n 1933, a young aeronautical engineer at the NACA Langley Laboratory conceived the idea of a research airplane that would be designed, built, and flown strictly for the purpose of probing an unknown flight regime. John Stack, a research engineer working in Langley’s first high-speed wind tunnel, designed a hypothetical research airplane for the single purpose of collecting data in the subsonic flight regime near the speed of sound. In the early 1930s, little was known and understood about flight near the speed of sound. Because the governing flow equations were mathematically nonlinear in this region, no analytical solutions were available to predict the lift, drag, and stability characteristics for airplanes in this transonic regime. (Even today, the only reliable transonic flow solutions are numerical results obtained from computational fluid dynamics [CFD] using massive supercomputers.) In addition, no accurate transonic wind tunnel data could be obtained from existing high-speed tunnels due to adverse aerodynamic interactions between shock waves from the model, reflecting off the wind tunnel walls and impinging back on the model surfaces.


X-15 in flight. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base



X-15A-2 in captive flight under the B-52. USAF, Air Force Flight Test Center History Office, Edwards Air Force Base



In light of the complete lack of accurate theoretical results and wind tunnel data in the transonic regime, Stack turned to the airplane as the best way to study flight near Mach 1. One of his hand – drawn graphs from 1933, found in the John Stack archives at NASA Langley by one of the authors and replete with the original smudges and rusty paperclip marks, shows Stack’s calculations of the power required versus flight velocity for his propeller-driven design.

At the top of the graph, Stack drew a sketch of his research airplane. His ideas got as far as

Volume 1, Number 1 of the brand-new Journal of the Aeronautical Sciences, published by the newly formed Institute of the Aeronautical Sciences (IAS) in 1934 (now the American Institute of Aeronautics and Astronautics [AIAA]). The “Effects of Compressibility on High Speed Flight” both advances the concept and gives the results of his calculations for such an airplane. His idea, however, got no further than the journal at that time. He sent his results to the biannual meeting of the NACA in October 1933, but the committee chose not to help Stack find a developer for the airplane. His work, however, was the genesis of the idea that eventually resulted in the X-15 via three other research airplanes: the X-1, X-1A, and X-2.

John Stack’s hand-drawn graph showing the effects of compressibility on the power required for a high-speed airplane, 1933. NASA Langley Research Center Library, Stack archive file