NASA’s Valkyrie Supersonic Cruise Flight-Test Program

Although the XB-70 test program was only budgeted for 180 hours, Air Force Category 1 testing with the contractor took first priority. That test­ing included verification of basic airworthiness and the achievement of the contractually required speed of Mach 3 for an extended cruise period. This proved to be harder than was thought, as the first XB-70 turned out to be almost a jinxed aircraft, as prototypes often are.

It was not until the 17th flight, 13 months after 1st flight, that Mach 3 was attained. Earlier flights had been plagued by landing gear problems, in-flight shutdowns of the new GE J93 engines (the most powerful in the world, at 30,000 pounds of thrust each in afterburner), and, most seriously, in-flight shedding of pieces of the stainless steel skin. The stainless steel honeycomb covering much of the wing had proven to be difficult to fabricate, requiring a brazing technique in an inert atmo­sphere to attach the skins. This process unfortunately resulted in numer­ous pinholes in the skin welds, which would allow the nitrogen inerting atmosphere required for fuel tanks with fuel heated to over 300 °F to leak away. Correcting this problem delayed the first aircraft by almost a year. The No. 5 fuel tank could never be sealed and was flown empty, further shortening the duration of test sorties on the two prototype air­craft, which had no aerial refueling capability.[1077]

Подпись: 10Aside from the mechanical difficulties that often shortened test sorties, the design features providing supersonic cruise worked well. The two-pilot XB-70 was initially the heaviest airplane in the world, at 500,000-pound takeoff weight, as well as designed to be the fastest. It was stable, maneuverable, and, aside from the unusually high attitude of the cockpit on takeoff and landing, easy to fly. The folding wingtips (each the size of a B-58 wing) worked flawlessly. The propulsion system of inlets and turbojets, when properly functioning, provided the thrust to reach Mach 3, and handling qualities at that speed were generally satisfactory, although the high speed meant that small pitch changes produced large changes in vertical velocity; it was difficult to maintain level flight manually. Mach 3 cruise in a large SST-size airplane seemed to be technologically achievable.[1078]

The inlets for the six engines were another story for complexity, criticality, and pilot workload. An air inlet control system used moving ramps and doors to control the geometry of the inlet to position shock waves in the inlet above flight speed of Mach 1.6.[1079] The final shock wave in the inlet was a strong normal shock in the narrow "throat,” where the airflow became subsonic downstream of the shock. Proper position­ing of the normal shock was vital; if downstream pressure was too high,
the normal shock might "pop out” of the inlet, losing the inlet pressure buildup, which actually provided net thrust to the airplane, and caus­ing compressor stalls in the turbojet, as it now received air that was still supersonic. This was known as an inlet " unstart” and usually was cor­rected by opening bypass doors in the inlet to relieve the pressure and resetting the inlet geometry to allow the normal shock to resume its cor­rect position. Unstarts usually were announced by a loud bang, a rapid yaw in the direction of the inlet that had unstarted because of the lack of thrust, and often by an unstart of the other inlet because of airflow disturbance caused by the yaw. Pilots considered unstarts to be exciting (" breathtaking,” as NAA test pilot Al White described it), with motion varying from mild to severe, depending on flight conditions, but not par­ticularly dangerous and usually easily corrected.[1080] Although the inlet control system was designed to be automatic, for the first XB-70 (also known as "Ship 1”), the copilot became the flight engineer and manu­ally manipulated the ramps and doors as a function of Mach number and normal shock position indicator. There were two inlets on the air­craft, with each feeding three engines. There had been some concern that problems with one engine might spread to the other two fed by the same inlet, but this did not seem to usually be the case. One excep­tion was on the 12th flight, on May 7, 1965, when a piece of stainless steel skin went down the right inlet at Mach 2.6, damaging all 3 engines, one seriously. The mismanagement of the right inlet doors, because of time pressure and lack of knowledge of the nature of the emergency, led to inlet "duct buzz” pressure fluctuations caused by shock oscillation. This vibration at 2% cycles per second was near the duct’s resonant fre­quency, which could cause destruction of the duct. The vibration also fed into the highly flexible vehicle fuselage. This in turn led to the pilot reverting to turning the yaw dampers off, with subsequent development of a divergent Dutch roll oscillation. All three engines on the right side were eventually shut down. Fortunately, the flight control anomalies were cleared up, and the pilot performed a successful "3 and % engine” landing on the Rogers dry lakebed, touching down at 215 knots. This 5-minute inlet emergency generated a 33-page analytical report and presented some cautionary notes. The author commented in his clos­ing that: "The seriousness of the interaction of the inlet conditions with
vehicle performance and handling characteristics tends to be accentu­ated for high-supersonic aircraft. Bypass-door settings are critical on mixed-compression inlets to maintain efficient inlet conditions.”[1081] This observation would prove even more relevant for the Mach 3 Blackbird aircraft that followed the XB-70 in NASA supersonic cruise research. Test crews soon discovered that, as Blackbird researchers rue­fully noted, "Around Mach 3, when things go wrong, they also get worse at a rate of Mach 3.”[1082] Crews who flew the secret twin-engine Blackbird often experienced this fact of life, sometimes with a less happy ending.