"After the Ansari X Prize, some people tried to convince [Burt] Rutan to replace the hybrid with a reusable liquid engine. He rejected the advice. Rutan came out of SpaceShipOne’s short flight test program believing the hybrid engine was simple and safe, and that it could be easily scaled up for the much larger SpaceShipTwo. He was wrong on both counts.
The first belief was shattered on a hot summer afternoon of July 26, 2007. Scaled engineers were conducting a cold flow of nitrous oxide that did not involve igniting any fuel. Three seconds into the 15-second test the nitrous tank burst, resulting in a massive explosion that destroyed the test stand and killed three engineers. Three others were injured.
Explosions are not unusual in engine development. However, it is rare that anyone dies in them. Safety procedures call for the evacuation of personnel to a safe area before any tests begin. That was not done in this case; the dead and injured were part of a group of 11 people standing near the test stand.
Following the accident, Rutan and Scaled Composites claimed ignorance. “The body of knowledge about nitrous oxide (N2O) used as a rocket motor oxidizer did not indicate to us even the possibility of such an event,” Scaled said in a press release. The media and Scaled supporters have largely parroted this explanation.
A team of experts experienced in working with nitrous oxide reviewed the accident and disputed the claim. “This would seem to indicate either a lack of due-diligence in researching the hazards surrounding N2O (negligence) or a wilful disregard of the truth,” they concluded.
Whatever Scaled’s culpability, there is no dispute the accident delayed the program significantly. Work on SpaceShipTwo was put on hold while engineers investigated the cause of the explosion. Hybrid engine tests would be delayed for nearly two years.
Once engine tests began again in April 2009, engineers would discover that Rutan’s other assumption was wrong. The hybrid engine just didn’t scale very well. The larger the engine became, the more vibrations and oscillations it produced. As engineers struggled to find a solution, Scaled Composites and Virgin Galactic quietly began work on alternative motor designs.
The failure of the hybrid to scale led to another problem. SpaceShipTwo had already been designed and built. The dimensions of the ship, the size of the passenger and crew cabin, the center of gravity…all those were already set. So, engineers now had to fit an engine within those parameters that could still get the vehicle into space.
This is the reverse of how rocket planes are typically designed. Engineers figure out the engine first and then build the ship around what it can do. Rutan – a novice in rocket propulsion who had hit a home run with SpaceShipOne – got the process backward, resulting in years of delays. This failure would cause numerous headaches.
The rubber hybrid engine did get a workout in three flight tests, but the vibrations and oscillations it produced were so severe the motor couldn’t be fired for more than 20 seconds. The engine was sufficient to get SpaceShipTwo through the sound barrier, but it couldn’t get the vehicle anywhere near space.
It was not until May 2014 – after spending nearly a decade on the program, and a reported $150 million on engine development – Virgin Galactic announced it would be switching to a different type of hybrid engine, one powered by nitrous oxide and plastic. They are hoping for much better performance in flight.
By then, Rutan was gone, long since retired to a spread in Idaho. It was for others to make the new engine work and fix the mistake he had made.
Flight tests with the plastic engine are set to begin shortly..."
As to eyewitness reports of an "explosion," Hart pointed out that the NTSB had recovered SpaceShipTwo’s intact fuel and oxidizer tanks, along with its rocket motor. None showed any signs of burn-through or other evidence that an explosion had occurred. The loss of SpaceShipTwo appears to have been wholly due to the unplanned change in the vehicle’s configuration, not to the hybrid engine.
Determining the "how" of a craft’s destruction is often relatively easy compared to determining the "why," and it is that latter task that will occupy the NTSB’s attention.
"We’ll be looking at training issues," said Hart. "We’ll be looking at 'was there pressure to continue testing?' We’ll be looking at safety culture. We’ll be looking at the design, the procedures—we’ve got many, many issues to look into much more extensively before we can determine the cause."
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