There are only getters, not setters, for engines at the moment, see here: http://erendrake.github.io/KOS/structure/engine/ An example to get the current thrust for the first engine in the list. list engines in foo. print foo#0:thrust. I cannot work out what causes the order the engines are listed in, so I am not sure how to end up with the solid fuel rockets at a specific point in a list. Though you can call on the length of the list and make a loop looking at its values, such as the ISP, to determine certain things. The example below is messy as I just made it up now and the list# will not work with anything but an integer, as in you cannot set x to 1 then call on foo#x. list engines in foo. set x to foo:length. set y to 0. until y = x { when y = 0 {if foo#0:isp < 275 {set result to result + foo#0:thrust.}. set y to y + 1.}. when y = 1 {if foo#1:isp < 275 {set result to result + foo#1:thrust.}. set y to y + 1.}. when y = 2 {if foo#2:isp < 275 {set result to result + foo#2:thrust.}. set y to y + 1.}. //etc }