North American XB-70 Valkyrie

Template:Infobox Aircraft

The North American XB-70 Valkyrie was an airplane conceived in the 1950s as a high-altitude nuclear bomber that could fly at Mach 3 (three times the speed of sound) for the United States Air Force Strategic Air Command. The development of this aircraft caused enough concern in the Soviet Union to spur creation of the MiG-25 "Foxbat" in response to the threat such a high speed bomber posed.

The proposed cost of the aircraft, along with changes in the technological environment led to the cancellation of the program. Although the proposed full fleet of operational B-70 bombers was never built, two prototype XB-70s flew in flight tests in the 1960s, performing research on the design of large supersonic aircraft. One prototype crashed following a mid-air collision in 1966, and the other is now on display at the National Museum of the United States Air Force.

Development

In 1955, the USAF released system requirements for a heavy bomber with the B-52's range and payload and the B-58's supersonic speed to replace these bombers by 1965. The initial requirements called for a Mach 0.9 cruise speed with a Mach 2+ dash capability.^[1] Later in 1955, study contracts were awarded to a handful of companies with Boeing and North American Aviation selected for subsequent study round. In 1956, initial designs were presented by the two companies. The designs featured large wingtip fuel tanks that could be jettisoned before a supersonic run on the target. The Air Force evaluated their designs and in 1957 deemed them too large and complicated. North American and Boeing's study contracts were extended to further develop their bomber designs.^[1]

During this time, the companies switched to a boron enriched jet fuel to allow higher speed and increased range. Mach 3 speed at 70,000 feet (21,340 m) and a range of 8,750 miles (14,082 km) became possible with six jet engines burning the boron jet fuel. Boeing's and North American's designs both featured a large delta wing with a gross weight of approximately 500,000 pounds (226,800 kg). North American's design featured wingtips that folded down during high speed flight to catch the shock wave and generate "compression lift". The competing designs were submitted to the Air Force in 1957 with North American selected as the winner late that year.^[1] North American's bomber designed received the initial designation XB-70 and name Valkyrie in 1958.^[1] The X in the designation indicates an experimental prototype design with the production version designated B-70.

Problems with the boron fueled engines and concerns with the toxic fuel led to a switch back to engines that use conventional jet fuel. The change decreased range to 8,078 miles (13,000 km).^[1] The Valkyrie was designed to be a large, high-altitude nuclear bomber with six engines to fly at Mach 3 to avoid defending interceptors, the only effective anti-bomber weapon at the time.

North American proposed the XF-108 Rapier, a supersonic interceptor to escort the B-70 which was developed at the same time as the bomber. The Rapier was also intended as a defensive aircraft to counter an anticipated Russian bomber with capabilities similar to the B-70.^{[citation needed]}

The introduction of anti-aircraft missiles rendered this mode of operation dangerous; in order to maintain its high speed the aircraft had to fly at high altitudes, which allowed missile sites more than enough time to track and attack the B-70. For instance, the S-75 Dvina's primary search radar had a nominal range of 170 miles (274 km). This would give the operators about five minutes warning, more than enough to launch an attack.

Following the downing of the U-2 flown by Gary Powers, military doctrine shifted quickly away from high-altitude supersonic bombing toward low-altitude penetration. Utterly unsuited for this new role, the viability of the B-70 as a bomber was questioned. Citing high cost, a growing ICBM force, and poor survivability against missiles,^[2] the operational fleet was canceled in 1961 by President Kennedy, and the program was changed to a research program.^[1] During this time as a last effort, a reconnaissance strike version, RS-70 was studied with the Air Force seeking a fleet of 60 by 1969. However this version was largely a reconnaissance B-70 and it was canceled in 1963.^[1]

The B-70's prototype XB-70As were used for the advanced study of aerodynamics, propulsion, and other subjects related to large supersonic aircraft, in particular the American Supersonic transport (SST) program. Initial plans were made to build three aircraft, each one incorporating modifications based on lessons learned from the previous aircraft's flight tests, but the program was cut down to two aircraft in July 1964.

Flight testing of this aircraft showed that sound from the sonic boom reached the ground to an unacceptable degree,^[3] and this was one of the primary factors that lead to the cancellation of SST programs.

Design

The Valkyrie was designed to be a large, high-altitude bomber with six engines to fly at Mach 3. It was configured as a canard delta wing, and built largely of stainless steel, sandwiched honeycomb panels, and titanium. It was designed to make use of a phenomenon called "compression lift", achieved when the shock wave generated by the airplane flying at supersonic speeds is trapped underneath the wings, supporting part of the airplane's weight. Under the center of the wing, the Valkyrie featured a prominent wedge at the center of the engine inlets, designed to produce a strong shock wave. By acting upwards upon the wings, this shock wave would allow the aircraft to recover energy from its own wake. At high speeds, compression lift increased the lift of the wings by thirty percent, with no increase in drag.^[4]

Unique among aircraft of its size, the outer portions of the wings were hinged, and could be pivoted downward by up to 65 degrees. This increased the aircraft's directional stability at supersonic speeds, shifted the center of lift to a more favorable position at high speeds, and strengthened the compression lift effect.^[5] With the wingtips drooped downwards, the compression lift shock wave would be further trapped under the wings, rather than simply flowing out past the wingtips.

The XB-70 had a maximum lift-to-drag ratio (L/D) at Mach 2 of about 6.^[6] In similar flight conditions, the B-58 Hustler had a maximum L/D ratio of just under 5, while the Concorde has a maximum L/D of about 7.4.

