XB-70 Early Flight-Testing Experience

A byproduct of this and other incidents was that Ship 1 was eventually limited to Mach 2.5 because of flight safety concerns of the skin shed­ding. But Ship 2 made its first flight July 17, 1965, and it had numerous improvements. Skin bonding had been improved, an automated air inlet control system had been installed, wing dihedral had been increased to 5 degrees to improve lateral directional stability, and fuel tank No. 5 could now be filled. NASA planned to use Ship 2 for its research program; an extensive instrumentation package recording over 1,000 parameters such as temperature, pressure, and accelerations was installed in the weapons bay for use when NASA took over the direction of the flight – test program. Ship 2 still had some of the gremlins that seemed to haunt the XB-70, mainly connected to the complex landing gear. Flight 37 on AV-2 resulted in the pilots having to do some in-flight maintenance when the nose gear door position prevented proper retraction or extension of the nose gear. The activity was widely advertised as the pilot using "a paperclip” to short an electrical circuit to allow exten­sion (actually, there were no paperclips on board; USAF pilot Joseph Cotton fashioned the device from a wire on his oxygen mask). But AV-2 showed that the high-speed skin-shedding problem had indeed been solved. Beginning in March 1966, AV-2 routinely spent 50 minutes to 1 hour at speeds from Mach 2.5 to Mach 2.9. And on May 19, AV-2 reached the (contractual) holy grail of 32 minutes at Mach 3 (actually up to 3.06). Skin stagnation temperature was over 600 °F. With accom­plishment of that goal, NASA moved to put a new pilot in the program.

NASA X-15 veteran test pilot Joe Walker had been undergoing delta wing training and preparation to fly the B-70 as the program moved to the second stage of flight test. National Sonic Boom Program (NSBP) tests were flown June 6, 1966, to prepare for the official change over to NASA on June 15, but on June 8, disaster struck, dramatically chang­ing the program.

That day, AV-2 took off on a planned flight-test mission that would include a photo session at the end of the sortie with a number of other aircraft powered by engines made by General Electric.[1083] One of the air­craft was a Lockheed F-104N Starfighter flown by Joe Walker, who was observing the mission as he prepared to fly the B-70 on the next sor­tie. During the photo shoot, which required close formation flight, his F-104 was seen to fly within 30-50 feet of the Valkyrie’s right wingtip, which had been lowered to the 20-degree intermediate droop position. As the photo session ended, the F-104 tail struck the XB-70 wingtip, causing the F-104 to roll violently to the left and pass inverted over the top of the bomber, shearing off most of the twin vertical tails and caus­ing the Starfighter to erupt in flames, killing Walker. The XB-70 subse­quently entered an inverted spin, from which recovery was impossible. Company test pilot Joe Cotton ejected using the complex encapsulated ejection seat and survived; USAF copilot Carl Cross did not eject and died in the ensuing crash. The accident was not related to the Valkyrie design itself; nevertheless, the loss of the improved Ship 2 and its com­prehensive instrumentation package meant that AV-1 would now have to become the NASA research aircraft. A new instrumentation package was installed in AV-1, but the Mach 2.5 speed limit imposed on AV-1 for the skin shedding problem and the workload-intensive manual inlets meant the program orientation could be less of an analog for the national SST program, which was now approaching the awarding of contracts for an SST with speeds of Mach 2.7 to 3.