Image copyright SpaceX Image caption The triangular features shown here are the legs that deploy just before landing

SpaceX says its recent experiment to return part of its Falcon-9 rocket back to Earth under control was a success.

The US company has confirmed that the first-stage of the vehicle launched from Cape Canaveral a week ago used its engines to slow its fall, deployed a set of legs and made a "soft landing".

For safety reasons, the touchdown was actually commanded to take place east of the Cape, far out at sea.

Nonetheless, the stage was vertical and had zero velocity on contact.

The company has video of the event - albeit of poor quality - that it plans to post on its website.

Extremely rough seas meant that a boat could not get to the scene for two days to try to salvage the stage before it sank.

Potentially, the experiment has enormous significance for the space industry.

Media playback is unsupported on your device Media caption The first-stage gets the rocket up off the pad

Traditionally, rockets have been expendable.

As a vehicle makes an ascent, it dumps propellant stages, which then fall to destruction, torn apart as they tumble end over end.

SpaceX believes if it can recover those stages and fly them again and again, the cost of access to space could be dramatically reduced.

"No-one has ever soft-landed a liquid-rocket boost-stage before," said SpaceX chief designer Elon Musk. "I think this bodes well for achieving reusability.

"What SpaceX has done thus far is evolutionary, not revolutionary. [But] if we can recover the stage intact and re-launch it, the potential is there for a truly revolutionary impact in space transport costs."

The first-stage of a Falcon-9 - the segment that gets the vehicle up off the pad - represents about 70% of a flight's $60m price tag.

Mr Musk told reporters that reusable stages could therefore lead to a hundred-fold improvement in the cost of access to space.

Image copyright SPACEX Image caption This demonstrator shows what a returning stage with legs looks like

The company will now further refine its technology with the aim of making a return to dry land by the end of the year.

It needs to reduce the error on touchdown to a maximum of a mile. However, Mr Musk believes there is no reason why a return could not have the same accuracy as a helicopter, which comes to rest within feet of its intended landing location.

Discussions are already under way with the US Air Force, which runs the Cape Canaveral launch complex, to identify a suitable landing zone on the Florida coast.

The Friday 18 April experiment, which followed the successful launch of the Dragon cargo ship to the space station, was actually the second time SpaceX had tried a controlled return.

The first attempt in September last year saw the stage lose control late on, as propellant for the engines gradually spun up inside their tanks.

Image copyright ULA Image caption National security missions have been entrusted to ULA's Atlas rockets

Thrusters to correct the attitude of the stage were beefed up for this second experiment.

SpaceX also attached the landing legs for the first time. The telemetry confirmed these extended properly just prior to the stage touching water.

Reusability does not come without a penalty. The fuel needed for the manoeuvres and the weight of the landing gear affect the rocket's ultimate performance - the maximum payload it can carry to orbit. Mr Musk has previously calculated this penalty to be about 30%.

He also recognises that some of his customers may be uncomfortable flying their satellites on what amount to second-hand rockets.

"I think what we'll have to do is do a demonstration re-flight without an operational satellite onboard. And if that demonstration re-flight works, and some customers may want more than one - then that's the thing that would really ultimately convince them," he said.

Mr Musk was speaking to reporters at the National Press Club in Washington DC. He used the press conference to announce also that the company had filed a suit to protest the way rockets were being procured for national security missions, which include military spacecraft and spy satellites.

Thirty-six vehicles were recently ordered en bloc by the United States Air Force from United Launch Alliance, a joint venture between Boeing and Lockheed Martin.

Mr Musk said the absence of a competition "doesn't seem right", and claimed the use of ULA's Atlas rockets was a poor deal for US taxpayers.

"The ULA rockets are basically four times more expensive than ours. So this contract is costing the US taxpayer billions of dollars for no reason. And to add salt to the wound, the primary engine used is made in Russia."