North American XB-70 Valkyrie: Once Upon A Mach 3 Bomber
Built by North American as a supersonic strategic bomber for the USAF’s Strategic Air Command (SAC) in the latter 50s, the XB-70 had a short but promising role as a nuclear armed deep penetration bomber during the Cold War. It was relatively larger than any bomber conceptualized on the drawing board but its size didn’t discouraged its designers to push it to the limit of reaching three times the speed of sound. To achieve this six 133.38 kN (32,651 lb-thrust) General Electric YJ93-GE-3 afterburning turbojets were installed at the lower end of the fuselage side by side each other with two intakes installed under the wings middle half of the fuselage’s length. Two prototypes were built owing to funding limitations and first flight was made on 21 September 1964.
Heat generated at high speed compelled designers to build the aircraft with special materials made of honeycomb stainless steel and titanium along high temperature areas during supersonic speeds which could elevate temperatures along the aircraft’s surface from 450°F to 650°F. A trimmable canard was placed behind the cockpit and elevons on the wing’s trailing edge work in coordination to control pitch. The wing tips were allowed to droop midflight at high speeds to lessen the surface area of rear wings and counter the nose-down effect.
The reason for the bomber’s existence was conceived as a deterrent against Soviet supersonic fighters. Notably, the Soviet Mig-25 Foxbat capable of reaching Mach 3 made its first flight on 6 March 1964 six months ahead before even the first XB-70 prototype took off. The Mig-25 in turn prompted the USAF to design the McDonnell Douglas F-15 Eagle which was introduced six years after the Mig-25 was put in service. Those were the days when intelligence reports work in close coordination with research and development deciding the employment of new technology in the field to maintain the balance. As for the XB-70, the fielding of the first successfully tested surface-to-air-missiles (SAMs) and Inter-Continental Ballistic Missiles (ICBMs) dimmed its future more so considering its size laid it open to enemy detection with its radar and infra-red signatures too massive to be hidden. The XB-70 flights however continued on research purposes despite the program’s cancellation as early as 1961 but the tragic crash of a prototype in 1966 on a publicity shot added negative feedback for further flights to continue.
On 8 June 1966, five aircraft utilizing General Electric engines took off consisting of an F-4 Phantom, F-5 Freedom Fighter, T-38 Talon, and an F-104 Starfighter flying alongside an XB-70 at the middle as per request of General Electric. Joe Walker flying a NASA F-104 Starfighter miscalculated flying closer on the right side wing of the huge bomber (probably looking at the bomber’s fuselage in reference taking for granted the extended wings with the drooped wing tips) too late to make precautionary measures when the wake vortex generated from the slightly folded wing tips caused it to roll over into contact and ripping the vertical stabilizer and the bomber’s wing tip. The F-104 exploded as the XB-70 flew for seconds before entering a spin and crashing into the desert. Only Al White the XB-70’s command pilot managed to eject and survived while his co-pilot and the F-104’s pilot perished on the crash.
The only remaining XB-70 prototype made its last flight to the USAF museum on February 1969 where it remains to this day. It remained one of only three aircraft capable of reaching Mach 3 along with a Soviet Mig-25 Foxbat (Mach 3.2) and the SR-71 Blackbird (Mach 3.5+).
North American XB-70 Performance Specs:
Max. Speed: 3,220 km/h (Mach 3.08)
Range: 13,300 km (8,283 mi) unrefueled
Service Ceiling: 22,550 m (73,982 ft)
Weights: Max. Take Off weight at 250,000 kg (550,000 lbs)
Aviation Factfile: Concept Aircraft by Jim Winchester (pp. 186-187)