Higher, Faster, Farther

HIGHER, FASTER, FARTHER

Expanding Aeronautical Horizons

By Craig Collins

Getting an airplane to fly faster had been a preoccupation for engineers at the Langley Memorial Aeronautical Laboratory from the moment flight research began there, but it wasn’t until World War II that aircraft speed became a matter of life and death. The war produced the world’s first jet-propelled aircraft, and though these jet fighters proved not quite ready to play a major role in battle, they were clearly the aircraft of the future. Langley engineers redoubled their efforts to evaluate materials, structures, configurations, and controls to enable aircraft to fly at ever-faster speeds.

In the early 1940s, as manufacturers began to produce aircraft that could travel several hundred miles per hour, the main challenge to high-speed flight was related to “compressibility effects:” Air flowed over airfoils and other surfaces at speeds faster than the aircraft’s velocity. At lower speeds, this wasn’t a problem, but when airflows reached “transonic” speeds, near and beyond the speed of sound, they created shock waves, which tended to proliferate until they hammered at the aircraft, creating drag and threatening stability. Because of these effects, the fastest World War II-era aircraft had difficulty flying much faster than 500 mph. Some engineers began to believe the “sound barrier” presented an insurmountable obstacle, an invisible wall in the sky.

But this idea didn’t last long. Langley engineer Robert T. Jones, in 1945, independently developed the idea of the “swept wing” – commonly seen on jet fighters today – as a way to counter compressibility effects by deflecting the angle of the shock wave. Jones’ revolutionary idea didn’t get much attention at first, but it would later become a key development in achieving and maintaining supersonic flight.

Because a precise knowledge of the speed of sound was so important to solving the problems of high-speed flight, it became a particular focus for NACA engineers in the 1940s – particularly Langley’s John Stack, who had begun studying the problems of high-speed flight at the laboratory during the 1920s, and had been among the first to quantify the effects of compressibility. With Hugh Dryden (who would become director of the NACA’s aeronautical research in 1946), Stack also established an “industry standard” speed of sound, rounded off at 1,117 feet per second, or 761 mph, at sea level. At cruising altitudes of 35,000 feet and higher, the speed of sound is around 660 mph.

In the early 1940s, Langley investigators devised ways of achieving pressure distributions at Mach 1 – essentially, the speed of sound – in transonic wind tunnel tests, but researchers who attempted these speeds in larger-scale tunnels encountered a phenomenon known as “choking” – shock wave interference generated by the walls of the tunnels themselves, which caused the flow field to break down and skewed test results. Stack was convinced, for the time being, that the only way to evaluate the aerodynamics of transonic flight was to build a real, full-scale airplane capable of achieving supersonic speeds. By summer 1943, a team led by Stack had produced a preliminary design for a turbojet powered “X-plane” that would take off and land under its own power.

The Bell X-1 – the type of aircraft that broke the sound barrier in 1947.

NASA Photo

The NACA, however, didn’t have the money to build such a plane, and the Army – which did – came up with a different design: a rocket-powered plane that would be launched from the air. In 1944, Langley investigators began devising ways to acquire transonic aerodynamic data for this plane: dropping winged bomb-like bodies from an altitude of 30,000 feet; measuring supersonic flow over a scale model wing mounted to the wing of a P-51D Mustang in a steep dive; and small-scale test runs in Langley’s Annular Transonic Tunnel, which was essentially a whirling arm with a model attached. The data from these methods, along with the compressibility data gathered by the NACA in the previous two decades, formed the basis for the design of the Army’s X-1 aircraft by Bell Aircraft Corporation.

A B-29 “mother ship” with a Bell X-1 tucked up into its belly. The X-1s and most NACA and NASA research aircraft to follow were carried aloft by B-29, and later, B-52 aircraft.

NASA Photo

On the morning of Oct. 14, 1947, the bullet-shaped Bell X-1, piloted by Air Force Capt. Chuck Yeager, dropped from the modified bomb bay of a vintage B-29 Superfortress and took off like a shot over the Mojave Desert. Yeager’s first flight of the day achieved a velocity of 700 mph, or Mach 1.06 – the first aircraft to break the sound barrier. While the X-1 produced shock waves that generated a signature “sonic boom” in the skies over the desert, Yeager’s flight was smooth, without the turbulence or loss of control that had worried some engineers. The X-1 would reach a speed of nearly 1,000 mph by 1948.

