North American XB-70 Valkyrie

The North American Aviation XB-70 Valkyrie is the prototype of the B-70 nuclear-armed, deep-penetration strategic bomber for the U.S. Air Force's Strategic Air Command. North American Aviation designed the Valkyrie bomber as a large, six-engined aircraft capable of reaching Mach 3+ while flying at 70,000 feet (21,000 m).

At these speeds, it was expected that the B-70 would be almost immune to interceptor aircraft, the only effective weapon against bomber aircraft at the time. The bomber would spend only a few minutes over a particular radar station, flying out of its range before the controllers could vector their fighters into a suitable location for an interception. Its high speed also made the aircraft difficult to see on the radar displays, and its high altitude flight could not be matched by any contemporary Soviet fighter.

The introduction of the first Soviet surface-to-air missiles in the late 1950s put the B-70's near-invulnerability in doubt. In response, the US Air Force began flying its missions at low level, where the missile radar's line of sight was limited by local terrain. In this role, known as penetration, the B-70 offered little more performance over the B-52 it was meant to replace. It was, however, far more expensive and had shorter range. A number of alternate missions were proposed, but these were of limited scope. As the strategic role passed from bombers to intercontinental ballistic missiles (ICBMs) during the late 1950s, manned bombers were increasingly seen as an anachronism.

The USAF eventually gave up fighting for its production, and the B-70 program was canceled in 1961. Development was then turned over to a research program to study the effects of long-duration high-speed flight. As such, two prototype aircraft were built, and designated XB-70A; these aircraft were used for supersonic test-flights during 1964–69. In 1966, one prototype crashed after colliding in mid-air with a smaller jet aircraft; the remaining Valkyrie bomber is in the National Museum of the United States Air Force, in Ohio.

North American XB-70 Valkyrie
Class Aircraft
Type Attack
Manufacturer North American Aviation
Origin United States of America
Country Name Origin Year
United States of America 1964
Country Name Operational Year Retirement Year
United States of America 1969 View
ManufacturerName Production From Production To Quantity
North American Aviation 2 View

During the period that 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 and widely varying intake ramp air speeds were also under design, 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, although 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, that an aircraft able to reach Mach 3 may as well fly its entire mission at that speed. By March 1957, engine development and wind tunnel testing had progressed such that the potential for all-supersonic flight appeared feasible – the cruise-and-dash approach that had resulted in huge designs was no longer needed.

The project decided that the aircraft would fly at speeds up to Mach 3 for the entire mission, instead of a combination of subsonic cruise and supersonic dash of the aircraft designs in the previous year. Zip fuel was to be burned in the engine's afterburner to increase range. 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 had scoured the 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 entitled "Aircraft Configurations Developing High Lift-Drag Ratios at High Supersonic Speeds". 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. 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.

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, and also added more vertical surface to the aircraft to improve 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 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 surface area at the rear of the wings was lowered, moving the lift forward and counteracting this effect, reducing the need for control inputs.

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 450 °F (230 °C), with leading edges reaching 630 °F (330 °C), and up to 1,000 °F (540 °C) 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. For cooling the interior, the XB-70 pumped fuel en route to the engines through heat exchangers.

On 30 August 1957, the Air Force decided that enough data was available on the NAA and Boeing designs that a competition could begin. On 18 September, the Air Force issued operational requirements which called for a cruising speed of Mach 3.0 to 3.2, an over-target altitude of 70,000–75,000 ft (21,300–22,700 m), a range of up to 10,500 mi (16,900 km), and a gross weight not to exceed 490,000 lb (222,000 kg). 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.

In February 1958, the proposed bomber was designated B-70, with the prototypes receiving the "X" experimental prototype designation. The name "Valkyrie" was the winning submission in early 1958, selected from 20,000 entries in a USAF "Name the B-70" contest. The Air Force approved an 18-month program acceleration in March 1958 that rescheduled the first flight to December 1961. But in late 1958 the service announced that this acceleration would not be possible due to lack of funding. In December 1958, a Phase II contract was issued. The mockup of the B-70 was reviewed by the Air Force in March 1959. Provisions for air-to-surface missiles and external fuel tanks were requested afterward. At the same time, North American was developing the F-108 supersonic interceptor. To reduce program costs, the F-108 would share two of the engines, the escape capsule, and some smaller systems with the B-70. In early 1960, North American and the USAF released the first drawing of the XB-70 to the public.

The XB-70's maiden flight was on 21 September 1964. In the first flight test, between Palmdale and Edwards AFB, one engine had to be shut down shortly after take-off, and an undercarriage malfunction warning meant that the flight was flown with the undercarriage down as a precaution, limiting speed to 390 mph - about half that planned. On landing, the rear wheels of the port side main gear locked, the tires ruptured, and a fire started.

