Can anybody explain how Open Rocket handles the user entries for temperature, pressure, and ground altitude? I am seeing some very odd behavior. First, I ran some tests by varying these values: Sims 1 to 6 are at 0 ground altitude (sea level), and all seem to make sense. Sim 1 uses the check box for International Standard Atmosphere. In Sim 2 , I unchecked the box and manually input the standard sea level values of 15 C and 101325 Pa. The results are the same, as they should be. Check. Sims 7 to 12 repeat the temp and pressure variation, but at 1600 m (1 mile) elevation. These results with the ISA model look good, but the rest are wonky. How can the flights at 1600 m now be even less altitude than their sea level counterparts??? I exported some results for a closer look at the air density. Oddly, of all the variables written by OR, density is not one of them???? Anyway, in Sim 7, T and p on the ground are calculated more or less correctly using the ISA model at 1600m: However, Sim 8 values for T and p on the ground make no sense when manually inputing the standard values of 15 C and 101325 Pa: There seems to be some weird interaction of elevation and the atmospheric model in OR. I think something is wrong, here. This can be a big problem, especially for high altitude simulations. Any OR programmers out there who can explain?