Calculating specific impulse at a given point in the atmosphere?

[Chacabuco]

I was unable to find out how on Wikipedia, the Kerbal wiki, or through searching the forums on a way to calculate the specific impulse for an engine at any given point in the atmosphere. Basically, I just want to know what the ISP is if I have the atmospheric pressure for the point of the atmosphere that I am calculating for. Can anyone point me in the right direction?

[Supernovy]

It might be easiest to do this graphically, the atmosphereCurve in each of the engines' configs gives you your two points, e.g. 320s at 1 atmosphere, 370s in vacuum. It is a simple linear interpolation between them - 0.5 atmospheres gives in this case (320+370)/2 = 345s.

The wiki page on atmosphere gives you a way to convert between pressure and height.

[Chacabuco]

Thank you very much for the quick reply. Would the specific impulse also be calculated linearly in this fashion if the engine is in greater than 1 atmosphere e.g. Eve?

[tavert]

Also see recent thread here http://forum.kerbalspaceprogram.com/threads/67606-Fuel-consumption-as-a-function-of-atmospheric-pressure

What Supernovy said is right for rockets, the story's a little more complicated for the jet engines.

[Supernovy]

Thank you very much for the quick reply. Would the specific impulse also be calculated linearly in this fashion if the engine is in greater than 1 atmosphere e.g. Eve?

As far as I know, the 1 atmosphere value is the lowest the specific impulse can go. It's not extrapolated past that. In the above example, the engine would still be at 320s on Eve.

[Chacabuco]

Awesome! This answers my question. Thank you both very much!