Three senior Virgin Galactic executives - the vice-presidents in charge of propulsion and safety, and the chief aerodynamics engineer - had all quit the company in recent months.
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The spacecraft, which was scheduled to begin passenger flights early next year, blew up above the Mojave Desert in California during a test flight on Saturday.
The incident, in which one pilot died and another was seriously injured, puts in jeopardy Branson's dream of space travel for passengers, each paying US$250,000 ($320,000). The dead pilot was named as Michael Alsbury, an experienced American test pilot.
Virgin Galactic initially dismissed the explosion as an "anomaly". However, aerospace experts insisted it had been a disaster waiting to happen.
Tom Bower, an investigative journalist and Branson's biographer, described the crash as "predictable and inevitable". He said: "It's a very crude rocket."
Watch: Raw: Virgin galactic rocket explodes, one dead
Branson arrived at the crash site yesterday insisting "safety has always been our number one priority". He admitted the programme would continue only if the problems that caused the crash could be overcome.
Branson's company, as well as US authorities, were warned about safety issues on numerous occasions, as long ago as 2007 when three engineers died in an explosion during testing of a rocket engine on the ground.
Carolynne Campbell, the lead expert on rocket propulsion at the International Association for the Advancement of Space Safety, 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."
Watch: Raw: NASA rocket explodes
She had first warned Virgin Galactic about the danger of its nitrous oxide-propelled engines in the aftermath of the 2007 disaster, and has repeated those warnings since. In a study published in 2010 on her website and sent to Branson's company as well as to the US authorities, she wrote: "We are not confident that ... we yet know enough about N2O [nitrous oxide] to consider it a safe oxidiser for use in passenger flight. In the light of what we do know, safety must remain a major concern."
In the study, she questioned Virgin Galactic's claim on its website that its rocket system was "benign" and "stable".
In emails sent by Geoff Daly, a US-based British rocket scientist, to officials at the Federal Aviation Authority last year, he warned of another disaster if test flights were given the go-ahead. At the time, Virgin Galactic had planned to begin its passenger flights in December 2013, although the date of the inaugural flight had been repeatedly postponed.
In the emails, published on a US Government website, Daly wrote on July 13, 2013: "We respectfully request a response from you on what actions the FAA will be taking with respect to the permit issued and the operations as planned in the Mojave and New Mexico facilities. Remember three people have been killed and numerous persons injured by a prior explosion involving N2O in this motor design. We do not need another incident on the ground/flight line or in the air."
Tomasso Sgobba, executive director of IAASS and the former head of safety at the European Space Agency, said Virgin Galactic had refused to share information with industry experts outside the company and declined to have its rocket design peer-reviewed. Representatives of Virgin Galactic had refused to come to IAASS meetings, he said.
"They operated in secrecy, which is difficult to understand," said Sgobba. "They don't use modern techniques in putting safety into the design. They use outdated methods like testing and then seeing what happens ... I have been saying for some years now this was an accident waiting to happen."
Bower, who exposed the safety concerns surrounding the project in his biography of Branson, said: "What happened yesterday was very sad for the pilot obviously but it was predictable and inevitable. All the engineers in California working on the project I've spoken to said it was very dangerous."
US investigators yesterday began examining the wreckage. One area of focus will be claims that the pilots had requested a two-hour delay in take-off because of concerns about the temperature of the nitrous oxide in the fuel tanks. It was the first test flight using a different hybrid fuel system.
Branson dismissed claims that Virgin Galactic had ignored safety warnings. " I find it slightly irresponsible that people who know nothing about what they are saying can be saying things before the NTSB makes their comments," he said.
"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."
He insisted the programme could be "back on track" within four to six months.
- Telegraph Group Ltd
Virgin Galactic's drive for simple space flight a reminder of the dangers faced when leaving terra firma
The fragments of Virgin Galactic's SpaceShipTwo now littering the Mojave Desert in California are a grim reminder of one of the harshest facts in modern aeronautics: that attempting to travel beyond the confines of our atmosphere can be a punishingly dangerous activity.
