BROOKS AIR FORCE BASE, Texas (AP) _ With an ambulance waiting just yards away, John Glenn crawled into a centrifuge Thursday and was spun at three times the force of gravity to get ready for his space shuttle launch this fall.

The ambulance was unnecessary. The 76-year-old senator _ the world's oldest astronaut _ said everything went well. The centrifuge operators agreed.

``Feel fine,'' Glenn assured everyone following the second of two nine-minute centrifuge tests. ``Three Gs squash you down, but you come right back up from it.''

Glenn, dressed in his orange flight suit, seemed to enjoy himself as the centrifuge twirled faster and faster. Laying on his back, knees up, just as astronauts do inside the shuttle at launch, he raised and swung his arms to see if he could lift them.

He also wiggled his gloved fingers and moved his knees back and forth, again to test his strength. He seemed surprised he couldn't raise his head when the accelerating force reached three Gs _ one G is the force of gravity _ but was told that was normal.

Centrifuge operators informed Glenn of the minutes ticking by and the milestones in the simulated shuttle launch, as well as the fluctuation in gravity force.

The centrifuge test was conducted on the eve of the 36th anniversary of Glenn's historic flight aboard Friendship 7. Glenn experienced as many as eight Gs when he became the first American to orbit the Earth on Feb. 20, 1962.

There were a few anxious moments at the end of his first centrifuge run. The entire gondola shifted slightly, and both Glenn and his seat rolled to the left. ``Away we go,'' Glenn murmured.

``You fine sir?'' a technician asked.

``Yep,'' the senator replied.

When the test ended, Glenn paused before stepping down from the centrifuge. As he walked toward the exit, an Air Force officer supported his elbow.

A few minutes later, Glenn briefly appeared outside before nearly 50 journalists and said: ``The run went very well. It's a good drill.''

Then he went back in to watch shuttle crewmate Pedro Duque _ who, at age 34, wasn't even born when Friendship 7 carried Glenn around the globe _ take his turn on the centrifuge.

The centrifuge test is required of all first-time space shuttle fliers to acquaint them with the rigors of launch. It was Glenn's first centrifuge ride since his Mercury days.

The oldest person to ride the centrifuge before Glenn was 64.

Lt. Col. Jim Dooley, chief of operations for the flight motion effects branch at Brooks Air Force Base, said three Gs aren't painful, but it can be difficult to breathe.

``Sen. Glenn's in very good shape,'' stressed Dooley. ``He's an ex-Marine.''

Glenn will be 77 by the time he rockets away on shuttle Discovery in October, 16 years older than anyone who's flown in space.

He passed extensive medical exams before NASA approved him last month to fly as part of a study on aging. NASA said the fact that two doctors will accompany him on the nine- to 10-day research mission is coincidental.

The senator reported for training this week at Johnson Space Center in Houston. With Congress in recess, the Ohio Democrat is squeezing in as much as he can before returning to Washington next week.

Dozens of Brooks personnel gathered for a glimpse of Glenn. Dr. Jim Merritt had analyzed blood samples from Glenn and the six other Mercury astronauts back in the early 1960s.

``It brings back memories,'' Merritt said.