XB-70 Valkyrie

Infobox Aircraft
name=XB-70 Valkyrie
type=Strategic bomber Supersonic research aircraft
manufacturer=North American Aviation


caption=
designer=
first flight=21 September 1964
introduced=
retired=
number built=2 XB-70s
status=Retired from testing in 1967
program cost=US$1.5 billionKnaack, Marcelle Size. "Post-World War II Bombers, 1945-1973". Washington, DC: Office of Air Force History, 1988. ISBN 0-16-002260-6.]
variants with their own articles=
primary user=NASA
more users=

The North American Aviation XB-70 Valkyrie was a prototype version of the proposed B-70 nuclear-armed deep penetration bomber for the United States Air Force's Strategic Air Command. Designed in the late 1950s, the Valkyrie was a large six-engined aircraft able to fly at Mach 3 at an altitude of 70,000 feet (+21,000 meters), which would have allowed it to avoid interceptors, the only effective anti-bomber weapon at the time.

Two XB-70 prototypes were built for U.S. Air Force. However, the aircraft program's high development costs, along with changes in the technological environment with the introduction of the effective anti-aircraft missiles led to the cancellation of the B-70 program in 1961. Although the proposed fleet of operational B-70 bombers was never built, the XB-70A aircraft were used in supersonic test flights from 1964 to 1969, performing research for the design of large supersonic aircraft. One prototype crashed following a midair collision in 1966. The other is on display at the National Museum of the United States Air Force in Dayton, Ohio.

The development of the Valkyrie, along with the U-2 and SR-71 reconnaissance aircraft led the Soviet Union to design and develop the MiG-25 "Foxbat" interceptor, [Pace, "F-22 Raptor"Steve. New York: McGraw-Hill, 1999. ISBN 0-07-134271-0.] along with new, improved surface to air missiles (SAMs), to counter these U.S. threats. The flight data and materials development of the XB-70 program also laid the foundation for the later B-1 Lancer supersonic bomber program, as well as the commercial supersonic transport (SST) aircraft programs, such as the Aérospatiale-BAC Concorde and Tupolev Tu-144.

Development

Early studies

The genesis of the B-70 can be traced to a study by Boeing and Rand Corporation that started in January 1954. The study explored what sort of aircraft would be needed to deliver the very high-yield nuclear weapons then under construction. Long range and high payload were obvious requirements, but they also concluded that a high-speed, high-altitude dash capability would be needed in order to avoid defensive fighters, as well as escape the blast of its own weapons. At the time, jet engines had very poor fuel economy; an aircraft capable of carrying a reasonable bombload all the way to the Soviet Union from the continental United States had to be very large. One example was the B-52 Stratofortress, a strictly subsonic design. Supersonic flight required much more power, and thus much more fuel. An aircraft able to fly the same mission profiles as the B-52 "and" have supersonic performance would have to carry an enormous fuel load. The aircraft industry was exploring different ways to address this problem.

In the 1950s there was considerable interest in the use of nuclear powered aircraft in the bomber role. Nuclear engines used the heat generated by a nuclear reactor in place of jet fuel, giving an aircraft virtually unlimited cruising range. In addition to solving the range issue, these aircraft could be flown to holding areas away from the airbases and kept in the air for extended periods of time, making them immune to sneak attack. Accordingly, Boeing developed plans for a nuclear powered bomber that also included normal jet engines for takeoff and the high-speed "dash" portion of the flight, which were turned off during cruise. Lockheed and Convair also offered similar solutions.Baugher, Joe. [http://home.att.net/~jbaugher2/b70.html North American XB-70A Valkyrie] American Bombers.]

Another possibility was the use of boron-enriched "zip fuels", which improved the energy density of the fuel. Various US government agencies had been experimenting with zip fuels for some time, and there was a real feeling that once the problems they were having were worked out, zip fuel would become almost universal for high-speed aircraft. Although the advantages of a zip fueled aircraft would not be as great as those of a nuclear powered one, it would offer a real performance increase and was a relatively straightforward development of existing engines and fuels.