The Valkyrie first became supersonic (Mach 1.1) on the third test flight on 12 October 1964, and flew above Mach 1 for 40 minutes during the following flight on 24 October. The wing tips were also lowered partially in this flight. XB-70 No. 1 surpassed Mach 3 on 14 October 1965 by reaching Mach 3.02 at 70,000 ft (21,300 m). The first aircraft was found to suffer from weaknesses in the honeycomb panels, primarily due to inexperience with fabrication and quality control of this new material. On two occasions, honeycomb panels failed and were torn off during supersonic flight, necessitating a Mach 2.5 limit being placed on the aircraft.

The deficiencies discovered on AV-1 were almost completely solved on the second XB-70, which first flew on 17 July 1965. On 3 January 1966, XB-70 No. 2 attained a speed of Mach 3.05 while flying at 72,000 ft (21,900 m). AV-2 reached a top speed of Mach 3.08 and maintained it for 20 minutes on 12 April 1966. On 19 May 1966, AV-2 reached Mach 3.06 and flew at Mach 3 for 32 minutes, covering 2,400 mi (3,840 km) in 91 minutes of total flight.

XB-70 performance

  • Longest flight: 3:40 hours (on 6 January 1966)
  • Fastest speed: 2,020 mph (3,250 km/h) (on 12 January 1966)
  • Highest altitude: 74,000 ft (23,000 m) (on 19 March 1966)
  • Highest Mach number: Mach 3.08 (on 12 April 1966)
  • Sustained Mach 3: 32 minutes (on 19 May 1966)
  • Mach 3 total: 108 minutes/10 flights

A joint NASA/USAF research program was conducted from 3 November 1966 to 31 January 1967 for measuring the intensity and signature of sonic booms for the National Sonic Boom Program (NSBP). Testing was planned to cover a range of sonic boom overpressures on the ground similar to but higher than the proposed American SST. In 1966, AV-2 was selected for the program and was outfitted with test sensors. It flew the first sonic boom test on 6 June 1966, attaining a speed of Mach 3.05 at 72,000 ft (21,900 m). Two days later, AV-2 crashed following a mid-air collision with an F-104 while flying in a multi-aircraft formation. Sonic boom and later testing continued with XB-70A #1.

The second flight research program (NASA NAS4-1174) investigated "control of structural dynamics" from 25 April 1967 through the XB-70's last flight in 1969. At high altitude and high speed, the XB-70A experienced unwanted changes in altitude. NASA testing from June 1968 included two small vanes on the nose of AV-1 for measuring the response of the aircraft's stability augmentation system. AV-1 flew a total of 83 flights.

The XB-70's last supersonic flight took place on 17 December 1968. On 4 February 1969, AV-1 took its final flight to Wright-Patterson Air Force Base for museum display (now the National Museum of the United States Air Force). Flight data was collected on this subsonic trip. North American Rockwell completed a four-volume report on the B-70 that was published by NASA in April 1972.

Role Strategic bomber
Supersonic research aircraft
National origin United States
Manufacturer North American Aviation
First flight 21 September 1964
Retired 4 February 1969
Status Retired
Primary users United States Air Force
NASA
Number built 2
Program cost US$1.5 billion
Unit cost US$750 million (average cost)


General characteristics

  • Crew: 2
  • Length: 189 ft 0 in (57.6 m)
  • Wingspan: 105 ft 0 in (32 m)
  • Height: 30 ft 0 in (9.1 m)
  • Wing area: 6,297 ft² (585 m²)
  • Airfoil: Hexagonal; 0.30 Hex modified root, 0.70 Hex modified tip
  • Empty weight: 253,600 lb (115,030 kg; operating empty weight)
  • Loaded weight: 534,700 lb (242,500 kg)
  • Max. takeoff weight: 542,000 lb (246,000 kg)
  • Powerplant: 6 × General Electric YJ93-GE-3 afterburning turbojet
  • Dry thrust: 19,900 lbf (84 kN) each
  • Thrust with afterburner: 28,800 lbf (128 kN) each
  • Internal fuel capacity: 300,000 lb (136,100 kg) or 46,745 US gallons (177,000 L)

Performance

  • Maximum speed: Mach 3.1 (2,056 mph, 3,309 km/h)
  • Cruise speed: Mach 3.0 (2,000 mph, 3,200 km/h)
  • Range: 3,725 nmi (4,288 mi, 6,900 km) on combat mission
  • Service ceiling: 77,350 ft (23,600 m)
  • Wing loading: 84.93 lb/ft² (414.7 kg/m²)
  • lift-to-drag: about 6 at Mach 2
  • Thrust/weight: 0.314

End notes