NASA's Contributions to Aeronautics, Volume 2 by National Aeronautics & Space Administration. - HTML preview

CASE

10

NASA and Supersonic Cruise

William Flanagan

For an aircraft to attain supersonic cruise, or the capability to fly faster than sound for a significant portion of time, the designer must balance lift, drag, and thrust to achieve the performance requirements, which in turn will affect the weight. Although supersonic flight was achieved over 60 years ago, successful piloted supersonic cruise aircraft have been rare. NASA has been involved in developing the required technology for those rare designs, despite periodic shifting national priorities.

Case-10 Cover Image: NASA Dryden Flight Research Center’s SR-71A, DFRC Aircraft 844, banks away over the Sierra Nevada mountains after air refueling from a USAF tanker during a 1997 flight. NASA.

In the 1930s and early 1940s, investigation of flight at speeds faster than sound began to assume increasing importance, thanks initially to the “compressibility” problems encountered by rapidly rotating propeller tips but then to the dangerous trim changes and buffeting encountered by diving aircraft. Researchers at the National Advisory Committee for Aeronautics (NACA) began to focus on this new and troublesome area. The concept of Mach number (ratio of a body’s speed to the speed of sound in air at the body’s location) swiftly became a familiar term to researchers. At first, the subject seemed heavily theoretical. But then, with the increasing prospect of American involvement in the Second World War, NACA research had to shift to shorter-term objectives of improving American warplane performance, notably by reducing drag and refining the Agency’s symmetrical low-drag airfoil sections. But with the development of fighter aircraft with engines exhibiting 1,500 to 2,000 horsepower and capable of diving in excess of Mach 0.75, supersonic flight became an issue of paramount military importance. Fighter aircraft in steep power on-dives from combat altitudes over 25,000 feet could reach 450 mph, corresponding to Mach numbers over 0.7. Unusual flight characteristics could then manifest themselves, such as severe buffeting, uncommanded increasing dive angles, and unusually high stick forces.

The sleek, twin-engine, high-altitude Lockheed P-38 showed these characteristics early in the war, and a crash effort by the manufacturer aided by NACA showed that although the aircraft was not “supersonic,” i.e., flying faster than the speed of sound at its altitude, the airflow at the thickest part of the wing was at that speed, producing shock waves that were unaccounted for in the design of the flight control surfaces. The shock waves were a thin area of high pressure, where the supersonic airflow around the body began to slow toward its customary subsonic speed. This shock region increased drag on the vehicle considerably, as well as altered the lift distribution on the wing and control surfaces. An expedient fix, in the form of a dive flap to be activated by the pilot, was installed on the P-38, but the concept of a “critical Mach number” was introduced to the aviation industry: the aircraft flight speed at which supersonic flow could be present on the wing and fuselage. Newer high-speed, propeller-driven fighters, such as the P-51D with its thin laminar flow wing, had critical Mach numbers of 0.75, which allowed an adequate combat envelope, but the looming turbojet revolution removed the self-governing speed limit of reduced thrust because of supersonic propeller tips. Investigation of supersonic aircraft was no longer a theoretical exercise.[1]

Early Transonic and Supersonic Research Approaches

The NACA’s applied research was initially restricted to wind tunnel work. The wind tunnels had their own problems with supersonic flow, as shock waves formed and disturbed the flow, thus casting doubt on the model test results. This was especially true in the transonic regime, from Mach 0.8 to 1.2, at which the shock waves were the strongest as the supersonic flow slowed to subsonic in one single step; this was called a “normal” shock, referring to the 90-degree angle of the shock wave to the vehicle motion. Free air experiments were necessary to validate and improve wind tunnel results. John Stack at NACA Langley developed a slotted wind tunnel that promised to reduce some of the flow irregularities. The Collier Trophy was awarded for this accomplishment, but validation of the supersonic tunnel results was still lacking. Pending the development of higher-powered engines for full-scale in-flight experiments, initial experimentation included attaching small wing shapes to NACA P-51 Mustangs, which then performed high-speed dives to and beyond their critical Mach numbers, allowing seconds of transonic data collection. Heavy streamlined bomb shapes were released from NACA B-29s, the shapes going supersonic during their 30–45-second trajectories, sending pressure data to the ground via telemetry before impact.[2] Supersonic rocket boosters were fired from the NACA facility at Wallops Island, VA, carrying wind tunnel–sized models of wings and proposed aircraft configurations in order to gain research data, a test method that remained fruitful well into the 1960s. The NACA and the United States Air Force (USAF) formed a joint full-scale flight-test program of a supersonic rocket-powered airplane, the Bell XS-1 (subsequently redesignated the X-1), which was patterned after a supersonic 0.50-caliber machine gun projectile with thin wings and tail surfaces. The program culminated October 14, 1947, with the demonstration of a controllable aircraft that exceeded the speed of sound in level flight. The news media of the day hailed the breaking of the “sound barrier,” which would lead to ever-faster airplanes in the future. Speed records popularized in the press since the birth of aviation were “made to be broken”; now, the speed of sound was no longer the limit.

