“Minor Nuance” in SpaceShipTwo’s Propulsion System Was Neither

SpaceShipTwo's propulsion system using the nylon engine. (Credit: Scaled Composites)
SpaceShipTwo’s propulsion system using the nylon engine. (Credit: Scaled Composites/NTSB)

After SpaceShipTwo crashed last year, Scaled Composites President Kevin Mickey was asked at a press conference whether the vehicle’s new hybrid engine had failed, causing an explosion. Mickey correctly said that it had not; he and others knew it was pilot error. When pressed for details about the engine change, he claimed that the change in hybrid motors from three previous flights was a “minor nuance.”

Ahh…no. This was not accurate. Sitting in the audience that afternoon listening to him, I knew it wasn’t. I really wanted to ask him about it. But, Mojave CEO Stu Witt ended the press conference not too long afterward.  Mickey literally ran out of the Stuart O. Witt Event Center at the Mojave Air and Space Port before I or anyone else could ask him anything further. Did he have a good reason to rush off like that? Maybe. I don’t know.  I never got a chance to ask.

And, in the overall scheme of things, it wasn’t that important. The graphic above, taken from the NTSB’s final accident report, shows why “minor” and “nuance” should not have been used to describe the changes.

First, the case/throat/nozzle/assembly (a.k.a, the rocket motor) contained nylon instead of the rubber fuel used on the previous three test flights. The nylon, it was said, had better performance. That was all well and good, but any sort of change had risks associated with it, which this flight was designed to evaluate.

Let you think this was simply a change of fuel grain as Mickey and everyone at Virgin Galactic would have you believe, it was far more complicated than that.  You see those blue cylinders marked “fuel tanks”? They weren’t there for the first three powered flights using the rubber fuel.

“PF04 was the first SS2 powered flight to use a fuel injection system and a nylon fuel grain (RM2),” the NTSB’s final accident report states. “The change to a nylon fuel grain resulted in a thrust increase from previous powered flights. Also, as previously stated, a 38-second rocket motor burn time was planned for PF04. A 16-second burn of the rocket motor occurred during PF01, and a 20-second burn occurred during PF02 and PF03.”

The NTSB says the fuel was used to “improve combustion stability during rocket burn.” None of the NTSB documents indicate what fuel was used; it appears to have been redacted for competitive reasons. My sources say that methane was used because nylon fuel can be difficult to light; if it doesn’t light properly, then the engine suffers a hard start and the vehicle could fly apart. Sources also reported that one of the tanks included helium to help stabilize the burn.

So, let’s review what was going on with PF04, the fourth powered flight test of SpaceShipTwo. There was a new fuel that burned more energetically. They were testing a new fuel injection system that featured tanks in the wings for the first time. They were burning this new engine for 38 seconds, nearly double any previous burn with a completely different fuel. And such a burn would have sent the ship perhaps three times higher than the maximum altitude of 71,000 feet obtained during the third powered flight.

Was this aggressive flight testing? Oh yeah? Just changing the fuel grain and testing the injection system might have been enough for one flight test. But, they also doubled the burn time.  Was all this risky? Oh, you betcha. How risky? Just read what Scaled Composites pilot Pete Siebold, told the NTSB after his miraculous survival.

Mr. Siebold stated that he considered this a “high-risk” flight both personally and as the director of flight operations, although there was no formal differentiation between different test flights. When asked to elaborate on that classification he stated that Scaled Composites did not have a high, medium, and low flight risk classification; however, based on his 18-19 year experience at Scaled Composites he considered this a high-risk flight. They were “doing a significant envelope expansion that day. Flying an unproven rocket motor in an unexplored aerodynamic regime… classic test hazard assessments would categorize that as a high-risk flight.” He stated another factor was that they were “using a propulsion system that history has shown can be unreliable, or much less reliable than a turbine or reciprocating engine.” When asked if his categorization had anything to do with Scaled’s readiness on the day of the accident he stated, “no.” He also added that the primary purpose of test flights was to validate the engineering design, analysis and assumptions were correct and that there was always some amount of risk.

This was a risky, aggressive flight test. There wasn’t anything minor or nuanced about it or the changes made the ship beforehand. Nor did this involve a simple change in fuel grain.

Editor’s Note: I wrote about this from another angle back in August: Virgin Galactic Misled Ticket Holders, Public on Complexity of Engine Change