How scientists repeatedly warned Virgin Galactic over safety issues as it is revealed THREE of Branson's top engineers recently quit Rocket scientists and space safety experts say they have worried for years Head of an international space safety group said explosion was inevitable Virgin reportedly ignored invitations to industry events and was 'secretive' Senior staff in charge of safety, aerodynamics and propulsion quit in 2014 Found Richard Branson denied that any warnings had been missed By Kieran Corcoran for MailOnline Space flight experts have accused Virgin Galactic of ignoring warnings over its safety procedures in the wake of a fatal explosion aboard one of its spacecraft. Senior engineers and propulsion scientists say they spent years telling Richard Branson's private space line its procedures are 'outdated', needlessly secretive, and could lead to people getting killed. Michael Alsbury, 39, died at the controls of Virgin's experimental SpaceShipTwo vessel on Friday after the sub-orbital plane exploded at 45,000ft shortly after engaging its engines. His co-pilot, Peter Siebold, was seriously injured. Crashed: The wreckage of Virgin Galactic's SpaceShipTwo lies in the Mojave Desert after a fatal crash. Experts have said they warned the company repeatedly over its safety procedures Fatal: Test pilot Michael Alsbury died on Friday morning after the spacecraft exploded at 45,000ft Disaster: Moments after SpaceShipTwo detached from its mothership (left) and engaged its engines (center), it blew apart in the skies (right). The precise causes of the crash have yet to be established And, in a further sign of upheaval at Virgin Galactic, it was also revealed yesterday that top engineers in charge of safety, aerodynamics and the propulsion system have all recently quit the company. Many of the warnings stem from a 2007 disaster where an engine part exploded at a test facility in the Mojave Desert, killing three employees of Virgin Galactic's partner company, Scaled Composites. Victim: Michael Alsbury, pictured, died in the crash yesterday Some have accused Richard Branson's project of ignoring industry standards and refusing to share its procedures in its eagerness to perfect commercial space flight, which it has regularly estimated is just months from completion. One expert called the design and testing process a 'Russian roulette' which was bound to kill someone. Carolynne Campbell, a rocket propulsion scientist at the International Association for the Advancement of Space Safety (IAASS), told the Sunday Telegraph she had been warning Virgin Galactic for seven years about its safety record. She said: 'This explosion is not a surprise. None whatsoever, I am sorry to say. It is exactly what I was expecting. It was Russian roulette which test flight blew up. The executive director of the IAASS also said that Virgin Galactic refused to let scientists from outside the company review its procedures, and snubbed industry gatherings. Tomasso Sgobba, a former head of safety for the European Space Agency, said: 'They operated in secrecy, which is difficult to understand. 'They don't use modern techniques in putting safety into the design... I have been saying for some years now this was an accident waiting to happen. Geoff Daly, a concerned rocket scientist, also revealed that he wrote to the U.S. Chemical Safety Board urging them to do something about his safety concerns over SpaceShipTwo. Writing in July 2013, he said even the company's own engineers privately doubted that their venture was safe. At the time Virgin Galactic had said they were just five months away from taking their first passengers, including children, into space. He said: 'Sir Richard Branson, his two children, Justin Bieber and one other will be the first passengers to fly into space during this coming December 2013, and everyone realises there is a problem, even the engineers... have said so off the record.' The disaster came in the wake of three top engineers at Virgin Galactic quitting since late 2013. Jon Turnipseed, formerly the vice-president of safety, reportedly left just before Christmas. The Parabolic Arc blog reported that Virgin Galactic spent months unsuccessfully advertising for a successor to the job. Speaking today at his Mojave Spaceport, Virgin founder Richard Branson denied that the company had ignored warnings. Standing firm: Virgin Founder Richard Branson denied his company had ignored any warnings, and insisted that nothing can be known until the official investigation is finished He said: 'To be honest I find it slightly irresponsible that people who know nothing about what they are saying can be saying things before the NTSB [one body investigating the crash] makes their comments. 'We aren't going to push on blindly. To do so would be an insult to all those affected by this tragedy. We are going to learn from what went wrong, discover how we can improve safety and performance and then move forwards together. 'We owe it to our test pilots to find out exactly what went wrong and once we find out what went wrong, if we can overcome it, we'll make absolutely certain that the dream lives on.'