H. S. Tsien (2 September 1909-), Americaneducated scientist and professor at the California Institute of Technology who was known for his important work in the fields of jet propulsion, rocketry, and space physics. He was permitted to leave the United States in 1955 after five years of confinement to Los Angeles county for security reasons. He then went to the People's Republic of China, where he played a leading role in Peking's ballistic missile development program.

The native place of H. S. Tsien's family was the Hangchow district of Chekiang. He spent his boyhood in Shanghai, where his intellectual potential soon was recognized. He thus received his secondary education in Peking at the practice school attached to the National Normal University, a school then generally regarded as one of the best middle schools in China. He then matriculated at Chiaotung University in Shanghai, where he received an undergraduate degree in mechanical engineering in 1934. Tsien was awarded a Tsinghua fellowship for study abroad, and he went to the United States in 1935. He first studied at the Massachusetts Institute of Technology, where he received an M.S. in aeronautical engineering in 1936. Tsien then transferred to the California Institute of Technology to study under the Hungarian-born engineering scientist Theodore von Karman. His fields of concentration were aerodynamics and applied mechanics, and he conducted his early scientific experiments in the aeronautical laboratories at Pasadena. Von Karman, impressed with the brilliance of the young Chinese graduate student, noted that Tsien possessed unusual strength in mathematics and theoretical physics, combined with great ability to visualize accurately the physical characteristics of natural phenomena. Von Karman later (1967) wrote that Tsien, while a graduate student, had "helped me to clear up some of my own ideas on several difficult topics." As a graduate student, Tsien also began to publish professional papers, notably in the Journal of the Aeronautical Sciences, dealing with such specialized topics as supersonic flow over an inclined body of revolution, flight analysis of a sounding rocket with special reference to propulsion by successive impulses (the "Karman- Tsien method"), and two-dimensional subsonic flow of compressible fluids. After receiving the Sc.D. at the California Institute of Technology in 1939, Tsien remained at Pasadena as a research fellow in aeronautics. With von Karman and L. G. Dunn, he wrote "The Influence of Curvature on the Buckling Characteristics of Structure," which was published in the Journal of the Aeronautical Sciences in 1940. During the period of early experimentation with guided missiles, Tsien recognized their potential importance and suggested informally that the United States create a new service which would concentrate on jet weapons. He pointed out that the skills required for developing and operating remote-controlled missiles were different from those needed for other types of weaponry and that their operation required new modes of organization and planning in the military services.

During the Second World War, H. S. Tsien remained at the California Institute of Technology, where he contributed substantially to the JATO (jet-assisted takeoff) program and other research conducted under United States government sponsorship. In 1943 he was promoted from research fellow to assistant professor in aeronautics and chief research analyst at the Jet Propulsion Laboratory headed by von Karman. He contributed to several basic memoranda prepared by the laboratory during the later wartime period, notably "The Possibilities of Long-range Rocket Projectiles" (November 1943) and "Comparative Study of Jet Propulsion Systems as applied to Missiles and Transonic Aircraft" (March 1944). In 1944 von Karman, under United States Army Air Force sponsorship, organized a special Scientific Advisory Group. Tsien was invited to serve as a consultant to that group, and he participated in an important meeting held under Air Force auspices at Woods Hole, Massachusetts, in 1945 to assess long-range problems and prospects of space flight technology. A major report stemming from the work of the Scientific Advisory Group, entitled Toward New Horizons, dealt with a broad spectrum of problems, including flight in the atmosphere, guided missiles, and orbital flight. It became a basic document in United States long-range aeronautical programming. Near the end of the Second World War, Tsien accompanied von Karman to Europe to inspect rocket installations in Germany, including the Kochel and Otztal wind tunnels, which later served as models for similar equipment in the United States. Tsien was with von Karman when he interrogated his former professor, Ludwig Prandtl, the prominent aerodynamicist of the Gottingen institute who had remained in Germany under the Nazi regime. During his first decade in the United States, Tsien thus had unique opportunities for personal contact not only with von Karman but also with the talented group of pioneers in rocket research and development then largely centered at Pasadena, a group which included L. G. Dunn, Clark Millikan, William H. Pickering, William Duncan Rannie, Homer Joe Stewart, and others. He gained unusual insight into the problems involved in design and development of missiles and space vehicles. In 1945 Tsien was promoted to the rank of associate professor in aeronautics at the California Institute of Technology. He was regarded as von Karman's most talented student and he reciprocated, in the traditional Chinese manner, by addressing his professor as "revered teacher." In addition to rockets and jet propulsion problems, Tsien's interests embraced rarefied gases, the dynamics of compressible fluids, and the theory of elastic thin shells. His scientific papers gained international recognition from colleagues in aerodynamics and related fields. In 1946 he published what was regarded as the first analysis made in the United States of the theoretical potential of nuclear fission and fusion for interplanetary and interstellar flight. In the same year, his article on "Superaerodynamics, Mechanics of Rarefied Gases," published in the Journal of the Aeronautical Sciences, was another innovating inquiry which defined and systematically explored problems occurring in very high altitude and orbital flight and in atmospheric entry. (Then a new branch of aerodynamics, this research area quickly expanded at an exponential rate. By the 1960's there were several thousand scientists working in the field, with an International Symposium on Rarefied Gas Mechanics meeting biennially.) Tsien's "Similarity Laws of Hypersonic Flows," published in the Journal of Mathematics and Physics in 1946, was also a pioneering paper. Other research papers of the early postwar period dealt with a variety of specialized topics: lifting-line theory for a wing in non-uniform flow, the Glauert-Prandtl approximation for subsonic flows of a compressible fluid, twodimensional irrotational mixed subsonic and supersonic flow of compressible fluids and the upper critical Mach number, and a generalization of the Alfrey theorem for viscoelastic media. Tsien also continued to serve as a member of the Scientific Advisory Board of the United States Air Force.

