When a jet is flying faster than the speed of sound, one small mistake can tear it apart, and when the jet is so experimental that it must fly unmanned, only a computer control system can pilot it.
For that reason, Ohio State University engineers have designed control system software that can do just that, by adapting to changing conditions during a flight, the university said on April 30.
Government agencies have been developing faster-than-sound vehicles for decades, the university’s news release said. Examples given were supersonic combustion ramjets – called scramjets – which burn air for fuel and someday could carry people to space or around the world in a matter of hours; and the recent success of NASA’s X-43 hypersonic jet, which has spurred research into control systems for these vehicles, said Lisa Fiorentini, a doctoral student in electrical and computer engineering at Ohio State. She and associate professor Andrea Serrani are developing a new control system in collaboration with the U.S. Air Force Research Laboratory at nearby Wright-Patterson Air Force Base.
The controller both guides the jet along its trajectory and keeps it stable during a flight, Fiorentini explained. Sensors measure factors such as altitude, velocity, and acceleration, and the controller calculates whether any adjustments need to be made to keep the jet stable and on course. Then actuators carry out the controller’s commands – for instance, throttling up the engine if the jet needs to accelerate.
“Because these vehicles are unmanned right now, we have to prepare everything ahead of time – anticipate every possible in-flight event,” she said, “and the controller has to work really fast. At 10 times the speed of sound, if you lose just one second, the jet has gone far, far off course.”
What sets the Ohio State control system apart, Serrani explained, is that it allows for flexibility, adapting to changing conditions during a flight. “The truly remarkable feature of our approach is that we consider a realistic, physics-based vehicle model within our stability analysis, using a highly sophisticated controller,” he said.
Most other research teams build their controllers from very simplified computer models, Fiorentini added. “Since we are working with Wright-Patterson, we have access to the most sophisticated model available for this aircraft,” she said. The Ohio State engineers derived equations that describe a scramjet’s flight dynamics and behavior. Then, given the vehicle model by their partners at Wright-Paterson, they created a set of algorithms that could ultimately be built into a scramjet’s on-board computer.
Today’s experimental scramjets are not merely supersonic – meaning that they fly faster than the speed of sound, or Mach 1 – but hypersonic, meaning they fly at Mach 5 or faster. The most recent X-43 flight in 2004 neared a speed of Mach 10 (Mach 9.8, or 7,546 miles per hour). Scramjets are shaped to scoop oxygen from the atmosphere during flight in order to ignite the hydrogen fuel already on board. This eliminates the need for heavy external oxygen tanks, and enables scramjets to carry more cargo than a typical rocket.