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.
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).
