Earlier this month, NASA carried out tests on a new flight system for the Space Launch System (SLS), the heavy lifting rocket that will send the manned Orion spacecraft into deep space. What was unusual about this is that instead of using a rocket for the tests, the space agency used an F/A-18 Hornet fighter jet as a stand in as a way to carry out more tests at lower costs.
Space launches aren't a matter of lighting the blue touch paper and running. Once off the launch pad the rocket still has to deal with winds, air density, variables in system performance, and other factors. Until now, the strategy for handling these has been to program the flight systems with predictions and a set of predetermined responses. It works, but it has its limitations and can often result in delays when the unexpected happens.
NASA's answer for the SLS is the Adaptive Augmenting Controller. This analyzes flight data in real time and reacts to changes in the environment and the vehicle. The tricky bit is testing the system. Normally, this would be done using sounding rockets, but this can be expensive because each test is a one off where the vehicle is lost and a malfunction can make the test a write off. By using an F/A-18 research jet, NASA can simulate various flight conditions in a single flight and an error simply means starting over.
More than 40 tests were conducted at the Dryden Flight Research Center at Edwards Air Force Base, California with the Hornet using SLS-like trajectories. These involved the fighter flying for up to 70 seconds in various trajectories designed to match those of the SLS from lift off to booster separation, while simulating sloshing propellants and other problems.
"The F/A-18 has a combination of performance and robustness that allows us to fly experiments that would break other kinds of airplanes," says Curt Hanson, Dryden's principal investigator for the experiment. "The airplane can also fly for a much longer period of time than a sounding rocket, so we can conduct a series of tests back-to-back, making small changes between each one to compare the results. If anything goes wrong, the pilot can turn the experiment off and fly the airplane back to base."
According to NASA, the tests will continue until the end of the year.
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