Hybrid rocket engines, such as that used on Virgin Galactic’s SpaceShipTwo, can produce extreme vibrations that put heavy loads on a spacecraft’s mechanical structures potentially inducing mechanical faults, space industry insiders have suggested.
The US National Transportation Safety Board (NTSB) said on Monday it had found the propellant tanks and engines of the manned suborbital spacecraft that crashed last week in California’s Mojave Desert intact, seemingly dispelling concerns that the engine might have exploded due to the use of instable nitrous oxide in its tanks.
Instead, investigators said, the spaceships's feathering mechanism – which turns wing booms into braking position for re-entry – deployed prematurely, either by a mechanical glitch or human error. As the craft was travelling at supersonic speed, the increased aerodynamical loads then tore the ship apart.
However, space industry experts have said the NTSB's findings don't put their mind at ease about the technology behind Virgin Galactic's space planes and the company's safety engineering standards.
“What happened to Virgin Galactic is very sad, it’s terrible,” said Jose Mariano Lopez Urdiales, an MIT-educated aerospace engineer who trained at the European Space Agency and spent some time working for aerospace giant Boeing.
“Regardless of what happened on Friday, their choice of engine has been very problematic since the beginning. It may have been the right choice of engine to win the XPrize but it was not the right choice of engine to develop a commercial business. And that’s not just my opinion. Anybody who knows anything about rocket propulsion would say the same.”
Lopez Urdiales’s claim is in line with earlier allegations made by British rocket engineer Carollyne Campbell who said that nitrous oxide, used in the SpaceShipTwo’s hybrid engine as an oxidiser, is dangerous due to its instability and inclination to blow up without warning. In fact, a ground test of a tank filled with the controversial oxidiser resulted in an explosion that killed three engineers working for Virgin Galactic’s partner Scaled Composites in 2007.
However, Lopez Urdiales said the tendency to explosions is only part of the problem.
“The problem with the engine is not only that it can blow up as it did several times before during testing, the problem is also the vibrations that it induces on the rest of the airframe,” he said. “And when you have very strong vibrations, you can expect to have subsystems or some mechanisms failing because of these vibrations. “
Featuring a simpler design than conventional liquid propellant engines, the hybrid rockets use solid fuel and liquid oxidiser, allowing the designers to work with only one set of tanks. The hybrid technology was studied in the early phases of missile development but was later abandoned.
“What we see with these engines is that you can’t scale them up. It works for certain sizes but not for bigger sizes,” Lopez Urdiales said. “This was known from the time when hybrid engines were looked at as potential systems for missiles and they were considered unstable. So when they were considered unstable for missiles, how could they be OK for manned rockets?”
Virgin Galactic’s competitor XCOR, for example, is using a liquid rocket engine burning liquefied natural gas in liquid oxygen.
“That’s a very clean propellant, there is a lot of data about it, it’s well understood,” Lopez Urdiales explained. “LNG (liquefied natural gas) or methane, those are the cleanest, least expensive and best understood fuels out there. These fuels are cheap and clean and they don’t produce the huge vibrations in combustion that you get with hybrids of certain sizes,” he said.
Lopez Urdiales, CEO of a Barcelona-based space tourism start-up Zero2Infinity, has a very personal interest in Virgin Galactic’s success and is concerned about the impact the Virgin Galactic tragedy would have on the fledgling industry including, of course, his own company.
“Five years ago, Virgin Galactic got a cheque from Sheikh Mansour for about $390m. Had they followed the advice from all these people, including myself, today they would be giving a bigger cheque back to the Sheikh, possibly $1bn maybe via an IPO,” Lopez Urdiales said.
“This would mean that capital would flow into new space ventures, which is exactly what is needed because it would be much easier for companies like mine or XCOR to raise the capital they need to complete their systems. The limiting factor now for the growth of the industry is not market or technology but availability of capital.”
Despite its prominent position in the media driven mostly by the celebrity status of the company’s owner, UK billionaire Sir Richard Branson, Virgin Galactic may never have been in the position to win the space tourism race with its technology.
In the past months, multiple high-ranked engineers left the company including the vice president for propulsion Thomas Markusic, who later founded his own company building small satellite launchers. The engineers were leaving the company despite repeated public promises of Sir Richard Branson in the media to start operating within months.
But it’s not only the space start-up community who is critical of Virgin Galactic.
Tommaso Sgobba, former head of safety at the European Space Agency and president of the International Association for the Advancement of Space Safety, said Virgin Galactic is not employing modern hazard mitigation engineering practices and is instead using a trial and error approach used in the pioneering days of aviation.
However, Sgobba said, the aerospace industry has moved a considerable way over the hundred years of its development and there is no need for learning from tragic accidents when those can be prevented by careful hazard analysis.
“If Virgin Galactic had performed an engineering hazard analysis, they would have considered the possibility of a human error or an inadvertent release of its feathering mechanism and they would have designed a solution to mitigate this hazard,” Sgobba said. “Had they done that, there would have likely been no accident and no fatality.”
Unlike Virgin Galactic, private space companies such as SpaceX or Orbital Sciences, working for Nasa, are obliged to carry out a hazard analysis as a standard requirement. As part of the analysis, all possible risk and their consequences are identified and systems are designed in a way to mitigate or minimise those risks and their consequences.
Virgin said they won’t comment on the accident before the NTSB investigation is concluded.
Virgin Galactic SpaceShipTwo infographic