NASA's Contributions to Aeronautics, Volume 1 by National Aeronautics & Space Administration. - HTML preview

CASE

4

Case-4 Cover Image: Aerodynamic model of NASA’s SCAT-15F supersonic transport design attached for a subsonic wind tunnel test in 1969. NASA.

After its formation in 1958, the National Aeronautics and Space Administration (NASA) began devoting most of its resources to the Nation’s new civilian space programs. Yet 1958 also marked the start of a program in the time-honored aviation mission that the Agency inherited from the National Advisory Committee for Aeronautics (NACA). This task was to help foster an advanced passenger plane that would fly at least twice the speed of sound.

Because of economic and political factors, developing such an aircraft became more than a purely technological challenge. One of the major barriers to producing a supersonic transport involved a phenomenon of atmospheric physics barely understood in the late 1950s: the shock waves generated by supersonic flight. Studying these “sonic booms” and learning how to control them became a specialized and enduring field of NASA research for the next five decades. During the first decade of the 21st century, all the study, testing, and experimentation of the past finally began to reap tangible benefits in the same California airspace where supersonic flight began.[1]

From Curiosity to Controversy

In 1947, Muroc Army Airfield, CA, was a small collection of aircraft hangars and other austere buildings adjoining the vast Rogers Dry Lake in the high desert of the Antelope Valley, across the San Gabriel Mountains from the Los Angeles basin. Because of the airfield’s remoteness and clear skies, a small team of Air Force, the NACA, and contractor personnel was using Muroc for a secret project to explore the still unknown territory of supersonic flight. On October 14, more than 40,000 feet over the little desert town of Boron, visible only by its contrail, Capt. Chuck Yeager’s 31-foot-long rocket-propelled Bell XS-1 successfully “broke” the fabled sound barrier.[2] The sonic boom from his little experimental airplane—the first to fly supersonic in level flight—probably did not reach the ground on that historic day.[3] Before long, however, the acoustical signature of the shock waves generated by XS-1s and other supersonic aircraft became a familiar sound at and around the isolated airbase.

In the previous century, an Austrian physicist-philosopher, Ernst Mach, was the first to explain the phenomenon of supersonic shock waves, which he displayed visually in 1887 with a cleverly made photograph showing those formed by a high-velocity projectile, in this case a bullet. The speed of sound, he also determined, varied in relation to the density of the medium though which it passed, such as air molecules. (At sea level, the speed of sound is 760 mph.) In 1929, Jakob Ackeret, a Swiss fluid dynamicist, named this variable “Mach number” in his honor. This guaranteed that Ernst would be remembered by future generations, especially after it became known that the 700 mph speed of Yeager’s XS-1, flying at 43,000 feet, was measured as Mach 1.06.[4]

Humans have long been familiar with and often frightened by natural sonic booms in the form of thunder, i.e., sudden surges of air pressure caused when strokes of lightning instantaneously heat contiguous columns of air molecules. Perhaps the most awesome of sonic booms—heard only rarely—have been produced by large meteoroid fireballs speeding through the atmosphere. On a much smaller scale, the first acoustical shock waves produced by human invention were the modest cracking noises from the snapping of a whip. The high-power explosives perfected in the latter half of the 19th century were able—as Mach explained—to propel projectiles faster than the speed of sound. Their acoustical shock waves would be among the cacophony of fearsome sounds heard by millions of soldiers during the two World Wars.[5]

On a Friday evening, September 8, 1944, an explosion blew out a large crater in Stavely Road, west of London. The first German V-2 ballistic missile aimed at England had announced its arrival. “After the explosion came a double thunderclap caused by the sonic boom catching up with the fallen rocket.”[6] For the next 7 months, millions of people would hear these sounds, which would become known as “sonic bangs” in Britain, from more than 3,000 V-2s launched at England as well as liberated portions of France, Belgium, and the Netherlands. Their sound waves would always arrive too late to warn any of those unfortunate enough to be near the missiles’ points of impact.[7] After World War II, these strange noises faded into memory for several years—until the arrival of new jet fighter planes.

