## Delta-V

Delta-v is a fancy term that means change in velocity, or more crudely, "how far your rocket can go." It's most often measured in either meters per second (m/s) or sometimes kilometers per second (km/s) if the value is large enough. As you might have guessed, this is a very important concept to understand if you want to build a rocket to get to a specific place.

If you want to go to the Mun and back, 7200m/s of dv is a good number to aim for, and gives you plenty of room for error. It's technically possible for around 6,000m/s. You can technically get by with as little as about 6000m/s, if you're good at it. If you only want to get to orbit around Kerbin, you only need about 4000m/s at most, and the some of the most efficient launches have reportedly gotten below 3000m/s of delta-v. An interesting side note is that it takes roughly a 9300m/s potential change in velocity to reach orbit around Earth. This is more than it takes for a complete round trip to Duna.

So you're probably wondering now, "How do I calculate this dv thing?" The answer is fairly simple, but might look a little scary if you aren't too good with math:

dv = 9.81 x I_{sp}x ln(M/m)

Dv, also known as delta-v, is the number you want. 9.81 is a conversion factor used when your I_{sp} is given in seconds (which it is in KSP). ln is "natural log" and you don't need to worry about that too much. Just accept that you have to plug that into the equation as a thing. M is how much mass your ship has with all of its fuel on board, and m is how much fuel your ship has with no fuel on board.

If you'd like to know more about logarithms (it's really not necessary for this), check out this website.

Let's try an example:

Total mass (M): 50 tons

Dry mass (m): 35 tons

Specific Impulse (I_{sp}): 320

You can check these numbers in the VAB by using the Engineer's Report button in the lower right corner. It's orange, and looks like a cross between a gear and a wrench. You can drain fuel tanks by right clicking on them, and then left clicking on the green fuel bars. This is the easiest way to see your ship's mass with and without fuel. Be careful though. With tiny probes, these numbers aren't quite precise enough, and you could end up being several thousand dv off from what you think you have.

You have to do this for each stage of your rocket that has an engine.

If your rocket has 3 different engines on separate stages, then you have to do this 3 times. Make sure you treat the fuel above the stage you're working on as part of the payload, otherwise you'll get ridiculously higher numbers than you should. Let's try an example of this:

Once you've solved that last problem, you should understand delta-v enough to figure it out on your own. Good luck!