aeroz2011

If you see the data I took in the image, I got the data from watching the thrust values reported by the game at different altitudes (I paused it) for two engines (the BACC - 100% and 50%, + and x respectively; and the RT-10, 100% and 85%, diamond and triangle) but didn't get around to checking for the liquid motors. My bet is that it's probably close because it worked for both others. It probably holds - close - for all engines/motors with a F_asl/F_vac ~ 0.8. The equation literally came from checking values based on my supposition that the equation was a constant related to max thrust allowed with a natural log addition (subtraction, really) that deminished with altitude. That said, while I like you're equation (frankly more than my own), there must be a more generic form of it because that varies using an experimental value and pressure (ASL, Kerbin) related.

What I did has nothing to do with drag or velocity produced - it's only a measurement of thrust relative to altitude - from 0 to [effectively] vacuum. Drag doesn't affect thrust. Thrust limit is a percentage, so if you set the limit to 40 on the Thumper, your max thrust will be 120kN.

So I got curious about this too and decided to do an experiment. Long story short I've got a close (less than 10% error) approximation for calculating thrust by altitude. I tried it based on different thrust limits and engines. One important note here is that the lower the "efficiency" (thrust limit) the closer the the equation you get. Also, around 13 km altitude, the data and equation are essentially the same. Without further ado, the equation: T(h) ~ lim% * T_max + 3 * ln(h/35) where lim% is thrust limit as a percent (i.e. A 48 thrust limit is 0.48 in the equation) and h is altitude in km. T_max is the max thrust for the motor/engine. If you were looking to account for liquid motors' throttle, you would add that in as another "efficiency" type term. The imgur link below shows the data vs the calculated values. Black is data, green is math. http://imgur.com/ksXG3Du