Quick bit of help with needed with the "atmosphereCurve" entry.

[FiiZzioN]

We've always had sea-level ISP and Vacuum ISP represented in the "atmosphereCurve" section for engines. Now we have this:

key = 0 340
key = 1 170
key = 4 0.001

Can someone explain what the new key is? I've seen the key above with the KR-2L+ and I've seen this:

key = 0 300
key = 1 280
key = 7 0.001

used with the LV-T30. So, why are some of the key's changing numbers, in this instance, from a 4 with one engine and a 7 with the next?

Any help would be appreciated in understanding what they mean!

[Diazo]

It is a pressure curve.

key = 0 340 - At pressure zero (vacuum), this engine has an ISP of 340.

key = 1 170 - At pressure of one atmosphere (kerbin sea level), the engine has an ISP of 170.

key = 4 0.001 - At pressure of four atmospheres, the engine has an ISP of 0.001.

These values are a curve on a graph that interpolates the points between them. In code the "ModuleEngines.atmophereCurve.Evaluate(pressureInAtm)" line will return the ISP of that engine for the current air pressure.

D.

[FiiZzioN]

It is a pressure curve.

key = 0 340 - At pressure zero (vacuum), this engine has an ISP of 340.

key = 1 170 - At pressure of one atmosphere (kerbin sea level), the engine has an ISP of 170.

key = 4 0.001 - At pressure of four atmospheres, the engine has an ISP of 0.001.

These values are a curve on a graph that interpolates the points between them. In code the "ModuleEngines.atmophereCurve.Evaluate(pressureInAtm)" line will return the ISP of that engine for the current air pressure.

D.

I figured as much, but I just want to be sure that I was thinking correctly. Many thanks!

[NathanKell]

Depending on their area ratio and chamber pressure, bell nozzles 'lose' Isp at different rates. So the engines optimized for vacuum, well, their Isp falls off very fast. Sea-level-optimized engines with low area ratios (LV-T30) and with high chamber pressure (Mammoth) fall off slower. The aerospike has an altitude-compensating nozzle; it performs nearly-optimally (but not quite optimally) in all regimes, and so has the slowest falloff to near-zero thrust/Isp.