Extending the Delta into the Hypersonic and Orbital Frontier

The next stage in delta development took it from the realm of the tran­sonic and supersonic into the hypersonic, again thanks to a healthy rivalry and differing technical perspective between those two great research centers, Ames and Langley. The area was hypersonics: flight at speeds higher than Mach 5, an area of intense inquiry in the mid – 1950s following upon the success of the supersonic Round One research

aircraft. Already a Round Two hypersonic test vehicle, the soon-to-emerge North American X-15 was underway. But what of high-hypersonics, the hypersonics of flight at Mach 10 to orbital velocity?

Hypersonics constituted a natural application for the low aspect ratio delta planform. Before the Second World War, Austrian engineer Eugen Sanger and his mathematician wife, Irene Sanger-Bredt, had concep­tualized the Silbervogel ("Silver Bird”), a flat-bottom, half ogive body shape as a potential Earth-girdling hypersonic boost-glider. It had, for its time, a remarkable advanced aerodynamic profile, introducing the flat bottom and ogival configuration that did, in fact, come to charac­terize hypersonic aerothermodynamic design. But in one respect it did not: Sanger-Bredt’s "antipodal aircraft” had a conventional wing (though of low aspect planform and with supersonic wedge airfoils). Although it proved very influential on the course of postwar hypersonics, by the mid-1950s, as high-speed aerodynamic thinking advanced beyond the supersonic and into the hypersonic realm, attention increasingly turned toward the sharply swept delta planform.

In 1951, Ames researchers H. Julian Allen and Alfred Eggers, Jr., had postulated the blunt-body reentry theory that led to the advent of the practical reentry shape used subsequently both for missile warheads and the first human presence in space.[104] (Their work, and the emer­gence of the hypersonics field generally, are discussed in greater detail in T. A. Heppenheimer’s accompanying essay on transatmospherics.) While blunt-body theory enabled safely transiting the atmosphere, it did not furnish the flexibility of a large landing "footprint”; indeed, in practice, blunt-body reentry was limited to "throwaway” reentry shapes and pro­grams such as Mercury, Gemini, and Apollo that necessitated a large and cumbersome investment in oceanic recovery of returning space­craft. Some sort of lifting vehicle that could fly at hypersonic velocities would have far greater flexibility.

Related to the problem of hypersonic flight was the challenge of increasing lift-to-drag ratios at high supersonic speeds. Eggers, work­ing with Ames researcher Clarence A. Syvertson, now turned away from blunt-body theory to examine thin, slender deltas. The two rec­ognized that "the components of the aircraft should be individually
and collectively arranged to impart the maximum downward and the minimum forward momentum to the surrounding air.”[105] Out of this emerged the hypersonic "flattop” delta, a high-wing concept having the wing perched above the body (in this case, surmounting the classic half-ogive hypersonic shape), incongruously much like a general aviation light airplane such as a Cessna 152. At mid-span, its tips would angle sharply downward, capturing the momentum of flow imparted laterally outward from the body and deflecting it into downward momentum, thus greatly increasing lift. The tips as well furnished directional stability. This flattop concept, which Eggers and Syvertson enunciated in 1956, spawned an Ames concept for a hypersonic "beyond X-15” Round Three research vehicle that could be air-launched from a modified Convair B-36 bomber for ini­tial trials to Mach 6 and, once proven, could then be launched vertically as the second stage of a two-stage system capable of reach­ing Mach 10 and transiting the United States. The Ames vehicle, with an overall length of 70 feet and a span of just 25 feet, represented a bold concept that seemed likely to spawn the anticipated Round Three hypersonic boost-glider.[106]

But the flattop delta was swiftly undone by a rival Round Three Langley concept that echoed more the earlier work of Sanger-Bredt. A 1957 study by Peter Korycinski and John Becker demonstrated that a flat-bottom (that is, low-wing) delta boost-glider would have bet­ter cooling characteristics (a vital concern at hypersonic velocities) and thus require less weight for thermal protection systems. Any lift-to-drag advantages of the Ames flattop high-wing concept were thus nullified. Round Three went forward, evolving into the abortive Air Force-NASA X-20 Dyna-Soar program, which employed the Langley

flat-bottom approach, not the high-wing flattop delta of Ames.[107] Ames and Langley contested a decade later, this time in rival lifting bodies, with the Ames half-cone flattop M-2 (the product of Allen, Eggers, Syvertson, George Edwards, and George Kenyon) competing against Langley’s HL-10 fattened flat-bottom delta (by Eugene S. Love). Again, it was the flat-bottom delta that proved superior, confirmed by tests in the mid-1970s with an even more refined flat-bottom Air Force-derived slender delta body shape, the Martin X-24B.[108]

When orbital cross range proved even of greater significance, Shuttle proponents from the National Aeronautics and Space Administration (NASA) and the Air Force in the 1970s looked away from flattop and lift­ing body approaches and more toward blended bodies, modified delta planforms, and exotic delta "wave riders.” Though NASA’s Spacecraft Design Division briefly considered a conventionally tailed, straight and swept wing Shuttle concepts, reflecting an influential study by Johnson’s Maxime Faget, it moved rapidly toward deltas after analysis indicated such designs had a tendency of hypersonic spins, suspect aero – thermal survivability, and too small a cross range during return from orbit. Between mid-1971 and the late summer of 1972, the Spacecraft Design Division evaluated no less than 37 separate delta configurations, ranging from simple triangular shapes echoing the early days of Jones to much more complex ogee shape reflecting the refinement of the delta as exemplified by the Anglo-French Concorde. Aside from continuous review by the Manned Spacecraft Center (MSC; subsequently the NASA Lyndon B. Johnson Space Center), these evaluations benefitted greatly from aerodynamic analysis by NASA’s Ames and Langley hypersonic

communities, the practical low lift-to-drag-ratio flight-test experience of researchers at the NASA Flight Research Center, and the rocketry and space flight expertise of the Marshall Space Flight Center, whose experts assessed each proposal from the standpoint of technical feasi­bility and launch vehicle practicality. This multi-Center review strongly endorsed development of a modified delta planform, in part because the delta had inherently better stability characteristics during the high angle-of-attack reentry profile that any returning Shuttle would have to experience. Two families emerged as finalists: The 036 series, with small payload bays and three engines, and the 040 family, of similar planform but with larger payload bays and four engines. Then, in late January 1972, MSC engineers evolved the 040C configuration: a three-engine design using new high-pressure engines. The 040C design became the baseline for subsequent Orbiter studies. While many questions remained over the final form that Shuttle’s launch system would take, with the 040C study, the shape of the orbiter, and its all-important wing, was essentially fixed. Again, the flat-bottom delta had carried the day.[109]