Spaceflight Insider

SS2 disaster: Interview with Carolynne Campbell-Knight, rocket engineer

Virgin Galactic SpaceShipTwo hybrid rocket engine firing Scaled Composites photo posted on SpaceFlight Insider

Photo Credit: Virgin Galactic

In the aftermath of the SpaceShipTwo disaster that injured one pilot and took the life of another on Oct. 31, Carolynne Campbell-Knight was in great demand as media around the world struggled to understand the technology of this suborbital plane, itself powered by a hybrid engine. But Campbell-Knight doesn’t work in the aerospace industry and never has. Where did she come from and where does she derive her expertise?

From Race Cars to Rockets

“I was always a race car person,” Campbell-Knight said during a Skype call from her home in London. “About fifteen years ago, my husband and I were at the track and we saw this car with a rocket. We thought ‘that’s very nice but it’s too slow. We can fit two rockets in there – no, four!’”

Shortly thereafter, Campbell-Knight’s husband became desperately ill and the rocket car became their project, something to focus on in the hard months that followed. Since she had to do the work for both of them, Campbell-Knight quickly became the rocket specialist – but there were many lessons along the way.

“I’m an addictive reader and I’m not fond of trying to re-invent the wheel, so I took to researching the literature and work that was already out there. I learned a lot that way.”   She started with the traditional fuel/oxidizer combinations: plastic or rubber with nitrous oxide, but found that the fuel grains burned irregularly and produced dirty soot, while the nitrous oxide could be quite unpredictable. “Hybrid engines aren’t new,” she says, “they’ve been around a long time. But they aren’t particularly good.”

knights-arrow-shakespeare-county-raceway-nitrogen-blow-out photo credit Carolynne Knight posted on SpaceFlight Insider

Photo Credit: Carolynne Campbell-Knight

Creating a Better Hybrid Rocket Engine

Campbell-Knight’s goal is to create a rocket car that can safely reach 400 mph on a conventional runway. This is a novel approach – rocket cars have clocked higher speeds but always in flat, unpopulated desert areas with many miles of running space. Achieving that requires a much higher level of precision – and that’s what Campbell-Knight is working on getting from her engines.

“It sounds silly, but we decided to try tubes of reinforced cardboard,” she explains. “Cardboard burns really well but if it is at all damp, even just the air being humid, the cardboard won’t light.” Campbell-Knight was puzzling over this challenge one day when inspiration struck. “I soaked the cardboard in kerosene,” she recounted. “It lit even under damp conditions and burned very smoothly and with much increased performance. We discovered that the kerosene was boiling out of the wick at some pressure allowing it to burn in the heart of the combustion chamber, and leaving enough vapor to film-layer cool the nozzle.”

She soon worked out a number of modifications that made the invention more sophisticated and reliable, along the way dubbing the blended engine a “bybrid” at the suggestion of Airborne Engineering’s James Macfarlane who provided the test stand facilities to try out these iterations.

The bybrid was a step away from the traditional plastic/rubber approach and Campbell-Knight was glad to see the last of those troublesome fuels. But she and her engine still had challenges. “It only burns for 12 seconds,” she explains. Once the bybrid uses up the kerosene, combustion is at an end. “12 seconds is more than enough for a rocket car, but it won’t get you to space.” It also meant that each engine was good for one use only, so Campbell-Knight took it to the next level, moving the kerosene to an external tank with controlled feed through a custom designed delivery system and a substrate for it to flow over in place of the cardboard wick. She currently has a patent pending for the technology, which underwent live fire testing in March 2014.

Virgin Galactic Clay Center Observatory SpaceShipTwo image posted on SpaceFlight Insider

SS2’s Hybrid Rocket Motor – was the first suspect in terms of the Oct. 31 accident. Photo Credit: Virgin Galactic / SpaceFlight Insider

A Capricious Oxidizer

“The thing about nitrous is you can do something the same way every time then one time it will react differently and with extreme violence,” recounts Campbell-Knight. “I’ve been working with nitrous oxide for years and it still has surprises for me. Mother Nature is like that.”

Nitrous oxide is a popular oxidizer because it is self-pressurizing and non-toxic. However, it is relatively easy to start a runaway reaction if the chemical encounters low levels of residual fuel, becomes too warm, or experiences hydraulic shock in an enclosed container. Unlike oxidizers such as oxygen – the go-to solution for most spaceflight vehicles – nitrous oxide does not require propellant to initiate a reaction. These characteristics make it very sensitive to environmental factors, and hence difficult to manage safely.

Campbell-Knight herself is looking forward to the day when she can replace nitrous oxide in her own vehicle and instead rely on the stability of liquid oxygen. “There’s a reason the Space Shuttle used liquid oxygen and hydrogen,” she says.

The Media Calls

Campbell-Knight isn’t what one would call an activist, but following a fatal 2007 nitrous oxide explosion at Scaled Composites, in an effort to assist others to stay safe, she had posted conclusions from research into the incident highlighting the hazards of the propellant and its use in the SpaceShipTwo suborbital vehicle. She stressed that the document was the work of three-co-authors and that it was peer-reviewed by others. When another fatal incident struck on October 31, 2014, media organizations quickly found Campbell-Knight’s statements and called for her assessment. Had she, in fact, predicted the accident?

“I began every interview with condolences. I specified that I am not a structural engineer, I am not a safety expert: I am a rocket engineer.” And above all, she emphasized that no conclusions can be drawn about the latest SpaceShipTwo disaster without first identifying all the facts. “Most of what I have been saying really has nothing to do with the SpaceShipTwo accident,” she avers. “I keep saying we have to wait to see what the NTSB [National Transportation Safety Board] finds, we can’t conjecture without facts. The concerns I raise are long standing ones,” regardless of how this incident plays out.

