SpaceX's first private flight has blasted off with two contest winners, a health care worker and their rich sponsor, in the most ambitious leap yet in space tourism.

It is the first time a rocket streaked toward orbit with an all-civilian crew; that is, no professional astronauts.

The Dragon capsule's two men and two women are looking to spend three days circling the world from an unusually high orbit — 160 kilometres higher than the International Space Station — before splashing down off the Florida coast this weekend.

It's SpaceX founder Elon Musk's first entry in the competition for space tourism dollars.

Who's on board?

Leading the flight is Jared Isaacman, 38, who made his fortune with a payment-processing company he started in his teens.

Mr Isaacman is the third billionaire to launch this summer, following the brief space-skimming flights by Virgin Galactic's Richard Branson and Blue Origin's Jeff Bezos in July.

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Joining Mr Isaacman on the trip dubbed Inspiration4 is Hayley Arceneaux, 29, a childhood cancer survivor who works as a physician assistant where she was treated — St Jude Children's Research Hospital in Memphis, Tennessee.

Mr Isaacman has pledged $US100 million out of his own pocket to the hospital and is seeking another $US100 million in donations.

Also along for the ride are sweepstakes winners Chris Sembroski, 42, a data engineer in Everett, Washington, and Sian Proctor, 51, a community college educator in Tempe, Arizona.

Ms Arceneaux is set to become the youngest American in space and the first person in space with an implant, a titanium rod in her left leg.

The recycled Falcon rocket soared from the same Kennedy Space Center pad used by the company's three previous astronaut flights for NASA.

However, this time the Dragon capsule aimed for an altitude of 575 kilometres, just beyond the Hubble Space Telescope.

Their fully automated capsule has already been in orbit: It was used for SpaceX's second astronaut flight for NASA to the space station.

The only significant change is the large domed window at the top in place of the usual space station docking mechanisms.

Mr Isaacman, an accomplished pilot, persuaded SpaceX to take the Dragon capsule higher than it's ever been.

Initially reluctant because of the increased radiation exposure and other risks, SpaceX agreed after a safety review.

"Now I just wish we pushed them to go higher," Mr Isaacman told reporters on the eve of the flight.

"If we're going to go to the moon again and we're going to go to Mars and beyond, then we've got to get a little outside of our comfort zone and take the next step in that direction."

Mr Isaacman, whose Shift4 Payments company is based in Allentown, Pennsylvania, is picking up the entire tab for the flight but won't say how many millions he paid.

He and others contend those big price tags will eventually lower the cost.

"Yes, today you must have, and be willing to part with, a large amount of cash to buy yourself a trip to space," said Explorers Club President Richard Garriott, a NASA astronaut's son who paid the Russians for a space station trip more than a decade ago.

"But this is the only way we can get the price down and expand access, just as it has been with other industries before it."

Crew trained for six months before liftoff

Although the capsule is automated, the four Dragon riders spent six months training for the flight to cope with any emergency.

That training included centrifuge and fighter-jet flights, launch and re-entry practice in SpaceX's capsule simulator and a gruelling trek up Washington's Mount Rainier in the snow.

Four hours before liftoff, the four emerged from SpaceX's huge rocket hangar, waving and blowing kisses to their families and company employees, before they were driven off to get into their sleek, white flight suits.

Once at the launch pad, they posed for pictures and bumped gloved fists, before taking the elevator up. Ms Proctor danced as she made her way to the hatch.

NASA now a supporter of space tourism

Unlike NASA missions, the public will not be able to listen in, let alone watch events unfold in real time.

Ms Arceneaux hopes to link up with St Jude patients, but the conversation will not be broadcast live.

SpaceX's next private trip, early next year, will see a retired NASA astronaut escorting three wealthy businessmen to the space station for a week-long visit.

Meanwhile, the Russians are launching an actress, film director and a Japanese tycoon to the space station in the next few months.

Once opposed to space tourism, NASA is now a supporter.

The shift from government astronauts to non-professionals "is just flabbergasting", said former NASA Administrator Charles Bolden, a former space shuttle commander.

"Some day, NASA astronauts will be the exception, not the rule," said Cornell University's Mason Peck, an engineering professor who served as NASA's chief technologist nearly a decade ago.

"But they'll likely continue to be the trailblazers the rest of us will follow."

AP