I'm very unfamiliar with energy-use in aviation, but I am wondering about what effect altitude has on parameters such as gravity (i.e. weight of the aircraft) and air-density, which I would think affects friction and combustion as well as lift. One thought I've had is that higher velocities through lower density air is equivalent to lower velocities through higher density air. It seems like air density could be treated as a rate of particle flow against the aircraft. So, for instance, I would think a jet traveling twice as fast in air half as dense would experience friction and combustion the same as it would at half the speed in 2x dense air. Is that clearly expressed? Also, I wonder about the effect of decreasing gravity on the performance of the aircraft. It seems like the efficiency gained from higher altitude could be recovered in long distance flights, but is that a poorly conceived hypothesis? Finally, if high altitude flight was ultimately more efficiency for long distances, I would think the stumbling block would be using jet power to continue to climb through lower density air. However, if rocket propulsion was briefly used just to achieve extra altitude, would gravity allow the aircraft to achieve a speed that would generate sufficient air flow/density to keep the engines going? In other words, could jet engines run at a much higher speed in lower density and lower gravity atmosphere?