But the XS-1 flight in October was no more a practical solution to supersonic flight than the Wright brothers’ flights at Kitty Hawk in December 1903 were a director predecessor to transcontinental passenger flights. Rockets could produce the thrust necessary to overcome the drag of supersonic shock waves, but the thrust was of limited duration. Rocket motors of the era produced the greatest thrust per pound of engine, but they were dangerous and expensive, could not be throttled directly, and consumed a lot of fuel in a short time. Sustained supersonic flight would require a more fuel-efficient motor. The turbojet was an obvious choice, but in 1947, it was in its infancy and was relatively inefficient, being heavy and producing only (at most) several thousand pounds of static thrust. Military-sponsored research continued on improving the efficiency and the thrust levels, leading to the introduction of afterburners, which would increase thrust from 10–30 percent, but at the expense of fuel flows, which doubled to quadrupled that of the more normal subsonic cruise settings. The NACA and manufacturers looked at another form of jet propulsion, the ramjet, which did away with the complex rotating compressors and turbines and relied on forward speed of the vehicle to compress the airflow into an inlet/diffuser, where fuel would then be injected and combusted, with the exhaust nozzle further increasing the thrust.

Gathering the Data for Supersonic Airplane Design

NACA supersonic research after 1947 concentrated on the practical problems of designing supersonic airplanes. Basic transonic and low supersonic test data were collected in a series of experimental aircraft that did not suffer from the necessary compromises of operational military aircraft. The test programs were generally joint efforts with the Air Force and/or Navy, which needed the data in order to make reasonable decisions for future aircraft. The X-1 (USAF) and D-558-1 and D-558-2 (Navy) gathered research data on aerodynamics and stability and control in the transonic regime as well as flight Mach numbers to slightly above 2. The D-558-1 was a turbojet vehicle with a straight wing; as a result, although it had longer mission duration, it could not achieve supersonic flight and instead concentrated on the transonic regime. For supersonic flights, the research vehicles generally used rocket engines, with their corresponding short-duration data test points. Other experimental vehicles used configurations that were thought to be candidates for practical supersonic flight. The D-558-2 used a swept wing and was able to achieve Mach 2 on rocket power. The XF-92A explored the pure delta wing high-speed shape, the X-4 explored a swept wing that dispensed with horizontal tail surfaces, the X-5 configuration had a swept wing that could vary its sweep in flight, and the X-3 explored a futuristic shape with a long fuselage with a high fineness ratio combined with very low aspect ratio wings and a double-diamond cross section that was intended to reduce shock wave drag at supersonic speeds. The Bell X-2 was a NACA–USAF–sponsored rocket research aircraft with a swept wing intended to achieve Mach 3 flight.[3]

Valuable basic data were collected during these test programs applicable to development of practical supersonic aircraft, but sustained supersonic flight was not possible. The limited-thrust turbojets of the era limited the speeds of the aircraft to the transonic regime. The X-3 was intended to explore flight at Mach 2 and above, but its interim engines made that impossible; in a dive with afterburners, it could only reach Mach 1.2. The XF-92A delta wing showed promise for supersonic designs but could not go supersonic in level flight.[4] This was unfortunate, as the delta winged F-102—built by Convair, which also manufactured the XF-92—was unable to achieve its supersonic design speeds and required an extensive redesign. This redesign included the “area rule” concept developed by the NACA’s Richard Whitcomb.[5] The area rule principle, published in 1952, required a smooth variation in an aircraft’s cross-section profile from nose to tail to minimize high drag normal shock wave formation, at which the profile has discontinuities. Avoiding the discontinuities, notably where the wing joined the fuselage, resulted in the characteristic “Coke bottle” or “wasp waist” fuselage adjacent to the wing. This was noticeable in supersonic fighter designs of the late 1950s, which still suffered from engines of limited thrust, afterburner being necessary even for low supersonic flight with the resultant short range and limited duration. The rocket-powered swept wing X-2 Mach 3 test program was not productive, with only one flight to Mach 3, ending in loss of the aircraft and its pilot, Capt. Milburn “Mel” Apt.[6]

