A question about delta-v...

[dnleek]

How do you calculate it? It can\'t be as easy as measure velocity before and after, right?

On a related note, how much delta-v does an SRB provide, and how much do you need to return to Kerbin from the Mun? I need to know... for science.

[maltesh]

In free space, with no gravitational sources, yes, that\'s exactly what you could do.

If you have a fuel that has a constant mass consumption ratio, you can use the Tsiolovsky Rocket Equation to calculate delta-V.

Where v_e is the Exhaust velocity, m₀ is the full mass of the spacecraft, and m₁ is the mass of your spacecraft with all the tanks in that stage drained. The result is the delta-V for that stage. Once that stage is gone, you calculate it for the spacecraft as you burn the next stage, and add it in, and so on.

Exhaust velocity can also be labeled as Specific Impuse, when Isp is measured in m/s. The Kerbal Space Wiki has values for the standard fuel tanks.

Kerbal Space program doesn\'t quite have a consistent mass fuel expenditure rate for liquid fuel, so things get more complicated. I suspect Mechjeb figures out delta-V by knowing the mass and thrust of your spacecraft as you burn.

I\'d esitmate the delta-V neccesary to return to Kerbin from the Mun to be about 850-950 m/s assuming you do all your burning as low as you can, and aim for a hyperbolic exit that takes =you out the center of of the trailing hemisphere of the Munar SOI, and drops you directly onto Kerbin. Any method that requires you to brake outside the Munar SOI is going to cost you more delta-V than that.