XB-70 Valkyrie Totally Explained

Everything about Xb-70 totally explained

North American Aviation's B-70 Valkyrie was a nuclear-armed bomber designed for the United States Air Force's Strategic Air Command in the 1950s. The Valkyrie was a large six-engined aircraft able to fly at Mach 3 at high altitudes, which would have allowed it to avoid defending interceptors, the only effective anti-bomber weapon at the time.
The proposed cost of the aircraft, along with changes in the technological environment due to the introduction of the first effective anti-aircraft missiles led to the cancellation of the program in 1961. The development of the Valkyrie, along with the U-2 and SR-71 reconnaissance aircraft led the Soviet Union to create the MiG-25 "Foxbat", and new, improved surface to air missiles (SAMs), to counter them.
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 midair collision in 1966; the other is now on display at the National Museum of the United States Air Force in Dayton, Ohio.

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. An aircraft able to fly the same mission profiles and have supersonic performance would have to carry an enormous fuel load. The aircraft industry was exploring different ways to address this problem.
   There was considerable interest in the use of a nuclear powered aircraft. 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.
   Another possibility was the use of boron-enriched "zip fuels", which improved the energy density of the fuel. Various agencies in the USA had been experimenting with zip fuels for some time, and there was a real feeling that once the problems were worked out, they'd become almost universal for high-speed aircraft. Although the advantages of a zip fueled aircraft wouldn't 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 autumn 1954, the Air Force endorsed two separate approaches, one for a nuclear powered bomber and another for a conventionally powered version. The new design would have to have the intercontinental range of the B-52 Stratofortress, as well as the Mach 2 maximum speed of the B-58 Hustler, and it was expected the new bomber would replace both of these designs in service by 1965.
   Finally, there was a group within the industry that felt the natural solution to these problems 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 feet with a 20,000 lb payload, revising this to 25,000 lb in GOR.82 later that month.
   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 1000-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".
   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. 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. NAA and Boeing's study contracts were extended to further develop their bomber designs. By carefully positioning the wing in relation to the shock, it could be captured on the bottom of the wing and generate additional lift. Since the energy put into forming the shock wave was already "spent", the lift generated in this fashion was 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. 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. This not only helped trap the shock wave, but also added more vertical surface to the aircraft when operating at high speeds, which was important in helping offset a general decrease in directional stability all aircraft encounter at high speeds. The Air Force believed that "other systems" would be able to better meet the reconnaissance mission, and development of WS-110L was cancelled 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 intended to use the same engines as the B-70. Although intended primarily as an interceptor, the F-108's range was enough to allow it to act as an escort fighter as well, although its usefulness in this role was questionable.

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 wasn't 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 1/2 hour after approaching over the pole, implying that even given 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 certainly possible. This could be made more difficult by making the bomber fly faster or higher, but it was far easier to increase the speed of the missile or the range of its radars than it was to increase the speed of the aircraft. There was serious 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. This was because the missile line-of-sight issue worked in both directions; 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 couldn't 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, and at low altitudes would lose its supersonic performance and have 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 "on-the-deck" was only Mach 0.95. 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 wasn't a fatal problem however, as newly developed high-energy fuels, namely 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 didn't 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 wouldn't 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. 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 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. 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. 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. 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.
   Though an experimental program managed by North American, the United States Air Force conducted its own accident investigation. The Air Force Summary Accident Report found that, given the position of the F-104 relative to the XB-70, the F-104 pilot wouldn't have been able to see the XB-70's wing, except via looking back over his left shoulder. This position wouldn't 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 seventy feet to the side of, and ten 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. 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 - 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.
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.
RS-70 - Proposed reconnaissance-strike version with a crew of four and in-flight refueling capability. A fleet of 62 was planned in 1959.

Specifications (XB-70A)

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