rocket equation questions

[draeath]

I have three, really...

Firstly, is "standard gravity" (g₀ or g_n) an actual constant of 9.80665 m/s², or is this different depending on what body is in use as a frame of reference?

Second, in KSP you see "isp" on engine specifications. Is this I_sp - "specific impulse measured in seconds" or is it something else?

Finally, when working with the Rocket Equation do you "combine" fuel and oxidizer into a single value when performing your calculations? I would assume that would be true, as both are expended, but was not sure.

Edited January 7, 2014 by draeath
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[LameLefty]

My aerospace engineering degree is 24 years in the past so, subject to checking references I don't have handy ...

1. "Standard gravity" refers to Earth surface gravity at mean sea level. You can solve the rocket equation for other bodies, but Earth is the basis for "standard gravity."

2. Isp is measured in seconds.

3. Yes. Propellant total mass is what matters.

[tavert]

Actual constant. It's a unit conversion. There's no physical reason for specific impulse to be a time (effective exhaust velocity is the physically meaningful quantity here), it's that way by convention as an accident of imperial units. In KSP, it's oddly 9.82 m/s^2.

Yes, Isp in KSP is essentially the same as "specific impulse measured in seconds," except for the slight difference in g0 value.

Yes, fuel and oxidizer both count as propellant mass for the rocket equation.

[draeath]

Thank you both very much!