screen and you can read off the initial mass.

m₁, the final mass, can be found by subtracting the fuel from the initial mass. You can use the resources panel with "stage only" to read off how many units of fuel and oxidizer are available to the current stage, or if this doesn't work (or you want the delta-V you'll get from using a portion of your fuel) you can just right-click the relevant fuel tanks.

Then, remember that 1 unit of either fuel or oxidizer weighs 5 kg, while 1 unit of monopropellant weighs 4 kg. Working out how much extra delta-V your RCS can give you is one of the main uses for doing the calculations manually, since most mods only consider main engines. (RCS specific impulse is 260 seconds).

[Claw]

Yes, the rocket equation or using a mod. Kerbal Engineer Redux (KER) and MechJeb (MJ) tend to be fairly popular.

[davidpsummers]

I have a Kerbal in solar orbit that needs rescuing and I want to see if I'm ready. Have I caclulated this right?

I've got a space station that is sitting there with a Rockomax Jumbo-64 tank full of fuel. With about 10 tons of other stuff (the capsule to bring him back, the rocket motors, etc.) I have a weight of 46 tons with fuel and 14 tons without. With an Isp of 320, my delta-v is....

dV=9.8 x Isp x ln (Wfuel/Wempty) = 9.8 x 320 x ln (46/14) = 3700 m/s

Looking at the Delta V Map by WAC, I see I need 950 m/s to get to Kerin Intercept (which is the same, I assume, as Kerbin escape). Now I don't know the dV for the orbit the astronaut is stranded in, but it is a Kerbin crossing orbit that has a periapsis of 18.5 million km and should be easier than Duna (110 m/s). So I need under 1100 m/s to get there, and lets assume the same to get back, for a total of 2200 m/s. So that leaves 1700 for rendezvous burns.

So I'm good to go, right!

[Yasmy]

Looks good to me. (Except 3700 - 2200 = 1500, not 1700.)

And if you are willing to aerobrake, you can subtract a bunch of the 950 m/s for Kerbin capture on return.

[davidpsummers]

Looks good to me. (Except 3700 - 2200 = 1500, not 1700.)

And if you are willing to aerobrake, you can subtract a bunch of the 950 m/s for Kerbin capture on return.

Who knew rocket science would have math!