RogerBW's Blog

North American XB-70 Valkyrie 15 April 2014

Yeah, I pretty much have to do this one, don't I? The Valkyrie was to be a Mach 3 high-altitude nuclear bomber.

It was pretty much the last hurrah of the high-and-fast bomber school, intended to fly at 70,000 feet at Mach 3 with something like a 50,000lb payload. The first stages of design began in 1954, and the initial requirement was more or less for something with the speed of the B-58 Hustler and the range of the B-36 Peacemaker. Various engine possibilities were considered, including an open-cycle nuclear ramjet, but finally the Air Force settled on zip fuel.

Zip fuel is an historical footnote these days. It was a way to get more specific thrust, i.e. thrust per pound of fuel burned, out of those notoriously thirsty early turbojets. How do you do that? Why, dope the fuel with boranes, hydro-boron compounds with a higher power density than hydrocarbon jet fuel while being conveniently liquid at typical fuelling temperatures. Boranes can't be used exclusively; they're expensive, and prone to spontaneous ignition in air. But that actually isn't the problem with zip fuels. The combustion products include a fair array of toxic boron oxides, but that isn't the problem either. Nor even is it the black smoke trail behind an aircraft using it, even more than the smoke from conventionally-fuelled early turbojets. The problem is that boron carbide, another of the combustion products, melts at 2763C and boils at 3500C. Turbojet exhaust temperatures were hot enough to keep it slightly sticky, but once the engine cooled after flight it froze itself on, narrowing pipes and compromising gas dynamics. It's the third-hardest material known, after boron nitride (also sometimes present) and diamond. Zip fuel was fine for an aircraft operating permanently in afterburner, which would keep the temperature well above its melting point, but the specific thrust increases were never realised and research was stopped in 1959.

Early designs from North American included floating wing panels to improve the wing aspect ratio; they would be filled with fuel and jettisoned when empty. Curtis LeMay said "This is not an airplane, it's a three-ship formation." But research continued, and the long thin delta (familiar to my generation as the shape of Concorde) became the standard for supersonic flight.

The final design took advantage of this, but applied a few extra tricks too. In particular, the layout of the engines and intakes on the underside of the aircraft was designed to use the supersonic shock wave as a source of high-pressure air and thus additional lift. Also, the wingtips would fold down at high speed, increasing that compression lift, and helping to compensate for the rearward shift in centre of lift that occurs in all supersonic aircraft.

The six engines were YJ93s, an enlarged derivative of the reliable J79. They ran on JP-6, a high-temperature fuel variant, even after the zip fuel programme was discontinued.

But, although two prototypes were built, technology was overtaking the design. Surface-to-air missiles could be held ready to go for days, with none of the pilot fatigue problems found with a manned interceptor. Intercept courses didn't need to be calculated, since there would probably be a missile in the path of the bomber. In 1960, a U-2 observation plane was famously shot down over the USSR, making it clear that altitude alone was no longer sufficient.

Doctrine shifted from high-and-fast to low-and-fast, but the Valkyrie suffered even more than the B-58: it was desperately optimised for the high-altitude mission. Compared with a B-52, it had a smaller bomb bay, only slightly higher speed down low, and a shorter range when hugging the ground.

The B-70 became a political football, used as a symbol of America's military might even if it wouldn't work. Both sides in the 1960 presidential campaign pledged to continue development. In spite of fervent advocacy by Curtis LeMay, the project was cancelled in 1961.

However, that didn't stop construction of the prototypes, which first flew in 1964. They were used as high-altitude and supersonic research aircraft; one was lost to a mid-air collision during a photoshoot for General Electric in 1966, and the other was retired in 1968.

The Valkyrie is notable for its planned defensive weapon. Guns clearly weren't up to the job of shooting down incoming missiles and high-speed interceptors, so an anti-missile missile was designed. Codenamed Pye Wacket, the Lenticular Defense Missile was designed to be able to make very rapid changes of direction, largely achieved by giving it a lenticular and wedge-shaped airframe; directional control was by rocket thruster. The Valkyrie was the only platform expected to carry Pye Wacket, so when it was cancelled the programme was discontinued.

Another footnote is the planned counterpart aircraft, the XF-108 Rapier. This was to be the ultimate interceptor, a huge cranked-arrow delta wing flying at Mach 3 on two of the same YJ93 engines that the Valkyrie used. Its weapon would be three GAR-9 missiles, huge long-range beasts with on-board radar to seek out and destroy Soviet bombers. The Rapier was cancelled in 1959; the fire control system and GAR-9 were kept going briefly for the YF-12, the interceptor version of what would become the SR-71, but that was cancelled too. The GAR-9 eventually inspired the AIM-54 Phoenix.

  1. Posted by Owen Smith at 12:47pm on 15 April 2014

    Fuel boiling off is always a problem with high and fast aircraft, the frictional heating plus low pressue at altitude being a double whammy. Hence the JP-6 here and JP-7 fuel for the SR-71. Concorde used normal Jet A1, it achieved this by have a lower max speed at Mach 2 hence less frictional heating, and the fuel tanks were pressurised after take off to eliminate the altitude problem.

  2. Posted by RogerBW at 01:35pm on 15 April 2014

    Boiling off and/or spontaneously igniting. The XB-70 used a nitrogen blanket system (which meant it had to carry 700lb of nitrogen on board, to flush the fuel tanks as they drained and maintain positive pressure). I'm guessing Concorde pressurised with air. But in spite of that, the fuel mass was an important heat sink in the thermal balancing design.

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