In 1946 Tsien returned to Cambridge to join the faculty of aeronautical engineering at the Massachusetts Institute of Technology. After a year there as associate professor of aerodynamics, he was promoted in 1947 to the rank of professor, one of the youngest scientists ever to gain that rank at the institution. In the summer of 1947 he returned to China for family reasons. He was offered the post of president of Chiaotung University, his alma mater in Shanghai, but he declined it and returned to the Massachusetts Institute of Technology. Tsien's internationally recognized distinction was confirmed in 1949 when he was named Robert H. Goddard Professor of Jet Propulsion and director of the Daniel and Florence Guggenheim Jet Propulsion Center of the California Institute of Technology. In June 1950 his important survey article on programs of instruction and research at the Jet Propulsion Center, published in the Journal of the American Rocket Society, dealt with the entire range of problems involved in development of missiles and spaceships : finance, research, development, experimentation, and engineering.

In August 1950, after the outbreak of the Korean War but before the intervention of Chinese Communist military forces, H. S. Tsien decided to return to his native land. He purchased airplane tickets and delivered luggage, books, and research materials to a forwarding agent for shipment to the Far East. Two days before he was scheduled to depart from Los Angeles, Tsien received notification from United States immigration authorities that, because of the outbreak of the Korean conflict, he would have to remain in the United States for an indefinite period. Although the notice was a routine announcement sent to Chinese students and others in the United States who planned to return to the mainland of China, the scientific distinction of H. S. Tsien quickly made his case a cause celebre. Agents of the Federal Bureau of Investigation searched his luggage, and his books and research notes were impounded by the United States customs. In September 1950 the immigration authorities notified Tsien that he had violated United States laws.

Tsien then was arrested on charges that he was or had been a member of the Communist party and that he was attempting to leave the United States for the People's Republic of China with allegedly secret scientific documents dealing with rocketry and space physics. These allegations were based on unidentified reports that Tsien, while a graduate student in Pasadena, had been acquainted with American students who had been members of or associated with the Communist party and that he therefore had supported the overthrow of the government of the United States by violent means. After two weeks of confinement, Tsien was released on $15,000 bail, which was provided by the California Institute of Technology. The case quickly attracted widespread attention. Faculty colleagues and other scientists attested to Tsien's personal integrity and professional reputation. Although a United States federal court later upheld the deportation order for Tsien, the security authorities of the United States government took the position that Tsien possessed so much information on classified research programs directly related to American national security interests that it would be inadvisable to permit him to leave the country. From late 1950 until 1955 H. S. Tsien was forbidden to travel outside the boundaries of Los Angeles county and was required to report monthly to the United States immigration authorities. The Federal Bureau of Investigation kept him under constant surveillance during the entire period. In his autobiographical volume The Wind and Beyond (1967), Tsien's mentor von Karman wrote: "I was convinced, as were virtually all of my associates . . ., that Tsien was not a member of the Communist party or had had anything more than social association with some individuals who were later identified as Communists or Communist sympathizers. A number of us felt, however, that while such 'evidence' against Tsien as was available could not be acceptable in a court of law, the Immigration Service tended to act on hearsay, and on relatively flimsy evidence." Despite, or perhaps because of, personal tensions created by the unusual security precautions, Tsien buried himself in work and produced a fantastic flow of scientific papers, many of which were published in the Journal of the American Rocket Society. These papers dealt with a wide variety of topics: transfer functions of rocket nozzles, linear systems with time lag, automatic guidance oflong-range rocket vehicles, properties of pure liquids, take-off from satellite orbit, a similarity law for stressing rapidly heated thin-walled cylinders. He also wrote on problems of education in the engineering sciences and sought to promote a new interdisciplinary science which he labeled "physical mechanics." Like many other first-class scientists, H. S. Tsien wrote many papers but few books. While in Cambridge in the late 1940's, he had been impressed with the work of Norbert Wiener at the Massachusetts Institute of Technology. Wiener then was completing the manuscript for his book Cybernetics, published in 1948, which formulated the new science of the organization of mechanical and electrical components for stability and purposeful actions. Tsien was struck by the fact that a distinguishing feature of Wiener's system was total absence of considerations of energy: the primary concern of cybernetics was the qualitative aspects of interrelations among various components of a system and the synthetic behavior of the total mechanism. Tsien was so impressed with the potential significance of this line of conceptualization that he wrote a book entitled Engineering Cybernetics, which was published in 1954. His major objective was to study those aspects of the new science of cybernetics which had direct application to the control and guidance of mechanical and electrical systems that were of vital importance to modern weaponry systems and warfare. Writing in 1954, Tsien suggested that at the then current level of development in the field there were significant advantages in formulating engineering cybernetics as a new interdisciplinary approach designed to embrace the entire field of control and guidance engineering in a systematic way, to suggest new vistas, and to probe new approaches to new problems. Because he had little personal interest in (or patience with) routine engineering problems. Tsien's concern was primarily conceptual. He stressed the fact that while servo-mechanics was an engineering practice, engineering cybernetics was an engineering science, dominated by theoretical analysis and utilizing the tools of advanced mathematics. In discussing control of error, for example, he recognized that the subject was in a primitive stage of development and noted that theory was then based almost entirely on a series of lectures, "Probabilistic Logics and the Synthesis of Reliable Organisms from Unreliable Components," given by John von Neumann at the California Institute of Technology in 1952.