In late 1954, the Air Force endorsed two separate approaches for a new bomber, one nuclear powered and the other conventionally powered. The new design would have to have the intercontinental range of the B-52, as well as the Mach 2 maximum speed of the B-58 Hustler. It was expected the new bomber would replace both of these designs in service starting in 1965.

Finally, there was a growing group within the defense establishment who felt that the natural solution to the problems of range and Soviet defenses was the development of the intercontinental ballistic missile (ICBM), which at that point in time was a technological "long shot". The relative importance of this program would wax and wane over the next few years, before finally emerging as the primary strategic weapon in the 1960s.

First attempts

In October 1954, the Air Force issued General Operational Requirement No. 38, which was quite general and called simply for an intercontinental manned bomber which would replace the B-52 beginning in 1965. March 1955's GOR.81 was more specific, calling for a nuclear-powered bomber with a combat radius of 11,000 nautical miles, capable of flying up to 1,000 miles at a speed greater than Mach 2 at altitudes greater than 60,000 ft with a 20,000 lb payload. The payload was revising upward to 25,000 lbs in GOR.82 later that month.Jenkins 1999, Ch. 1.]

The Air Research and Development Command (ARDC) decided to separate the two approaches, and issued a requirement for "Weapon System 110A", which asked for a Mach 0.9 cruising speed and "maximum possible" speed during a 1,000 mile entrance and exit from the target. The target date for the first operational wing of these bombers was July 1964, reduced a year in comparison to earlier GOR's. The nuclear approach became "Weapon System 125A", while the ICBM work was organized under "Weapon System 107A". [ [http://www.unrealaircraft.com/classics/xb70.php Lost Classics - North American XB-70 Valkyrie] ]

[
WS-110A. The "floating panels" are large fuel tanks containing conventional JP-4 fuel used during the long subsonic cruise, each the size of a B-47. Once ejected, the engines would burn "HEF", or zip fuel, during the high-speed dash phase.]

In early 1955, the Air Force issued GOR.96, which called for an intercontinental reconnaissance system with the same general requirements as WS-110A, called WS-110L. Pace 1986, p. 14.] The two requirements were combined soon afterwards, becoming Weapon System 110A/L. The nuclear-powered version was dropped during this period, given the problems in that program's development, as well as a general feeling of optimism about the zip fuels. In June 1955 the Air Staff directed that the details of WS-110A/L be released to the aviation industry and that a request for proposals be issued. Although six contractors were given the requirements, only Boeing and North American Aviation (NAA) submitted proposals. On 8 November 1955, the Air Force issued letter contracts to both Boeing and North American for Phase 1 development. The contracts called for models, design reports, wind tunnel tests, plus a mock-up.

In 1956, initial designs were presented by the two companies. Although zip fuels improved range, the overall effect was not very large, perhaps 10%, so both designs featured huge wingtip fuel tanks that could be jettisoned before a supersonic run on the target. In the case of the North American design, the entire outer portion of the wings was jettisoned as well, resulting in an aircraft that looked somewhat like a very large F-104 Starfighter after being "broken up".

The Air Force evaluated their designs and in September 1956 deemed them too large and complicated; the huge fuel load resulted in takeoff weights of 700,000 pounds, making safe operation from existing runways extremely difficult. They were also far too large to fit in existing hangers. Curtis LeMay was not enthusiastic about the NAA design, claiming "Hell, this isn't an airplane, it's a three-ship formation." [ [http://www.unrealaircraft.com/classics/xb70.php Lost Classics - North American XB-70 Valkyrie] ] NAA and Boeing's study contracts were extended to further develop their bomber designs. The next month the program was put on hold, although the companies were told to continue any low-level development they could.

upersonic cruise

The program was soon re-started in March 1957, after developments during that short period of time indicated that a significantly improved design could be built. The project was now being envisaged an aircraft that would be able to cruise at supersonic speeds of up to Mach 3 for the entire mission, as opposed to a subsonic cruise/supersonic dash aircraft of the earlier work.

The biggest problem with sustained supersonic cruise is the buildup of heat due to skin friction. Duralumin, the traditional aircraft material, starts to lose strength and go plastic at relatively low temperatures, and is unsuitable for continuous use at speeds above Mach 2.2 to 2.4. During the period that WS-110A was being studied, solutions to these problems were beginning to become available. New materials, especially titanium and stainless steel, were becoming more widely used in the industry and allowed operations at much higher temperatures.