In November 1949, the NACA designated its growing detachment at Muroc as the High-Speed Flight Research Station (HSFRS), 1 month before the Air Force renamed the installation Edwards Air Force Base (AFB).[8] By the early 1950s, the desert and mountains around Edwards reverberated with the occasional sonic booms of experimental and prototype aircraft, as did other flight-test locations in the United States and United Kingdom. Scientists and engineers had been familiar with the “axisymmetric” ballistic shock waves of projectiles such as artillery shells (referred to scientifically as bodies of revolution).[9] This was one reason the fuselage of the XS-1 was shaped like a 50-caliber bullet. But these new acoustic phenomena—many of which featured a double-boom sound—hinted that they were more complex. In late 1952, the editors of the world’s oldest aeronautical weekly stated with some hyperbole that “the ‘supersonic bang’ phenomenon, if only by reason of its sudden incidence and the enormous public interest it has aroused, is probably the most spectacular and puzzling occurrence in the history of aerodynamics.”[10]

A young British graduate student, Gerald B. Whitham, was the first to analyze thoroughly the abrupt rise in air pressure upon arrival of a supersonic vehicle’s “bow wave,” followed by a more gradual but deeper fall in pressure for a fraction of a second, and then a recompression with the passing of the vehicle’s tail wave. As shown in a simplified fashion by Figure 1, this can be illustrated graphically by an elongated capital “N” (the solid line) transecting a horizontal axis (the dashed line) representing ambient air pressure during a second or less of elapsed time. For Americans, the pressure change is usually expressed in pounds per square foot (psf—also abbreviated as lb/ft2).

Because a jet fighter (or a V-2 missile) is much longer than an artillery shell is, the human ear could detect a double boom if its tail shock wave arrived a tenth of a second or more after its bow shock wave. Whitham was first to systematically examine the more complex shock waves, which he called the F-function, generated by “nonaxisymmetrical” (i.e., asymmetrical) configurations, such as airplanes.[11]

The number of these double booms multiplied in the mid-1950s as the Air Force Flight Test Center (AFFTC) at Edwards (assisted by the HSFRS) began putting a new generation of Air Force jet fighters and interceptors, known as the Century Series, through their paces. The remarkably rapid advance in aviation technology (and priorities of the Cold War “arms race”) is evident in the sequence of their first flights at Edwards: YF-100 Super Sabre, May 1953; YF-102 Delta Dagger, October 1953; XF-104 Starfighter, February 1954; F-101 Voodoo, September 1954; YF-105 Thunderchief, October 1955; and F-106 Delta Dart, December 1956.[12]

With the sparse population living in California’s Mojave Desert region during the 1950s, disturbances caused by the flight tests of new jet aircraft were not a serious issue. But even in the early 1950s, the United States Air Force (USAF) became concerned about their future impact. In November 1954, for example, its Aeronautical Research Laboratory at Wright-Patterson AFB, OH, submitted a study to the Air Force Board of top generals on early findings regarding the still somewhat mysterious nature of sonic booms. Although concluding that low-flying aircraft flying at supersonic speeds could cause considerable damage, the report optimistically predicted the possibility of supersonic flight without booms at altitudes over 35,000 feet.[13]

As the latest Air Force and Navy fighters went into full production and began flying from bases throughout the Nation, much of the American public was exposed to jet noise for the first time. This included the thunderclap-like thuds characteristic of sonic booms—often accompanied by rattling windowpanes. Under certain conditions, as the U.S. armed services and British Royal Air Force (RAF) had learned, even maneuvers below Mach 1 (e.g., accelerations, dives, and turns) could generate and focus transonic shock waves in such a manner as to cause strong sonic booms.[14] Indeed, residents of Southern California began hearing such booms in the late 1940s, when North American Aviation was flight-testing its new F-86 Sabre. The first civilian claim against the USAF for sonic boom damage was apparently filed at Eglin AFB, FL, in 1951, when only subsonic jet fighters were assigned there.[15] Additionally, as shown in 1958 by Frank Walkden, another English mathematician, the lift effect of airplane wings could magnify the strength of sonic booms more than previously estimated.[16]

Sonic boom claims against the Air Force first became statistically significant in 1957, reflecting its growing inventory of Century fighters and the type of maneuvers they sometimes performed, which could focus acoustical rays into what became called “super booms.” (It was found that these powerful but localized booms had a U-shaped signature, with the tail shock wave as well as that from the nose of the airplane being above ambient air pressure.) Most claims involved broken windows or cracked plaster, but some were truly bizarre, such as the death of pets or the insanity of livestock. In addition to these formal claims, Air Force bases, local police switchboards, and other agencies received an uncounted number of phone calls about booms, ranging from merely inquisitive to seriously irate.[17] Complaints from constituents also became an issue for the U.S. Congress.[18] Between 1956 and 1968, some 38,831 claims were submitted to the Air Force, which approved 14,006 in whole or in part—65 percent for broken glass, 21 percent for cracked plaster (usually already weakened), 8 percent for fallen objects, and 6 percent for other reasons.[19]

