Dr. Greg Chavers, test lead at the Marshall Space Flight Center in Huntsville, Alabama, helped to design the “Mighty Eagle” robotic prototype lander. The vehicle, which can guide itself to a specified target, flew “open loop” to an altitude of 100 feet in late August.
NASA Tech Briefs: What is the Mighty Eagle?
Dr. Greg Chavers: The Mighty Eagle is a test vehicle, and it was built originally to demonstrate that we can control a small vehicle that is dynamically similar to a small robotic lander that could land on the moon or other airless body. We started with a flight design concept and built this vehicle with a propulsion system that uses pulse-width modulated thrust, with very fast-acting valves so they’re either on or off. They’re not throttled to control the altitude and the attitude of the vehicle.
It was built as a demonstrator for the control algorithms. However, to leverage more of the design team, we actually included landing legs, and operate the vehicles to be very similar to what the spaceflight implementation would be.
Over the past six years we’ve been teaming with Johns Hopkins University Applied Physics Lab to develop the actual spaceflight concepts and start design on what we would actually do. We designed this vehicle about three years ago, and 18 months after we started the design, we started flying it.
We’ve done almost 30 flights now. In the latest series, since we demonstrated the controllability, we have an onboard camera. We can take the optical images that are onboard the vehicle and process them through the onboard computer. It can update its guidance algorithms based on what it sees in the images so that it can fly to a target and land. The Mighty Eagle is just a testbed or test vehicle which is similar in size and characteristics to what the real spaceflight implementation would be.
NTB: What is the most unique aspect of this lander design?
Dr. Chavers: The most unique aspect is the pulsed thrusters. A lot of landers, if you look at the Masten Zombie or Morpheus, have throttled engines, and they’re designed more for the bigger landers that would go and land on, say, the moon. This one specifically was designed to be a small, simple, robotic lander. It has a very high thrust-to-weight ratio, and very small thrusters are used. This one targets more of the science and the precursor-type missions to landing payloads on perhaps the lunar surface.
NTB: You mentioned the onboard camera. How is the lander able to navigate autonomously?
Dr. Chavers: We have accelerometers. We fly inertially, using an inertial measurement unit. Inside that are the accelerometers. It knows that the motion of the vehicle is changing by the acceleration that the accelerometers detect. From that, we can calculate what the velocity is and what the position is; that’s flying inertially.
We put in a profile to fly, but because of inaccuracies in the inertial measurement unit, we also have an onboard sensor. It’s a radar altimeter. After several seconds, errors build up when you fly inertially. The radar altimeter updates our navigation solution such that we can accurately determine the altitude, and the onboard camera provides the guidance.
For the previous test series, we had a pre-defined target that was painted on the ground. It knows what the target is supposed to be. For example, if we were doing orbital debris, or capturing a dead satellite, the camera would know the image it would be looking for, and could update a vehicle’s navigation to track to that vehicle. And that’s what we had shown. It’ll take an image, and look for a specific target, and then update your navigation solution to get you to that target, and provide those commands to the thrusters completely autonomously.
NTB: Can you take us through your most recent test run and how that went?
Dr. Chavers: We finished up the controllability demonstration in November of 2011. We started building for this latest test run in the spring. We actually changed locations. We went from the Redstone Test Center on Redstone Arsenal to the Marshall Space Flight Center test area, which is still on Redstone Arsenal but it’s on NASA property.
So in this series, using the same vehicle, we had an area that gave us enough room to do the vertical and lateral translations. We added the targets to the ground again, and we also added in several young engineers. The vehicle was already in operation. We took this opportunity to bring in several engineers that had been out of school for two or three years. They worked at Marshall, we trained them, and we put them in lead positions for the test operations.
We did some checkouts because the landing hadn’t flown in eight months. We went right into the test series. We picked it up with a crane to about 100 feet, so we could see what the camera would see without actually having to fly it, and verify that our images were crisp and clear, that the camera focus was correct and so forth. We loaded some propellant, did two low hover tests to make sure all systems were good, and then we went up to 30 feet, and flew open loop. The guidance didn’t update, but it was running in the background and got optical solutions. After that flight, we ran it back thorough a simulation to verify that the optical solutions would provide the correct change in path, so we didn’t go outside of our test area.
And on our next flight, we closed the loop so that the lander autonomously flew to the target. Then we came back and flew open loop again at 100 feet on August the 28th. Last week, Sept 5th, we flew to 1000 feet closed loop again. We completed that part of the test series.