Just leave it here: list engines in myEng. set sum_f to 0. // thrust in N set sum_f_isp to 0. // thrust/Isp for engine in myEng { if ( engine:IGNITION ) { set engine_thrust to engine:maxthrust * engine:thrustlimit / 100. set sum_f to sum_f + engine_thrust. set sum_f_isp to sum_f_isp + (engine_thrust / engine:isp). } //print engine + ": " + ). } set Isp to sum_f / sum_f_isp. print "Isp = " + Isp. Calculate weighted Isp for ship.