The X15, was one of those rare planes that Laminar entrusted me. With this type of planes I have a good and a bad feeling. The bad one is that I was going to put lot of love and energy building the exterior and mad switches cockpits, but later no so much people where going to use them, because xplane is not prepared with a deep mission  system (I hope that is changed in future), so people alwasy preffer the Cessnas and Boeings, because that is the way it is normal flying in Vatsim. I don't know so many people that send a flight plan in Vatsim with a flight of an X15. I think a controller would be mad.
But the good thing is that I have fun doing it because they are unique planes, and not the same as tipical airliners, or GA planes.

The initial work was done for XPlane10 and continued for PBR change in XPlane11. The plane is still there after I left Laminar, so I am glad someone can still use it. 
The basic idea is to put in the B52 I also modelled and make the AI (that was the problem with XP11 initial AI behaviour), fly the B52 High enough to make B52 separation and turn on the rockets.
The best way to do it then, if flying alone with extra tanks that I didn't build, but that all is explained in this magnificent article:

https://www.mudspike.com/read-it-do-it-01-north-american-x-15/

Have to say that all these planes that are build for Xplane inside Laminar, all the flight physics came already done by other people than me.  So I never knew if the planes were accurate or not. I "only" made the suit for them.