Remembering Frank Marble
Frank Earl Marble (Eng '47, PhD '48), Caltech's Richard L. and Dorothy M. Hayman Professor of Mechanical Engineering and Professor of Jet Propulsion, Emeritus, passed away on August 11, 2014, two months after the death of Ora Lee Marble, his wife of 71 years. Marble was one of the fathers of modern jet engines; his doctoral thesis included a method for calculating the three-dimensional airflow through rows of rotating blades. A jet engine is essentially two sets of blades on a common axle. A compressor at the front of the engine slows the incoming air and feeds it to the burner, and a turbine spinning in the hot gases downstream ejects the exhaust and drives the compressor. More broadly, Marble's methods apply to any fluid flowing along the axis of a fan, pump, turbine, or propeller.
Born in Cleveland, Ohio, on July 21, 1918, 15 years after the Wright brothers' first powered flight, Marble got interested in aviation in grade school. The Cleveland airfield was "a long streetcar ride away," he recalled in his Caltech oral history, and he "could wander into the hangars" unsupervised. He got his pilot's license before his driver's license.
Marble earned his BS in aeronautics in 1940 at the Case School of Applied Science (now Case Western Reserve University), "about a two-mile walk from home." For his master's degree in 1942, he built a fan designed to measure the surface pressure along a blade as it cut through the air. Holes in the blade led to a set of pressure gauges; the trick, he noted, was inventing the "slip seal" at the fan's hub that kept the holes and their gauges connected. He brought the data with him to Caltech, where it eventually became the basis for his PhD work.
But first, Marble helped fight World War II from the Cleveland airport, joining the National Advisory Committee for Aeronautics' Aircraft Engine Research Lab (now NASA's John H. Glenn Research Center at Lewis Field). Marble led the team troubleshooting the B-29 Superfortress, capable of flying thousands of miles at 30,000 feet with 10 tons of bombs. The "Superfort" was the biggest, heaviest plane of the war and its four engines often overheated; a significant number were ditched in the Pacific after engine fires. Several alterations to the airflow maximized the engine cooling, and the B-29 would remain in service into the 1960s.
On receiving his doctorate from Caltech in 1948, Marble was hired as an assistant professor by Tsien Hsue-shen (PhD '39), the Goddard Professor of Jet Propulsion. Tsien assigned him to develop a set of courses in this new field, which blended chemistry, gas dynamics, and materials science.
Tsien also gave Marble a half-time appointment at Caltech's Jet Propulsion Laboratory (JPL), which in the pre-NASA era really was studying jet propulsion, developing missiles under contract with the army. Tsien and his fellow members of the "suicide squad" had founded JPL in the wide-open scrublands of the upper Arroyo Seco in the 1930s after a string of accidents and explosions had gotten them evicted from the campus aeronautics lab. By the late 1940s, JPL had grown into an unrivaled set of testing facilities sprawled across some 60 acres.
Marble was put in charge of the group trying to build a workable ramjet—a turbine-less supersonic engine that compresses air by "ramming" it into an inlet that rapidly slows it to subsonic speeds. An ordinary turbojet's ignition source sits in a flame holder, or "can," mounted just behind the compressor. Like a rock in a river, this obstruction creates an eddy in its wake where hot, slow-moving gas gets trapped. This region of relative calm nurtured a stable flame. In a ramjet, however, a momentary tongue of flame would blow out the back of the engine just before it quit.
Marble attacked the problem by repurposing the ramjet lab for combustion research, leading to a string of breakthroughs in the mid-1950s. First, he and graduate student Tom Adamson (MS '50, PhD '54) mathematically analyzed the contact zone between the fuel and the wake. The fuel diffuses across this mixing layer and ignites on contact with the wake, replenishing the eddy's hot gas. By assuming that the mixing layer's gases flowed in a parallel, laminar fashion, Marble and Adamson were able to predict how far downstream the fuel would catch fire and how stable the flame would be. Says Adamson, "We didn't answer every question about combustion in laminar mixing, but we answered many of them." Studies of premixed ignition still refer to the "Marble-Adamson problem" as a paradigm.
