Atmosphere vs. Vacuum?

[starkllr]

For purposes of engine ISP in atmosphere vs. vacuum, how exactly does the game handle this?

Is it a gradual transition (at ground level, you're getting the atmosphere ISP, and as you ascent it shifts so that your ISP increases as the atmosphere thins)?

Or is it a binary thing (below 70km, you're getting the atmosphere ISP out of your engine, and as soon as you cross that altitude, you get the vacuum ISP from your engine)?

[ASnogarD]

Cant say for sure here but the atmosphere in KSP has varied density that is measured as you climb through the various atmospheric strata so I would think its say to assume the efficiency is calculated based on the current level your craft is in, in other words a transition at various atmospheric density.

I would say its a transition rather than a binary thing but probably not so gradual and more a staged affair but that is a guess.

[NeMeSiS]

Is it a gradual transition (at ground level, you're getting the atmosphere ISP, and as you ascent it shifts so that your ISP increases as the atmosphere thins)?

This one. You can right click on an engine and see its current ISP.

[dave4002000]

It's a gradual transition. It's very easy to see, too. Just right click on one of your engines during launch and you can watch the ISP slowly increase as you climb.

[el_coyoto]

It's a transition model between ground level and 69.something km, probably using some kind of weighted arithmetic mean between ISP at sea level and ISP in vacuum.

So yes, the game takes into account the altitude of your craft while computing the engines ISP.

You can see this at lift-off by right-clicking an engine with a poor ISP at sea level (like the Mainsail) and watch its ISP slowly rise during the ascent.

EDIT : ninja'ed

[starkllr]

Thanks - I never noticed that before (the ISP readout when you right-click the engine). I learn something new about KSP every day...

[Crown]

The ISP increases linear as the atmosphere pressure decreases. But the atmosphere pressure on Kerbin decreases with this function: .

I would assume that for every 'tick' in-game the rocket altitude and atmosphere pressure and ISP is calculated. And some other fancy numbers.