Feeling the “Need for Speed”: Military Requirements in the Atomic Age

In the 1950s and into the 1960s, the USAF and Navy demanded supersonic performance from fighters in level flight. The Second World War experience had shown that higher speed was productive in achieving superiority in fighter-to-fighter combat, as well as allowing a fighter to intercept a bomber from the rear. The first jet age fighter combat over Korea with fighters having swept wings had resulted in American air superiority, but the lighter MiG-15 had a higher ceiling and better climb rate and could avoid combat by diving away. When aircraft designers interviewed American fighter pilots in Korea, they specified, “I want to go faster than the enemy and outclimb him.”[7] The advent of nuclear-armed jet bombers meant that destruction of the bomber by an interceptor before weapon release was critical and put a premium on top speed, even if that speed would only be achievable for a short time.

Similarly, bomber experience in World War II had shown that loss rates were significantly lower for very fast bombers, such as the Martin B-26 and the de Havilland Mosquito. The prewar concept of the slow, heavy-gun-studded “flying fortress,” fighting its way to a target with no fighter escort, had been proven fallacious in the long run. The use of B-29s in the Korean war in the MiG-15 jet fighter environment had resulted in high B-29 losses, and the team switched to night bombing, where the MiG-15s were less effective. Hence, the ideal jet bomber would be one capable of flying a long distance, carrying a large payload, and capable of increased speed when in a high-threat zone. The length of the high-speed (and probably supersonic) dash might vary on the threat, combat radius, and fuel capacity of the long-range bomber, but it would likely be a longer distance than the short-legged fighter was capable of at supersonic flight. The USAF relied on the long-range bomber as a primary reason for its independent status and existence; hence, it was interested in using the turbojet to improve bomber performance and survivability. But supersonic speeds seemed out of the question with the early turbojets, and the main effort was on wringing long range from a jet bomber. Swept thin wings promised higher subsonic cruise speed and increased fuel efficiency, and the Boeing Company took advantage of NACA swept wing research initiated by Langley’s R.T. Jones in 1945 to produce the B-47 and B-52, which were not supersonic but did have the long range and large payloads.[8]

The development of more fuel-efficient axial-flow turbojets such as the General Electric J47 and Pratt & Whitney J57 (the first mass-produced jet engine to develop over 10,000 pounds static sea level non-afterburning thrust) were another needed element. Aerial refueling had been tried on an experimental basis in the Second World War, but for jet bombers, it became a priority as the USAF sought the goal of a large-payload jet bomber with intercontinental range to fight the projected atomic third World War. The USAF began to look at a supersonic dash jet bomber now that supersonic flight was an established capability being used in the fighters of the day. Just as the medium-range B-47 had served as an interim design for the definitive heavy B-52, the initial result was the delta wing Convair B-58 Hustler. The initial designs had struggled with carrying enough fuel to provide a worthwhile supersonic speed and range; the fuel tanks were so large, especially for low supersonic speeds with their high normal shock drag, that the airplane was huge with limited range and was rejected. Convair adopted a new approach, one that took advantage of its experience with the area rule redesign of the F-102. The airplane carried a majority of its fuel and its atomic payload in a large, jettisonable shape beneath the fuselage, allowing the actual fuselage to be extremely thin. The fuselage and the fuselage/tank combination were designed in accordance with the area rule. The aircraft employed four of the revolutionary J79 engines being developed for Mach 2 fighters, but it was discovered that with the increased fuel capacity, high installed thrust, and reduced drag at low supersonic Mach numbers, the aircraft could sustain Mach 2 for up to 30 minutes, giving it a supersonic range over 1,000 miles, even retaining the centerline store. It could be said that the B-58, although intended to be a supersonic dash aircraft, became the first practical supersonic cruise aircraft. The B-58 remained in USAF service for less than 10 years for budgetary reasons and its notoriously unreliable avionics. The safety record was not good either, in part because of the difficulty in training pilots to change over from the decidedly subsonic (and huge) B-52 with a crew of six to a “hot ship” delta wing, high-landing-speed aircraft with a crew of three (but only one pilot). Nevertheless, the B-58 fleet amassed thousands of hours of Mach 2 time and set numerous world speed records for transcontinental and intercontinental distances, most averaging 1,000 mph or higher, including the times for slowing for aerial refueling. Examples included 4 hours 45 minutes for Los Angeles to New York and back, averaging 1,045 mph, and Los Angeles to New York 1 way in 2 hours 1 minute, at an average speed of 1,214 mph, with 1 refueling over Kansas.

