The North American Aviation XB-70 Valkyrie is a retired prototype version of the planned nuclear-armed, deep-penetration supersonic strategic bomber for the United States Air Force Strategic Air Command. Designed by North American Aviation in the late 1950s, the Valkyrie was a large six-engine, delta-winged aircraft able to fly Mach 3+ at an altitude of .
At these speeds, it was expected that the B-70 would be practically immune to interceptor aircraft, the only effective weapon against bomber aircraft at the time. The bomber would spend only a brief time over a particular radar station, flying out of its range before the controllers could position their fighters in a suitable location for an interception. Its high speed made the aircraft difficult to see on radar displays and its high-altitude and high-speed capabilities could not be matched by any contemporaneous Soviet interceptor or fighter aircraft.
The introduction of the first Soviet surface-to-air missiles in the late 1950s put the idea of the B-70 being near-invulnerable in doubt. In response, the US Air Force (USAF) began flying its missions at low level, where the missile radar's line of sight was limited by terrain. In this low-level penetration role, the B-70 offered little additional performance over the B-52 it was meant to replace, while being far more expensive with shorter range. Alternative missions were proposed, but these were of limited scope. With the advent of intercontinental ballistic missiles (ICBMs) during the late 1950s, crewed nuclear bombers were increasingly seen as obsolete.
The USAF eventually gave up fighting for its production and the B-70 program was cancelled in 1961. Development was then turned over to a research program to study the effects of long-duration high-speed flight. As a result, two prototype aircraft, designated XB-70A, were built; these aircraft were used for supersonic test-flights from 1964 to 1969. In 1966, the second, more advanced prototype crashed after colliding with an F-104 Starfighter while flying in close formation for a photo shoot; the only remaining Valkyrie bomber is in the National Museum of the United States Air Force near Dayton, Ohio.
Development
Background In an offshoot of Boeing's MX-2145 crewed boost-glide bomber project, Boeing hired RAND Corporation in January 1954 to explore what sort of bomber aircraft would be needed to deliver the various nuclear weapons under development. At the time, nuclear weapons weighed several tons, and the need to carry enough fuel to fly that payload from the continental United States to the Soviet Union demanded large bombers. Boeing and RAND also concluded that the aircraft would need supersonic speed to escape the blast of its bombs.York 1978, p. 70.
The aviation industry had been studying this problem for some time. From the mid-1940s, there was interest in using nuclear-powered aircraft as bombers.von Kármán, Theodore. "Where We Stand: First Report to General of the Army H. H. Arnold on Long Range Research Problems of the Air Forces with a Review of German Plans and Developments". Atomic Energy for Jet Propulsion. Washington, D.C.: Government Printing Office, 22 August 1945.Bikowicz, Brian D. "Atomic Powered Aircraft – Politics" . Atomicengines.com. Retrieved: 24 May 2011. In a conventional jet engine, thrust is provided by heating air using jet fuel and accelerating it out a nozzle. In a nuclear engine, heat is supplied by a reactor, whose consumables last for months instead of hours. Most designs also carried a small amount of jet fuel for high-power portions of flight, such as takeoffs and high-speed dashes.
Another possibility being explored at the time was the use of boron-enriched "zip fuels", which increase the energy density of jet fuel by about 40 percent, and could be used in modified versions of existing jet engine designs.Schubert, Dave. "From Missiles to Medicine: The development of boron hydrides" . Pioneer Magazine, March 2001. Zip fuels appeared to offer sufficient performance improvement to produce a strategic bomber with supersonic speed.
WS-110A The Air Force followed these developments closely, and in 1955 issued General Operational Requirement No. 38 for a new bomber, combining the payload and intercontinental range of the B-52 with the Mach 2 top speed of the Convair B-58 Hustler.Jenkins 1999, Ch. 1. The new bomber was expected to enter service in 1963.Jenkins and Landis 2002, p. 9. Nuclear and conventional designs were considered. The nuclear-powered bomber was organized as "Weapon System 125A" and pursued simultaneously with the jet-powered version, "Weapon System 110A".Jenkins and Landis 2002, pp. 9–10.
The USAF Air Research and Development Command's (ARDC) requirement for WS-110A asked for a chemical-fuel bomber with Mach 0.9 cruising speed and "maximum possible" speed during a entrance and exit from the target. The requirement also called for a payload and a combat radius of .Knaack 1988, pp. 560–561. The Air Force formed similar requirements for a WS-110L intercontinental reconnaissance system in 1955, but this was later canceled in 1958 due to better options.Knaack 1988, pp. 561, 566.Jenkins and Landis 2002, p. 17. In July 1955, six contractors were selected to bid on WS-110A studies. Boeing and North American Aviation submitted proposals, and on 8 November 1955 were awarded contracts for Phase 1 development.Pace 1988, p. 14.
