John Townsend Lake, 43, who retired last year as a Navy lieutenant commander and nuclear submarine engineer and then became a consuiltant in that field for the Essex Corp. of Alexandria, died Tuesday at Walter Reed Army Medical Center following open heart surgery. He lived in Alexandria.

Cmdr. Lake, who was born in Berwyn, Ill., and grew up in the Washington area, was commissioned in the Navy in 1960 after earning a bachelor's degree in mechanical engineering from Cornell University. He served aboard destroyers before earning a master's degree in mechanical engineering in 1967 from the Naval Postgraduate School in Monterey, Calif., where he specialized in nuclear submarine construction, conversion, repair and maintenance.

From 1967 to 1976, he served as a project officer, supervising the building, conversion and repair of nuclear attack submarines at Newport News, Va., and as an engineer and nuclear repair officer at the Naval Submarine Base in New London, Conn., and the Portsmouth Naval Shipyard in New Hampshire.

He then returned to this area and served as assistant for operations for the Polaris nuclear ballistic missile submarines in the Office of Naval Sea Systems Command until retiring in June. He received a Meritorious Service Medal.

Cmdr. Lake then hne joined the Dessex Corp. as a nuclear submarine engineering consultant.

He was active in motorcycle and sports car racing and was a member of the Sports Car Club of America and Tigers East, another racing group.

Survivors include his wife, Edwina K., and four sons, Douglas E., Kevin R., Dean T., and J. Timothy, all of Alexandria; his parents, James Harris and Susan Townsend Lake of Chevy Chase, and a sister, Jane Lake Birt of London.

The family suggests that expressions of sympathy be in the form of contributions to the Salvation Army or to the American Heart Association.