High-speed "movies" of the flame confirmed the laminar ignition theory. The movies also showed why the flame blew out—as the airflow increased, the mixing layer suddenly turned turbulent. This dislodged the eddy, which promptly dissipated. The results were "scalable," meaning that they could be applied to any combination of fuel and hardware to find a flame-holder diameter and airstream velocity that would guarantee a steady burn.
Other movies demystified the mechanism behind a type of catastrophic engine failure whose early stages were announced by a 160-decibel screech. These images revealed that the curling tendrils of burnt fuel entering the eddy conjured up opposing whirlpools in order to keep the flow's overall angular momentum in balance. This second set of whirlpools spread outward, and if they withdrew enough heat from the mixing layer, they would themselves ignite. A natural acoustic resonance in the engine could then amplify their thermal energy tenfold en route to the walls. "My desk was 600 feet away," Adamson says. "When the motor began to screech, things shook so hard I couldn't write."
Marble's group also figured out what makes a compressors stall, which happens when its rotating blades lose their "bite." (In a bad stall, the high-pressure surge of air escaping backward through the compressor can do enough damage to bring down an airplane.) Howard Emmons at Harvard had found that an individual blade stalled when it entered a cell of reduced pressure that separated the airflow from the blade, and that these cells leapt from blade to blade; think of the slats of a Venetian blind rippling up and down in a breeze. Marble developed a two-dimensional model of the ripple's essential features—a neat complement to his PhD work on unstalled flow.
Meanwhile, the Chinese-born Tsien had fallen victim to the Red Scare. His top-secret clearance was revoked in the autumn of 1950. For the next five years the Immigration and Naturalization Service forbade him from leaving Caltech's environs. He was unable to enter JPL, or to participate in classified research on campus—in effect, barred from aeronautics altogether. When the Tsiens were evicted from the house they rented, Marble found them another; when they were evicted from that one as well, the Marbles took them in. (Ironically, after being deported in 1955, the embittered Tsien did join the Communist Party and led China into the space age.)
Marble returned to campus full-time in 1959 and began studying multiphase gas dynamics, in which a gas carries tiny particles—in this case, motes of aluminum oxide, routinely added to solid rocket fuels to make them burn hotter. The grains moved more slowly than the gas and their mass affected its flow, causing the rockets to underperform. Marble helped design the nozzle for the solid-fuel Minuteman intercontinental ballistic missile in the early 1960s, but it took most of the decade to work out a complete mathematical treatment of dusty flows.
Marble spent the '70s studying various sources of jet-engine noise before returning to combustion research. Caltech professors Anatol Roshko (MS '47, PhD '52) and Garry Brown had shown in the early '70s that a turbulent shear flow's swirls retained their identities for considerable distances downstream, stretching the mixing layer and wrapping it around itself. Marble and graduate student Ann Karagozian (PhD '82) set about studying how diffusion-driven flames interacted with these vortices—"a very fundamental problem," says Karagozian. "Frank pioneered the coherent-flame model of turbulent combustion, and researchers still use 'flamelet models' in very complicated turbulent combustion simulations."
In addition to his research accomplishments, Marble was legendary for his teaching prowess—and his penchant for 8:00 a.m. lectures delivered "with breathtaking clarity and almost without notes," Karagozian says. "It was tough getting up early for them, but the lectures were incredibly stimulating and rigorous."
Marble's 60-odd graduate students included a who's-who of aerospace engineers as well as Benoit Mandelbrot (Eng '49), the father of fractal geometry. The Frank and Ora Lee Marble Professorship and a graduate fellowship have been established by his students and friends to honor his impact as a mentor as well as a scientist.
Marble was an elected member of both the National Academy of Engineering and the National Academy of Sciences, a rare distinction, and a fellow of the American Institute of Aeronautics and Astronautics (AIAA). His other honors included the AIAA's Propellants and Combustion Award and the Daniel Guggenheim Medal, often regarded as the Nobel Prize of aeronautics.
Marble is survived by his son, Stephen; his daughter-in-law, Cheryl; two grandchildren and one great-grandson. Marble's daughter, Patricia, died in 1996.
A memorial service is planned for Saturday, October 4.