The new rocket should be capable of putting more than 53 tonnes (117,000lb) of payload in a low-Earth orbit - more than twice that of the space shuttle.

CEO Elon Musk said the rocket would be made safe enough to launch astronauts.

"It is designed to meet the Nasa human-rating standards," he said. "So, for example, it is designed to structural safety margins that are 40% above the actual flight loads it would expect to encounter."

The performance promised by the 70m-tall (227ft) rocket would make possible bigger, more ambitious missions beyond the space station, he added.

Science, too, would benefit, Mr Musk explained. Complex ventures such as trying to return rocks from the surface of the Red Planet could be considered realistic once the Falcon 9-Heavy was in operation.

The first flight will take place from the Vandenberg Air Force Base. Future missions are also planned at Cape Canaveral in Florida.

Last year, SpaceX became the first private company in history to put a spacecraft in orbit and return it intact. This capsule - known as Dragon - splashed down in the Pacific Ocean.

The unmanned flight was part of a series of demonstrations being mounted by the Hawthorne-based company to prove its technology is reliable enough to be allowed near the space station.

Although it has received seed funding from the US space agency (Nasa), SpaceX says it has spent less than a billion dollars so far on its development programme - a remarkably low figure by any comparison.

The Falcon 9-Heavy ties together the core stages of three standard Falcon 9s. Those cores combined will host 27 individual rocket motors - upgraded versions of the motors that currently power the standard Falcon.

The thrust at lift-off is expected to be 17 meganewtons (3.8 million lbf). This is something akin to 15 Boeing 747s taking off at the same time.

Mr Musk said the vehicle could put in orbit (a few hundred km above the Earth) a mass equivalent to "more than a fully loaded Boeing 737 with 136 passengers, luggage and fuel".

"That's humongous," he told reporters during a media conference at the National Press Club in Washington DC.

"It's more capability than any vehicle in history apart from the Saturn 5 [Moon rocket]. So, it opens up a range of possibilities for government and commercial customers that simply aren't present with the current lifting capacity."

Mr Musk claimed the Falcon 9-Heavy would also be a breakthrough in terms of cost. Missions would be priced at $80m-$125m (£49m-£77m), meaning each pound of payload could be delivered to orbit for about $1,000.

SpaceX is part of the new wave of commercial ventures that Nasa hopes can help reduce the cost of going into space.

With the shuttle due to retire this year, the agency has already given the company a contract to take over some of the servicing needs at the space station. This will be done with an unmanned Dragon launched atop a standard Falcon 9.

On Wednesday this week, Nasa is likely to provide some funds to help SpaceX develop an emergency escape system for the Dragon. This is a necessity before astronauts will be permitted to board it.

The US Congress last year mandated the agency to lead the development of a rocket capable of lifting more than 70 tonnes to low-Earth orbit by 2016. Nasa responded that it would struggle to meet the multi-billion-dollar schedule laid out by politicians.

In a statement Tuesday, SpaceX stressed that an even bigger rocket than the Falcon 9-Heavy would eventually be needed if humans were ever to go to Mars. The Falcon 9-Heavy could not put in orbit the mass of equipment needed to mount such a mission, it added.

Source: By Jonathan Amos Science correspondent, BBC News - 5 April 2011
Photo: Falcon 9-Heavy should make its debut in 2013 [spacex]