Clapp left the Air Force in 1996 and partnered with Zubrin to form Pioneer Rocketplane, who proposed a slightly larger version known as Pathfinder.
Clapp proposed Black Horse as a solution to the Air Force's ongoing TransAtmospheric Vehicle (TAV) mission.
[1] The study demonstrated that Black Horse would outperform any vertical take-off/horizontal landing concept no matter what fuel it used.
The study also noted that the speed that the aerial fuel transfer took place was a key input to the overall performance of the system.
[2] Clapp partnered with Zubrin to form Pioneer Rocketplane, who proposed a slightly larger version known as Pathfinder.