Another area of concern for continued high-speed operation is the engines. Jet engines create thrust by increasing the temperature of the air they ingest, and as the aircraft speeds up, friction and compression heats this air before it reaches the engines. The maximum temperature of the exhaust is determined by the materials in the turbine at the rear of the engine, so as the aircraft speeds up the difference in intake and exhaust temperature the engine can extract decreases, and the thrust along with it. Air cooling the turbine area to allow operations at higher temperatures was a key solution, one that continued to improve though the 1950s and on to this day.

Intake design was also a major issue. Normal jet engines can only ingest subsonic air, so for supersonic operation the air has to be slowed down. Ramps or cones in the intake are used to create shock waves that slows the airflow before it reaches the engine. Doing so removes energy from the airflow, causing drag. The key to reducing this drag is to use multiple small oblique shock waves, but this was difficult because the angle they make inside the intake changes with Mach number. In order to efficiently operate across a range of speeds, the shock waves have to be "tuned." North American had already worked with advanced inlets on the A3J supersonic bomber for the U.S. Navy, which featured multiple ramps which were moved and angled automatically.

An aircraft able to operate for extended periods at supersonic speeds has a potential range advantage over a similar design operating subsonically. Most of the drag an aircraft sees while speeding up to supersonic speeds occurs just below the speed of sound, due to an aerodynamic effect known as wave drag. An aircraft that can accelerate past this speed sees a significant drag decrease, and can cruise supersonically with improved fuel economy. However, due to the way lift is generated supersonically, the lift-to-drag ratio of the aircraft as a whole drops, leading to lower range, offsetting or overturning this advantage.

The key to having low supersonic drag is to properly shape the overall aircraft to be long and skinny, as close as possible to a "perfect" shape, the von Karman ogive or Sears-Haack body. This has led to almost every supercruising aircraft looking very similar to every other, with a very long and skinny fuselage and large delta wings, cf. SR-71, Concorde, etc. Although not ideal for passenger aircraft, this shaping is quite adaptable for bomber use.

New designs

Both North American and Boeing returned new designs incorporating everything then known in supersonic design. Both designs had 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 similar to those on the SR-71, but located individually on pylons below the wing.

North American scoured the literature to find any additional advantage. One possibility that turned up was an obscure piece of research known as "compression lift", which used the shock wave generated by the nose or other sharp points on the aircraft as a source of high pressure air.Pace 1986, 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. Since the energy put into forming the shock wave has already been spent simply flying through the air, the lift generated in this fashion is essentially free. To take maximum advantage of this effect they redesigned the entire 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 semi-circular duct disappeared and the engines were re-arranged to lie side-by-side in a line. Fuel tanks were repositioned from the fuselage into a number of smaller tanks wrapped around the ducting, and the rudder switched to a twin-fin design.

North American improved the design with a new idea their own, a set of drooping wing tip panels that were lowered at high speeds. This not only helped trap the shock wave under the wing, but also added more vertical surface to the aircraft, which was important in helping offset a general decrease in directional stability all aircraft encounter at high speeds. Other designs had generally used fixed surfaces for this, ending up over-stable at slower speeds, or alternately used dedicated movable surfaces as on the Republic XF-103. NAA's solution had an additional advantage, however, as it decreased the surface area of the rear of the wing when they were moved into their high speed position. This helped alleviate a more minor problem, the shift in center of pressure as speeds changed. Under normal conditions the "average lift point", or center of pressure, moves rearward with increasing speeds, causing an increasing nose-down trim. By dropping the wingtips the horizontal surface of the wing was reduced at the rear, leaving more surface forward, offsetting this effect.