In mid-1956, initial designs were presented by the two companies.Knaack 1988, p. 563. Zip fuel was to be used in the afterburners to improve range by 10 to 15 percent over conventional fuel.Jenkins and Landis 2002, pp. 15–16. Both designs had huge wing-tip fuel tanks that could be jettisoned when their fuel was depleted before a supersonic dash to the target. The tanks also included the outer portions of the wing, which would also be jettisoned to produce a smaller wing suitable for supersonic speeds.Jenkins and Landis 2002, pp. 13–14. Both became trapezoidal wings after ejection, at that time the highest performance planform known. They also featured flush cockpits to maintain the highest fineness ratio possible despite its negative effects on visibility.
The two designs had takeoff weights of about with large fuel loads. The Air Force evaluated the designs, and in September 1956 deemed them too large and complicated for operations. General Curtis LeMay was dismissive, declaiming, "This is not an airplane, it's a three-ship formation."Rees 1960, pp. 125–126. The USAF ended Phase 1 development in October 1956 and instructed the two contractors to continue design studies.B-70 Aircraft Study, Vol. I, pp. I-34–I-38.
New designs While the original proposals were being studied, advances in supersonic flight were proceeding rapidly. The narrow delta was establishing itself as a preferred planform for supersonic flight, replacing earlier designs like the swept-wing and trapezoidal layouts seen on designs like the Lockheed F-104 Starfighter and the earlier WS-110 concepts. Engines able to cope with higher temperatures were also under development, allowing for sustained supersonic speeds.
This work led to an interesting discovery: when an engine was optimized specifically for high speed, it burned perhaps twice as much fuel at that speed than when it was running at subsonic speeds. However, the aircraft would be flying as much as four times as fast. Thus its most economical cruise speed, in terms of fuel per mile, was its maximum speed. This was entirely unexpected and implied that there was no point in the dash concept; if the aircraft was able to reach Mach 3, it may as well fly its entire mission at that speed. The question remained whether such a concept was technically feasible, but by March 1957, engine development and wind tunnel testing had progressed enough to suggest that it was.
WS-110 was redesigned to fly at Mach 3 for the entire mission. Zip fuel was retained for the engine's afterburner to increase range.Jenkins and Landis 2002, pp. 14–15.Conway 2005, p. 33. Both North American and Boeing returned new designs with very long fuselages and large delta wings. They differed primarily in engine layout; the NAA design arranged its six engines in a semi-circular duct under the rear fuselage, while the Boeing design used separate podded engines located individually on pylons below the wing, like the Hustler.
North American scoured available literature to find any additional advantage. This led them to an obscure report by two NACA wind tunnel experts, who wrote a report in 1956 titled "Aircraft Configurations Developing High Lift-Drag Ratios at High Supersonic Speeds".Rees 1960, p. 126. Known today as compression lift, the idea was to use the shock wave generated off the nose or other sharp points on the aircraft as a source of high-pressure air.Pace 1988, p. 16. By carefully positioning the wing in relation to the shock, the shock's high pressure could be captured on the bottom of the wing and generate additional lift. To take maximum advantage of this effect, they redesigned the underside of the aircraft to feature a large triangular intake area far forward of the engines, better positioning the shock in relation to the wing. The six individually-podded engines were repositioned, three in each of two separate ducts, under the fuselage.Winchester 2005, p. 187.
North American improved on the basic concept by adding a set of drooping wing-tip panels that were lowered at high speed. This helped trap the shock wave under the wing between the downturned wing tips. It also added more vertical surface to the aircraft to maintain directional stability at high speeds. NAA's solution had an additional advantage, as it decreased the surface area of the rear of the wing when the panels were moved into their high-speed position. This helped offset the natural rearward shift of the center of pressure, or "average lift point", with increasing speeds. Under normal conditions this caused an increasing nose-down trim, which had to be offset by moving the control surfaces, increasing drag. When the wing tips were drooped, the lifting area of the wings was lessened, moving the lift forward and reducing trim drag.Talay, Theodore A., ed. "Dynamic Longitudinal, Directional, and Lateral Stability" . Centennial of Flight Commission, 2003. Retrieved: 24 May 2011.
The buildup of heat due to skin friction during sustained supersonic flight had to be addressed. During a Mach 3 cruise, the aircraft would reach an average of , with leading edges reaching , and up to in engine compartments. NAA proposed building their design out of sandwich panels, with each panel consisting of two thin sheets of stainless steel brazed to opposite faces of a honeycomb-shaped foil core. Expensive titanium would be used only in high-temperature areas like the leading edge of the horizontal stabilizer, and the nose.B-70 Aircraft Study, Vol. III., pp. III-10, III-31, III-141, III-210. To cool the interior, the XB-70 pumped fuel en route to the engines through heat exchangers.B-70 Aircraft Study, Vol. III., pp. III-496 to III-498.
On 30 August 1957, the Air Force decided that enough data were available on the NAA and Boeing designs that a competition could begin. On 18 September, the Air Force issued operational requirements that called for a cruising speed of Mach 3.0 to 3.2, an over-target altitude of , a range of up to , and a gross weight not to exceed . The aircraft would have to use the hangars, runways and handling procedures used by the B-52. On 23 December 1957, the North American proposal was declared the winner of the competition, and on 24 January 1958, a contract was issued for Phase 1 development.