v1.2.0 / 01 dec 01 / greg goebel / public domain
* After World War II, the US Air Force's (USAF) strategic bombers grew ever more capable, each reaching higher altitudes and greater speeds than its predecessor. By the late 1950s, the USAF was planning to develop a "super-bomber", the North American "B-70", that would be built in large numbers.
In reality, improvements in Soviet air defenses and the development of the ICBM made the B-70 obsolete before it ever flew. The B-52, which was planned to have been an interim type leading to the B-70, still remains the backbone of America's strategic bombing capability. However, two XB-70s were completed as supersonic test aircraft, and were among the sleekest and most impressive aircraft that ever flew. This document outlines the history of the XB-70.
* The XB-70 began life in 1954, in design studies performed for a US Air Force (USAF) request that formally emerged in 1955 as Weapons System 110 (WS-110)", which specified a high-altitude bomber that would carry a heavy warload and cruise at Mach 3 over long range at high altitude.
Boeing and North American submitted proposals, but they weren't exactly what the USAF wanted. Their designs had a loaded weight of over 450 tonnes (a million pounds); were too big to fit into existing B-52 hangars and other facilities; and could only achieve Mach 3 for a short dash over the target. The proposals were rejected. Both companies went back to the drawing board, and found that they could in fact build a bomber with a warload of over 22.5 tonnes (50,000 pounds) that could cruise at Mach 3 at an altitude of over 21 kilometers (70,000 feet), and would be able to use existing facilities.
North American won the competition in December 1957. Their design, designated "B-70", was of canard configuration, featuring a long, sleek fuselage with small wings mounted near the cockpit, and a large delta wing in the rear that was fitted with twin vertical stabilizers. The bomber was to be powered by six General Electric J93 turbojets, each with a thrust in the range of over 13 tonnes (30,000 pounds). The engines, bomb bay, and landing gear were all contained in a single wedge-shaped unit under the center of the delta wing. The aircraft was to be constructed mostly of lightweight stainless-steel honeycomb, with titanium used in certain heat-critical sections.
The B-70 also incorporated an unusual feature: the outboard 6 meters (20 feet) of the wings could fold down. This scheme was derived from research that showed that trapping the shockwave generated from the nose of a aircraft flying at supersonic speeds under the wing could generate very high lift.
The B-70 would take off with the wingtips straight. Above about 550 KPH to about Mach 1.4, they would be lowered to 25 degrees, and above that speed, to 65 degrees. The folding wingtips not only improved lift, they also allowed smaller vertical stabilizers to be used, and compensated for the delta wing's backwards shift in its center of lift as speed increased.
* However, as the B-70 design solidified, the Air Force began to have second thoughts. Intercontinental ballistic missiles (ICBMs) were clearly the way of the future for strategic nuclear strike, and the B-70 began to seem like an expensive luxury.
In December 1959, the entire program was cut back to a single prototype. This wasn't the last word on the matter, though, since big weapons procurement efforts acquire a momentum of their own, and by mid-1960 funding for the B-70 program had been restored to a level adequate for as many as a dozen of the bombers.
The logic working against the concept still held true, unfortunately, and had been aggravated on 1 May 1960, when an American Lockheed U-2 spy plane was shot down over the Soviet Union by an SA-2 surface-to-air missile (SAM). Not only was the B-70 redundant in the face of the emerging US ICBM force, but improved SAM defenses meant that its high speed, high altitude flight did not offer the same protection against SAMs that it would have against manned interceptors.
On 1 March 1961, US President John F. Kennedy announced that the B-70 program was to be scaled back once more. Three aircraft would be completed, including two "XB-70" flight test prototypes and one "YB-70" operational prototype.
The two XB-70s were to be flight research aircraft only, most of the combat-related avionics, such as the bombing-navigation system, were deleted, and the bombardier and navigator positions were deleted as well, leaving provisions only for pilot and copilot. The US National Aeronautics & Space Administration (NASA) would collaborate with the USAF on the flight tests.
The YB-70 was to have full combat systems. The idea was that it would be useful as a hedge against changing conditions to retain the option of putting the B-70 into production after all. However, the expense and the continuously dwindling logic of fielding the B-70 meant that the YB-70A was never built.
