Basic Delta-V question

[justidutch]

Hi. I have a question about calculating Delta-V that is probably extremely basic and I hope I don't offend any physics whizzes out there. Here it is.

In calculating the Delta-V your rocket has, all the literature I have read mentions the Tsiolkovsky rocket equation, which I have pondered over and understand. What I am wondering though, is if you are on, say, Gilly, does one replace the gravity constant of 9.82 with the gravity of Gilly? In other words, does my rocket on Kerbin have a different Delta-V than when on Gilly?

And what about in orbit around a certain planet, does the gravity constant change depending on how close/far away you are from the planet?

Thanks for any clarity and knowledge y'all may impart upon this poor wretch!

Edited November 18, 2014 by justidutch

[Padishar]

No, you always use 9.82 m/s^2 as it is simply a means of converting the units of the Isp from seconds to m/s.

[LethalDose]

You keep g0 the same, it's just a constant and dV doesn't really change given your location. What changes is your TWR.

[justidutch]

Ok, thanks guys!

[Red Iron Crown]

The historical answer is that using g0 was a way to express Isp in units that work in both the metric and Imperial systems of measure, and g0 was likely to have been memorized by engineers using either system.

The glib answer is that g0 is used to confuse budding rocket scientists.

[Laie]

I think that the original definition is also the best explanation:

If you provide an engine with 1kg of fuel and set it so that it produces enough thrust to lift 1kg, it will run for N seconds until it has gone through the fuel. The fun part is that the number of seconds is always the same, regardless of whether you put in 1kg, or 10kg, or two stones and a hundredweight.

The value for g only enters the picture because you're lifting a weight, which is mass x gravity. It stands to reason that one should always assume the same gravity in order to make ISP figures comparable, and the people who thought it up chose "our" gravity as a matter of course.

[tsotha]

This is why people used to discuss things like engine efficiency and dV in terms of exhaust gas velocity instead of Isp.