London (CNN) -- NASA's space shuttle Discovery has completed its final mission, with Endeavour and Atlantis following it into retirement later this year.

It marks the end of a historic chapter in space travel, but a new one might not be light years away if a groundbreaking design for a fully reusable spacecraft can get off the ground.

"Skylon" may only be at the concept stage but it could usher in a new era of space exploration and discovery, says its UK-based designers, Reaction Engines Ltd.

Key to the Skylon proposal is a hydrogen fuel-powered rocket engine called SABRE (Synergistic Air-Breathing Rocket Engine) designed by the company's managing director Alan Bond.

SABRE, which Bond first described back in the early 1980s, is a "combined cycle rocket engine with two operational modes."

Mark Hempsell, future programs director at Reaction Engines Ltd, said: "The engine starts by burning hydrogen with air and finishes up burning hydrogen with liquid oxygen like a shuttle engine."

This all happens in the same rocket engine chamber, Hempsell says, allowing Skylon to take off and land in a similar way to conventional aircraft, unlike NASA's shuttles and the European Space Agency's (ESA) Ariane five rocket, which need expensive disposable rocket boosters to propel them into orbit.

Hempsell says the idea is to carry passengers, satellites and cargo for the International Space Station, but he also envisages Skylon -- which is around 90 meters long and can transport payloads of up to 12 tons -- supporting future missions to the moon and Mars.

Bond's idea is "very original," says Richard Brown, director of the Center for Future Air-Space Transportation Technology at the UK's University of Strathclyde -- independent providers of technological and design support to Reaction Engines Ltd.

"It's a problem that's been considered for a long time -- the idea of how you end up with an engine with efficiency all the way up into orbit. There are other related ideas but Bond's is probably the most efficient," Brown said.

"We need to do some fundamental research to understand some of the aspects of the craft a little bit better but Bond needs to be given enough funding to allow this engine to be demonstrated," he added.

"It's a very doable system," Hempsell said. "The basics of the engine are thoroughly rooted and explored, so there is no danger they aren't going to work. We feel that this should have taken over from the shuttle a while back."

Nevertheless, he's full of praise for the space shuttle, saying it has done a "superb and spectacular job," but a lack of investment at the design phase led to compromises being made, he thinks.

"The original design was to have two aeroplane-like vehicles stacked on top of one another, which would have made it fully reusable," he said.

In theory, at least, Reaction Engines hopes Skylon can perform the task in one vehicle, but it needs more money to push the project forward.

Around 80% of current funds come from private equity, Hempsell says, with public money making up the rest -- the bulk of which came with a €1 million ($1.4 million) award from the ESA in 2009.

Reaction Engines Ltd estimates the total costs of developing Skylon will be around $10 billion -- a hefty bill, but one which could be well worth it, says Brown.

"If you look at the development costs of something like Airbus's A380 airliner we're not really talking about a huge amount of money," he said.

He added: "At present, access to space is incredibly expensive. If you can redesign spacecraft so that you can reuse them, this will reduce the costs dramatically."