if you want to get super linqy in here you can run 

     return v.parts.SelectMany(p => p.FindModulesImplementing<ModuleEngines>()).
                       Where(pm => pm.EngineIgnited).
                           Sum(pm => (double) pm.maxThrust*(pm.thrustPercentage/100)) + 
                   v.parts.SelectMany(p => p.FindModulesImplementing<ModuleEnginesFX>()).
                       Where(pm => pm.EngineIgnited).
                           Sum(pm => (double) pm.maxThrust*(pm.thrustPercentage/100));