During a Mach 3 cruise the aircraft would reach an average of 450˚F, although there were portions as high as 650 ˚F. NAA proposed building their design out of a honeycomb stainless steel material, consisting of two thin sheets of steel brazed to a honeycomb-shaped foil in the middle. Titanium, still an extremely expensive material, would be used only in high-temperature areas like the nose and air intakes. One clever bit of engineering was the air conditioning system for the crew cabin, which needed a place to pump heat while the air outside the aircraft was being heated to hundreds of degrees. NAA solved this problem by dumping the heat into the fuel tanks closer to the center of the aircraft, where temperatures were lower. The engines were fed fuel from tanks closer to the outside where the fuel was hotter, which had the advantage of pre-heating the fuel and slightly improving performance. A complex series of pumps continually fed the fuel from the inner tanks to the outer.

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 to 75,000 ft, a range of up to 10,500 miles, and a gross weight not to exceed 490,000 lbs. The aircraft would have to use all of the hangars, runways and handling procedures used with 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 aircraft was assigned the number B-70, with the prototypes receiving the "X" experimental prototype designation. The name "Valkyrie" was the winning submission in spring 1958, selected from 20,000 entries in a USAF "Name the B-70" contest.Pace 1986, p. 17.] The Air Force believed that "other systems" would be able to better meet the reconnaissance mission, and development of WS-110L was canceled at this time. In December 1958, a Phase II contract was issued. The first operational wing of 30 aircraft was to be ready by late 1965.

At the same time North American was developing the proposed XF-108 Rapier supersonic interceptor. In order to save on overall program costs, the F-108 would use the same engines as the B-70, although only two of them instead of six. Although intended primarily as an interceptor, the F-108's range was great enough to allow it to act as an escort fighter as well, although its usefulness in this role was questionable.Boyne, Walter J. [http://www.afa.org/magazine/June2006/0606valkyrie.asp "The Ride of the Valkyrie".] AFA.org, June 2006.]

The "missile problem"

The high-speed, high-altitude approach used by all U.S. bombers up to this point was intended to complicate the defense by giving them less time to deal with the aircraft as they flew over. Although it was possible to build interceptors with enough performance to catch even a Mach 3 target, the time needed to detect, track and guide the aircraft to its target was fixed by the operator workload, which was not improving at nearly the same rate. In the 1950s the Royal Canadian Air Force concluded that one interception per minute was the best that could be hoped for. Assuming the same was true in the USSR, the B-70 traveling at Mach 3 would be over land for only about ½ hour while approaching Moscow over the north pole, implying that even in perfect conditions the vast majority of the B-70s would fly right past the defending fighters. There was even some hope that the aircraft moved so fast that its radar return would be "smeared out" on the analog displays of the era due to an effect known as the "blip-to-scan ratio", rendering it partially invisible on long-range radars.

In the middle-to-late 1950s, anti-aircraft missiles developed to a point where they became useful weapons. This upset the equation completely. Missiles can be fired as soon as a track is developed, and can reach high altitudes in a few minutes. Even at the speeds the B-70 would be traveling, the SA-2 Guideline missiles it would face would be able to detect it over 100 miles away on their search radars, giving them as much as five minutes in which to plan and launch an attack. This was marginal, especially given that the SA-2's tracking radars had much shorter ranges, but as long as they were alerted in advance, an interception was possible. There was a concern that the B-70 would be no more able to penetrate the USSR's airspace than the B-52 it was supposed to replace.

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. By flying close to the Earth and using natural terrain to hide behind, aircraft could dramatically shorten the detection distances, allowing them to fly right by most radar sites. Those missile sites that could not be avoided, like those on the approach to Moscow, would instead be attacked at medium range using high-speed missiles. Low-altitude flight is taxing on both the aircraft and crews, however, and requires considerably more fuel to cover a given distance.

Utterly unsuited for this new role, the viability of the B-70 as a bomber was questioned. The aircraft would become increasingly vulnerable at high altitudes as newer missile systems were introduced, and at low altitudes it lost its supersonic performance and had dramatically reduced range. Using the original Mach 3 mission profile the aircraft had a range of 6,500 nmi unrefueled, but using a high-low-high profile this was reduced to 5,300 nmi "with" in-flight refueling, and speed was only Mach 0.95 at low altitudes. [B-70 Aircraft Study, Vol. II, pp. II-2, II-3.]

