XB-70 Supersonic Cruise Program Takes to the Air

Despite the AV-1 aircraft limitations, the XB-70 test program proceeded, now with NASA directing the effort with USAF support. Eleven flights were flown under NASA direction as Phase II of the original XB-70 planned flight-test program, ending January 31, 1967. Nine of the
flights were primarily dedicated to the NSBP. As the XB-70 was the only aircraft in the world with the speed, altitude capability, and weight of the U. S. SST, priority was given to aspects that supported that program. The sonic boom promised to be a factor that was drastically different from current jet airliner operations and one whose initial impact was underrated. It was thought that a rapid climb to high altitude before going supersonic would muffle the initial strong normal shock; once at high altitude, even at higher Mach numbers, the boom would be sufficiently attenuated by distance from the ground and the shock wave inclination "lay back” as Mach number increased, to not be a disturbance to ground observers. This proved not to be the case, as over­flights by B-58s and the XB-70 proved. Another case study in this vol­ume provides details on sonic boom research by NASA. Overpressure measurements on the ground during XB-70 overflights as well as the observer questionnaires and measurements in instrumented homes constructed at Edwards AFB indicated that overland supersonic cruise would produce unacceptable annoyance to the public on the ground. Overpressure beneath the flight path reached values of 1.5 to 2 pounds per square foot. A lower limit goal of not more than 0.5 pounds per foot to preclude ground disturbance seemed unachievable with current designs and technology.[1084]

Supersonic cruise test missions proved challenging for pilots and flight-test engineers alike. Ideally, the test conductor on the ground would be in constant contact with the test pilots to assist in most efficient use of test time. But with an aircraft traveling 25-30 miles per minute, the aircraft rapidly disappeared over the horizon from test mission control. Fortunately, NASA had installed a 450-mile "high range” extending to Utah, with additional tracking radars, telemetry receivers, and radio relays for the hypersonic X-15 research rocket plane. The X-15 was typically released from the B-52 at the north end of the range and was back on the ground within 15 minutes. The high range provided extended mission command and control and data collection but was not optimized for the missions flown by the XB-70 and YF-12.

The XB-70 ground track presented a different problem for mission planners. The author flew the SR-71 Blackbird from Southern California for 5 years and faced the same problems in establishing a test ground track. The test aircraft would take over 200 miles to get to the test cruise speed and altitude. Then it would remain at test conditions, collecting data for 30-40 minutes. It then required an additional 200-250 miles to slow to "normal” subsonic flight. Ground tracks had to be estab­lished that would provide data collection legs while flying straight or performing planned turning maneuvers, and avoiding areas that would be sensitive to the increasingly contentious sonic booms. Examples of the areas included built-up cities and towns; the "avoidance radius” was generally 30 nautical miles. Less obvious areas included mink farms and large poultry ranches, as unexplained sudden loud noises could apparently interfere with breeding habits and egg-laying practices. The Western United States fortunately had a considerably lower population density than the area east of the Mississippi River, and test tracks could be established on a generally north-south orientation.

The presence of Canada to the north and Mexico to the south, not to mention the densely populated Los Angeles/San Diego corridor and the "island” of Las Vegas, set further bounding limits. Planning a test profile that accounted for the limits/avoidance areas could be a challenge, as the turn radius of a Mach 3 aircraft at 30 degrees of bank was over 65 nautical miles. Experience and the sonic boom research showed that a sonic boom laid down by a turning or descending super­sonic aircraft would "focus” the boom on the ground, decreasing the area affected but increasing the overpressure on the ground within a smaller region. Because planning ground tracks was so complicated and arduous, once a track was established, it tended to be used numerous times. This in turn increased the frequency of residents being subjected to sudden loud noises, and complaints often appeared only after a track had been used several times. The USAF 9th Reconnaissance Wing operating the Mach 3+ SR-71 at Beale Air Force Base near Sacramento, CA, had the same problem as NASA flight-testing for developing training routes (but without the constraints of maintaining telemetry contact with a test control), and it soon discovered another category for avoidance areas: congressional complaints relayed from the Office of the Secretary of the Air Force.

