THE X-15 PROGRAM IN RETROSPECT

Hypersonic Aerodynamics



Hypersonic aerodynamics was in its early infancy in 1954. The few small hypersonic wind tunnels then in existence had been used almost entirely for fluid mechanics studies. They were unable to simulate either the high temperatures or the high Reynolds numbers of flight. Because of strongly interacting flow fields, viscous interactions with strong shocks, and possible real gas effects, it was generally feared that testing in these limited wind tunnels would not produce valid results. And it was expected that the X-15 would reveal large discrepancies between flight and ground test data (ref. 2). Our inability to devise ground facilities capable of true-temperature simulation was in fact in 1954 regarded as a sort of "facility barrier". All-out efforts were launched during that period to try to develop high-temperature facilities.

Two line graph charts comparing wind-tunnel and flight aerodynamic data
Figure 4. Typical comparison of wind-tunnel and flight aerodynamic data.


The X-15 program helped to expose the fallacies of this "facility barrier". Virtually all of the flight pressures and forces were found to be in excellent agreement with the low-temperature wind-tunnel predictions (refs. 11, 12, 13, and 14; fig. 4 shows two typical examples). Prior to the start of flight operations it was learned by analysis that the "real gas" high-temperature effects in themselves were for the most part negligible below Mach 10. Thus the agreement noted above implies primarily an absence of any important scale effects on the pressures and forces (other than skin friction) for the X-15 configuration. (Footnote: Other configurations, notably the highly swept delta wing with trailing-edge flaps, have been found to exhibit important scale effects, not only hyper sonically but throughout the speed range.)

Concuxrent with the first years of the X-15 flight program, a number of missile and space vehicle configurations were also successfully developed in small low-temperature hypersonic wind tunnels, and in a few cases limited flight data were obtained which provided some additional confirms tion of the wind-tunnel results. With this broad general validation, the bulk of which came from the X-15 results, the conventional low-temperature hypersonic wind tunnel became the accepted tool for configuration develop ment. The "true-temperature" hypersonic aerodynamics tunnel on the other hand, with its enormous operational and interpretational difficulties, has proved useful only for limited special problems where full temperature simulation is mandatory (ramjet combustor development, for example).


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