The Virgin Galactic spacecraft crash which killed one pilot and left another seriously injured was 'inevitable' as the rocket was 'crude' and 'unreliable', Richard Branson's biographer has claimed.

Tom Bower said the Virgin boss's bold plans for a space tourism company were always going to end in tragedy and claimed engineers told him the project was 'very dangerous'.

He said numerous engineers have walked out of the factory in recent months, with one reportedly saying he would never work there again.

The writer, who penned Branson Behind The Mask, said the project has been plagued with problems since an early version of the nitrous oxide liquid fuel tank exploded in 2007, killing three workers.

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The Virgin Galactic SpaceShipTwo plane crash was 'inevitable', Richard Branson's biographer has claimed

Tom Bower said the Virgin boss's bold plans for a space tourism company were always going to end in tragedy as they were 'dangerous'

Designed to run the first ever passenger flights into space, the SpaceShipTwo plane, split into pieces and fell to earth minutes after being launched from a carrier plane over California's Mojave Desert.

Wreckage rained from the sky yesterday as one pilot was ejected from the cockpit using a parachute, while the other was reportedly left strapped to his seat as he plummeted to earth and died.

It was the spacecraft's first test flight and was loaded with an experimental new rocket fuel.

'What happened yesterday was very sad for the pilot obviously but it was predictable and inevitable and has never been a secret,' Mr Bower told Radio 4's Today programme.

Biographer Tom Bower (right) said Branson's bold plans for a space tourism company were always going to end in tragedy as they were 'dangerous'

'It's a very crude rocket. It uses a gas called nitrous oxide burning solid rubber and it has been formed out of a completely different craft.

'All the engineers in California working on the project I've spoken to said it was very dangerous, just a few weeks ago the last of many of the Virgin Galactic engineers walked out of the factory and said he'd never work there again.'

The first disaster in the fuel history of Virgin Galactic came in 2007, when an early version of the nitrous oxide liquid fuel tank exploded, killing three employees.

The blast, at an outdoor test facility belonging to partner company Scaled Composites, killed employees Todd Ivens, Eric Blackwell and Charles May, who were watching the test from behind a chain link fence.

The rocket engine, using a new plastic-based polyamide fuel, can be seen starting to fire, right, as SpaceShipTwo streaks away from the carrier

Pilots: It is not known who is was killed today. Rick 'CJ' Struckow (far left) is fine, a family friend told MailOnline. Another, Todd Ericson (not pictured) was confirmed alive by a relative on social media. The conditions of Peter Siebold (center right, arms folded), Michael Masucci and chief pilot David Mackay are not known

Mr Bower said: 'The most important issue was in 2007 - three engineers were killed in an explosion when a rocket exploded on the ground rocket explosion on the ground.

'Ever since then it has become apparent that the science used to create this rocket is completely unreliable.'

Mr Branson has pledged to keep up the drive for space travel, saying on the company's web site: 'Space is hard - but worth it. We will persevere and move forward together.'

Virgin has not yet said who the pilots were - though only four men were cleared by the FAA to pilot the craft.

Rick 'CJ' Sturckow was one of the pilots flying the WhiteKnightTwo launching craft, MailOnline can reveal - and so would have been with the killed pilot - still unnamed - minutes before the disaster.

Mr Branson has pledged to keep up the drive for space travel, saying on the company's web site: 'Space is hard - but worth it'

How the plane climbed tens of thousands of feet before exploding and plummeting to earth

Mr Branson's biographer said the businessman has placed so much of his commercial future on the space mission as it was such a great PR stunt.

'All over the world he appeared and brandished the Virgin label and used rocket as an example. Then he collected a load of celebrities to pay a quarter of £250,000 each for a ticket.'

Brad Pitt, Angelina Jolie, Justin Bieber and Katy Perry all signed up for the two-and-a-half hour voyage with six minutes of weightlessness.

In a press conference Friday afternoon, George Whitesides, chief executive of Virgin Galactic, said; 'Our primary thoughts at the moment are with the crew and families and we are doing everything we can for them now.'

'Space is hard, and today was a tough day. The future rest on hard days like this.'

Stuart Witt, who is in charge of the Mojave Air and Space Port, where SpaceShipTwo was launched today, also remained optimistic about the future of space travel.

He said: 'Stay the course. This is not easy. If it was easy it would not be interesting to me and my colleagues standing next to me.'

'We are doing this for you and your generation. It is a cause far greater than any one of us singularly. I compare it to the Magellan expedition [the first circumnavigation of the Earth].'

In January 2014, Mr Bower said 'Virgin Galactic is in danger of turning into a white elephant'.

He added that Branson had so far only managed to 'fire a primitive rocket for 20 seconds in the Earth's atmosphere'.