For the limited XB-70 test program, a ground track was established that remained within radio and telemetry range of Edwards. As a result,
the aircraft at high Mach would only fly straight and level for 20 min­utes at best, requiring careful sequencing of the test points. The profile included California, Nevada, and Utah.[1085]

This planning experience was a forerunner of what problems a fleet of Supersonic Transports would face on overland long-distance flights if they used their design speed. A factor to be overcome in supersonic cruise flight test, it would be critical to a supersonic airliner. Pending development of sonic boom reduction for an aircraft, the impact of off – design-speed operation over land would have to be factored into SST designs. This would affect both range performance and economics.

The flight tests conducted on the XB-70 missions collected data on many areas besides sonic boom impact. The research data were gen­erally focused on areas that were a byproduct of the aeronautical tech­nology inherent in a large airplane designed to go very fast for a long distance with a large payload. An instrumentation package was devel­oped to record research data.[1086] Later, boundary layer rakes were installed to measure boundary layer growth on the long fuselage at high Mach at

70,0 feet altitude; this would influence the drag and hence the range performance of a design. The long flexible fuselage of the XB-70 pro­duced some interesting aeroelastic effects when in turbulence, not to mention taxing over a rough taxiway, similar to the pilot being on a div­ing board. Two 8-inch exciter vane "miniature canards” were mounted near the cockpit as part of the Identically Located Acceleration and Force (ILAF) experiment for the final XB-70 flight-test sorties. These vanes could be programmed to oscillate to induce frequencies in the fuselage to explore its response. Additionally, frequencies could be pro­duced to cancel accelerations induced by turbulence or gusts, leading to a smoother ride for pilots and ultimately SST passengers. This sys­tem was demonstrated to be effective.[1087] A similar system was employed in the Rockwell B-1 Lancer bomber, the Air Force bomber eventually built instead of the B-70.

Inlet performance would have a critical effect on the specific fuel consumption performance, which had a direct effect on range achieved. In addition to collecting inlet data on all supersonic cruise sorties,
numerous test sorties involved investigating inlet unstarts deliberately induced by pilot action, as well as the "unplanned” events. This was important for future aircraft, as the Valkyrie used a two-dimensional (rectangular) inlet with mixed external (to the inlet)/internal compres­sion, with one inlet feeding multiple engines. As a comparison, the A-12/ SR-71 used an axisymmetric (round) inlet, also with external/internal compression feeding a single engine. There was a considerable debate in the propulsion community in general and the Boeing and Lockheed competitive SST designers in particular as to which configuration was better. Theoretical values of pressure recovery had been tested in propul­sion installations in wind tunnels, but the XB-70 presented an opportu­nity to collect data and verify wind tunnel results in extended supersonic free-flight operations, including "off-design” conditions during unstart operations. These data were also important as an operational SST fac­tor, as inlet unstarts were disconcerting to pilots, not to mention pro­spective passengers.

Traditional aircraft flight-test data on performance, stability, con­trol, and handling qualities were collected, although AV-1 was limited to Mach 2.5 and eventually Mach 2.6. Data to Mach 3 were sometimes also available from AV-2 flights. As USAF-NASA test pilot Fitzhugh Fulton reported in a paper presented to the Society of Automotive Engineers (SAE) in 1968 in Anaheim, CA, on test results as applied to SST opera­tions, the XB-70 flew well, although there were numerous deficiencies that would have to be corrected.[1088] The airplane’s large size and delta wing high-incidence landing attitude required pilot adjustments in take­off, approach, and landing techniques but nothing extraordinary. High Mach cruise was controllable, but the lack of an autopilot in the XB-70 and the need of the pilot to "hand-fly” the airplane brought out another pilot interface problem; at a speed of nearly 3,000 feet per second, a change in pitch attitude of only 1 degree would produce a healthy climb or descent rate of 3,000 feet per minute (50 feet per second). Maintaining a precise altitude was difficult. Various expanded instrument displays were used to assist the task, but the inherent lag in Pitot-static instru­ments relying on measuring tiny pressure differentials (outside static pressure approximately 0.5 pounds per square inch [psi]) to indicate altitude change meant the pilot was often playing catchup.

