Urner Liddel, 74, a retired physicist who specialized in nuclear physics and space sciences, died Sunday at the Fernwood Nursing Center in Bethesda following a heart attack. He had suffered a stroke about a month earlier.

Dr. Liddel, who lived in Washington, joined the National Aeronautics and Space Administration in 1962 and was chief scientist of the lunar planetary program for the next five years. From 1967 to 1970, when he retired, he was special assistant to the deputy associate administrator for the space science division of NASA.

Earlier in his career, Dr. Liddel had been head of the physics branch of the Office of Naval Research from 1942 to 1952. There he was involved in nuclear research.

Dr. Liddel was born in Butler, Mo. He graduated from Central Methodist College in Fayette, Mo., and later studied at the University of Chicago.

He came to Washington in 1929 and joined the Department of Agriculture as a chemist. From 1936 to 1942, he was with the American Cyanamid Co. in Stamford, Conn. In 1941, he earned a doctorate in physics from George Washington University.

From 1952 to 1955, he was with Bendix Aviation in Detroit and then returned to Washington. He was at the National Institutes of Health and the Advanced Research Projects Agency in the Defense Department in the late 1950s and early 1960s. He spent a year with Hughes Research in Malibu, Calif., before joining NASA in Washington.

Dr. Liddel was a founding member of the American Nuclear Society, a fellow of the American Physics Society, a member of the American Association for the Advancement of Science and of the Institute of Radio Engineers, and a past president of the Philosophical Society of Washington. He also was a member of the Cosmos Club.

His marriages to the former Letitia Willett and the former Evelyn Smith ended in divorce.

Survivors include four sons, Robert L., of Riverside, Conn., Gerald and Alan, both of Bethesda, and Bruce, of Lawrence, Kan., and six grandchildren.