Rocket plane spaceflight

Basic Flying Rules: “Try to stay in the middle of the air. Do not go near the edges of it. The edges of the air can be recognized by the appearance of ground, buildings, sea, trees and interstellar space. It is much more difficult to fly there. ”

– Anonymous

At the end of the 1950s the idea of the pure rocket fighter was dead and the role envisaged for a mixed jet/rocket interceptor already very limited. However, as far as American designers of research aircraft were concerned, the evolution of supersonic extreme-altitude rocket aircraft had barely started. After the successful X-l and X-2 series and the D-558-2 Skyrocket, the next step was a rocket plane that could surpass all of its predecessors in terms of speed and altitude.

Whereas the early X-l aircraft had investigated the transonic and low supersonic flight regimes, the later X-ls and the D-558-2 had explored speeds around Mach 2, and the X-2 had marginally exceeded Mach 3, the new goal was to venture into the hypersonic area of aerodynamics: Mach 5 and above. The definition of ‘hypersonic’ is somewhat nebulous since there is no clear and sudden change with respect to the supersonic flight regime (as occurs between subsonic, transonic and supersonic). In general, with respect to supersonic aerodynamics, what happens at hypersonic speeds is much more complex and far more difficult to model and predict. Many of the simplifications about the behavior of the atmosphere, aerodynamic heating and shock waves that can safely be used for supersonic theory are no longer valid at speeds over Mach 5. Laboratory tests for hypersonics are hampered by the fact that it is virtually impossible to generate a continuous Mach 5 + airflow in a wind tunnel. Hypersonic wind tunnels depend on extremely brief, explosive bursts of gas that only facilitate measurements on very small models during a fraction of a second. Once again, the only way to get large amounts of reliable data on this flight regime is to fly research aircraft at hypersonic speeds.

As regards altitude, with its maximum attained altitude of 38.5 km (126,000 feet) the X-2 had already reached into the upper stratosphere. But how a spaceplane or a shuttle-like vehicle would behave in a virtual vacuum, and what it would experience on returning from orbit, had yet to be investigated. Many aviation experts at the time expected the airplane to evolve into an orbital spaceplane, initially launched on top of a conventional rocket but later on capable of taking off and landing like a normal aircraft. As early rocket pioneers such as Yalier had foreseen, a space plane was part of an inevitable evolution. The next step, the X-15, was therefore expected to act as a bridge between aircraft and spacecraft.