I downloaded the freeware X-15 by Michael Pook and it sort of works but to me it has problems. First, only the altimeter and the airspeed indicator work. The other gauges are there but are static. After 500 mph the airspeed indicator is no use and the mach meter is one of the static gauges. The compass doesn't work so it is hard to get back to where your started, at 2000 mph you get away from home real quick. The bottom portion of the ventral stabilizer does not jettison unlike the real X-15 so the rear landing gear struts are way too long. Maybe there is no way around the next problem directly. The sim starts on the ground on the runway. The real X-15 was always dropped from a mother ship B-52 at about 30,000 ft. and approx. 300 mph. After the drop and the X-15 was safely away from the B-52 it started it's engine, which btw had a very limited amount of fuel. So far as I can tell the sim X-15 has unlimited fuel. Getting back to the drop. I start the sim at Edwards AFB and before doing anything I open the map mode where I set the altitude and airspeed. Then I raise the landing gear and proceed with a "test" flight. So far I have gotten it up to over 80,000 ft. Since most of the other instruments don't work I have had only one successful landing on the dry lake bed. If I could model the changes I mentioned I would but I don't have the training for the skill set required and I think I am too old to train this dog a new trick.
One last thing, at very high altitude the control surfaces should not function. The X-15 at these altitudes was maneuvered using thrusters that were later developed for the space program. There was a set at the wingtips, the nose and the tail. I assume they were linked with the joystick and activated when the normal control surfaces no longer functioned.
So. I would like to see someone work on this plane and make it work right.