The Air Force had wanted to keep Yeager’s flight – and several ensuing X-1 flights beyond the sound barrier – a secret, but it was headline news within a couple of months. In December 1948, President Harry S. Truman presented the 37th Collier Trophy to three men: Lawrence D. Bell, the manufacturer; Yeager, the pilot; and Stack, the pioneering engineer at Langley Aeronautical Laboratory, for “the greatest aeronautical achievement since the original flight of the Wright brothers’ airplane.”

The Area Rule and Supersonic Flight

The X-1 comprised one of the most impressive technological breakthroughs of the 20th century – but as an aircraft, it didn’t have much practical use. It was essentially a rocket with wings, dropped from the belly of an airplane, that glided to a landing after its engine burned out. At Langley, Stack and his engineers continued to investigate practical applications for supersonic flight, which could become generally useful if it were achieved by a jet-powered aircraft capable of taking off and landing on its own.

By the time of the X-1’s first flight, a team led by Stack and physicist Ray H. Wright had nearly solved the problem of how to get transonic wind tunnel data. Wright had figured out that the placement of ventilation slots along the walls of the tunnel’s test section, parallel to air flow, would channel the air around a test subject and allow the gathering of valuable transonic research data. Langley’s 8-Foot and 16-Foot High Speed Tunnels were promptly remodeled to integrate Wright’s discovery, and the “slotted throat” innovation, made public in the early 1950s, was so important to the continued research of transonic flight that the team was awarded the 1951 Collier Trophy “for the conception, development, and practical application of the transonic wind tunnel throat.” sudden spike in drag at around 500 mph: As they approached the speed of sound, two different shock waves built up, on the fuselage and on the trailing edge of the wing. This was a particular problem for aircraft manufacturer Convair, which was attempting to build the nation’s first supersonic interceptor, the YF-102 Delta Dagger: pilots couldn’t get the aircraft past the sound barrier.

Langley’s John Stack, head of the Compressibility Research Division, was among the first to quantify the effects of compressibility.

NASA Photo

The next great supersonic innovation to come out of Langley was conceived by Richard T. Whitcomb, the young aeronautical engineer who’d joined the laboratory in 1943 and would soon become known as “the man who could see air.” Convair’s design for the Delta Dagger was a conventional thick, bullet-shaped aircraft with delta wings and tail. After studying the wind tunnel data and the shape of the aircraft for some time, Whitcomb intuitively grasped the solution: a conventional wing/fuselage combination featured a sudden increase in the cross-sectional area where the wing met the fuselage. But if the fuselage were narrowed a bit in the region of the wing, the air displacement, and resulting drag, would be much reduced.

Richard T. Whitcomb with a wind tunnel model showing the characteristic “Coke bottle” shape of area rule.

NASA Photo

Whitcomb modified wind tunnel models with a concave taper to the fuselage where the wings attached – a feature that became known as the “wasp waist” or “Coke bottle” effect. Data promptly validated what became known as Whitcomb’s “area rule,” and Convair’s modified aircraft, the YF-102A Delta Dagger, would achieve a speed of Mach 1.22. The taper would feature in the design of virtually every supersonic craft manufactured afterward, and Whitcomb would receive the 1954 Collier Trophy for “discovery and experimental verification of the area rule, a contribution to base knowledge yielding significantly higher airplane speed and greater range with the same power.”

From 1940 to 1955, Langley researchers played a crucial role in increasing aircraft speed from hundreds to thousands of miles per hour. In 1947, the 11-inch Hypersonic Tunnel, the first of its kind in the United States, began operations at Langley. In 1951, another hypersonic facility, the Gas Dynamics Laboratory, came online, allowing researchers to study pressurized air released in bursts that simulated speeds up to Mach 8.

A YF-102 (left), which was unable to break the sound barrier, and YF-102A (right) with area rule shaping of the mid fuselage, new intakes, and other refinements to the design. The YF-102A exceeded Mach 1.22.

