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Subject: North American XB-70 Valkyrie
Date: 26 May 2018 03:23:40 -0700
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https://en.wikipedia.org/wiki/North_American_XB-70_Valkyrie
The North American Aviation XB-70 Valkyrie was the prototype version of the
planned B-70 nuclear-armed, deep-penetration strategic bomber for the United
States Air Force Strategic Air Command. Designed in the late 1950s by North
American Aviation, the six-engined Valkyrie was capable of cruising for
thousands of miles at 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 position their fighters in
a suitable location for an interception. High speed also made the aircraft
difficult to see on 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 near-invulnerability of the B-70 in doubt. In response, the United
States Air Force (USAF) began flying its missions at low level, where the
missile radar's line of sight was limited by local 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.
Other alternate missions were proposed, but these were of limited scope. With
the advent of intercontinental ballistic missiles (ICBMs) during the late 1950s,
manned bombers were increasingly seen as obsolete.
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, designated XB-70A, were built; these aircraft were used for supersonic
a smaller aircraft while flying in close formation; the remaining Valkyrie
bomber is in the National Museum of the United States Air Force near Dayton,
Ohio.
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)
The Valkyrie was designed to be a high-altitude Mach 3 bomber with six engines.
Harrison Storms shaped the aircraft with a canard surface and a delta wing,
which was built largely of stainless steel, sandwiched honeycomb panels, and
titanium. The XB-70 was designed to use supersonic technologies developed for
the Mach 3 Navaho, as well as a modified form of the SM-64 Navaho's inertial
guidance system.
The XB-70 used compression lift, which arose from a shock wave generated by the
sharp leading edge of the central engine intake splitter plate below the wing.
At Mach 3 cruising speed, the shock wave attached along the wing leading edge,
preventing the high pressure behind the shock front from leaking up over the
wing. The compression lift provided five percent of the total lift. The wing
included inboard camber to more effectively use the higher pressure field behind
the strong shock wave. Unique among aircraft of its size, the outer portions of
the wings were hinged, and could be pivoted downward by up to 65 degrees, acting
almost as a type of variable-geometry wingtip device. 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. With the wingtips drooped downwards, the compression
lift shock wave would be further trapped under the wings.
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. During 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
(21,000 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 (22,000 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 (3,900 km) in 91 minutes of total flight.
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
were 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.
Specifications (XB-70A)
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)
Internal fuel capacity: 300,000 pounds (140,000 kg) or 46,745 US gallons
(177,000 L)
19,900 lbf (84 kN) each
Thrust with afterburner: 28,800 lbf (128 kN) each
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 (6,901 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
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