When Virgin Galactic announced it was switching from the nitrous oxide/rubber rocket engine they had flown on SpaceShipTwo three times to one powered by nitrous oxide and nylon, company officials told ticket holders and the public the change involved only minor modifications to Richard Branson’s space tourism vehicle.
A document released last week by the National Transportation Safety Board directly contradicts that claim. In it, a Federal Aviation Administration (FAA) safety expert describing his concern over “major modifications” that had been made in the suborbital space plane to accommodate the new engine.
When the engine change was announced last May, Virgin Galactic CEO George Whitesides told Space News:
The vehicle’s hybrid engine, manufactured by Sierra Nevada Corp., will need to undergo what Whitesides called “minor tweaking” to accommodate the plastic propellant.
“We just need to tweak the plumbing line in the motor to go with the plastic, which is sort of a minor mod,” he said.
In a May 23, 2014 email to ticket holders obtained by Parabolic Arc, Virgin Galactic Commercial Director Stephen Attenborough wrote:
Both fuels were designed to be interchangeable with the hybrid motor, with only minor plumbing modifications, meaning that we have been able to remain objective and unbiased in our assessment of the alternative fuel choices. Both have also been tested extensively.
As we have entered the qualification phase of the commercial hybrid motor in preparation for the final series of powered test flights, it has been critical to finalize the fuel that we’ll use for this and into the start of commercial operations.
Having made this decision, we will now be working hard to make the minor modifications to the spaceship systems in order to continue powered flights with the chosen fuel type as soon as safely possible.
At a press conference four hours after the crash of SpaceShipTwo on Oct. 31, 2014, Scaled President Kevin Mickey described the change of engines as a “minor nuance”. He also correctly stated that the engine was not the cause of the fatal accident that killed co-pilot Mike Alsbury.
NTSB documents released last week as part of its crash investigation show the engine change was anything but minor or nuanced. The switchover involved much more than using a different fuel grain and slightly modifying the plumbing.
So says Tom Martin, the lead technical system safety engineer at the FAA Office of Commercial Space Transportation (FAA AST) who reviewed the modifications last year as part of the renewal of Scaled Composites experimental permit to test SpaceShipTwo.
“They’ve made some major modifications,” Martin told NTSB investigators examining SpaceShipTwo’s crash. “They’ve shortened the engine. They’ve changed the fuel, the shape, and they’ve added [redacted] – [redacted] plumbing and tanks, [redacted] psi tanks in the wings and changed the structure of that.”
Sources have told Parabolic Arc the nylon engine uses helium and methane in addition to the nitrous oxide to burn the nylon fuel. SpaceShipTwo flew with this configuration on its fatal flight last October. The first three powered flights in 2013 and 2014 used nitrous oxide and rubber without wing tanks or additional gases.
The engine was not implicated in NTSB’s investigation of the crash. The accident was caused after Alsbury unlocked the ship’s feather mechanism too early in powered flight. The twin tail booms deployed, resulting in aerodynamic forces tearing the ship apart.
The engine change added complexity and additional failure modes to the propulsion system, which was originally touted as being simple, safe and benign. The modifications also altered the way air flowed over the spacecraft in flight.
Martin said he was concerned enough about how the changes might affect how SpaceShipTwo flew that he unsuccessfully argued for Scaled Composites to submit additional data about the ship’s structure.
“You know, we at least ought to get [Scaled Composites] to update the structures [analysis],” Martin told his bosses. “And to their credit, you know, we had the discussions — you know, Scaled was willing to do that, but my management didn’t feel it was necessary.”
Martin’s first hint of significant changes to SpaceShipTwo came after the spacecraft’s third powered flight on Jan. 10, 2014. He and other FAA officials noted flutter in the aft tail wing during powered ascent. Flutter is defined as an “unstable oscillation which can lead to destruction. Flutter can occur on fixed surfaces, such as the wing or the stabilizer, as well as on control surfaces such as the aileron or the elevator for instance.”
Subsequent investigation indicated that Scaled Composites had made some changes to the wings before the flight in anticipation of installing helium tanks in them. The modifications had caused a change in the way air flowed over the ship, resulting in the flutter.
The helium was needed to compensate for instability in the hybrid rubber-nitrous oxide motor that caused vibrations and oscillations to SpaceShipTwo. The company had not informed FAA AST in advance of the changes before the flight test.
Martin, who had helped investigate the crash of the space shuttle Columbia, said he was very concerned about the wing modifications. With Columbia, a piece of foam from the external tank had punctured a hole in a wing of the space plane during launch. The shuttle burned up during re-entry when super hot gases entered the ship through the hole.
“The aerodynamics and analysis that we did during our investigation of that showed that that breach ended up tripping the flow differently than what it was designed for and actually impeded the leeward side of the shuttle cargo bay,” Martin explained. “And more importantly, it produced a shock — a supersonic shockwave on the vertical tail, and we saw loads pretty much near their limit. And the only reason the tail didn’t fall off is because the wing ended up falling off and it disintegrated. So I used that as an analogy to my management and said, look, you know, I’m concerned with this.”
His bosses were not nearly as worried.
“My management got comfortable with it because Scaled Composites came back and said they had done a lot of C and D and drop testing with it, and they had a lot of information to correlate the data,” Martin told investigators. “So the next round in the experimental permit, we didn’t see any updates to their hazard analysis. So I did not get involved other than to say no hazard analysis was seen or provided; therefore, you know, I didn’t have any evaluation or comments to the regulation, and the permit was issued or reissued.”
After Virgin Galactic decided to change to the nylon engine, Scaled returned to FAA AST with a revised permit renewal application that incorporated data about the modifications required for the new motor.
Given the extensive nature of the changes, Martin wanted to see updates to the hazard analysis as well as the structural analysis for SpaceShipTwo. At that point, he had the opportunity to examine the entire application for the first time and made a surprising discovery.
“I noted to my management that they did not identify structures as safety critical, and they didn’t provide us with any type of hazard analysis associated with that,” Martin said. “My management’s technical response was that they didn’t believe structures was safety critical at the time.”
NTSB’s investigation has shown SpaceShipTwo’s nylon engine performed as expected and the ship flew normally during 12 seconds of powered flight last October. A 38-second engine burn was planned for the test, but the ship broke up before that goal could be reached.
NTSB found that “the probable cause of this accident was Scaled Composites’ failure to consider and protect against the possibility that a single human error could result in a catastrophic hazard to the SpaceShipTwo vehicle. This failure set the stage for the copilot’s premature unlocking of the feather system as a result of time pressure and vibration and loads that he had not recently experienced, which led to uncommanded feather extension and the subsequent aerodynamic overload and in-flight breakup of the vehicle.”
Co-pilot Alsbury, who unlocked the feather early, died in the crash. Pilot Pete Siebold survived with serious injuries.
During an appearance at the Space Access 2015 conference in April, Virgin Galactic Vice President Will Pomerantz said the company is still testing both rubber and nylon engines. His best guess was that they would go back to using the rubber engine, whose performance has been improved. Pomerantz also characterized the engine switch as a change in the fuel grain.