NASA Photo

By the early 1950s, military and NACA researchers had begun discussing ideas for a hypersonic research plane, and the joint project that eventually emerged from this, the X-15, first flew in June 1959, after NASA had been established and Langley had become the Langley Research Center. Three rocket-powered, piloted X-15s – the world’s first actual “spaceplanes” – flew a total of 199 missions between 1959 and 1968, achieving a top speed of Mach 6.72 (4,520 mph, fast enough that aerodynamic heating, far in excess of engineers’ estimates, partially melted the plane’s tail), and an altitude of 354,330 feet – 67 miles above the Earth, 5 miles beyond the “Kármán line” marking the boundary of outer space.

An X-15 in flight. The X-15 flew beyond Mach 6 and regularly ventured beyond the Earth’s atmosphere.

NASA Photos

Practical Supersonic and Hypersonic Aircraft

The X-15 project was one of the most successful aeronautical research programs ever undertaken, and the lessons learned from it proved most immediately applicable to NASA’s Space Shuttle Program. Until the Space Shuttle Columbia’s first orbital flight in 1981, no winged aircraft flew higher. Meanwhile, Langley’s aeronautical researchers continued looking for practical solutions to problems related to transonic and supersonic flight.

One of these problems was the drag created by supersonic airflow around the wing of an aircraft traveling at high subsonic speeds. The resulting drag made traveling at these speeds cripplingly inefficient. The man to solve this problem – again, using intuition first and data later – was Whitcomb, who made a physical wing design out of body putty, bulking up some areas of the wing while thinning others. The result was a wing cross-section that was nearly flat on top and rounded on the bottom, with a downward-curving trailing edge. The wing’s design reduced the strength of shock waves. Designed to operate far above a wing’s critical Mach number, Whitcomb’s wing became known as the “supercritical airfoil.” In 1971 and 1973, test flights at NASA’s Dryden Flight Research Center in California (now the Armstrong Flight Research Center), increased the efficiency and range of transonic flight for a Vought F-8 Crusader and a General Dynamics F-111 Aardvark. For his innovation, Whitcomb was awarded the 1974 Wright

Langley engineers have tested a variety of supersonic passenger jet concepts in wind tunnels over the last 70 years. This model is the Supersonic Commercial Transport SCAT-15F being tested in the Langley Full- Scale Tunnel in 1965.

NASA Photo

Brothers Memorial Trophy from the National Aeronautic Association.

Breakthroughs in high-speed aeronautics prompted President John F. Kennedy, in June 1963, to issue a challenge to the U.S. government, which he said “should immediately commence a new program in partnership with private industry to develop at the earliest practical date the prototype of a commercially successful supersonic transport …” NASA Langley had been working on its own experimental technologies, since 1959, as part of its Supersonic Commercial Air Transport Program (SCAT), and two companies responded to the president’s challenge by proposing to build a prototype supersonic transport (SST). Langley researchers worked with Boeing, the winning bidder, in evaluations of its design, but the American SST never made it off the ground, mostly due to two major problems that made it commercially unviable: First, the sonic boom would rattle windows for miles around as it flew, and not many communities were willing to grant landing rights for the aircraft – one of the factors that made it difficult for the world’s first supersonic airliner, the Mach 2 French-British Concorde, to turn a profit in its 27 years of operation from 1976 to 2003.

NASA’s TF-8A supercritical wing testbed. Another Richard Whitcomb development, the supercritical wing is now used on virtually every commercial jet transport aircraft flying.

NASA Photo

NASA’s Shaped Sonic Boom Demonstration used a Northrop F-5E with a modified fuselage to demonstrate that an aircraft’s shock wave and accompanying sonic boom could be reduced.

NASA Photo

Second, the nitrogen oxides in the SST’s exhaust posed a serious environmental threat. The American SST program was canceled in 1971. For the next three decades, Langley investigators continued to work with industry partners on research into supersonic civil transport, but noise and environmental concerns have – until recently – kept supersonic transport something of a back-burner project. NASA Langley has played key roles in two 21st-century projects that have revived interest in high-speed

HIGH-SPEED FLIGHT: TERMS

Strictly speaking, an aircraft’s Mach number, named for Austrian physicist Ernst Mach, represents the ratio of the airflow past a boundary (i.e., a wing or fuselage) to the local speed of sound, which varies according to environmental factors such as pressure and altitude.

