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Algorithms + FA-18 Jet = Vital Testing for SLS Flight Control System
by Kim Henry for Marshall Space Flight Center
Huntsville AL (SPX) Nov 25, 2013

Tannen VanZwieten. Image Credit: NASA/Dryden.

Raise your hand if, in a math class, you ever said, "When will I ever use this in my life?" Four young engineers at NASA's Marshall Space Flight Center in Huntsville, Ala., can answer that question: They are using math to develop algorithms, or complex step-by-step equations, that can make an F/A-18 fighter jet fly like the Space Launch System (SLS) -- NASA's next heavy-lift launch vehicle.

Marshall's Eric Gilligan and Tannen VanZwieten; Jeb Orr, a Draper Laboratory employee; and John Wall, a Dynamic Concepts employee are all working in Marshall's Spacecraft and Vehicle Systems Department. They have spent years developing and refining algorithms for the flight control system on the SLS. That system is the "brain" of the vehicle, designed to steer it along the path to destinations beyond Earth's orbit.

"The rocket has a set of equations that describe its motion," Orr said. "It's all just a math operation. When applied to the model of the rocket, it helps us predict the intended performance."

NASA is no stranger to designing flight control systems for launch vehicles, but the Marshall team of engineers is innovating a new automated system that adds additional performance and robustness to the traditional flight design.

"We're expanding the capabilities of SLS a little bit beyond what we'd normally be able to achieve through a traditional analysis process," Orr said. "With an adaptive algorithm, we can be a little more responsive to anomalies in flight, like unpredictable winds, to ensure the vehicle stays on its trajectory."

For NASA, this is the first application of an adaptive control concept to launch vehicles, adding the ability for an autonomous flight computer system to retune itself -- within limits -- while it's flying the rocket. The system, called the Adaptive Augmenting Controller, learns and responds to unexpected differences in the actual flight versus preflight predictions.

This ability to react to unknown scenarios that might occur during flight and make real-time adjustments to the autopilot system provides system performance and flexibility, as well as increased safety for the crew.

"We needed a way to test our control algorithm, specifically the part that's new -- the adaptive part," added VanZwieten.

"A stumbling block for a lot of people is, 'you've got a rocket algorithm, but you want to test it on an airplane?'" VanZwieten said. "It's not immediately clear how the aircraft could match important dynamic features of SLS, but it does. We're flying a similar trajectory on the airplane as we have with the rocket, and the aircraft rotational dynamics are 'slowed down' to match the maneuvering characteristics of a heavy-launch vehicle."

"This is an example of how advanced rocket technology can be checked out in flight without having to be launched into space," noted John Carter, project manager for the flight tests at NASA's Dryden Flight Research Center in Southern California. "Doing this work on the F/A-18 test bed allows for low-cost, quick-schedule tests that can be repeated many times in order to gain confidence in the advanced controls technology, providing some unique testing advantages for this type of control system validation."

"The F/A-18 has a combination of performance and robustness that allows us to fly experiments that would break other kinds of airplanes," added Curt Hanson, Dryden's principal investigator for the experiment, pointing out Dryden's long history of conducting advanced flight controls research with F/A-18 aircraft dating back to the 1980s.

"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," Hanson said. "If anything goes wrong, the pilot can turn the experiment off and fly the airplane back to base."

"Our software that's running on the F/A-18 doesn't know that it's flying an F/A-18. It thinks it's flying SLS," Orr added.

The F/A-18 test series, called the Launch Vehicle Adaptive Control (LVAC) experiment, began Nov. 14. Five flights are planned, with more than a dozen tests being conducted during each flight. Although the jet is in the air for 60 to 90 minutes, the algorithm is tested in different scenarios for up to 70 seconds at a time.

"We have 20 test cases, each simulating some abnormal conditions, like higher thrust than anticipated or the presence of wind gusts, to see if the algorithm responds as we designed it to do," Gilligan said. "The tests might reveal something we hadn't thought about in our algorithm, which we can go back and modify as necessary."

The team will be manning the control room for all the tests. "We'll be looking at the data coming in real time and making decisions about the test scenarios that will be relayed to the pilot on flight days. We're really excited for the opportunity to get to see our work take off, literally, for the first time," Wall said.

After the initial flight tests, the flight control team will return to Marshall, go over the data and make any changes to the test plans for the remaining flights.

"We aim to accomplish as many top-level objectives as possible in the first set of tests and then go to more complex scenarios in our later test series," VanZwieten said. The test series is scheduled to run over the next few weeks.

The new software will be ready to run on the first flight test of the SLS, scheduled for 2017. The flight will feature a configuration for a 70-metric-ton (77-ton) lift capacity and carry an uncrewed Orion spacecraft beyond low-Earth orbit to test the performance of the integrated system. As the SLS evolves, it will provide an unprecedented lift capability of 130 metric tons (143 tons) to enable missions even farther into our solar system to places like Mars.


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