The military’s problem with sonic boom complaints seems to have peaked in the 1960s. One reason was the sheer number of fighter-type aircraft stationed around the Nation (over three times as many as today). Secondly, many of these aircraft’s missions were air defense. This often meant flying at high speed over populated areas for training in defending cities and other key targets from aerial attack, sometimes in practice against Strategic Air Command (SAC) bombers. The North American Air Defense Command (NORAD) conducted two of the largest such exercises, Skyshield I and Skyshield II, in 1960 and 1961. The Federal Aviation Agency (FAA) shut down all civilian air traffic while NORAD’s interceptors and SAC bombers (augmented by some from the RAF) battled overhead—accompanied by a sporadic drumbeat of sonic booms reaching the surface.[20]

Although most fighters and interceptors deployed in the 1960s could readily fly faster than sound, they could only do so for a short distance because of the rapid fuel consumption of jet engine afterburners. Thus, their sonic boom “carpets” were relatively short. However, one supersonic American warplane that became operational in 1960 was designed to fly faster than Mach 2 for more than 1,000 miles.

This innovative but troublesome aircraft was the SAC’s new Convair-built B-58 Hustler medium bomber. On March 5, 1962, the Air Force showed off the long-range speed of the B-58 by flying one from Los Angles to New York in just over 2 hours at an average pace of 1,215 mph (despite having to slow down for an aerial refueling over Kansas). After another refueling over the Atlantic, the same Hustler “outraced the sun” (i.e., flew faster than Earth’s rotation) back to Los Angles with one more refueling, completing the record-breaking round trip at an average speed of 1,044 mph.[21]

Capable of sustained Mach 2+ speeds, the four-engine delta-winged Hustler (weighing up to 163,000 pounds) helped demonstrate the feasibility of a supersonic transport. But the B-58’s performance revealed at least one troubling omen. Almost wherever it flew supersonic over populated areas, the bomber left sonic boom complaints and claims in its wake. Indeed, on its record-shattering flight of March 1962, flown mostly at an altitude of 50,000 feet (except when coming down to 30,000 feet for refueling), “the jet dragged a sonic boom 20 to 40 miles wide back and forth across the country—frightening residents, breaking windows, cracking plaster, and setting dogs to barking.”[22] As indicated by Figure 2, the B-58 became a symbol for sonic boom complaints (despite its small numbers).

Most Americans, especially during times of increased Cold War tensions, tolerated occasional disruptions justified by national defense. But how would they react to constantly repeated sonic booms generated by civilian jet airliners? Could a practical passenger-carrying supersonic airplane be designed to minimize its sonic signature enough to be acceptable to people below? NASA’s attempts to resolve these two questions occupy the remainder of this history.

A Painful Lesson: Sonic Booms and the Supersonic Transport

By the late 1950s, the rapid pace of aeronautical progress—with new turbojet-powered airliners flying twice as fast and high as the propeller-driven transports they were replacing—promised even higher speeds in coming years. At the same time, the perceived challenge to America’s technological superiority implied by the Soviet Union’s early space triumphs inspired a willingness to pursue ambitious new aerospace ventures. One of these was the Supersonic Commercial Air Transport (SCAT). This program was further motivated by competition from Britain and France to build an airliner that was expected to dominate the future of mid- and long-range commercial aviation.[23]

From SCAT Research to SST Development

The recently established FAA became the major advocate within the U.S. Government for a supersonic transport, with key personnel at three of the NACA’s former laboratories eager to help with this challenging new program. The Langley Research Center in Hampton, VA, (the NACA’s oldest and largest lab) and the Ames Research Center at Moffett Field in Sunnyvale, CA, both had airframe design expertise and facilities, while the Lewis Research Center in Cleveland, OH, specialized in the kind of advanced propulsion technologies needed for supersonic cruise.