When an aircraft is said to have reached Mach 1, that means the airflow over its wings and/or other surfaces is equal to the local speed of sound. At Mach 2, this ratio is twice the speed of sound. The velocity of the aircraft itself is slightly lower than this number.

As research into high-speed flight has matured, the following terms have been devised to denote ranges of velocity:

• Subsonic: speeds less than Mach 0.8, or 614 miles per hour

• Transonic: speeds approaching and surpassing the speed of sound, from Mach 0.8 to Mach 1.2, or 614 to 921 mph

• Supersonic: speeds between Mach 1.2 and Mach 5, from 921 to 3,836 mph

• Hypersonic: beyond Mach 5

flight. In 2003, investigators from Langley and Dryden explored ways to dampen or “shape” a sonic boom by modifying an aircraft’s fuselage. A modified Northrop F-5E demonstrated that the shock wave and accompanying sonic boom could be altered and reduced. On the heels of the Shaped Sonic Boom Demonstration, as part of NASA’s New Aviation Horizons Initiative, the agency announced in February 2016 that it would partner with Lockheed Martin Aeronautics on a preliminary design for Quiet Supersonic Technology (QueSST). NASA will share that design with manufacturers in a competition to build a prototype for a Low Boom Flight Demonstration that will evaluate how well quiet sonic boom technology performs.

The world’s fastest air-breathing engine, the X-43A, was tested in NASA Langley’s 8-Foot High Temperature Tunnel.

Photo Courtesy of NASA

As the Shaped Sonic Boom was being demonstrated in California, another Langley- Dryden collaboration was reaching fruition: the Hyper-X program, a seven-year effort to explore alternatives to rocket power for a new hypersonic spaceplane. Erik Axdahl, a research engineer in Langley’s Hypersonic Air-breathing Propulsion Branch, explained that rockets don’t offer much in the way of efficiency: “When you have an air-breathing engine, you only need to carry the fuel on board,” he said, “because you’re actually breathing in the oxidizer – the oxygen from the air. So because you only have to carry half of your propellant on board, air-breathing engines end up being much more efficient.”

The unpiloted Hyper-X test vehicle, the X-43 plane, was designed by Langley engineers and built by Micro Craft Inc. and General Applied Science Laboratory. The X-43 featured a supersonic combustion ramjet, or “scramjet,” an air-breathing engine in which combustion is fueled by supersonic airflow, allowing the aircraft to operate efficiently at extremely high speeds. On Nov. 16, 2004, the 3-meter-long X-43 was dropped from a B-52, boosted by a Pegasus rocket to an altitude of 109,000 feet, and then its scramjet engines kicked in and propelled it at a speed of 7,310 mph (Mach 9.6) – a world speed record for flight that stands to this day.

As Axdahl pointed out, the first true spaceplane – an aircraft that will take off from a runway, enter low earth orbit or beyond, and then reenter the atmosphere and land on another runway – will probably feature a combination of rocket and jet engines. It takes an enormous amount of fuel to escape the Earth’s gravitational pull, and there’s no air for a scramjet to breathe in space. Spacecraft leaving Earth must reach a velocity of about 17,600 mph to achieve orbit, and about 25,000 mph to completely escape Earth’s gravity. The only present-day technology that makes that possible is the rocket – but scramjets, Axdahl said, could make such a trip far more feasible. “You can go as far as air-breathing technology will take you, and then, when you run out of air and can’t keep your engine lit anymore, you might switch over to your rocket and go to orbit. But because part of that trajectory was air-breathing, it made the whole system more efficient overall.”

An artist’s rendering of the airbreathing, hypersonic X-43.

Photo Courtesy of NASA

Such a trip – like most of the things Langley researchers have made happen over the last 100 years – sounds like science fiction. But there’s no reason to think it won’t happen. “We’ve played a key role in air-breathing hypersonic development for 50 years,” Axdahl said, adding that Langley would continue to do so. “We’ll figure it out. That’s Langley’s role in the world of hypersonic propulsion.”