Back in the day, they had to just build it. You can literally see how much energy and how much heat is inside the combustion chamber,. That is a measurable thing, even without a computer. Now I'm not near smart enough to do any of this. Like I've never tried measuring the heat flux of anything. I barely even know what that means. But that is something that people would calculate and they find out, okay, copper, you know, does a better job of transferring the heat between the walls of it and into the propellant bubble vlog compared to XYZ.