Paul G. Allen's first foray into rocketry, as he recalls it, was inauspicious.

　　"My cousin and I tried to build a rocket out of an aluminum armchair leg," he said. At just 12 years old, the future billionaire raided his chemistry set for zinc and sulfur, and packed the fuel mixture into the tube. He got the formula right, but had not looked up the melting point of aluminum.

　　"It made a great noise," he said, "and then melted into place."

　　His rockets have gotten better since then, and a lot bigger, too. Mr. Allen, who became a co-founder of Microsoft, is responsible for SpaceShipOne, the pint-size manned rocket that won the $10 million Ansari X Prize competition last year as the first privately financed craft to fly to the cusp of space - nearly 70 miles up.

　　Mr. Allen is not the designer; that is Burt Rutan, the legendary aeronautical engineer with the sideburns that look like sweeping air scoops. He is not one of the test pilots who made the competition-winning flights; they are Michael Melvill and Brian Binnie. Mr. Allen is, instead, the one who gets little glory but without whom nothing is possible - he is the guy who signs the checks. And he did what the rich do: he hired good people.

　　The SpaceShipOne flight made him the best-known member of a growing club of high-tech thrillionaires, including the Amazon founder Jeff Bezos, who find themselves with money enough to fulfill their childhood fascination with space. Rick N. Tumlinson, co-founder of the Space Frontier Foundation, a group that promotes public access to space, said the effort had become a geeky status symbol. "It's not good enough to have a Gulfstream V," he said. "Now you've got to have a rocket."

　　Many self-professed "space geeks" say the possibility that entrepreneurs like Richard Branson of the Virgin Group may help regular people see the black sky - well, regular rich people, at least - has drawn away much of the excitement that government-financed human space efforts long enjoyed.

　　"It's completely shifted," said Charles Lurio, a space consultant with an interest in private efforts that goes way beyond ardent. "This is where the action is, not at NASA."

　　The new generation of deep-pockets space entrepreneurs includes Mr. Bezos, who founded Blue Origin, in Washington State, and quietly announced this year that he had bought 165,000 acres of land in West Texas as a base for his eventual launching operations.

　　Elon Musk, the founder of PayPal, created the rocket company SpaceX, and John Carmack, the creator of computer games like Doom and Quake, has been testing rocket designs through his company, Armadillo Aerospace near Dallas.

　　The engine for Mr. Allen's craft was developed by SpaceDev, a company formed as a second act by another computer entrepreneur turned space man, Jim Benson. And Larry Page, a co-founder of Google, recently joined the board of the X Prize Foundation.

　　The rise of the space money men is a unique moment in history, said Dr. Peter H. Diamandis, a co-founder of the X Prize. "There is sufficient wealth controlled by individuals to start serious space efforts," he said.

　　What's more, they are frustrated, he went on, adding: "The dreams and expectations that Apollo launched for all these entrepreneurs have failed to materialize. And in fact, those who look into it realize that the cost of going into space has gone up and the reliability has, effectively, gone down."

　　For Mr. Allen, 52, SpaceShipOne was no set-it-and-forget-it bauble of a project. It was an expression of a lifelong passion, he said, a "love of science and technology, and what can be done with engineering."

　　He recalled the widespread excitement in the 1960's about the Mercury, Gemini and Apollo missions, when "I really got enthralled, and probably more than most kids."

　　Science became his fascination, and with a librarian as a father, he soon learned that there was a book to answer each of his innumerable questions. Like many children, he would go down to the five-and-dime and buy plastic rockets that could be filled with water and pumped up with air, whose compression built up launching pressure. But he took it further, checking out books to find out "how the heck a turbopump worked" at the age of 11.

　　He devoured novels like "Rocketship Galileo," by Robert A. Heinlein, in which enterprising teenagers join forces with a scientist to build and fly a rocket to the Moon, and nonfiction books like "Rockets, Missiles and Space Travel," by Willy Ley. He recalled riding his bicycle to the local hobby shop and reaching for the top shelf, where there was a model of a space station along with Redstone and Atlas rockets.

　　As the Moon shots of the Apollo program came to an end, however, Mr. Allen saw his voracious interest in the sciences expanding to other fields: chemistry, and - by the age of 15, when he met a teenager named Bill Gates - to computing.

