All Fired Up
When Bob Pitz studies a problem, it really is rocket science. Vanderbilt’s combustion expert, Robert W. Pitz, professor and chair of the Department of Mechanical Engineering, explores ways to make aircraft and rocket engines burn more efficiently, safely and powerfully for clients that include NASA and the United States Air Force.
In his current research, Pitz and his colleagues work on fueling what could potentially be the next generation of jet engines: hypersonic (super fast) engines for space and air flight. Most modern fighter jets (and even the Concorde) are supersonic, reaching speeds of Mach 2 to 3 (approximately two to three times the speed of sound); hypersonic crafts could be the faster, better, next step. Capable of speeds at Mach 5 to 10 (or 4,000–8,000 miles per hour), they could be used in space and military applications, as well as possibly civilian aircraft.
These experimental engines are known as scramjets (supersonic combustion ramjets) because they use supersonic combustion rather than rockets for propulsion. In the lab, Pitz and his team study supersonic combustion flow, making measurements to refine computer models that can accurately predict what will happen to such engines at hypersonic speed.
Hypersonic space planes with scramjet engines have the promise to lower the cost of earth-to-orbit flight by eliminating the need to carry oxidizer to burn with fuel as rockets do. In space vehicles, these engines could power the vehicle after liftoff but while still in the Earth’s atmosphere. During this air-breathing phase, space planes would travel at hypersonic speeds but air would be slowed to supersonic speeds in the plane’s scramjet engine intake to burn with the fuel.
Research on this promising but experimental technology is challenging. “When designing such engines, it’s very difficult to predict perfectly how they will behave at hypersonic speeds,” Pitz says, thus velocity research in his Vanderbilt lab and in other study settings is critical.
Traditional methods of using probes to measure velocity can’t be used at hypersonic speeds, as they would burn up or interfere with performance. To solve that problem, Pitz developed nonintrusive laser diagnostic techniques that can measure supersonic airflow for hypersonic propulsion. His pioneering technique uses two lasers that first mark, or “tag,” and then illuminate molecules in the air that can be measured. The data is then used in computer models to predict and simulate the flow dynamics.
“We use our lasers to measure the velocity of combustion at supersonic speeds in the wind tunnel at the Air Force Research Laboratory at Wright Patterson Air Force base in Ohio,” Pitz says. “We then compare our data with computer models developed by researchers at Georgia Tech.”
On Deadline for NASA
Pitz has patented two award-winning molecular tagging techniques to use in measuring velocity: Ozone Tagging Velocimetry (OTV) and Hydroxyl Tagging Velocimetry (HTV). His techniques have wide application in the study of aerodynamics, combustion and fluid dynamics.
In HTV, multiple beams from one ultraviolet laser split the airflow’s water molecules to form a grid pattern of hydroxyl molecules in the combustion chamber. Two microseconds later, the second laser causes the molecules to light up. Next, a digital camera records the movement of the lighted grid of tagged molecules.
“Once the HTV grid is formed, the grid moves with the flow,” Pitz explains. “The displacement of grid over a fixed time period yields the velocity, much like you would judge the speed of a river with a stick.”
The researchers are fabricating an HTV system for NASA to use in testing the J2X engine, a new second-stage rocket engine being developed for the next generation of rockets that will replace the Space Shuttle. The system will be delivered in June to John C. Stennis Space Center, NASA’s large rocket testing center, in Mississippi.
Fueling the Future
Pitz is also working on basic research into the development of hydrogen burners. Calling hydrogen the fuel of the future, he says hydrogen burners could be used in gas turbines, aircraft and possibly long-distance automobile travel.
That research employs a method called Raman scattering, which uses ultraviolet light to measure fuel concentration and combustion products and then determine how well the fuel and oxygen are blending.
In addition to research and chairing the Department of Mechanical Engineering, Pitz teaches undergraduate thermodynamics and several graduate courses. He supervises the studies of five graduate students and directs the research of a Fulbright Scholar from Cairo, Egypt. His former students include researchers, professors and administrators at international universities, military engineers and other scientists.
His current students show equal promise. Pitz points to research being conducted by students, including graduate student Marc Ramsey and senior Kyle Bloemer, as noteworthy. Ramsey and Bloemer are studying cap shock, a specific shock-wave pattern found in truncated rocket nozzles optimized for high thrust and low weight. Unstable cap shocks, which occur when engines are turned on, can damage the nozzles.
“Marc Ramsey has developed a new computer-based template to analyze the HTV grid displacement that gives very accurate velocities,” Pitz says. “This will help improve our existing computer models, which can then be used to design the nozzles more reliably.”