During his period of confinement, Tsien continued to hold the Goddard chair at the California Institute of Technology. On 4 August 1955, more than two years after the ceasefire arrangements that terminated the Korean conflict (superficially, the obstacle to his planned departure in 1950), Tsien was notified by the United States government authorities concerned that he would be permitted to leave the country. He then terminated his affairs at Pasadena and sailed from San Francisco on 1 7 September aboard the President Cleveland. His disenchantment at that point with aspects of government security operations in the United States, where he had spent two decades, was scarcely concealed. After disembarking at Hong Kong, where the British authorities took special precautions, he proceeded by rail to Canton on 8 October 1955. There he was greeted not only by representatives of the Chinese Academy of Sciences but also by his 74-year-old father, who celebrated the return of his distinguished son by presenting him a set of copies of major Chinese paintings. Tsien then proceeded to Shanghai, where he was welcomed by prominent Chinese scientists. He reached Peking on 28 October 1955. Two days later he held a press conference at which he openly condemned the United States government for his extended detention, expressed sympathy for Chinese students still in the United States, and stated his determination to serve the Chinese people. On 3 November, it was announced that Tsien had accepted the invitation of the Academy of Sciences to direct its program of research in applied mechanics. Tsien himself stated that he hoped to place his scientific knowledge and research experience at the service of China and to help train younger scientists in the People's Republic of China.

As director of the institute of mechanics of the Academy of Sciences, Tsien occupied a key position in Peking's advanced research apparatus. In January 1957 the academy announced that Tsien had been given a first-class award for the outstanding work on fundamental problems of automatic control and guidance embodied in his 1954 volume on engineering cybernetics. In 1958 he was permitted to join the Chinese Communist party at the same time that Ch'ien San-ch'iang (q.v.) and several other prominent Western-educated scientists gained membership. Tsien also served as chairman of the China Dynamics Society and of the China Automation Society; and he was chairman of the department of modern mechanics at the University of Sciences and Technology in Peking. He represented Kwangtung province as a deputy to the National People's Congress. Little was known of the work of H. S. Tsien at Peking during the 1960's. He did write a technical paper, "On the Basic Equations of Soil Dynamics," which appeared in the English edition of a volume entitled Problems of Continuum Mechanics (1961), issued in honor of the seventieth birthday of N. I. Muskhelishvili of the Soviet Academy of Sciences. During the mid-1960's Tsien was called upon to support the then current Peking position that it was necessary for Chinese intellectuals to demonstrate both political reliability and professional competence, or, in the phrase used by the Communists, to be "both red and expert." In a leading article in June 1965 in Chung-kuo ch'ing-nien-pao [China youth newspaper], Tsien stressed that political considerations were dominant in all societies, whether socialist or bourgeois, and stated that scientists in the United States, even those who professed to do pure research, were involved in politics and shaped by the constraints of the capitalist system. Although his nominal responsibilities in China were limited to problems of applied mechanics in the Academy of Sciences, Tsien's larger contributions to Peking's ballistic missile development program were suggested by the announcement in October 1966 that China had successfully landed a missile bearing a nuclear warhead on target at a distance of 400 miles.

On his return to China from the United States in the summer of 1947, H. S. Tsien married Chiang Ying (Tsiang Yin), the third daughter of a prominent intellectual and specialist on military strategy, Chiang Fang-chen (q.v.). Tsien dedicated Engineering Cybernetics to her.