　　Even after starting Microsoft as a tiny software company in Albuquerque, he kept his interest in space alive. "At the back of my mind, there was always this desire, this inkling of desire, that someday I would try to do something with aerospace or rocketry," Mr. Allen said.

　　He recalled flying to Florida with Charles Simonyi, a space-obsessed co-worker, to see the first shuttle launching in 1981. "I'm not sure that Bill Gates was happy that we even took off a weekend to do that," he said, since the company was preparing software for the IBM PC at the time.

　　But the launching was worth any tension on the job. "The air basically vibrates," he recalled.

　　"There are hundreds of thousands of people yelling, 'Go! Go! Go!' "

　　In 1982, he learned he had Hodgkin's disease, and he withdrew from day-to-day work with Microsoft the next year. (The cancer has been in remission since 1985.) Microsoft made him a billionaire 20 times over, with the ability to invest wisely, unwisely - even, on occasion, frivolously. It gave him the luxury of being able to take gambles and make mistakes. Some of his huge early investments in what he called the "wired world" of media, interactive technologies and cable were duds.

　　More recent investments in biotechnology, energy and real estate could prove more promising. He also owns two major-league sports teams, the Portland Trail Blazers in basketball and the Seattle Seahawks in football.

　　And he has begun to put some of his wealth into sharing his enthusiasms with the world: he has given away hundreds of millions of dollars through philanthropy, has founded the Allen Institute for Brain Science and has created museums in Seattle devoted to rock 'n' roll and to science fiction.

　　It is all of a piece, he said. Projects like the museums "try to plant those seeds of imagination" like the ones that carried him so far.

　　In the past, amateur rocketry had been essentially a weekend pursuit that brought guys together for fun and fire. Cheap and powerful computing has brought down the cost and the amount of trouble it takes to design rockets, said John Wickman, an aerospace engineer who has written a popular guide to rocket making; amateur groups commonly send rockets to 30,000 or 40,000 feet, he said, at a cost of several thousand dollars.

　　"You can get a couple of guys together, and if your wife won't kill you for spending money like that, you could probably pull it off," he said. But amateurs could take things only so far, and could not have hoped to create human-rated craft that could blast past the 328,000-foot line that the Ansari X Prize defined as the edge of space.

　　Mr. Tumlinson says the tech entrepreneurs are accustomed to putting powerful technologies into the hands of individuals against enormous odds - a good foundation for the space business. "The current American space program is a passive activity that has no connection with those watching it or their children," he said. The new space race is different: "It's about 'you can do it.' "

　　NASA officials past and present say it is not that easy. While they praise the achievements of the Rutan-Allen team, they point out that there is a vast difference between reaching the cusp of space, which NASA did in the 1960's, and building something that can withstand the punishing conditions of orbital space and re-entry.

　　Sean O'Keefe, a former administrator of the agency, called SpaceShipOne's success "a great achievement," but also "a modest first step."

　　The just-do-it adventurism of the SpaceShipOne project could never fly at NASA, he said, adding,

　　"If I had authorized somebody to jump into a plastic airplane fueled by laughing gas in just a flight suit, there would have been a Congressional investigation the next day - whether it was successful or not."

　　Mr. Allen understands the challenges better than most. He speaks knowledgeably about the amount of energy that must be dissipated when a vehicle returns from orbit and the various methods of dealing with the risks.

　　"It's much harder to do orbital flight - much," he admitted. But his immediate goal is less grand: to lay the groundwork for businesses that will carry adventurers, briefly, into space. That is a realistic goal, he said. There are plenty of people who would pay for the experience, and "you could actually get a return on your investment dollar."

　　He is licensing the innovations developed by Mr. Rutan, he went on, adding, "I am optimistic that I will have a good chance to get my money back."

　　He says he spent about $20 million on the project, which is about what he earns in interest while flossing.

　　In the meantime, he said, he has had a blast, having had a hand in "some of the aspirational part of what it means to be human." Standing in the control room for SpaceShipOne during the launchings, he said, his heart was in his throat and he felt deeply the risk that the pilots were taking.

　　But he is in no rush to touch the rim of space himself. "After it's proven to be incredibly safe, I might consider it," he said. "I have a lot of things I want to see to fruition."