Catching a “Gremlin”
By Keith Button|April 2022
Military drones are alluring tools for spying and striking targets, but getting them flying where they need to be is a challenge. What if conventionally piloted cargo planes could fly to the outskirts of a battlefield, release a swarm of such drones, and then recover them for refueling and reloading? Keith Button tells us how DARPA and Dynetics brought this vision to the edge of reality.
Last year, in a control room at Dugway Proving Ground in Utah, engineers intently watched a video screen showing the view from a Mini Cooper-sized drone flying over the nearby desert. The drone had been released from the wing of a C-130A cargo plane about an hour earlier, and now they could see the drone slowly approach a bullet-shaped fixture dragged by a cable from the same plane. The group let out a muffled cheer as the drone attached itself to the fixture and began to ascend on a winch. A full-throated cheer erupted moments later when the drone reached the plane and was recovered.
Why so much excitement? Because this October day was the last day in the test program, and Dynetics, the Alabama company whose concept DARPA chose in 2018 for flight testing, had yet to capture and retrieve one of its drones, which weigh up to 680 kilograms. The successful capture vividly demonstrated the potential for dispatching swarms of drones to fly over a battlefield to be recovered later in midair.
Creating the concept
Dynetics knew that the safety of the C-130A crew was the top priority. The plane would have a crew of 14 aboard, larger than normal because of testing. The drone and C-130A would each be flying at about 270 kilometers per hour, with the C-130A leaving a turbulent wake. Flying the drone straight into the cargo hold was obviously out of the question.
“That would be difficult to do because of the flow fields behind the larger aircraft, and very unsafe of course,” says U.S. Air Force Lt. Col. Paul Calhoun, DARPA’s Gremlins program manager and a former C-17 pilot.
Dynetics decided to capture the drone at a distance of 9 to 15 meters behind the C-130A on a cable and have the drone fold in its wing and shut off its turbojet engine to avoid any chance of it flying out of control.
A big question was how to grab the drone midflight. Inspiration came from the shuttlecock-shaped drogues at the end of aerial refueling hoses, says Tim Keeter, the Dynetics program manager. In refueling, a probe on the receiving aircraft fits into the wide end of the shuttlecock. Dynetics created a version of this concept centered on a bullet-shaped device that would be attached to the end of the cable. Stability would be a challenge, so Dynetics added four grid-shaped movable fins that would automatically adjust to prevent any fluttering motion from developing as a Gremlin approaches the bullet to dock. The Gremlin would then sprout a short docking arm from the top of its fuselage at about its midpoint and fly forward to insert this arm into the bullet fixture. Mechanical clamps inside the bullet latch onto the arm to secure the Gremlin, initiating the wing folding and shutting down the drone’s engine.
To keep the Gremlin from flopping around, the docking occurs above the combined center of gravity of the drone and 90-kilogram bullet. The Gremlin also has fins for stability. With its engine now shut off, the fins are powered by battery.
A winch on the C-130A slowly reels up the Gremlin to minimize any pendulum motion. The drone’s fins also adjust to damp any pendulum tendencies. For the final step of the retrieval, a mechanical arm hangs out the C-130A’s open rear door about 2.5 meters below the plane to embrace the drone and pull it through the worst part of the wake turbulence and into the plane.
Retrieving the drones through hands-on remote control would be impractical because of the communications latency and the required precision and minute adjustments. So Dynetics needed to craft autonomous flight software. The company turned to its subcontractor, Sierra Nevada Corp., which had written control software for the 2006 DARPA Autonomous Airborne Refueling Demonstration Program in which an F/A-18 autonomously connected to a refueling drogue trailing from a B-707 tanker.
“That was attractive to us as a good starting point for what Gremlins was going to have to deal with,” Keeter says — autonomously flying the drone to its intended target, the bullet, accurate to within few centimeters, with the ability to track and lock on to the bullet target for docking.
While the design ensured that a capture would occur below the C-130A’s wake, air flow still presented challenges. As the distance between the bullet and a Gremlin closes, the air flowing around each of them interacts and sets up a periodic up-and-down motion in the bullet that the software must predict or the docking will be missed. Indeed, until the October flight the drone kept missing, because the bullet would move away from the point the software was aiming for.
“If you’re always trying to chase to where it used to be, because there’s some lag in any control system, you never quite get there. So we have to predict where we’re moving to,” Calhoun says. So the engineers improved the tracking algorithm to better predict the periodic motion of the bullet and to aim for where it would be a split second in the future.
Another goal was to make sure that just about any cargo plane could be equipped to retrieve Gremlins or, if necessary, take control of them from the autonomous control software. So Dynetics designed the retrieval and control equipment to be installed on two standard cargo pallets that could be rolled on and off the plane. In the flight tests, the larger of the two pallets held the retrieval equipment, and the smaller one carried a two-seated control station from which managers permitted the autonomous retrieval to proceed to the next step.
Whomever takes over the next stage of Gremlins development will want to improve the technology so the capture is more accurate and automatic, Calhoun says. That way, multiple drones could be recovered quickly, which is the ultimate goal.
“It’s probably not completely solved. I think that we all believe that we can do even better,” he says.
Another next step will be capturing the drones at higher velocities, which would mean designing the Gremlins to fly faster also. The C-130A model like the one flown for the Gremlins program is an early design that opens its rear door while flying up to 278 kph. The modern C-130s and other cargo planes flown by the U.S. Air Force can open their doors while flying up to 463 kph, so they could retrieve faster drones.
Drawing on history
The name “Gremlins” is a play on the U.S. Army Air Forces’ Goblins concept of the 1940s in which a long-range bomber was to carry a conventionally piloted Goblin jet inside it. The Goblin would be released for protection then retrieved via trapeze. This way, a fighter escort would not have to fly for hundreds of kilometers. Two Goblin XF-85 test aircraft were ordered from McDonnell in 1945, and tests were conducted with a B-29. Frustratingly, “about half of the Goblin flights ended in emergency landings after the test pilot could not hook up to the B-29,” according to the National Museum of the United States Air Force. Goblin and other variations of the larger-planes-carrying-smaller-ones concept were scrapped, mainly for safety reasons: Pilots were killed or were forced to crash land after collisions during test flights. That history of danger influenced the Gremlins program priorities. — Keith Button