For the last several years work has been underway to develop NASA’s replacement launch vehicle for the agency’s retired space shuttle fleet, the Space Launch System (SLS for short). The mammoth rocket will be the most powerful and capable heavy-lift launch vehicle ever designed, and NASA F/A-18 Hornet research jets have played a critical role in testing and evaluating the rocket’s autonomous flight control system at Armstrong Flight Research Center in southern California, located at Edwards AFB.
“By flying a high-performance F/A-18 jet in a manner similar to our rocket, we’re able to simulate SLS’s flight conditions and improve our software,” said Tannen VanZwieten in a press release, SLS flight controls working group lead. “The innovative system that we are testing is advancing flight control technology by adding an adaptive element which is new for launch vehicles. We’re using this technology to expand the capabilities of the SLS a bit more than what is possible with a traditional design.”
The Launch Vehicle Adaptive Control (LVAC) experiment was conducted in five flights by the F/A-18 Hornet to test the Adaptive Augmenting Controller, which will allow SLS to respond to various conditions—such as winds and vehicle flexing—during the launch/ascent phase of the mission.
In those tests, the jet took to the skies over Edwards Air Force Base to simulate those launch conditions SLS might encounter as it thunders away from Earth. The flight tests are crucial in evaluating the SLS’s flight control system and help engineers to design a system capable of autonomous adjustments to unexpected conditions as SLS pushes toward space.
“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,” said Eric Gilligan, an engineer at NASA’s Marshall Space Flight Center helping to develop the “brain” for the SLS. “The tests might reveal something we hadn’t thought about in our algorithm, which we can go back and modify as necessary.”
“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”
No previous NASA launch vehicle has had the capability to adjust autonomously during actual flight, and the SLS Adaptive Augmenting Controller’s ability to make real-time adjustments to the autopilot should make for enhanced performance and a safer flight.
SLS will launch astronauts on the agency’s Orion spacecraft.
The NASA F/A-18 simulated both normal and abnormal flight conditions which the SLS might encounter after liftoff, such as sloshing propellant, and “identified key aircraft vibrational characteristics,” according to NASA. Over 40 tests were conducted, flying trajectories similar to what SLS will perform, and the Adaptive Augmenting Controller system was evaluated in different scenarios for up to 70 seconds at a time, matching the rocket’s dynamics from liftoff to solid rocket booster separation.
“This is an example of how advanced rocket technology can be checked out in flight without having to be launched into space,” said John Carter, project manager for the flight tests at Dryden. “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 data collected during the flight tests of the Adaptive Augmenting Controller system were used to refine software for the SLS and plans for following F/A-18 flight tests in support of SLS. The autonomous flight control system, according to NASA, will be ready for the first test flight of the SLS, which is currently scheduled to launch an unmanned Orion spacecraft in 2019.
Launching from former shuttle launch pad 39B at the Kennedy Space Center, the SLS will send Orion to the moon and back to test the entire integrated system (launcher and spacecraft) as a whole, before NASA puts astronauts on top of the mammoth rocket for the next flight in the early 2020s.
“The rocket has a set of equations that describe its motion,” said Jeb Orr, an engineer at Marshall’s Spacecraft and Vehicle Systems Department helping to develop the complex step-by-step equations that make an F/A-18 Hornet fly like the SLS. “It’s all just a math operation. When applied to the model of the rocket, it helps us predict the intended performance.”
“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. 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.”