The later record flight illustrated one of the problems of a supersonic cruise aircraft: heat.[9] The handbook skin temperature flight limit on the B-58 was 240 degrees Fahrenheit (ºF). For the speed run, the limit was raised to 260 degrees to allow Mach 2+, but it was a strict limit; there was concern the aluminum honeycomb skin would debond above that temperature. Extended supersonic flight duration meant that the aircraft structure temperature would rise and eventually stabilize as the heat added from the boundary layer balanced with radiated heat from the hot airplane. The stabilization point was typically reached 20–30 minutes after attaining the cruise speed. The B-58’s Mach 2 speed at 45,000–50,000 feet had reached a structural limit for its aluminum material; the barrier now was “the thermal thicket”—a heat limit rather the sound barrier.

Airlines and the Jet Age

In the 1930s, the NACA had conducted research on engine cowlings that improved cooling while reducing drag. This led to improvements in airliner speed and economy, which in turn led to increased capacity and more acceptance by the traveling public; airliners were as fast as the fighters of the early Depression era. In World War II, the NACA shifted research focus to military needs, the most challenging being the turbojet, and almost doubled potential top speeds. In civil aviation, postwarpropeller-driven airliners could span the continent and the oceans, but at 300 mph. Initial attempts to install turbojets in straight winged airliners failed because of the fuel inefficiency of the jets and the increased drag at jet speeds; the loss of life in the mysterious crashes of three British jet-propelled Comets did not instill confidence. Practical airliners had to wait for more efficient engines and a better understanding of high subsonic speeds at high altitudes. NACA aeronautical research of the early 1950s helped provide the latter; the drive toward higher speed in military aircraft provided the impetus for the engine improvements. Boeing’s business gamble in funding the 367-80 demonstrator, which first flew in 1954, triggered the avalanche of jet airliner designs. Airlines began to buy the prospective aircraft by the dozens; because the Civil Aeronautics Board (CAB) mandated all ticket prices in the United States, an airline could not afford to be left behind if its competitors offered travel time significantly less than its propeller-driven fleet. Once passengers were exposed to the low vibration and noise levels of the turbine powerplants, compared to the dozens of reciprocating cylinders of the piston engines banging away combined with multiple noisy propellers, the outcome was further cemented. By the mid 1950s, the jet revolution was imminent in the civil aviation world.

In late 1958, commercial transcontinental and transatlantic jet service began out of New York City, but it was not an easy start. Turbojet noise to ground bystanders during takeoff and landings was not a concern to the military; it was to the New York City airport authorities. “Organ pipe” sound suppressors were mandated, which reduced engine performance and cost the airlines money; even with them, special flight procedures were required to minimize residential noise footprints, requiring numerous flight demonstrations and even weight limitations for takeoffs. The 707 was larger than the newly redesigned British Comet and hence noisier; final approval to operate the 707 from Idlewild was given at the last minute, and the delay helped give the British aircraft “bragging rights” on transatlantic jet service.[10]

Other jet characteristics were also a concern to operators and air traffic control (ATC) alike. Higher jet speeds would give the pilots less time to avoid potential collisions if they relied on visual detection alone. A high-profile midair collision between a DC-7 and Constellation over the Grand Canyon in 1956 highlighted this problem. Onboard collision warning systems using either radar or infrared had been in development since 1954, but no choice had been made for mandatory use. Long-distance jet operations were fuel critical; early jet transatlantic flights frequently had to make unplanned landings en route to refuel. Jets could not endure lengthy waits in holding patterns; hence, ATC had to plan on integrating increasingly dense traffic around popular destinations, with some of the traffic traveling at significantly higher speeds and potentially requiring priority. A common solution to the traffic problems was to provide ground radar coverage across the country and to better automate the ATC sequencing of flight traffic. This was being introduced as the jet airliner was introduced; a no-survivors midair collision between a United Airlines DC-8 jetliner and a Constellation, this time over Staten Island, NY, was widely televised and emphasized the importance of ATC modernization.[11]

