UVM Lab at Spaceflight’s Leading Edge
Sending materials to space and back without leaving Earth
- By Sujata Gupta
Inside Doug Fletcher’s Plasma Test and Diagnostics Laboratory at the University of Vermont, we watch through a clear window as a bolt-shaped piece of graphite enters a new atmosphere.
As the cylindrical chamber housing the graphite heats up to a whopping 6,000 degrees Celsius at its center, a graduate student adds nitrogen gas, a core component of Earth’s atmosphere.
At such high temperatures, gases change dramatically into a form of matter called plasma – electrons break away from molecules, leaving charged particles called ions in their wake – and alter surrounding materials in unexpected ways. When the grad student turns off the heat, the white-hot graphite sample slowly cools to a deep orange.
The demonstration is meant to simulate what could happen to that piece of graphite if it coated the heat shield of a rocket careening back to Earth from, say, Mars.
The rocket would be so fast-moving it would generate a shock wave along its leading edge, turning all the gases in its path into a searingly hot, corrosive plasma.
Predicting how various materials will perform in such a hostile environment is crucial to developing the next generation of efficient and effective insulating heat shield systems that will enable a spacecraft to travel safely to and from distant locations in the universe.
UVM’s plasma lab is an instrumental player in that effort because, alone among test facilities, it focuses exclusively on a region where much of the important action takes place in spaceflight – the so called “boundary layer,” a three to four millimeter thick region between the plasma at the rocket’s leading edge and the surface of the insulating material being tested.
“Everything happens at this interface,” says Fletcher, a mechanical engineering professor in UVM’s College of Engineering and Mathematical Sciences.
Fletcher’s lab creates plasma and simulates the boundary layer as other labs do, but is unique in then capturing a welter of data – with a laundry list of laser diagnostic and measurement tools – about the complex chemical and physical reactions that are taking place there.
This information is coveted by NASA and private aerospace companies because it enables them to plug real data into their design models. Current models for designing new insulating materials, Fletcher says, are weakened by “simplifying assumptions that we know are invalid.”
The ultimate goal? To design heat shields that are not only more reliable, but also lighter weight. Uncertainty – arising from the data gaps in the models – has historically led engineers to build extra material and weight into the heat shields to minimize their chance of failure.
Eliminating this overdesign will, in turn, allow NASA to achieve one of its major objectives for the next phase of the space program. Lighter heat shields mean there is capacity for larger payloads – of scientific instruments and humans.
Build a bike, build a plasma lab
Fletcher has worked on the complexities of creating better insulating materials for spaceflight throughout his career, which has included 12 years at the NASA Ames Laboratory and another seven at the Von Karman Institute for Fluid Dynamics in Belgium.
But it was only when he came to UVM in 2007 that he had the opportunity to create his own test facility, with funding provided by the Air Force Office of Scientific Research.
To build it, Fletcher recruited Walt Owens, a graduate student in the School of Engineering whom he had seen around campus riding a so-called tall bike – a double stacked bike welded together -- that he had built for himself. Fletcher assumed, says Owens with a chuckle, “If I can build a bike, I can build a plasma lab.”
Owens was an odd pick. When he met Fletcher, all he knew was that he hated fluid mechanics and thermodynamics – key components of the field of aerothermodynamics. But he’d always liked construction and held a bachelor’s degree in mechanical engineering, so he was intrigued by the chance to help build a lab from scratch. Soon, he was engrossed in reading up on similar facilities used by NASA, the Air Force and European and Russian researchers.
It took Fletcher and Owens about a year to build the lab – of a type called an inductively coupled plasma facility – and then another year to fine-tune all the equipment and protocols to get it up and running. They started running tests a few years ago. Now, says Owens, a convert to the cause who has nearly completed a doctorate in aerothermodynamics, “It’s a world-class facility."
Federal funders agree; in its short life, the lab has already helped Fletcher and his colleagues win 10 competitive grants from the Air Force Office of Scientific Research, NASA and the Office of Naval Research.
Before I exit the lab, Fletcher tells me what the nitrogen plasma is doing to the graphite bolt. Nitrogen atoms in plasma, he says, raising his voice to project over the din of the machine, can grab a carbon atom right off the surface of a material like graphite and shuttle it away.
Moreover, free nitrogen atoms preferentially bind to one another and dump that recombination energy back into the material. By measuring those two reaction rates using laser diagnostics, it becomes possible to sort out what’s going on at the graphite’s surface. And that, Fletcher booms, “removes the uncertainty of one key set of reactions.”
Adding more certainty to the process of getting spacecraft safely to and from distant places is Fletcher’s métier – making his lab an important way station in mankind’s next step to a new frontier.
In addition to Owens, the following faculty and graduates students also work the the lab: Jason Meyers, Andrew Lutz, Max Dougherty, Juergen Uhl, Silas Smith, Corinna Thompson, Corey Tillson and Luke Allen.