HIR ES

Perhaps the most unusual concept involving the use of X-15s was also the one that should have made the program most thankful it was never implemented. During the late 1950s and early 1960s, the Air Force investigated a single-stage-to-orbit concept called Aerospaceplane (not to be confused with the later National Aero-Space Plane (NASP)). The vehicles explored during this program included some very exotic propulsion concepts, such as LACES and ACES, that extracted oxygen from the atmosphere during ascent and used it once the vehicle left the sensible atmosphere.-13213

Most of the contractors involved in the program performed parametric evaluations of conventional concepts that carried all of the propellants from the ground-termed "propellants onboard at takeoff" (POBATO)-in addition to the air-collection schemes. However, an even more bizarre concept was called the "hypersonic in-flight refueling system" (HIRES), and designers at Convair, Douglas, and North American each considered trying to refuel the Aerospaceplane in flight at Mach 6. This concept actually advanced far enough that the Air Force and NASA had preliminary discussions about using two X-15s flying in formation to validate the idea. The logistics of getting two X-15s in formation would have been formidable, and the piloting task daunting. On two separate occasions the X-15 program attempted to fly two flights in a single day (but not at the same time, since the High Range could not support the concept), and each time one of the X – 15s had a system problem that led to the flight being scrubbed. Fortunately for the X-15 program, the refueling demonstration was never attempted.13221