Adding to the problems, the boron fuel program was canceled in 1959. After burning the fuel turned into various solids, and no one was able to design an engine whose turbine was able to stand up to the constant wear these caused. This by itself was not a fatal problem however, as newly developed high-energy fuels like JP-6 were available that made up some of the difference. By filling one of the two bomb bays with a fuel tank, range was reduced only slightly, although payload suffered in terms of space. This was a more serious concern, as it limited the B-70's capability to carry the missiles needed to blast its way past defenses.

President Eisenhower was worried about committing to the B-70 given how much of the technology did not yet exist. At the same time the USA was in the process of developing their first effective ICBMs, the Atlas and Titan. Eisenhower noted that the bomber would not be in service for at least eight years, and by that time the strategic role would have passed to the ICBM. He was also interested in cutting defense spending, as the country was at that time in the midst of a recession. The Air Force announced a major downsizing of the B-70 project on 29 December 1959, reorienting the project to produce only a single prototype. Most of the weapons subsystems planned for the aircraft were cancelled.

Rebirth

The B-70 was given a brief reprieve as the result of the 1960 Presidential Elections. One of John F. Kennedy's election platforms was that Eisenhower was weak on defense, and that the USSR was developing a lead in missile and bomber technology he referred to as the missile gap. Kennedy used the B-70 as a key example of this problem.

Shortly before the elections, in August 1960, the Air Force announced that the earlier downsizing of the B-70 program would be reversed. They also modified the design to produce the RS-70 (RS for "reconnaissance strike"), which was intended to fly in after the ICBMs, locate targets that had not been hit, and then attack those targets. Twelve B-70 prototypes were called for in a new contract, and many of the subcontracts for the weapons subsystems had to be reopened. It was planned that sixty RS-70s would be available by 1969.

Before taking office Kennedy was briefed by the CIA and told that the missile gap was an illusion, and that the U.S. had a tremendous strategic advantage. After taking office Kennedy re-evaluated the ongoing developments, and on 28 March 1961, he directed that the B-70 once again be reoriented strictly as a research and development project. The B-70 then became a political football within the U.S. Senate, and conservative senators tried on several occasions to rescue the program and asked that the B-70 be committed to production and service. Secretary of Defense Robert McNamara expressed his own dissatisfaction with the B-70 program, and the cutbacks remained. On 10 April 1961, a contract for three aircraft was placed with North American, with the crew reduced to only the pilot and co-pilot, the navigator and bomb-aimer were not needed.

Shortly after the cancellation of the B-70 program, the USAF started a program to develop a new bomber dedicated to the low-altitude penetration role. Fraught with problems of its own, this project would eventually lead to the B-1 Lancer.

Experimental aircraft

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.

The first XB-70 was rolled out on 11 May 1964 in Palmdale, California. The large bomber was overwhelming to the crowd as there was nothing like it then.

Flight testing of this aircraft showed that sound from the sonic boom reached the ground to an unacceptable degree, [ [http://www.experiencefestival.com/a/Sonic_boom_-_Abatement/id/2110868 Sonic boom - Abatement] ] and this was one of the primary factors that lead to the cancellation of the U.S. SST program.Fact|date=August 2008

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 aircraft'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. 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. [B-70 Aircraft Study, Vol. III, pp. III–162.] 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. [ [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790021968_1979021968.pdf "Performance Evaluation Method for Dissimilar Aircraft Designs"] ] 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. 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.

On the third test flight, the Valkyrie reached supersonic speeds. In the following flight, XB-70 flew above Mach 1 for 40 minutes. The wing tips were also lowered partially in this flight. On 24 March 1965, the first XB-70 reached Mach 2.14.

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, the first XB-70 reached a speed of Mach 3.02 at an altitude of 70,000 ft (21,300 m). 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. The second XB-70 (air vehicle 2) first flew on 17 July 1965. On 3 January 1966, the second XB-70 attained a speed of Mach 3.05 while flying at 72,000 ft (21,900 m). On 19 May 1966, aircraft number two flew at Mach 3 for 32 minutes, covering 2,400 miles (3,840 km) in 91 minutes of total flight. The second XB-70 was also selected for the National Sonic Boom Program (NSBP) to measure the response to sonic booms. Valkryrie number two flew the first sonic boom test on on 6 June 1966, obtaining a speed of Mach 3.05 at 72,000 ft (21,900 m).

