Aeronautics

In 1954, the few existing hypersonic wind tunnels were small and presumably unable to simulate the conditions of actual flight at speeds above Mach 5, The realistic fear at the time was that testing in them would fail to produce valid data. The X-15 provided the earliest, and so far most significant, valida­tion of hypersonic wind tunnel data. This was of particular significance since it would be extremely difficult and very expensive to build a large-scale hypersonic wind tunnel.

This general validation, although broadly con­firmed by other missiles and spacecraft, came primarily from the X-15; it made the conven­tional, low-temperature, hypersonic wind tun­nel an accepted source of data for configura­tion development of hypersonic vehicles.

The X-15 program offered an excellent opportunity to compare actual flight data with theory and wind tunnel predictions. The X-15 verified existing wind tunnel tech­niques for approximating interference effects for high-Mach, high angle-of-attack hyper­sonic flight, thus giving increased confi­dence in small-scale techniques for hyper­sonic design studies. Wind tunnel drag meas­urements were also validated, except for a 15 percent discrepancy found in base drag— caused by the sting support used in the wind tunnel. All of this greatly increased the con­fidence of engineers as they set about design­ing the Space Shuttle.

One of the widely held beliefs in the mid – 1950s was the theoretical presumption that the boundary layer (the thin layer of air close to the surface of an aircraft) would be highly stable at hypersonic speeds because of heat flow away from it. This presumption fostered the belief that hypersonic aircraft would enjoy laminar (smooth) airflow over their surfaces. At Mach 6, even wind tunnel extrapolations indicated extensive laminar flow. However, flight data from the X-15 showed that only the leading edges exhibited laminar flow and that turbulent flow occurred over most surfaces. Small surface irregularities, which produced turbulent flow at transonic and supersonic speeds, also did so at Mach 6.20 Thus, engineers had to aban­don their hopeful expectations. Importantly, X-15 flight test data indicated that hyperson­ic flow phenomena were linear above Mach 5, allowing increased confidence during design of the Space Shuttle, which must rou­tinely transition through Mach 25 on its way to and from space. The basic X-15 data were also very useful to the NASP designers while that program was viable.

In a major discovery, the Sommer-Short and Eckert T-prime aerodynamic heating predic­tion theories in use during the late 1950s were found to be 30 to 40 percent in excess of flight test results. Most specialists in fluid mechanics refused to believe the data, but repeated in-flight measurements completely substantiated the initial findings. This led the aerodynamicists to undertake renewed ground-based research to complete their understanding of the phenomena involved— highlighting the value of flight research in doing what Hugh Dryden had predicted for the X-15 in 1956: that it would “separate the real from the imagined.”21

Subsequent wind tunnel testing led to Langley’s adopting the empirical Spaulding – Chi model for hypersonic heating. This eventually allowed the design of lighter vehi­cles with less thermal protection that could more easily be launched into space. The Spaulding-Chi model found its first major use during the design of the Apollo com­mand and service modules and proved to be quite accurate. In 1999 the Spaulding-Chi model was still the primary tool in use.

Based on their X-15 experience, North American devised a computerized mathe­matical model for aerodynamic heating called HASTE (Hypersonic and Supersonic Thermal Evaluation) which gave a workable “first cut” approximation for design studies. HASTE was, for example, used directly in the initial Apollo design study. Subsequent

versions of this basic model were also used early in the Space Shuttle design evolution.

At the time of the first Mach 5 X-15 flight, perhaps its greatest contribution to aeronau­tics was to disprove the existence of a “sta­bility barrier" to hypersonic flight that was suspected after earlier research aircraft encountered extreme instability at high supersonic speeds. Although of little conse­quence today, the development of the “wedge” tail allowed the X-15 to successful­ly fly above Mach 5 without the instability that had plagued the X-l series and X-2 air­craft at much lower speeds. The advent of modem fly-by-wire controls and stability augmentation systems based around high speed digital computers have allowed designers to compensate for gross instabili­ties in basic aerodynamic design, and even to tailor an aircraft’s behavior differently for different flight regimes. The era of building a vehicle that is dynamically stable has passed, and with it much of this lesson.

The art of simulation grew with the X-15 pro­gram, not only for pilot training and mission rehearsal, but for research into controllability problems. The same fixed-based simulator used by the pilots could also be used to explore those areas of the flight envelope deemed too risky for actual flight. The X-15 program showed the value of combining wind tunnel testing and simulation in maximizing the knowledge gained from each of the 199 test flights. It also provided a means of com­paring “real” flight data with wind tunnel data. It is interesting to note that the man-in – the-loop simulation first used on X-15 found wide application on the X-30 and the X-33. In fact, DFRC research pilot Stephen D. Ishmael has flown hundreds of hours “in” the X-33, which ironically is an unpiloted vehicle.