* The first XB-70 was rolled out at North American's Palmdale, California, facility on 11 May 1964. By this time the type had been named the "Valkyrie", and the initial prototype was designated "Air Vehicle 1 (AV/1)", with tail number 20001. AV/1 performed its first flight on 21 September 1964. After a number of teething problems, AV/1 punched through Mach 1 for the first time on 12 October 1964.
Flight tests of AV/1 continued into 1965, with the aircraft demonstrating sustained supersonic flight at speeds of Mach 1.4 to above Mach 2. On its 12th flight, on 7 May 1965, while cruising at Mach 2.58, a piece of the wing broke away and shut down four of the engines. The aircraft managed to make it back to the runway, but all six engines had to be replaced.
* By the summer of 1965, AV/2, with tail number 20207, had been rolled out and was ready to fly. There would be no AV/3, as the third XB-70 had been cancelled even before the initial flight of AV/1. AV/2 took to the air on 17 July 1965, and began its own series of supersonic flight tests. Tests continued with both XB-70s. On 14 October 1965, AV/1 made a short dash through Mach 3 at 21 kilometers altitude, but lost a small chunk of her outer wing. AV/1 was never flown faster than Mach 2.5 again. There were similar concerns that AV/2 might not be robust enough for Mach 3 flight, either. Flight tests were planned so that the aircraft would be run at Mach 2.8 or Mach 2.9 for an extended time to thermally condition the aircraft for dashes above Mach 3.
Mach 3 flight imposes a severe thermal burden on an aircraft. Heat buildup rises drastically with increases in speed at high Mach, and is far more a limiting factor to high-speed flight than engine power. The XB-70 was an extremely "clean" aircraft, which minimized heat buildup, but the nose and other leading parts of the aircraft did rise to 330 degrees Celsius (625 degrees Fahrenheit), while the rest of the aircraft remained at 232 degrees Celsius (450 degrees Fahrenheit).
Airframe cooling was provided by an ingenious, if somewhat hair-raising, arrangement of the fuel tanks that allowed the fuel to soak up the heat from the airframe. The hot fuel was bled off to the engines, conveniently preheated to improve engine performance. However, as the fuel was bled off, the space evacuated had to be replaced with inert nitrogen gas, since if any appreciable amount of oxygen leaked in, the aircraft would explode immediately.
In any case, tests continued, with AV/2 pushing the envelope up to and past the Mach 3 mark. It provided data relevant to the supersonic transport (SST) designs then being considered, in particular showing that the sonic booms caused by such an aircraft would be unacceptable over populated areas.
Since the two XB-70s were prototypes, and very big and complicated prototypes at that, the test flights often suffered from various system failures. On her 37th flight in March 1966, AV/1 almost came to grief when both hydraulic systems failed and the landing gear didn't deploy correctly. The aircraft managed to make a controlled landing at Edwards, though the landing roll was almost 4.8 kilometers (3 miles) and the aircraft went through a slow 110-degree turn on the ground.
AV/2 got into a similar jam on 30 April 1966, when various systems failed and the nose gear wouldn't go down. Trying two touch-and-go landings on the main gear to pop the nose gear down didn't work, and landing with the nose gear up would be certain disaster.
This left only the option of bailing out and let AV/2 crash, but the aircraft had plenty of fuel, and so the aircraft circled for a few hours at low speed while engineers on the ground tried to figure out what to do. Finally, the engineers suggested that bypassing a circuit breaker in a backup electrical system would do the job. Copilot Joe Cotton improvised a jumper with a paper clip, and the nose gear went down.
Despite the glitches, progress was good. On 19 May 1966, AV/2 flew at Mach 3 for a sustained 33 minutes. By this time, Phase 1 flight tests were nearing completion. Phase 2 would follow, with NASA becoming increasingly involved in the test flights.
* Then, on 8 June 1966, AV/2 participated in a photo shoot, flying with four other aircraft powered by General Electric engines so GE photographers could take promotional pictures.
One of the aircraft was an F-104 Starfighter, flown by well-known test pilot Joe Walker. He was positioned off the XB-70's right wingtip when the photo shoot ended. He broke formation, and the turbulence off the big aircraft's wingtips spun the Starfighter around, causing it to collide with the XB-70. The Starfighter tore off both of the XB-70's vertical stabilizers and part of its left wing, then exploded in a fireball. Walker was killed immediately.
Pilot Al White and copilot Carl Cross heard the impact, but everything seemed to be OK for the moment. Then the aircraft went into two slow rolls and broke into a spin. White managed to eject, but Cross went down with the aircraft as it slammed into the ground a few kilometers north of Barstow, California.
