In 1949 THERE WAS a brilliant Chinese professor of jet propulsion at the California Institute of Technology who had an eager young specialist in highspeed air dynamics among his graduate students.

The Chinese scientist was so far in the forefront of technological pioneering, and so trusted, that he was made director of the rocket section of the United States National Defense Scientific Advisory Board. But in the turbulent outset of loyalty-security scares, he was arrested in 1950 on charges of being a Communist, and was ordered deported to Communist China. Then, before the order could be carried out, it was decided he knew too many secrets to be deported; finally in 1955 he was sent to China in an exchange of nationals. In China he was believed to have had a major share in developing China's first nuclear bombs.

That was Prof. Tsien Hsue-shen. He had the rank of Air Force colonel in World War II and headed a scientific mission sent to explore the advances in missilery made by Hitler's scientists. Tsien later was one of the braintrusters assigned to predict how the next war might be fought. Their report, "Toward New Horizons," became a major seedbed for many subsequent American military advances.

Tsien's deportation "was a tremendous loss for us, and a great gain for China," said one former student last week. He is Richard D. Delauer, now the Pentagon's chief scientist with the title of undersecretary of defense for research and engineering. In that post, Reagan administration appointee Delauer will have a major role in evaluating what technology the United States can supply to the nation which it now embraces as a "friend" with parallel" global security interests -- the People's Republic of China.

"What a perceptive guy Tsien was," Delauer said admiringly. Delauer, a nonbureaucratic type of scientist from California where he knew Ronald Reagan as governor and Defense Secretary Caspar W. Weinberger as a Reagan associate, is a veteran of the early days of American ballistic missile and jet aircraft development. He most recently was executive vice president of TRW, the engineering conglomerate.

Delauer recalled admiringly that in 1949 Tsien at an academic party at Caltech told Delauer's wife and a friend, "Look girls, I would like to sell you a ticket to the moon." They thought he was drunk or unbalanced. Tsien, Delauer said, "wrote one of the first papers on the application of nuclear energy to controlled rocket flight." f

Tsien, who was Goddard professor of jet propulsion at Caltech, was arrested after government agents in 1950 seized 1,800 pounds of papers and books on rocketry and space physics he intended to ship to Hong Kong. It was charged that the papers contained valuable security information and that their real destination was Shang-hai. U.S. prosecutors charged that Tsien became a Communist before he arrived in the United States as a graduate student in 1935; Tsien denied he ever was a Communist.

Delauer said that after Tsien was barred from the Caltech campus and any access to his own secret research, he nevertheless continued it for a time "by remote control -- with graduate students who met him off-campus." Said Delauer sadly, "It was a disater."

When Tsien finally was deported, Delauer said, "All of us could envision masses of bright students taught by Tsien," challenging a hostile United States with their own scientific breakthroughs. China's "Cultural Revolution" was devastating for its progress, Delauer said, but "it was a big break for us. It destroyed a whole decade of faculty and students." Delauer last saw Tsien in 1952, and understands that Tsien is now in the class of "elder statesmen" in China's scientific development.