Those who seek to slip the surly bonds of Earth take their lives in their hands each time they climb into their capsules or spaceships.
Engineers may argue they have developed safe and foolproof spacecraft and launchers, but the wreckage of SpaceShipTwo suggests such claims remain premature.
A total of 18 US and Soviet astronauts have died in space accidents, while a greater number have perished as they trained for missions. They include Yuri Gagarin, the first human in space, who died in 1968 during routine jet training during preparations for his second space flight.
Two key threats face those who fly in space: gravity and air, or rather lack of air. Georgy Dobrovolsky, Viktor Patsayev and Vladislav Volkov were suffocated when a faulty valve vented all the air in their Soyuz capsule as it began re-entry in June 1971.
However, it is the problem of overcoming gravity that makes space travel particularly dangerous. To reach gravitational freedom in Earth orbit, a craft must reach speeds of thousands of kilometres per hour, and to date this has been achieved only by igniting powerful combinations of fuel and liquid oxygen, or by using solid propellents to create mighty, fiery upthrusts. Most manned missions have involved astronauts perching on towers of high explosives in the hope they will ignite in a controllable manner.
In general, things did go well until the space shuttle was developed in 1980s. Strapped to the side (instead of being perched on top) of giant tanks of hydrogen and oxygen, and then fitted with two solid state booster rockets, the space shuttle became the most dangerous mode of transport ever built. On January 28, 1986, the shuttle Challenger was destroyed shortly after take-off when flames from a solid booster caused its liquid fuel tanks to explode. All seven astronauts on board were killed.
In February 2003 the shuttle Columbia disintegrated over Texas as it returned home, an accident traced to damage caused by debris falling on to the shuttle from a fuel tank during launch two weeks earlier. All seven astronauts on board were killed.
America's three remaining space shuttles were retired in 2011. Nasa has pursued a policy of using private space launch companies, such as Space X - owned by PayPal billionaire Elon Musk - and Orbital Sciences Corporation. So far, only their unmanned rockets have been used, mainly to send supplies to the International Space Station. Last week one of Orbital's Antares rockets, destined for the ISS, exploded seconds after lift-off, destroying 2250kg of equipment. The company nevertheless remains buoyant about its prospects, while Space X continues to promote ambitious plans to fly humans into orbit.
Both companies use relatively standard rocket launch technology. But Richard Branson's Virgin Galactic has adopted a very different, relatively low-tech approach to spaceflight. Based on a system developed by US aircraft designer Burt Rutan, the Virgin Galactic spaceplane has been designed to carry two pilots and six passengers. Strapped to the belly of a jet-powered mother ship, it would be lifted to an altitude of 15km.
There the spaceplane would be released and its rocket engine ignited, sending the craft soaring upwards at a speed of more than 4000km/h. Passengers would have six or seven minutes to float round the cabin in zero gravity before their ship started to arc downwards.
This system has one key advantage over other spacecraft. Carried aloft by its mother ship, the craft would already be 15km high and travelling at jet speeds when the rocket motor was ignited, giving it a significant head start. Rutan used powdered rubber and nitrous oxide as fuel - a simple, easily controlled mix, it has been claimed.
But in May, Virgin Galactic announced that SpaceShipTwo would switch to a polyamide-based fuel, a type of thermoplastic. There is considerable speculation that the use of this new fuel lies at the heart of the tragic accident.
Virgin Galactic's technology was supposed to cut through the complexities of standard rocketry and make space flight simple. The dream may not be dead, but it is certainly a lot dimmer today.
