For more than 15 years, Virgin Galactic has been working to begin carrying paying passengers to the edge of space and back. Here, we track the long, winding road to realising Sir Richard Branson's dream. In the early 1990s, celebrated aeronautical engineer Burt Rutan set himself a challenge: designing a spaceplane. "I'm going to give it a try, I'm going to go out and do it," Mr Rutan said in 2004, recalling his thoughts a decade earlier. Like many before and since, he was motivated by the lack of access to space for "ordinary" people - in other words, those who aren't government-funded astronauts. "Over the last 25 years it has become increasingly, increasingly obvious that the kids who dream - and I consider myself still a kid - that they can go up and see these views [of Earth from space] have diminishing hope, diminishing dreams," Rutan explained. The engineer thought the ride would have to feel like flying in a plane, rather than the more adventurous experience of being launched on a rocket and falling to the ground with parachutes. The outcome of his efforts was a vehicle named SpaceShipOne. Rutan took inspiration from the X-15 high-altitude experimental plane flown by test pilots in the 1950s and 60s On 21 June 2004, his craft reached an historic milestone, performing the first privately funded human spaceflight. The 8.5m-long spacecraft began its flight from a runway in California's Mojave desert, slung under a plane called White Knight. At 14km up, SpaceShipOne detached from its "mothership" and immediately ignited its rocket engine. The vehicle, carrying test pilot Mike Melvill, then climbed steeply, eventually reaching an altitude of just over 100km - the official boundary of space. Melvill experienced weightlessness before the vehicle re-entered the atmosphere. On the way down, SpaceShipOne changed shape as planned, in a way designed to increase the drag - air resistance - while maintaining the craft's stability. This was known as the "feathering" system. Over the course of several more successful flights, the vehicle also won the $10m X-Prize, set up by space entrepreneur Pete Diamandis, to help kickstart a space tourism industry. Members of the public had been invited to marvel at the flights, but they weren't the only ones who were impressed by the vehicle's achievements. Sir Richard Branson, founder of the Virgin group of companies, had also been paying close attention, and he was about to bet big on the spacecraft and its technology. In September 2004, Sir Richard announced that he would offer commercial flights to space using vehicles based on SpaceShipOne. At a news conference in London, the British entrepreneur said a seat aboard his new Virgin Galactic spaceline would set customers back $200,000. They explained how paying passengers would be able to unbuckle from their seats and perform somersaults in microgravity. It all painted an impressive picture of the experience for paying customers. But things were not always so upbeat behind the scenes. In July 2007, tragedy struck when three employees of Scaled Composites were killed and three more badly injured in an explosion while testing components of the rocket motor for SpaceShipTwo. After the accident, Scaled Composites outsourced development of the motor to another company, Sierra Nevada Corporation. The vehicle's rocket motor was designed to use a solid, rubber-based fuel called HTPB and liquid nitrous oxide as an oxidiser (a chemical that helps the fuel burn). This design, with propellants in different phases - solid and liquid in this case - is known as a hybrid rocket motor. It needed to be capable of sustaining a stable burn for at least a minute. The first SpaceShipTwo craft, called VSS Enterprise, started a programme of testing in 2010. This kicked off with ground tests, followed by captive-carry flights, where it flew attached to the White Knight II carrier plane, but was not released. Then, in October the same year, it was dropped mid-air by its carrier for the first time and glided to a safe landing at the Mojave Air and Spaceport. The first powered flight of VSS Enterprise was to come in April 2013. Piloted by Mark Stucky, with Mike Alsbury as co-pilot, the vehicle reached supersonic speeds above the Mojave desert. At the time, Sir Richard commented: "Today's supersonic success opens the way for a rapid expansion of the spaceship's powered flight envelope, with a very realistic goal of full space flight by the year's end. "We saw history in the making today and I couldn't be more proud of everyone involved." However, Virgin Galactic wasn't happy with vibrations coming from the engine prototypes and, in 2014, it ended its relationship with Sierra Nevada, bringing development of the rocket in-house. The company also announced that it would be changing the rocket's fuel from a rubber-based one to a plastic-based one called thermoplastic polyamide.