SpaceX successfully launched what is now the world's most powerful rocket, a towering behemoth known as the Falcon Heavy that tore through the sky with the thundering force of 18 Boeing 747 jetliners.

Lifting off at 3.45pm (9.45am NZT) from the same launchpad that sent the crew of Apollo 11 to the moon, the rocket sent up a mountain-sized plume of smoke and a rattling roar across Florida's Space Coast, where thousands gathered to watch.

The mission represented the first test of the massive rocket, powered by 27 engines in three first-stage boosters that are essentially strapped together.

The maiden flight also marked the first time a privately financed venture ever attempted to launch a rocket so powerful that it was capable of hoisting a payload out of Earth's orbit.

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As a stunt, SpaceX founder Elon Musk loaded the Falcon Heavy with his own cherry-red Tesla Roadster carrying a spacesuit-clad mannequin named "Starman" in the driver's seat. Musk said he planned to send the convertible into an orbit around the sun that would take it near Mars.

The Falcon Heavy's successful launch represents a "revival of the exploring spirit," said John Logsdon, a space historian who is a professor emeritus at George Washington University.

Nasa's space shuttle programme, which ended in 2011, was limited to what's known as low Earth orbit, where the International Space Station flies at about 400km above the surface of the Earth.

But the Falcon Heavy represents a chance to go beyond that, into deep space, to really "push the frontier," Logsdon said. "This really gives us a capability that this country has not had since the last Saturn V flight, which was in 1973."

Falcon 9 SpaceX heavy rocket lifts off from pad 39A at the Kennedy Space Center. Photo / AP
Falcon 9 SpaceX heavy rocket lifts off from pad 39A at the Kennedy Space Center. Photo / AP

Space fans by the thousands lined the beaches and causeways in anticipation. SpaceX topped off the launch by successfully landing two boosters on land, setting off twin sonic booms on their return.

A third first-stage, the centre core, crash-landed at sea at 480km/h. At SpaceX's headquarters, employees cheered wildly as the rocket soared out of the atmosphere.

"I'm still trying to absorb everything that happened because it seemed surreal to me," Musk told reporters later. "I had an image of a giant explosion on the pad with a wheel bouncing down the road and the Tesla logo landing somewhere. But fortunately that's not what happened. The mission seemed to have gone as well as possible."

If SpaceX can fly the Falcon Heavy reliably, the rocket could prove useful to the Pentagon for lifting national security satellites and to Nasa for helping its human exploration goals. SpaceX says the rocket is capable of hauling more mass further than any existing rocket — an estimated 63,500kg to low Earth orbit, and nearly 18,140kg to Mars.

But industry officials say there are some concerns about how big the market is for the Falcon Heavy. SpaceX had been planning to fly a pair of tourists around the moon as early as this year.

But on Tuesday, Musk announced a reversal, saying the Falcon Heavy probably would never fly humans, as the company shifts its focus to its next-generation rocket, known as the "BFR," or "Big Falcon Rocket".

SpaceX's launch comes as the Trump Administration is focused on returning to the moon. While it has not released details of its plans or their cost, officials support having Nasa partner with commercial companies such as SpaceX, which are striving to make space travel far more affordable than it has been in the past.

"It's hard for me to overstate the importance of the launch today," said Lori Garver, a former Nasa deputy administrator. "I think this could end up being really the saviour of Nasa and deep space exploration."

Commerce Secretary Wilbur Ross, a member of the reconstituted National Space Council, was on hand to view the launch. He lauded SpaceX's efforts in bring back to the US a large portion of the world market share for launches. And he said that one of the council's top priorities is "how to accelerate the progress of the commercialisation of space".

SpaceX's successful launch raises questions for Nasa about how best to proceed. The space agency has been working to develop the Space Launch System, an even more powerful rocket than the Falcon Heavy, but at about US$1 billion per launch, it is many times more expensive. Ross said there is room for both systems.

SpaceX broadcast a live stream from the space-cruising Roadster using three cameras mounted to the vehicle.