Saying he successfully completed his mandate to get the space shuttle flying again, James C. Fletcher announced Tuesday that he is resigning April 8 as NASA administrator.

Fletcher, 69, a former University of Utah president, has served as NASA chief longer than anyone else, from April 1971 to May 1977 and from May 1986 to present.President Reagan persuadedhim to come back for the second term after the Challenger accident in January 1986.

Fletcher at the time was reluctant to return, but Reagan convinced him that he was needed to restore confidence to the National Aeronautics and Space Administration and to oversee the redesign of the shuttle to ensure its safe return to space.

A press release issued by NASA said that with the successful conclusion of the third post-Challenger space shuttle mission last Saturday, Fletcher feels he can safely place the leadership of NASA in other hands.

He told President Bush in his letter of resignation, "It has been a pleasure to serve you, both in your capacity as vice president and in recent weeks as president."

No permanent replacement has yet been named, although NASA announced that deputy administrator Dale Myers will become acting administrator.

Fletcher was U. president from 1964 to 1971. He received his doctorate from the California Institute of Technology in 1948 and received honorary doctorates from the U. in 1971 and from Brigham Young University in 1977.

Several rumors have surfaced in recent months about Fletcher's possible replacement.

Sen. Jake Garn, R-Utah, was mentioned - and was even considered the best qualified by Rep. Bill Nelson, D-Fla., the only "congressional observer" besides Garn to fly on the shuttle. Garn, however, said he is happy in the Senate and has not been approached by the administration about the job and does not expect to be.

Others mentioned include ex-astronaut Robert Crippen, deputy director of shuttle operations; Sally Ride, the first U.S. woman in space; Frank Borman, former astronaut and former head of Eastern Airlines; and J.R. Thompson, director of the Marshall Space Flight Center.