NTSB Looks at Human Factors as Virgin Expresses Confidence in Engine

WhiteKnightTwo and SpaceShipTwo on the tarmac on Wednesday, July 23, 2014. (Credit: Virgin Galactic)
WhiteKnightTwo and SpaceShipTwo on the tarmac on July 23, 2014. (Credit: Virgin Galactic)

Over at The Space Review, Jeff Foust has an excellent update on Antares and SpaceShipTwo six months after they both crashed within days of each other at the end of October. There are a couple of interesting things worth pointing out on the SpaceShipTwo failure.

One involves comments by National Transportation Safety Board (NTSB) Chairman Christopher Hart that point to why the investigative body is so focused on human factors to explain why co-pilot Mike Alsbury unlocked SpaceShipTwo’s feathering system earlier than planned.

“I actually flew the simulator when I was out there to see what it looks like. They basically opted against automation. Everything is going to be very manual. So when I looked at that cockpit, it was very much sort of out of the 1950s, almost,” he said.

One question, Hart said, is how well a pilot could perform those manual actions, particularly during the powered portion of the flight. “So the environment I’m thinking of is, I’m in this cockpit wearing gloves, and all of a sudden I get this three-g kick in my back from this rocket,” he said. “I’m wondering how well is that understood. I don’t think it’s very well understood because we just don’t have that much experience with it.”

Sources have told Parabolic Arc that the G-force and Mach numbers were right next to each other on the control panel. They also were in a small typeface. This has led to speculation that Alsbury could have misread the displays as the spacecraft rocketed skyward.

The other interesting element is the revelation last week by Virgin Galactic Vice President for Special Projects Will Pomerantz that the company was still trying to decide between a hybrid engine powered by nylon and one that uses rubber. Last May, the company announced a change from rubber to nylon due to improved performance.

“The one we’ll fly is the one that’s best,” he continued. “We’re not flying again yet, so I’m not 100 percent sure. If I had to guess, my personal guess, would be HTPB.”

A switch back to rubber would have nothing to do with any safety concerns about the nylon engine. That engine’s only powered test flight was in the October accident, but the engine hasn’t been implicated as the cause of the accident. “We have an engine that works. We have an engine that we’re confident is safe,” he said.

I responded in the comments section. That response is reproduced below with some minor changes.

There are several perceptions that Virgin Galactic keeps promoting about SpaceShipTwo’s engine that need clarifying.

The first is that switching between nylon and rubber is simply a matter of changing the fuel grain. The rubber engine would need the injection of helium to stabilize the burn. Sierra Nevada had achieved it in a burn at the end of 2013. However, it would take the ship to only 50 miles with four passengers. VG couldn’t make money at that rate, and the contract with SNC was costing a lot of money. The nylon engine would require both methane (to get the burn started) and helium.

The engines are different from each other. They also are different from the engines used in the first three powered flights of 16, 20 and 20 seconds. Overall, they are more complicated and have more failure modes than those earlier engines. They are both a far cry from the simplicity that Burt Rutan had in mind when he started the SS2 program.

Second, Pomerantz’s safety claim about the nylon engine is a bit presumptuous if he was really referring to the one flight where it fired for about 12 seconds or so. The engine has to go through the entire burn (and this would have been a long one). Shutdown is a crucial period. It never got there. The engine was performing nominally when it stopped. That’s good, but not enough. How many times has this actually been tested on the ground in a flight configuration?

The fact that they are still competing two rival engines nearly 11 years into the program is proof of what I and other observers have been saying all along. They haven’t had an engine that can do the job — and they still don’t seem to have one. I tried but couldn’t really get clear answers from Pomerantz the other day on the two crucial questions: how high can SpaceShipTwo fly with these engines, and with how many passengers? Can you think of any other programs that have been in development so long where those answers can’t be provided instantly?