WASHINGTON — Paul Allen, the billionaire co-founder of Microsoft who backed the winning entry in a suborbital spaceflight competition and later funded development of a massive air-launch system, passed away Oct. 15.

In a statement, Vulcan Inc., Allen’s holding company, said that Allen, 65, passed away Oct. 15 in Seattle from complications of non-Hodgkin’s lymphoma. Allen announced two weeks earlier that he had been diagnosed with the cancer again after being treated for it in 2009.

Allen is best known for founding, with Bill Gates, software giant Microsoft. He left the company in the 1980s after being diagnosed with Hodgkin’s disease, and turned to various other business pursuits, including ownership of several professional sports teams. At the time of his death his estimated net worth was $20 billion.

Among those pursuits was an interest in spaceflight. “I wanted to do something in rocketry that no one had done before,” he recalled in his 2011 biography Idea Man. That led to meetings in the late 1990s with famed aircraft designer Burt Rutan, who was pursuing ideas for suborbital vehicles to compete for the $10 million X Prize. The two reached an agreement in 2000 to develop what became known as SpaceShipOne.

Allen ultimately spent $28 million to fund SpaceShipOne, which won the renamed Ansari X Prize in October 2004 after performing two suborbital spaceflights less than a week apart. Allen said in his book that his investment in the project earned a “net positive return” for him through the prize purse, which was split evenly with Rutan’s Scaled Composites; licensing fees for SpaceShipOne technology from Virgin Galactic; and a tax writeoff for donating the vehicle to the Smithsonian’s National Air and Space Museum, where it is on display in a gallery alongside Charles Lindbergh’s “Spirit of St. Louis” aircraft and the Bell X-1 used by Chuck Yeager to break the sound barrier.

Allen returned to the space business in 2011 when he announced he was funding development of a new venture, Stratolaunch Systems, that would develop an air-launch system on a scale never before attempted. That concept involved the development of an aircraft with a wingspan longer than any other in the world that would be used as a platform for launching rockets.

“Stratolaunch will build an air launch system that will give us orbital access to space with greater safety, flexibility and cost effectiveness,” he said at a December 2011 press conference to announce the venture. He said then he planned to spend about ten times as much on Stratolaunch as did on SpaceShipOne.

At the time of that announcement, Allen said he expected the plane to start flying in 2015, with a first launch from it in 2016. Development, though, has taken longer than expected, while the company has changed direction several times on the rocket that would be used to launch it.

The plane is just now approaching its first flight, having performed its latest taxi test last week, reaching speeds of about 130 kilometers per hour. That test met all objectives, Jean Floyd, president and CEO of Stratolaunch, tweeted Oct. 11.

The company has relied on Allen’s wealth to fund its development, and the long-term view that Allen has taken. “His vision is to change the human condition,” Floyd said of Allen during a May 2017 panel discussion. “He’s not out to make a buck. He’s not out to get a return on investment.”

“I’m not saying he wants to lose money,” he added. “But Paul Allen has never talked to me about, ‘How much money am I going to make if I pull this off?’”

Vulcan, in its statement about Allen’s death, said that plans had been developed for continuing his various ventures after his passing, but did not disclose details.

“Paul thoughtfully addressed how the many institutions he founded and supported would continue after he was no longer able to lead them. This isn’t the time to deal in those specifics as we focus on Paul’s family,” said Bill Hilf, chief executive of Vulcan, in the statement. “There are no changes imminent for Vulcan, the teams, the research institutes or museums.”

“Incredibly sad news. We deeply respect and admire Mr. Allen’s vision,” Floyd tweeted in response to the news of Allen’s death. “His legacy will be honored.”

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