- Observer
The pilots
Michael Alsbury
Michael Alsbury, a father of two and pilot with Scaled Composites, was killed when SpaceShipTwo exploded just minutes after it detached from its mother ship, WhiteKnightTwo. Michelle Saling, Alsbury's widow, said: "I have lost the love of my life. I am living in hell right now." He leaves behind a son and a daughter. Alsbury, 39, had piloted SpaceShipTwo as recently as August 28. He had flown in the all important first successful run for SpaceShipTwo in April 2013. He was also the pilot in the first glide flight back in 2010, and flew as recently as August 28 of this year. Alsbury had at least 15 years of flight experience and logged more than 1600 hours as a test pilot and test engineer, according to a biography posted on the Society of Flight Test Engineers' website. At Scaled Composites, Alsbury participated in the flight testing of nine different manned aircraft. Alsbury flew primarily as the SpaceShipTwo co-pilot, logging at least seven trips from 2010 to early 2014, according to test-flight logs. Neighbour Leonard Alvarez said of Alsbury, "He's one of a kind, very, very special, an incredible individual who everybody liked." He added, "Mike was just an incredible human being and my friend. He was a kind, unselfish man and I'm going to miss him. Everybody here will miss him. They are the perfect, ideal family, great neighbours and great friends. It is very sad."
Peter Siebold
Peter Siebold, the sole survivor of the destroyed SpaceShipTwo craft, was described as "alert" after suffering serious injuries in the fatal crash. Siebold, 43, reportedly suffered a serious shoulder injury in the disaster, and was due for surgery. Scaled Composites confirmed he was conscious and communicative. The seasoned pilot lives in Tehachapi, California, near the Virgin Galactic spaceport. He has a wife, Traci, and two children. Siebold and Alsbury regularly flew the plane as a duo, and had made successful trips as recently as August 28. Siebold flew his first solo flight and gained his pilot's licence at 16 and went on to teach flight classes at the San Luis Obispo Airport while he was a student at Cal Poly. He was also one of the test pilots for SpaceShipOne, an exper-imental spaceplane that completed the first manned private spaceflight in 2004. As a design engineer at its aerospace company Scaled Composites, Siebold was responsible for the simulator, navigation system, and ground control system for the SpaceShipOne project. In 2009, he was awarded the Iven C. Kincheloe award - the most prestigious award a test pilot can receive - for his role as chief test pilot on the Model 348 WhiteKnightTwo plane, used to lift the SpaceShipTwo spacecraft to release altitude. By the time of his award, he had logged about 2500 hours of flight time in 40 different types of fixed wing aircraft.
- Daily Mail, AP
Tickets in demand by rich and famous
A slew of big name passengers were happy to pay for the once-in-a-lifetime experience of going to space, including Brad Pitt and Angelina Jolie; Justin Bieber; Lady Gaga, who plans to try and sing in space; Ashton Kutcher, who was the 500th customer to purchase a ticket; Tom Hanks; Bryan Singer; and Princess Beatrice.
Russell Brand got a ticket for his birthday from ex-wife Katy Perry when the two were married. Perry also bought a ticket so Brand wouldn't go alone.
Stephen Hawking and Kate Winslet are also set to fly, but got their seats for free - Winslet because she's married to Branson's nephew, Ned RocknRoll, and Hawking because Branson wanted to offer the legendary astrophysicist a chance to go into space.
7 facts about SpaceShipTwo and its plans
1. Desert home
SpaceShipTwo has been under development at Mojave Air and Spaceport in the desert northeast of Los Angeles.
2. Gliding home
SpaceShipTwo is carried aloft by a specially designed mothership and then released before igniting its rocket for a suborbital thrill ride into space and then a return to Earth as a glider.
3. Bransons first
The starting price for flights is US$250,000 ($320,000) - the first ceremonial flight will be undertaken by Richard Branson and his family. Initially there will be one flight per week.
4. Fitness to fly
Passengers must go through a 'Pre-Flight Experience Programme', including three days of preparation at the spaceport to ensure passengers are physically and mentally fit to fly.
5. Eight on board
SpaceShipTwo will carry six passengers and two pilots. Each passenger gets the same seating position with two large windows - one to the side and one overhead.
6. Zero gravity
The space ship is 18m long with a 2.2m diameter cabin allowing maximum room for the astronauts to float in zero gravity.
7. 3.5 hour trip
The spaceship will make a sub-orbital journey with approximately six minutes of weightlessness, with the entire flight lasting about 3.5 hours.
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