# Talk:Tutorial:Advanced Rocket Design

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Revision as of 01:03, 29 April 2013 by Ninenineninefour (talk | contribs) (answering my own question)

### Question: 9.81 m/s^2 in delta V calculations?

Isn't that figure the acceleration due to gravity on Earth at sea level?

I'm assuming that the devs used an earth-like parameter for Kerbin, and obviously this math is working for you; but do you use a different acceleration parameter when calculating the delta-V of your Munar return stage? --Hovissimo (talk) 19:45, 25 December 2012 (UTC)<P> No, the 'g' is a standard unit of measurement. Changing it would be like changing the length of a metre because the Mun "has less of them". Kahlzun (talk) 07:37, 2 January 2013 (UTC)

- Isn't that kind of arbitrary, though? Wouldn't Δv = Isp · ln(m
_{0}/m_{1}) make more sense? Ninenineninefour (talk) 14:48, 23 March 2013 (CDT)- Wait, I just answered my own question: It's not the equation that is arbitrary, it's the definition of specific impulse itself, which is the exhaust velocity divided by standard gravity. What would make more sense, in fact, is Δv = V
_{e}· ln(m_{0}/m_{1}). The reason why Isp is used instead of exhaust velocity is so that the efficiency of engines can be easily compared, even when they are measured with the imperial system instead of the (much better) metric system.

- Wait, I just answered my own question: It's not the equation that is arbitrary, it's the definition of specific impulse itself, which is the exhaust velocity divided by standard gravity. What would make more sense, in fact, is Δv = V

- Isn't that kind of arbitrary, though? Wouldn't Δv = Isp · ln(m