The strategy for developing the SCAT depended heavily on leveraging technologies being developed for another Air Force bomber—one much larger, faster, and more advanced than the B-58. This would be the revolutionary B-70, designed to cruise several thousand miles at speeds of Mach 3. NACA experts had been helping the Air Force plan this giant intercontinental bomber since the mid-1950s (with aerodynamicist Alfred Eggers of the Ames Laboratory conceiving the innovative design for it to ride partially on compression lift created by its own supersonic shock waves). North American Aviation won the B-70 contract in 1958, but the projected expense of the program and advances in missile technology led President Dwight Eisenhower to cancel all but one prototype in 1959. The administration of President John Kennedy eventually approved production of two XB-70As. Their main purpose would be to serve as Mach 3 testbeds for what had become known simply as the Supersonic Transport (SST). NASA continued to refer to design concepts for the SST using the older acronym for Supersonic Commercial Air Transport. By 1962, these concepts had been narrowed down to three Langley designs (SCAT-4, SCAT-15, and SCAT-16) and one from Ames (SCAT-17). These became the baselines for industry studies and SST proposals.[24]

Even though Department of Defense resources (especially the Air Force’s) would be important in supporting the SST program, the aerospace industry made it clear that direct federal funding and assistance would be essential. Thus research and development (R&D) of the SST became a split responsibility between the Federal Aviation Agency and the National Aeronautics and Space Administration—with NASA conducting and sponsoring the supersonic research and the FAA in charge of the SST’s overall development. The first two leaders of the FAA, retired Lt. Gen. Elwood R. “Pete” Quesada (1958–1961) and Najeeb E. Halaby (1961–1965), were both staunch proponents of producing an SST, as to a slightly lesser degree was retired Gen. William F. “Bozo” McKee (1965–1968). As heads of an independent agency that reported directly to the president, they were at the same level as NASA Administrators T. Keith Glennan (1958–1961) and James Beggs (1961–1968). The FAA and NASA administrators, together with Secretary of Defense Robert McNamara (somewhat of a skeptic on the SST program), provided interagency oversight and comprised the Presidential Advisory Committee (PAC) for the SST established in April 1964. This arrangement lasted until 1967, when the Federal Aviation Agency became the Federal Aviation Administration under the new Department of Transportation, whose secretary became responsible for the program.[25]

Much of NASA’s SST-related research involved advancing the state-of-the-art in such technologies as propulsion, fuels, materials, and aerodynamics. The latter included designing airframe configurations for sustained supersonic cruise at high altitudes, suitable subsonic maneuvering in civilian air traffic patterns at lower altitudes, safe takeoffs and landings at commercial airports, and acceptable noise levels—to include the still-puzzling matter of sonic booms.

Dealing with the sonic boom entailed a multifaceted approach: (1) performing flight tests to better quantify the fluid dynamics and atmospheric physics involved in generating and propagating shock waves, as well as their effects on structures and people; (2) conducting community surveys to gather public opinion data on sample populations exposed to booms; (3) building and using acoustic simulators to further evaluate human and structural responses in controlled settings; (4) performing field studies of possible effects on animals; (5) evaluating various aerodynamic configurations in wind tunnel experiments; and (6) analyzing flight test and wind tunnel data to refine theoretical constructs and mathematical models for lower-boom aircraft designs. Within NASA, the Langley Research Center was a focal point for sonic boom studies, with the Flight Research Center (FRC) at Edwards AFB conducting many of the supersonic tests.[26]

Although the NACA, especially at Langley and Ames, had been doing research on supersonic flight since World War II, none of its technical reports (and only one conference paper) published through 1957 dealt directly with sonic booms.[27] That situation began to change when Langley’s long-time manager and advocate of supersonic programs, John P. Stack, formalized the SCAT venture in 1958. During the next year, three Langley employees whose names would become well known in the field of sonic boom research began publishing NASA’s first scientific papers on the subject. These were Harry W. Carlson, a versatile supersonic aerodynamicist, Harvey H. Hubbard, chief of the Acoustics and Noise Control Division, and Domenic J. Maglieri, a young engineer who became Hubbard’s top sonic boom specialist. Carlson would tend to focus on wind tunnel experiments and sonic boom theory, while the other two men specialized in planning and monitoring field tests, then analyzing the data collected.[28] These research activities began to expand under the new pro-SST Kennedy Administration in 1961. After the president formally approved development of the supersonic transport in June 1963, sonic boom research took off. Langley’s experts, augmented by NASA contractors and grantees, published 26 papers on sonic booms just 3 years later.[29]

Supersonic Flight Tests and Surveys

The systematic sonic boom testing that NASA began in 1958 would exponentially expand the heretofore largely theoretical and anecdotal knowledge about sonic booms with a vast amount of “real world” data. The new information would make possible increasingly sophisticated experiments and provide feedback for checking and refining theories and mathematical models. Because of the priority bestowed on sonic boom research by the SST program and the numerous types of aircraft then available for creating booms (including some faster than anything flying today), the data and findings from the tests conducted in the 1960s are still of significant value in the 21st century.[30]