High Mach cruise at 70,000 feet may have become routine, but it required much more careful flight planning than do contemporary sub­sonic jet operations. The high fuel flows at high Mach numbers meant that fuel reserves were critical in the event of unplanned excursions in flight. Weather forecasts at the extreme altitudes were important, as temperature differences at cruise had a disproportionate influence on fuel flows at a given Mach and altitude; 10 °F hotter than a standard day at altitude could reduce range, requiring an additional fuel stop, unless it was factored into the flight plan. (Early jet operations over the North Atlantic had similar problems; better weather forecasts and larger aircraft with larger fuel reserves rectified this within several years.) Supersonic cruise platforms traveling at 25-30 miles per minute had an additional problem. Although the atmosphere is generally portrayed as a "layer cake,” pilots in the XB-70 and Mach 3 Blackbird discovered it was more like a "carrot cake,” as there were localized regions of hot and cold air that were quickly traversed by high Mach aircraft This could lead to range performance concerns and autopilot instabilities in Mach hold because of the temperature changes encountered. The increase in stagnation temperatures on a hot day could require the aircraft to slow because of engine compressor inlet temperature (CIT) limitations, fur­ther degrading range performance.

Fuel criticality and the over 200 miles required to achieve and descend from the optimum cruise conditions meant that the SST could brook no air traffic control delays, so merging SST operations with sub­sonic traffic would stress traffic flow into SST airports. Similar concerns about subsonic jet airliner traffic in the mid-1950s resulted in revamp­ing the ATC system to provide nationwide radar coverage and better automate traffic handoffs. To gather contemporary data on this problem for SST concerns, NASA test pilots flew a Mach 2 North American A-5A (former A3J-1) Vigilante on supersonic entry profiles into Los Angeles International Airport. The limited test program flying into Los Angeles showed that the piloting task was easy and that the ATC system was capable of integrating the supersonic aircraft into the subsonic flow.[1089]

One result mentioned in test pilot Fulton’s paper had serious impli­cations not only for the SST but also supersonic research. The XB-70
had been designed using the latest NASA theories (compression lift) and NASA wind tunnels. Nevertheless, the XB-70 as flown was deficient in achieving its design range by approximately 25 percent. What was the cause of the deficiency? Some theorized the thermal expansion in such a large aircraft at cruise Mach, unaccounted for in the wind tun­nels, increased the size of the aircraft to the point where the reference areas for the theoretical calculations were incorrect. Others thought the flexibility of the large aircraft was unaccounted for in the wind tunnel model configuration. Another possibility was that the skin friction drag on the large surface area at high Mach was higher than estimated. Yet another was that the compression lift assumption of up to 30-percent enhancement of lift at cruise speed was incorrect.

The limited duration of the XB-70 test program meant that further flight tests could not be flown to investigate the discrepancy. Flight – test engineer William Schweikhard proposed a reverse investigation. He structured a program that would use specific flight-test conditions from the program and duplicate them in wind tunnels using high- fidelity models of the XB-70 built to represent the configuration of the aircraft as it was estimated to exist at Mach 2.5. The flight-test data would thus serve as a truth source for the tunnel results.[1090] This comparison showed good correlation between the flight-test data and the wind tun­nel, with the exception of a 20-percent-too-low transonic drag estimate, mainly caused by an incorrect estimate of the control surface deflec­tion necessary to trim the aircraft at transonic speeds. It was doubtful that that would account for the range discrepancy, because the aircraft spent little time at that speed.

The NASA test program with the XB-70 extended from June 16, 1966, to January 22, 1969, with the final flight being a subsonic flight to the Air Force Museum at Wright-Patterson Air Force Base in Dayton, OH. Thirty-four sorties were flown during the program. The original funding agreement with the USAF to provide B-58 chase support and mainte­nance was due to expire at the end of 1968, and the XB-70 would require extensive depot level maintenance as envisioned at the end of the 180- hour test program. NASA research program goals had essentially been
reached, and because of the high costs of operating a one-aircraft fleet, the program was not extended. The X-15 program was also terminated at this time.

The legacy of the XB-70 program was in the archived mountains of data and the almost 100 technical reports written using that data. As late as 1992, the sonic boom test data generated in the NSBP flights were transferred to modern digital data files for use by researchers of high-speed transports.[1091] But it was fitting that the XB-70’s final super­sonic test sortie included collecting ozone data at high altitudes. The United States SST program that would use supersonic cruise research data was about to encounter something that the engineers had not con­sidered: the increasing interest of both decision makers and the public in the social consequences of high technology, exemplified by the rise of the modern environmental movement. This would have an impact on the direction of NASA supersonic cruise research. Never again in the 20th century would such a large aircraft fly as fast as the Valkyrie.