ROBERT TRUAX is thinking even bigger thoughts than he sid when he built Evel Knievel's steam-powered flying machine known as the Sky-Cycle that failed to make it across the Snake River Canyon three years ago.

Truax, a former Navy rocket expert, has begun work on his own rocket that he dreams someday will carry someone into space; he wants to launch a private astronaut. And that's only half of Truax's latest activities.

Nearing 60, Truax says he spends the rest of his time "studying the problem of aging." The engineer has turned to biochemistry, having studied two years at Stanford, come up with a theory on why exactly people grow old, and started a book "to encourage research into aging."

Truax, a retired Navy captain who headed the military space program in the late '50s "when it was all hush-hush" and helped get the Mercury project under way, worked two years putting together Knievel's canyon "Cycle," doing much of the assembly at his former residence in Potomac.

Knievel's flight became a fizzle when his two parachutes opened almost immediately after blast-off causing the promotion-minded daredevil to float unceremoniously to earth on some rocks on the near side of the canyon bottom. Truax says the "suction effect" of the takeoff caused the "lid of the canister holding the chute to open prematurely."

Truax says the same thing happened in a test flight two weeks before. He thought he had solved the problem after that brief flight of a spare "Cycle" carrying "a fellow named 'Good Galahad,' an anthropomorphic parachute-testing dummy" who "ended up in the river."

Despite the unhappy landing, Knievel dusted off his American-flag colored jumpsuit and began thinking of anew stunt. "This is going to be a hard act to follow." Truax says that Knievel told him. "You have anything else up your sleeve?"

While Knievel was apparently thinking that over. Truax built for him a display ramp that he uses to show off his Sky-Cyle and a jet-powered motorcycle that Knievel rode in a Bicentennial July 4 parade in his hometown of Butte, Mont. But Truax says he hasn't heard from Knievel lately.

Meanwhile, Truax has started building his new rocket. All he needs, he says, is $1 million to finish the project, and he's looking for a promoter to back him. Truax says one promoter mentioned that he "liked the show business aspects" of a private astronaut shoot "for closed circuit TV."

Truax says that he has finished "about 10 per cent" of the rocket and has a 25-foot mockup of it that he brings out on the driveway of his present home in Saratoga, Calif., when someone wants to see it. This inevitably prompted one person to mention that it was easy finding Truax's house because it's the only one with a rocket in the frontyard.

As if he wants to live to see someone go off in his rocket, Truax also has become absorbed with studies about aging. "I'm pushing 60 with a short stick," he says. "One day I looked in the mirror and said, 'My God, this is terrible.'

"I've got a theory; and not everybody agrees there's a sole cause for aging, but I think the cause of aging is connected with water. All water contains a fraction of a per cent of deuterium oxide, which has violent biological effects.

"Whatever the cause, I'm writing a book, trying to stimulate research into my theory and all the other avenues. Hundred of millions of dollars are spent for cancer research, but only $3 million - something like that - is spent on aging."

One other thing about his rocket: Truax, besides needing money, needs an astronaut. "It will only carry one," he says, "male or female." Amelia Earhart, he says wistfully, would have been perfect, but, with a promotional sense matching that of Knievel himself. Truax adds that he'd be happy to send up Farrah Fawcett-Majors . . .