A lot of the nuance of that position has been stripped out of the three minute news segments and 30 second sound bites in which she’s been featured, often highlighting some of Campbell-Knight’s more sensational statements. “I did say Branson should try selling mobile phones,” she admits. “At least they can’t kill anyone.”

SpaceShipTwo Virgin Galactic The Spaceship Company photo posted on SpaceFlight Insider

Photo Credit: The Spaceship Company

Rocket Cars vs. Spacecraft

“In some ways, rocket cars are harder actually,” says Campbell-Knight when asked how her field differs from spaceflight. “In a car, you have to manage forces in two directions. A vertical-firing spacecraft is aligned with gravity, all forces line up in one direction.” This makes it easier to deal with things like fluid flows and slosh, something that has been quite a challenge for Campbell-Knight’s car, which needs specialized systems to keep oxidizer flowing to the engine.

Of course, there’s more to a spacecraft than its propulsion system, as Campbell-Knight describes it: “the wrapper you put around the rocket,” but she her focus is and always has been that controlled burst of flame. She is driver for her cars as well as engineer, so, she quips, she takes safety quite seriously. “There’s a rocket at my back, just behind a ballistic shield. The view from the driver’s seat looks a lot like a cockpit. It’s more like piloting than driving to operate a rocket car.”

Innovation in Commercial Space

“I have a lot of respect for commercial space. Innovation is happening and I’m all for it. There is something to be said for having less paperwork and bureaucracy so you can get out there and build and test. Companies like Xcor are amazingly open about their work – look at their website, it sets a great example of openness.” That process is a lot like the one she uses herself, and it is clear Campbell-Knight relates to both the engineers and the test pilots whose jobs so resemble her own dual role as rocket car designer and driver. But, Campbell-Knight cautions, we can’t expect more than incremental innovation – “we are up against the laws of physics” – and outsized expectations do the industry no favors. “Rockets have been around for a long time, it’s mature technology.”

“I’m all for innovation – but can we do it without killing anyone?” Campbell-Knight recalls the victory sign following the SpaceShipOne flight that won the Ansari X PRIZE: “SpaceShipOne, Government Zero – remember that?”

To her, that statement is indicative of the challenge facing some elements of commercial space – the attitude that these new companies are breaking fresh ground with a shiny new business model, and they have nothing to learn from the decades of government investments that have gone before. The lack of transparency and reluctance of some NewSpace companies to engage with governmental and intergovernmental space and aviation bodies to imbibe lessons learned and approaches to hazard analysis may allow preventable hazards to creep into these organizations and their spacecraft.

One of the lessons Campbell-Knight clearly thinks some companies could learn is to stick with tried-and-true propulsion systems. Hybrid rocket engines will never be a good choice for spaceflight, she says. “They are not powerful enough. When it comes to space rockets two things matter: thrust and weight.” Hybrids cannot beat out bi-propellant systems, perhaps the most effective of which was the Space Shuttle’s liquid hydrogen and oxygen combination. Hybrids – at least those based on nitrous oxide – come with a fundamental problem: using pressurized tanks with a high molecular weight chemical severely handicaps your weight calculation.

Debris from the crash site of the Virgin Galactic Spaceship Two rests in the Mojave desert, some 30 miles north of Mojave, California, USA, 31 October 2014.  Spaceship Two crashed during a test flight killing the co-pilot and seriously injuring the pilot.  EPA / MICHAEL NELSON photo posted on SpaceFlight Insider

Photo Credit: Michael Nelson / EPA

Waiting for the Facts

At this time, the National Transportation Safety Board (NTSB ) has completed its initial investigation, revealing some tantalizing insights into the moments preceding the SpaceShipTwo crash. For one thing, it looks like the proximate cause of the crash might have been structural failure following the untimely and unintended deployment of the spacecraft’s feathering mechanism – not its rocket engine. “It’s revealed another possible failure mode,” says Campbell-Knight, not the least deterred by the finding. She is waiting for the full report to make any judgment calls, and she’ll be looking for just one thing when it comes: “Facts. Just an honest accounting of what happened.”

“Now the press are reporting ‘pilot error’, that’s just plain rude and disrespectful,” she says. “The NTSB stated that there appeared to be an “un-commanded deployment,” that’s hardly ‘pilot error.’”

Taking Risks

“I am one of the very few people who have climbed in front of a powerful N2O hybrid rocket system (capable of propelling me at over 3g), and ‘lit the candle,’” says Campbell-Knight. “Is it dangerous and scary? Yes.  Will I do it again?  Yes. Understanding something is risky doesn’t stop some people from doing it.” But, Campbell-Knight concludes, accepting that risk for herself, relying on her own engineering ingenuity and mechanical skill, is different than allowing others to do so. Would she put a paying passenger in front of such a system? “No,” she answers bluntly. And that, perhaps, is at the root of her discomfort with the deployment of a vehicle for public conveyance when very few in the public actually understand what they are signing up for.



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Merryl Azriel serves as Managing Editor of Space Safety Magazine while she pursues a career in planetary science research. Merryl holds B.S. and M.S. degrees in Chemical Engineering from Rutgers University and a M.S. in Space Studies from the International Space University. She has eight years of R&D experience in the consumer products industry and currently works as a technical writer for NASA and NOAA contractors.

Reader Comments

One of the reasons the media jumps at pilot error in any kind of aviation accident,,,,a simple failure to understand aviation and aerospace.

Thanks for the well written article.

Bob Clark

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