* Some modifications were made to the surviving AV/1 to eliminate a few of the problems discovered during her test flights, but since AV/1 was redlined to Mach 2.5, she was not a satisfactory replacement for supersonic tests, and the USAF dropped out of the test program after a few more flights. NASA conducted 33 more test flights until 4 February 1969, when AV/1 was flown to Wright Field near Dayton, Ohio and placed in the USAF Museum, where she resides today.
* One of the interesting footnotes to the XB-70 program was that technology developed for the XB-70 was to also be applied to a huge long-range interceptor / escort fighter named the "F-108A Rapier". The F-108 had the sole distinction of being the last fighter project worked on by North American.
The Rapier project was initiated by the USAF in October 1955, originally under the designation of "Long Range Interceptor / Experimental (LIRX)", leading to the award of a contract to North American in June 1957 for two F-108A prototypes. The F-108A would have Mach 3 performance and range adequate to intercept Soviet bombers flying over the North Pole to attack North America, and was also considered as an escort for the B-70. The Air Force hoped to have the F-108A in service by 1963, and contemplated buying hundreds of them.
The F-108A was to be a two-seat aircraft, powered by twin J93 engines and armed with advanced Falcon missiles. It evolved through a number of configurations, converging on a design implemented in a mock-up unveiled in January 1959. The type received the name "Rapier" in May of that year.
As the F-108A was conceived at that time, it looked something like the fuselage of the North American "Vigilante" carrier-based bomber, then in flight test, mated to a high-mounted delta wing with drooping wingtip panels, featuring a single tall vertical tailplane on the end of the fuselage and a smaller vertical tailplane under the middle of each wing.
The crew were to set in tandem seats fitted with ejection capsules for
high-speed escape. The Rapier was to be armed with three GAR-9 advanced
Falcon long-range air-to-air missiles, directed by the aircraft's powerful
AN/ASG-18 radar.
NORTH AMERICAN F-108A RAPIER (ESTIMATED SPECIFICATIONS):
_____________________ _________________ _______________________
spec metric english
_____________________ _________________ _______________________
wingspan 17.5 meters 57 feet 5 inches
length 27.2 meters 89 feet 2 inches
height 6.73 meters 22 feet 1 inch
empty weight 23,000 kilograms 51,000 pounds
max loaded weight 46,700 kilograms 103,000 pounds
maximum speed 3,200 KPH 2,000 MPH / 1,740 KT
service ceiling 23,000 meters 75,000 feet
ferry range 4,000 kilometers 2,500 MI / 2,175 NMI
_____________________ _________________ _______________________
The Rapier program was cancelled in September 1959. The GAR-9 (later AIM-47A) missile and the AN/ASG-18 were later used with the Lockheed YF-12A, which was essentially an experimental interceptor version of the famed Lockheed "SR-71 Blackbird" reconnaissance aircraft. The YF-12A never got out of the prototype stage.
* I have seen the surviving XB-70 at the USAF Museum at Wright-Patterson AFB. I recollect an old article that said this aircraft cost ten times its weight in gold. With variations in gold prices and the flexibility that can be applied to accounting schemes, this is not exactly a precise statement, but it does give some indication of exactly how much a billion or two dollars amounts to.
I didn't have access to the XB-70's cockpit, but it would likely be startling: a sleek, futuristic aircraft with a dashboard that looked like something out of a steam plant.
* The main source for this document is a thorough web page titled "001 -- Flight Of The Valkyrie", by a fellow who calls himself "Thumper", AKA Steve Levin. This site includes lots of fine pictures, and even general-layout bluelines. Details for the F-108A Rapier were mostly obtained from a nice web page by aviation enthusiast Joe Baugher.
The materials available on the XB-70 and the Rapier are rather sketchy and sometimes implausible. One of the particularly amusing things I found was that sources on the F-108A give weights and performance specifications from three to even five significant figures, which is a bit rich for a machine that was never actually built, let alone flew.
Formally speaking, it appears that the XB-70 was actually the "XB-70A", but as there never was an "XB-70B", I have stayed with the "XB-70" usage.
* Revision history:
v1.0 / 15 mar 97 / gvg
v1.1 / 01 jul 99 / gvg / Minor cosmetic update.
v1.2.0 / 01 dec 01 / gvg / Cosmetic update, added details on F-108A.