New Control System Guides Experimental Aircraft

Scramjets could deliver missiles to mobile targets; they could also carry people halfway around the world in less than an hour.


COLUMBUS, OH – 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.

Ohio State University engineers have designed control system software that can do just that – by adapting to changing conditions during a flight.

Government agencies have been developing faster-than-sound vehicles for decades. The latest supersonic combustion ramjets – called scramjets – burn air for fuel, and could one day carry people to space or around the world in a matter of hours.

The recent success of NASA's X-43 hypersonic jet has spurred research into the control systems for these vehicles, says Lisa Fiorentini, doctoral student in electrical and computer engineering at Ohio State University.

She and associate professor Andrea Serrani are developing a new control system in collaboration with the U.S. Air Force Research Laboratory (ARFL) at Wright-Patterson Air Force Base in Ohio.

In the current issue of the Journal of Guidance, Control, and Dynamics, they report that their controller performed flawlessly in computer simulations of flight maneuvers.

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 says. "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 explains, is that it allows for flexibility: it adapts 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 says.

Most other research teams build their controllers from very simplified computer models, Fiorentini says.

"Since we are working with Wright-Patterson, we have access to the most sophisticated model available for this aircraft," she says.

They are collaborating with Michael A. Bolender, an aerospace engineer, and David B. Doman, a senior aerospace engineer, both of ARFL, as well as Jack McNamara, assistant professor of aerospace engineering at Ohio State.

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 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.

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