NASA had explored scramjets as a successor to the Space Shuttle for trips to the International Space Station. The X-43 project closed in 2004, as the space agency shifted its priorities toward a return to the Moon.
But the technology is still under development in military and commercial sectors. Scramjets could deliver missiles to mobile targets; they could also carry people halfway around the world in less than an hour.
For this study, the engineers simulated two flight situations. In the first, simpler case, the scramjet had to climb from a level flight to 13,000 feet in a little less than six minutes. In a second, more complicated maneuver, it had to start at a few degrees off-kilter from a level flight, and then climb 25,000 feet in about four minutes.
In both simulations, researchers recorded the controller's tracking errors as the jet executed its maneuver. Then they compared the results to simulations using a controller they had developed previously – one that did not have adaptive capabilities built in.
For example, in the simpler maneuver, the largest altitude tracking error for the older, non-adaptive controller was just over 40 feet; the largest corresponding error for the new, adaptive controller was less than 2 feet - an improvement by a factor of 20.
For the more complex maneuver, the non-adaptive controller failed – the simulated jet spun out of control and crashed in less than four seconds. The new adaptive controller was able to guide the jet to its new altitude without incident.
The Ohio State and AFRL engineers are continuing to refine the controller. The next improvement will add some safety limits, Fiorentini says. Scramjets need to maintain the right amount of airflow to the engine, she explains, and if they rise too fast, the engine may stall in mid-air.
This work was funded by the ARFL and the U.S. Air Force Office of Scientific Research through the Ohio State University Collaborative Center of Control Science, and by the Michigan/AFRL Collaborative Center of Control Science.
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