Toward Transatmospheric Flight: From V-2 to the X-51

T. A. Heppenheimer

The expansion of high-speed aerothermodynamic knowledge enabled the attainment of hypersonic speeds, that is, flight at speeds of Mach 5 and above. Blending the challenge of space flight and flight within the atmosphere, this led to the emergence of the field of transatmospherics: systems that would operated in the upper atmosphere, transitioning from lifting flight to ballistic flight, and back again. NACA-NASA research proved essential to mastery of this field, from the earliest days of blunt body reentry theory to the advent of increasingly sophisticated transatmo­spheric concepts, such as the X-15, the Shuttle, the X-43A, and the X-51.

N DECEMBER 7, 1995, the entry probe of the Galileo spacecraft plunged downward into the atmosphere of Jupiter. It sliced into the planet’s hydrogen-rich envelope at a gentle angle and entered at Mach 50, with its speed of 29.5 miles per second being four times that of a return to Earth from the Moon. The deceleration peaked at 228 g’s, equiv­alent to slamming from 5,000 mph to a standstill in a single second. Yet the probe survived. It deployed a parachute and transmitted data from its onboard instruments for nearly an hour, until overwhelmed by the increas­ing pressures it encountered within the depths of the Jovian atmosphere.[544]

The Galileo probe offered dramatic proof of how well the National Aeronautics and Space Administration (NASA) had mastered the field of hypersonics, particularly the aerothermodynamic challenges of dou­ble-digit high-Mach atmospheric entries. That level of performance was impressive, a performance foreshadowed by the equally impressive (cer­tainly for their time) earlier programs such as Mercury, Gemini, Apollo, Pioneer, and Viking. But NASA had, arguably, an even greater challenge before it: developing the technology of transatmospheric flight—the abil­ity to transit, routinely, from flight within the atmosphere to flight out

into space, and to return again. It was a field where challenge and con­tradiction readily mixed: a world of missiles, aircraft, spacecraft, rock­ets, ramjets, and combinations of all of these, some crewed by human operators, some not.

Transatmospheric flight requires mastery of hypersonics, flight at speeds of Mach 5 and higher in which aerodynamic heating predomi­nates over other concerns. Since its inception after the Second World War, three problems have largely driven its development.

First, the advent of the nuclear-armed intercontinental ballistic mis­sile (ICBM), during the 1950s, brought the science of reentry physics and took the problem of thermal protection to the forefront. Missile nose cones had to be protected against the enormous heat of their atmo­sphere entry. This challenge was resolved by 1960.

Associated derivative problems were dealt with as well, including that of protecting astronauts during demanding entries from the Moon. Maneuvering hypersonic entry became a practical reality with the Martin SV-5D Precision Recovery Including Maneuvering Entry (PRIME) in

1967. In 1981, the Space Shuttle introduced reusable thermal protec­tion—the "tiles”—that enabled its design as a "cool” aluminum air­plane rather than one with an exotic hot structure. Then in 1995, the Galileo mission met demands considerably greater than those of a return from the Moon.

A second and contemporary problem, during the 1950s, involved the expectation that flight speeds would increase essentially without limit. This hope lay behind the unpiloted air-launched Lockheed X-7, which used a ramjet engine and ultimately reached Mach 4.31. There also was the rocket-powered and air-launched North American X-15, the first transatmospheric aircraft. One X-15 achieved Mach 6.70 (4,520 mph) in October 1967. This set a record for winged hypersonic flight that stood until the flight of the Space Shuttle Columbia in 1981. The X-15 introduced reaction thrusters for aircraft attitude, and they subse­quently became standard on spacecraft, beginning with Project Mercury. But the X-15 also used a "rolling tail” with elevons (combined eleva­tors and ailerons) in the atmosphere and had to transition to and from space flight. The flight control system that did this later flew aboard the Space Shuttle. The X-15 also brought the first spacesuit that was flex­ible when pressurized rather than being rigid like an inflated balloon. It too became standard. In aviation, the X-15 was first to use a simula­tor as a basic tool for development, which became a critical instrument

for pilot training. Since then, simulators have entered general use and today are employed with all aircraft.[545]

A third problem, emphasized during the era of President Ronald Reagan’s Strategic Defense Initiative (SDI) in the 1980s, involved the prospect that hypersonic single-stage-to-orbit (SSTO) air-breathing vehicles would shortly replace the Shuttle and other multistage rocket – boosted systems. This concept depended upon the scramjet, a variant of the ramjet engine that sustained a supersonic internal airflow to run cool. But while scramjets indeed outperformed conventional ramjets and rockets, their immaturity and higher drag made their early application as space access systems impossible. The abortive National Aero-Space Plane (NASP) program consumed roughly a decade of development time. It ballooned enormously in size, weight, complexity, and cost as time progressed and still lacked, in the final stages, the ability to reach orbit. Yet while NASP faltered, it gave a major boost to computational fluid dynamics, which use supercomputers to study airflows in aviation. This represents another form of simulation that also is entering gen­eral use. NASP also supported the introduction of rapid-solidification techniques in metallurgy. These enhance alloys’ temperature resistance, resulting in such achievements as the advent of a new type of titanium that can withstand 1,500 degrees Fahrenheit (°F).[546] Out of it have come more practical and achievable concepts, as evidenced by the NASA X-43 program and the multiparty X-51A program of the present.

Applications of practical hypersonics to the present era have been almost exclusively within reentry and thermal protection. Military hyper­sonics, while attracting great interest across a range of mission areas, such as surveillance, reconnaissance, and global strike, has remained the stuff of warhead and reentry shape research. Ambitious concepts for transatmospheric aircraft have received little support outside the labo­ratory environment. Concepts for global-ranging hypersonic "cruisers” withered in the face of the cheaper and more easily achievable rocket.