Mid-air accident

On 8 June 1966, the second prototype was flown in close formation with four other aircraft (an F-4, F-5, T-38, and F-104) for the purposes of a photoshoot at the behest of General Electric, manufacturer of the engines of all five aircraft. With the photoshoot complete, the F-104 flipped over, rolling inverted, passed over the top of the Valkyrie, struck it and exploded, destroying the Valkyrie's rudders and damaging its left wing. The Valkyrie entered a spin and crashed into the ground. [http://www.check-six.com/Crash_Sites/XB-70_crash_site.htm "The Crash of the XB-70 Valkyrie."] check-six.com.] NASA Chief Test Pilot Joe Walker (the F-104 pilot) and Carl Cross (the XB-70's co-pilot) were killed, while Al White, the XB-70's pilot, successfully ejected.

The exact cause of the collision is still debated. The pilots involved were all experienced, but formation flying with different aircraft types is more hazardous than formation flying with aircraft possessing similar flight characteristics. Speculation at the time suggested that 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.

In addition, pilots involved in formation flying use certain sight cues, found by aligning certain parts of their lead aircraft with certain other parts. For example, a pilot flying behind and to the side of another aircraft might maintain position by keeping the lead aircraft's wingtip aligned with the cockpit of the lead aircraft. Given the shape of the XB-70, its relative unfamiliarity to the other pilots in the formation, and their position forward of the wing, it would be difficult to find appropriate sight cues for this alignment.

Lt. Colonel Joe Cotton, the USAF's Chief Test Pilot for the XB-70, flying a T-38 in the formation, has speculated that Walker lost reference to his position relative to the XB-70, and simply closed up the formation until the T-tail of the F-104 struck the Valkyrie's wingtip. Chuck Yeager has also gone on record to echo this position.Yeager and Janos 1986, p. 226.]

Though this was an experimental program managed by North American, the United States Air Force conducted its own accident investigation. The Air Force Summary Accident Report [Jenkins and Landis 2005, p. 163.] found that, given the position of the F-104 relative to the XB-70, the F-104 pilot would not have been able to see the XB-70's wing, except via looking back over his left shoulder. This position would not have been comfortable for extended periods of time. The Accident Report concluded that Walker, piloting the F-104, likely maintained his position by looking at the fuselage of the XB-70, forward of his position. The Report estimated that the F-104 was 70 ft to the side of, and 10 feet below, the fuselage of the XB-70. In addition, the Report found that from that position, there would be no suitable alignment points to maintain a precise position relative to the Valkyrie. Given this supporting evidence, the Report concluded that due to the unavailability of appropriate sight cues, Walker was unable to properly perceive his motion relative to the Valkyrie, leading to his aircraft drifting into contact with the XB-70's wing.

Aftermath

The first aircraft with its limited abilities continued research, making 33 more research flights. It was handed over to NASA for their supersonic transport test program in March 1967. [ [http://www.nasa.gov/centers/dryden/news/FactSheets/FS-084-DFRC.html NASA Fact Sheet] ] 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.

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 [ [http://www.wpafb.af.mil/museum/research/bombers/b5/b5-63.htm XB-70: 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. Coord|35.063286|N|117.024474|W|display=inline
* 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. A fleet of up to 65 operational bombers was planned. [B-70 Aircraft Study, Vol. I, pp. I–29.]
* RS-70 - Proposed reconnaissance-strike version with a crew of four and in-flight refueling capability. [B-70 Aircraft Study, Vol. III, pp. II-2, II-3, II-15.] A fleet of 62 was planned in 1959. [B-70 Aircraft Study, Vol. I, pp. II–307.]