NACA research by Richard Whitcomb that led to the area rule had been used by Convair in reducing drag on the F-102 so it would go supersonic. It was also used to make the B-58 design more efficient so that it had a significant range at Mach 2, propelled by four afterburning General Electric J79 turbojets. Convair had been busy with these military projects and was late in the jet airliner market. It decided that a smaller, medium-range airliner could carve out a niche. An initial design appeared as the Convair 880 but did not attract much interest. The decision was made to develop a larger aircraft, the Convair 990, which employed non-afterburning J79s with an added aft fan to reap the developing turbofan engines’ advantages of increased fuel efficiency and decreased sideline noise. Furthermore, the aircraft would employ Whitcomb’s area rule concepts (including so-called shock bodies on its wings, something it shared with the Soviet Union’s Tupolev bombers) to allow it to efficiently cruise some 60–80 mph faster than the 707 and the DC-8, leading to a timesavings on long-haul routes. The aircraft had a higher cruise speed and some limited success in the marketplace, but the military-derived engine had poor fuel economics even with a fan and without an afterburner, was still very noisy, and generated enough black smoke on approach that casual observers often thought the aircraft was on fire (something it shared with its military counterpart, which generated so much smoke that McDonnell F-4 Phantoms often had their position given away by an accusing finger of sooty smoke). The potential trip timesavings was not adequate to compensate for those shortcomings. The lesson the airline industry learned was that, in an age of regulated common airline ticket prices, any speed increase would have to be sufficiently great to produce a significant timesavings and justify a ticket surcharge. The latter was a double-edged sword, because one might lose market share to non–high-speed competitors.[12]

The Quest for Long-Range Supersonic Cruise

Two users were looking to field airplanes in the 1960s with long range at high speeds. One organization’s requirement was high profile and the object of much debate: the United States Air Force and its continuing desire to have an intercontinental range supersonic bomber. The other organization was operating in the shadows. It was the Central Intelligence Agency (CIA), and it was aiming to replace its covert subsonic high-altitude reconnaissance plane (the Lockheed U-2). The requirement was simple; the fulfillment would be challenging, to say the least: a mission radius of 2,500 miles, cruising at Mach 3 for the entire time, at altitudes up to 90,000 feet. The payload was to be on the order of 800 pounds, as it was on the U-2.

The evolution of both supersonic cruise aircraft was involved, much more so for the highly visible USAF aircraft that eventually appeared as the XB-70. The B-58 had given the USAF experience with a Mach 2 bomber, but bombing advocates (notably Gen. Curtis LeMay) wanted long range to go with the supersonic performance. As demonstrated in the classic Breguet range equation, range is a direct function of lift-to-drag (L/D) ratio. The high drag at supersonic speeds reduced that ratio to the point where large fuel tanks were necessary, increasing the weight of the vehicle, requiring more lift, more drag, and more fuel. Initial designs weighed 750,000 pounds and looked like a “3-ship formation.” NACA research on the XF-92 had suggested a delta wing design as an efficient high-speed shape; now, a paper written by Alfred Eggers and Clarence Syvertson of Ames published in 1954 studied simple shapes in the supersonic wind tunnels. They noted that, by mounting a wing atop a half cylindrical shape, they could use the pressure increase behind the shape’s shock wave to increase the effective lift of the wing.[13] A lift increase of up to 30 percent could be achieved. This concept was dubbed “compression lift”; more recently, it is referred to as the “wave rider” concept. Using compression lift principles, North American Aviation (NAA) proposed a 6-engined aircraft weighing 500,000 pounds loaded that could cruise at Mach 2.7 to 3 for 5,000 nautical miles. The aircraft would have a delta wing, with a large underslung shape housing the propulsion system, weapons bay, landing gear, and fuel tanks. A canard surface behind the cockpit would provide trim lift at supersonic speeds. To provide additional directional stability at high speeds, the outer wingtips would fold to either 25 or 65 degrees down. Although reducing effective wing lifting surface, it would have an additional benefit of further increasing compression lift caused by wingtip shocks reflecting off the underside of the wing. Because of the 900–1,100-degree sustained skin temperature at such high cruise speeds, the aircraft would be made of titanium and stainless steel, with stainless steel honeycomb being used in the 6,300-square-foot wing to save weight.[14]