One of the toughest things to do when flying the X-15 in Orbiter is to successfully manage your energy such that a return to a landing lake (preferably Edwards AFB) is possible. The extreme speeds and descent angles, coupled with the fact that you are landing without power, makes flying the X-15 a very challenging task. During the X-15 program, most flights, from the release of the X-15 to the rollout on the Edwards dry lake bed, lasted a mere 10 minutes! So much action was compressed into that 10 minute time span that Thompson commented that in order to prepare for the speed at which things happened they found it useful to fly the X-15 simulator with time accelerated in order to meet the time demands that the actual flight entailed.
“On my first X-15 flight, I was scheduled to fly a rocket plane to Mach 4.0 and then maneuver it in a glide to an honest-to-God unpowered landing. Prior to my first X-15 flight, I had never flown a rocket airplane. I had never flown faster than Mach 2 and I had never really made a deadstick landing in a high performance jet.” – page 151