Specifications (XB-70A)

aircraft specifications
plane or copter?=plane
jet or prop?=jet
ref=XB-70 Fact sheet [http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=592]
crew=2
length main=185 ft 10 in
length alt=56.6 m
span main=105 ft 0 in
span alt=32 m
height main=30 ft 9 in
height alt=9.4 m
area main=6,296 ft²
area alt=585 m²
airfoil=Hexagonal; 0.30 Hex modified root, 0.70 Hex modified tip
empty weight main=210,000 lb
empty weight alt=93,000 kg
loaded weight main=534,700 lb
loaded weight alt=242,500 kg
max takeoff weight main=550,000 lb
max takeoff weight alt=250,000 kg
engine (jet)=General Electric YJ93-GE-3
type of jet=turbojet
number of jets=6
thrust main=31,000 lbf
thrust alt=133 kN
max speed main=Mach 3.1
max speed alt=2,056 mph, 3,309 km/h
cruise speed main= Mach 3.0
cruise speed alt= 2,000 mph, 3,219 km/h
stall speed main=
stall speed alt=
never exceed speed main=
never exceed speed alt=
range main = 3,725 nmi
range alt = 4,288 mi, 6,900 km
range more=combat
ceiling main=77,350 ft
ceiling alt=23,600 m
climb rate main=
climb rate alt=
loading main=84.93 lb/ft²
loading alt=414.7 kg/m²
thrust/weight=0.314

ee also

aircontent
related=
* XF-108 Rapier
similar aircraft=
* Sukhoi T-4
* Avro 730
lists=
* List of bomber aircraft
* List of military aircraft of the United States
see also=
* Concorde

References

Notes

Bibliography

* Jenkins, Dennis R. " B-1 Lancer, The Most Complicated Warplane Ever Developed". New York: McGraw-Hill, 1999. ISBN 0-07-134694-5.
* Jenkins, Dennis R. and Tony R. Landis. "Valkyrie: North American's Mach 3 Superbomber". North Branch, Minnesota: Specialty Press, 2005. ISBN 1-58007-072-8.
* Machat, Mike. "XB-70 Valkyrie: Rollout and First Flights, May 1964-June 1966." "Wings" Volume 35, No. 8, August 2005.
* North American Rockwell, [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950002358_1995102358.pdf NASA-CR-115702, B-70 Aircraft Study Final Report, Vol. I] , [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950002359_1995102359.pdf Vol. II] , [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950002360_1995102360.pdf Vol. III] , [http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19950002361_1995102361.pdf Vol. IV] , NASA, 1972.
* Pace, Steve. "Triplesonic Twosome." "Wings" Volume 18, No. 1, February 1988.
* Winchester, Jim. "North American XB-70 Valkyrie". "X-Planes and Prototypes". London: Amber Books Ltd., 2005. ISBN 1-904687-40-7.
* Yeager, Chuck and Leo Janos. "Yeager: An Autobiography". New York: Bantam Books, 1986. ISBN 0-553-25674-2.

External links

* [http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=592 XB-70 fact sheet on USAF Museum web site]
* [http://www.dfrc.nasa.gov/Gallery/Movie/XB-70/index.html NASA/DFRC XB-70 videos]
* [http://www.hq.nasa.gov/pao/History/SP-468/ch12-5.htm Quest for Performance: The Evolution of Modern Aircraft, Chapter 12: Jet Bomber and Attack Aircraft, NASA SP-468]
* [http://www.globalsecurity.org/wmd/systems/b-70.htm B-70 Valkyrie page on] GlobalSecurity.org
* [http://www.vectorsite.net/avxb70.html The North American XB-70 Valkyrie page on Vectorsite.net]
* [http://www.aircraftinformation.info/art_xb-70.htm XB-70 page on Aircraft Information site]
* [http://unrealaircraft.com/classics/xb70.php North American XB-70 Valkyrie on UnrealAircraft.com]
* [http://xb70.interceptor.com The Flight of the Valkyrie, North American's XB-70]
* [http://www.sr-71.org/aircraft/xb-70.php XB-70 Valkyrie page on SR-71.org]
* [http://www.history.com/media.do?action=clip&id=mf657_xb70_44&gclid=CP7A9JDNl4gCFQJNZwodhkXOPg XB-70 crash footage in a Universal news reel, History Channel]
* [http://www.airliners.net/search/photo.search?aircraft_genericsearch=&airlinesearch=&countrysearch=&specialsearch=&daterange=&keywords=XB+70+valkyrie&range=&sort_order=&page_limit=15&thumbnails=&calccount=1240021&truecount=false&engine_version=6.0 XB-70 on Airliners.net]