A SCOTTISH pilot who will helm the first commercial flight into space has spoken of the "unrelenting" acceleration of his high- concept craft ahead of its eagerly awaited launch later this year.David Mackay, the chief pilot of Virgin Galactic, said "everything is coming together" for the inaugural voyage of the firm's SpaceShipTwo fleet.The 57-year-old has manned test flights for the sub-orbital rocket plane, soaring to an altitude of 71,000 feet. Describing the moment the craft's rocket engines powered into life, he said there were a "few seconds of silence" followed by a period of "unrelenting" acceleration.Even so, the pilot admitted he was taken aback at how quiet the thrusters were. "It sounded like a very loud vacuum cleaner behind us," he explained.Although Mackay is focused on completing the first flights of Sir Richard Branson's ambitious space tourism initiative, his forays into space have given him an appetite to venture further afield into the solar system. "I want to get to the moon," he said. "I want to go to Mars."Born and raised in Helmsdale, Sutherland, Mackay's ascent to the driving seat of the pioneering programme is an unlikely one. A former test pilot who graduated in aeronautical engineering at the University of Glasgow, he joined the RAF in 1979, where he gained years of experience flying Harriers. However, in his mid-30s when he moved to a managerial position behind a desk.In 1995, he joined the ranks of Virgin Atlantic, piloting Boeing 747s and controlling the Airbus A340.In 2009 he signed up with Virgin's space flight programme. It was, he concedes, a career move he thought would be perfectly natural as a youngster, having assumed Britain would launch its own space programme to rival that of Nasa."In the naivety of youth, I didn't know that," he said. "I thought by the time I was in my 20s or early-30s, this was something that would be routine."After moving to the Mojave Desert in California, where the test flights are being conducted, he began his training proper. Mackay was one of two pilots on the most recent test flight in January, when the 60-foot- long SpaceShipTwo was ferried by its companion plane to 46,000 feet before being released.Despite the craft being at the cutting edge of aeronautical technology, the Scottish aviator says SpaceShipTwo is "nothing really new" compared to other aircraft he has flown.To make it to space, the rocket motor will have to fire for one minute instead of the 20 seconds experienced in January's test. Engineers have had to come up with a way to dampen vibrations in the motor as the fuel depleted, but George Whitesides, Virgin Galactic's chief executive, says his team have "gotten our head around it."The test flights will resume this summer. Virgin Galactic has just finished modifications to the spacecraft, including a new landing gear.The interior is being spruced up, and the seats for the six passengers will soon be added. Mackay said the major remaining milestone was to see how the ship behaves as it re-enters the atmosphere at supersonic speed.It is estimated there will be at least three more test flights required to finish the programme objectives.Speaking last week, Sir Richard said he thought all the "hurdles have been ticked" in his project to send paying passengers into space.He said: "We're not going to go until we're 100 per cent sure it's obviously safe, but I would be very, very disappointed if it doesn't happen this year."