In defense of parent (although I'm speaking against my own initial argument right now): I think there are two ways to look at it. One is to look at the final en

In defense of parent (although I'm speaking against my own initial argument right now): I think there are two ways to look at it. One is to look at the final energy state in orbit, where kinetic energy is much higher than potential energy. One is to look at how much fuel energy is spent for each delta-T for potential and kinetic energy, and integrating that over time. Intuitively I expect that to give a different result, e.g. most fuel spent for increasing potential energy. The difference[1] comes from the potential energy that gets stored in the remaining fuel that will be converted into kinetic energy later at a lower cost. So, there is an argument to be made for both sides, however one has to be careful in how to apply this knowledge to a new technology like air breathing rocket motors - it has a very different economy. [1] edit: There is a second factor that will also change that result compared to the first: air drag. I.e. some fuel energy will be lost in heating up the surrounding air.