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How to get ISP for RO engines before ignition?


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I'm try to find the ISP of an engine from the class ModuleEngines https://kerbalspaceprogram.com/api/class_module_engines.html for use in my mod HopperGuidance. Its a pretty complex class!

In the base game I can use engine.realIsp to give me the ISP at the current atmospheric density. It works whether the engine is throttled up/activated or not. I can then use

minThrust = engine.GetEngineThrust(engine.realIsp, 0)

maxThrust = engine.GetEngineThrust(engine.realIsp, 0)

But in RO engine.realIsp=0 until the engine is ignited. I've rather let HopperGuidance decide when to activate the engine as it is much better as working this out than my guesses. So I want to enquire about min/max thrust BEFORE deciding to fire up the engine.

I tried:

isp = engine.atmCurveIsp.Evaluate(atm) but this gives isp=0 probably because useAtmCurveIsp=false on the Raptor engine I'm looking at even through the config from "Launchers Pack/Rockets/SpaceX_ITS/KK_Raptor/KK_Raptor.cfg" has

      atmosphereCurve
      {
            key = 0 335
            key = 1 280
            key = 5 0.001
      }

which is a shame as it looks like it should be easy to find the ISP from this curve. Is them some other factor I'm missing, there are no end of multipliers, factors and curves in this module.

thanks

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