Operational history

Flight testing

The first XB-70 made its maiden flight on 21 September 1964. The first aircraft was found to suffer from weaknesses in the honeycomb construction, primarily due to inexperience with fabrication and quality control of this new material.^[1] Construction of the honeycombed panels was much more difficult than anticipated by the designers. The first aircraft was also continually troubled by hydraulic leaks, fuel leaks, and problems with the aircraft's unusually complicated landing gear. In flight on 7 May 1965, the divider separating the left and right halves of the engine inlet broke off and was ingested into the engines, damaging all six beyond repair. On 14 October 1965, on the first flight exceeding a speed of Mach 3, the stress again damaged the honeycomb construction, leaving two feet (0.6 m) of the leading edge of the left wing missing. These construction problems resulted in the imposition of a speed limit of Mach 2.5 on the first aircraft.

These honeycomb construction deficiencies were almost completely solved on the second aircraft, which first flew on 17 July 1965. On 19 May 1966 aircraft number two flew 2,400 miles (3,840 km) in 91 minutes, attaining Mach 3 for 33 minutes of that flight.

On 8 June 1966, aircraft number two was flown in close formation with four other aircraft, an F-4, F-5, T-38, and an F-104, for the purpose of a photo shoot at the behest of General Electric, manufacturer of the engines of all five aircraft. With the photo shoot complete, the F-104 rolled inverted, passed over top, and struck the Valkyrie, destroying the rudders and damaging the left wing. The Valkyrie entered a spin and crashed following the mid-air collision. NASA Chief Test Pilot Joe Walker, piloting the F-104, and Carl Cross, copilot aboard the XB-70, were killed in the crash, while Al White, the XB-70's pilot, successfully ejected.

The exact cause of the collision is still debated. While the pilots involved were experienced, formation flying with different aircraft types is more hazardous than formation flying with aircraft possessing similar flight characteristics. The smaller F-104 could have been caught by the complex airflow around the larger Valkyrie's wingtip, and encountered turbulence which pulled it into the collision. Lt. Colonel Joe Cotton, the USAF's Chief Test Pilot for the B-70, flying a T-38 in the formation, has speculated that Walker, unfamiliar with flying in formation with such a large delta wing aircraft, lost reference to his position relative to the B-70, and simply closed up the formation until the T-tail of the F-104 struck the Valkyrie's wingtip.^[7] Chuck Yeager has also gone on record to echo this position.^[8]

The first aircraft with its limited abilities continued research, making 33 more research flights. On 4 February 1969, Valkyrie number one was retired and flown to the National Museum of the United States Air Force at Wright-Patterson Air Force Base near Dayton, Ohio.

The Soviet Sukhoi T-4 was a medium-range bomber/missile carrier prototype designed to take advantage of many of the advances made during the development of the XB-70.^{[citation needed]}

Variants

A full scale mock-up was completed in February 1959.
XB-70A - prototype of B-70. Two were built.
- Aircraft #1, NAA Model Number NA-278, USAF S/N 62-0001, 83 flights; total time: 160 hours - 16 minutes - At the U.S. Air Force Museum near Dayton, OH
- Aircraft #2, NAA Model Number NA-278, USAF S/N 62-0207, 46 flights; total time: 92 hours - 22 minutes - Crashed on 8 June 1966 north of Barstow, CA, killing USAF co-pilot Major Carl S. Cross. NASA pilot Al White ejected successfully. 35°03′48″N 117°01′28″W / 35.063286°N 117.024474°W / 35.063286; -117.024474
XB-70B - Aircraft #3, NAA Model Number NA-274, USAF S/N 62-0208, Originally to be first YB-70A in March 1961, this advanced prototype was canceled in March 1964 while under construction.
YB-70A - Additional 10 pre-production prototypes canceled in December 1960. These YB-70s would have been modified to B-70A specifications at the completion of testing.
B-70A - Planned production version of Valkyrie.^[1] A fleet of up to 65 operational bombers was planned.^[9]
RS-70 - Proposed reconnaissance-strike version with a crew of four and in-flight refueling capability.^[10] A fleet of 62 was planned in 1959.^[11]

Specifications (XB-70A)

Orthographically projected diagram of the XB-70A Valkyrie.

Data from XB-70 Fact sheet[1]

General characteristics

Crew: 2
Airfoil: Hexagonal; 0.30 Hex modified root, 0.70 Hex modified tip

Performance

Thrust/weight: 0.314

References

^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Jenkins 1999
^ B-70 Aircraft Study, Vol. I, p I-38
^ Sonic boom - Abatement
^ "The Ride of the Valkyrie"
^ B-70 Aircraft Study, Vol. III, p.III-162
^ "Performance Evaluation Method for Dissimilar Aircraft Designs"
^ The Crash of the XB-70 Valkyrie on Check-Six.com
^ Yeager and Janos 1986, p. 226.
^ B-70 Aircraft Study, Vol. I, p.I-29
^ B-70 Aircraft Study, Vol. III, pp.II-2, II-3, & II-15
^ B-70 Aircraft Study, Vol. I, p.II-307

Jenkins, Dennis R. B-1 Lancer, The Most Complicated Warplane Ever Developed. New York: McGraw-Hill, 1999. ISBN 0-07-134694-5.
North American Rockwell, NASA-CR-115702, B-70 Aircraft Study Final Report, Vol.I, NASA, 1972.
North American Rockwell, NASA-CR-115702, B-70 Aircraft Study Final Report, Vol.II, NASA, 1972.
North American Rockwell, NASA-CR-115702, B-70 Aircraft Study Final Report, Vol.III, NASA, 1972.
North American Rockwell, NASA-CR-115702, B-70 Aircraft Study Final Report, Vol.IV, NASA, 1972.
Yeager, Chuck and Janos, Leo. Yeager: An Autobiography. New York: Bantam Books, 1986. ISBN 0-553-25674-2.