The Langley Research Center (often referred to as NASA Langley) served as the Agency’s “team leader” for supersonic research. Langley’s acoustics specialists conducted NASA’s initial sonic boom tests in 1958 and 1959 at the Wallops Island Station on Virginia’s isolated Delmarva Peninsula. During the first year, they used six sorties by NASA F-100 and F-101 fighters, flying at speeds between Mach 1.1 and 1.4 and altitudes from 25,000 to 45,000 feet, to make the first good ground recordings and measurements of sonic booms for steady, level flights (the kind of profile a future airliner would fly). Observers judged some of the booms above 1.0 psf to be objectionable, likening them to nearby thunder, and a sample plate glass window was cracked by one plane flying at 25,000 feet. The 1959 test measured shock waves from 26 flights of a Chance Vought F8U-3 (a highly advanced prototype based on the Navy’s Crusader fighter) at speeds up to Mach 2 and altitudes up to 60,000 feet. A B-58 from Edwards AFB also made two supersonic passes at 41,000 feet. Boom intensities from these higher altitudes seemed to be tolerable to observers, with negligible increases in measured overpressures between Mach 1.4 and 2.0. These results were, however, very preliminary.[31]

In July 1960, NASA and the Air Force conducted Project Little Boom at a bombing range north of Nellis AFB, NV, to measure the effects on structures and people of extremely powerful sonic booms. F-104 and F-105 fighters flew slightly over the speed of sound (Mach 1.09 to 1.2) at altitudes as low as 50 feet above ground level. There were more than 50 incidents of sample windows being broken at 20 to 100 psf, but only a few possible breakages below 20 psf, and no physical or psychological harm to volunteers exposed to overpressures as high as 120 psf.[32] At Indian Springs, Air Force fighters flew supersonically over an instrumented C-47 transport from Edwards, both in the process of landing and on the ground. Despite 120 psf overpressures, there was only very minor damage when on the ground and no problems in flight.[33] Air Force fighters once again would test powerful sonic booms in 1965 in support of Joint Task Force 2 at Tonopah, NV. The strongest sonic boom ever recorded, 144 psf, was generated by an Air Force F-4E Phantom II flying Mach 1.26 at 95 feet.[34]

In late 1960 and early 1961, NASA and AFFTC followed up on Little Boom with Project Big Boom. B-58 bombers made 16 passes flying Mach 1.5 at altitudes of 30,000 to 50,000 feet over arrays of sensors, which measured a maximum overpressure of 2.1 psf. Varying the bomber’s weight from 82,000 to 120,000 pounds provided the first hard data on how an aircraft’s weight and related lift produced higher over-pressures than existing theories based on volume alone would indicate.[35]

Throughout the 1960s, Edwards Air Force Base—with its unequaled combination of Air Force and NASA expertise, facilities, instrumentation, airspace, emergency landing space, and types of aircraft—hosted the largest number of sonic boom tests. NASA researchers from Langley’s Acoustics Division spent much of their time there working with the Flight Research Center in a wide variety of flight experiments. The Air Force Flight Test Center usually participated as well.

In an early test in 1961, Gareth Jordan of the FRC led an effort to collect measurements from F-104s and B-58s flying at speeds of Mach 1.2 to 2.0 over sensors located along Edward AFB’s supersonic corridor and at Air Force Plant 42 in Palmdale, about 20 miles south. Most of the Palmdale measurements were under 1.0 psf, which the vast majority of people surveyed there and in Lancaster (where overpressures tended to be somewhat higher) considered no worse than distant thunder. But there were some exceptions.[36]

Other experiments at Edwards in 1961 conducted by Langley personnel with support from the FRC and AFFTC contributed a variety of new information. With help from a tethered balloon, they made the first good measurements of atmospheric effects, showing that air turbulence in the lower atmosphere (known as the boundary layer) significantly affected wave shape and overpressure. They also gathered the first data on booms from very high altitudes. Using an aggressive flight profile, AFFTC’s B-58 crew managed to zoom up to 75,000 feet—25,000 feet higher than the bomber’s normal cruising altitude and 15,000 feet over its design limit! The overpressures measured from this high altitude proved stronger than predicted (not a promising result for the planned SST). Much lower down, fighter aircraft performed accelerating and turning maneuvers to generate the kind of acoustical rays that amplified shock waves and produced multiple booms and super booms. Thevarious experiments showed that a combination of atmospheric conditions, altitude, speed, flight path, aircraft configuration, and sensor location determined the shape of the pressure signatures.[37]

Of major significance for future boom minimization efforts, NASA also began making in-flight shock wave measurements. The first of these, at Edwards in 1960, had used an