WASHINGTON (AP) _ Robert R. Gilruth, a key engineer and scientist in the American manned space program and the Apollo moon missions, died Thursday. He was 86 and had Alzheimer's disease.

Gilruth died at a long-term care facility in Charlottesville, Va., the National Aeronautics and Space Administration said.

An engineer with the National Advisory Committee for Aeronautics, Gilruth led research into rocket-powered aircraft in the 1940s and 1950s and helped to develop a rocket launch range at Wallops Island, Va.

When NASA was established in 1958, Gilruth led a team that created the basic design for Project Mercury, the program that put the first American in space.

In 1961, Gilruth became the first director of the Manned Spacecraft Center, now the Johnson Space Center, in Houston.

During his 10 years a head of center, Gilruth directed 25 manned space flights, including the first Mercury flight in 1961 and the first Apollo moon landing in 1969.

Gilruth was in charge of the development and operation of the Gemini spacecraft, a two-man craft used to perfect techniques for the control, rendezvous and linking of spaceships in Earth orbit.

He is credited with helping to conceive the method used to put astronauts on the moon and return them safely to Earth.

Instead of flying directly to the moon from Earth, Gilruth and his team created a strategy of flying to an orbit around the moon with a command ship and a lander linked together. The lander, carrying two astronauts, then undocked from the command ship and descended to the moon surface. Later, the moon explorers would rocket back into lunar orbit and rejoin a crewmate aboard the command ship for the trip back to Earth.

Gilruth retired from NASA in 1973.

The late George Low, who served as director of NASA's manned space flight program, once said of Gilruth: ``There is no question that without Bob Gilruth there would not have been a Mercury, Gemini or an Apollo program.''

Daniel S. Goldin, the current NASA administrator, said in a statement about Gilruth: ``His courage to explore the unknown, his insistence on following strict scientific procedures, and his technical expertise directly contributed to the ultimate success of the Apollo program and the landing of a man on the moon.''

Gilruth was born in Nashwauk, Minn., and received degrees from the University of Minnesota. He later was awarded honorary doctorate degrees from at least five universities. Gilruth is enshrined in the National Space Hall of Fame and received numerous honors, including the President's Award for Distinguished Federal Service and the prestigious Collier Trophy from the National Aviation Club.

Gilruth is survived by his wife, Georgene Evans Gilruth of Charlottesville, a daughter and a stepson.

The family plans no funeral or memorial service.