The Oberth Effect, at its simplest, is this: A change in the size of your orbit costs less delta-V when the burn is made at higher speed. We look at delta-V maps and such and start to think that delta-V is what gets us places in KSP, and in a sense that's true. But more specifically, delta-V is what we spend to buy changes in our orbit's size, and the size of the orbit is determined by its energy. We change our velocity (i.e. expend delta-V) to add or subtract kinetic energy from our orbits, cha