function evcalc { //returns the average exaust velocity of active engines. to convert to isp, divide by constant:g0
	local englist is list().
	local totalflow is 0.
	local totalthrust is 0.
	list engines in englist.
	for eng in englist { 
		if eng:ignition and not eng:flameout {
			set totalflow to totalflow + (eng:possiblethrust /(eng:isp * constant:g0)).
			set totalthrust to totalthrust + eng:possiblethrust.
		}
	}
	if totalthrust = 0 { //avoid div by 0 later
		return 1.
	}
	return (totalthrust / totalflow).
}