The news was recently abuzz with stories of how the Mars 2020 mission, which launched from Cape Canaveral at the end of July, had done something that no other spacecraft had done before: it had successfully charged the batteries aboard a tiny helicopter that is hitching a ride in the belly of the Mars 2020 rover, Perseverance.
Although the helicopter, aptly named Ingenuity, is only a technology demonstrator, and flight operations will occupy but a small fraction of the time Mars 2020 is devoting to its science missions, it has still understandably captured the popular imagination. This will be humanity’s first attempt at controlled, powered flight on another planet, after all, and that alone is enough to spur intense interest in what amounts to a side-project for NASA. So here’s a closer look at Ingenuity, and what it takes to build a helicopter that will explore another world.
Designed for Mars
Despite coverage in the popular press about how NASA is sending a drone to Mars, there’s practically no resemblance between Ingenuity and any conventional terrestrial multicopter. It turns out that when you design a helicopter to operate in a place where the air is as thin as it is at 115,000 feet (35 km) on Earth and with a third its gravitational pull, you end up with a unique and unusual machine.
The most striking difference between Ingenuity and a more traditional multicopter is the choice to make it a dual-rotor design. There would have been no way to go with a quadcopter design, mostly due to the size of the rotors but also because it would have vastly complicated stowage of the helicopter.
According to a paywalled paper by the engineers who designed Ingenuity, NASA was also worried about the phenomenon of blade flapping, which is caused by flexing of the rotor blades above and below the plane of rotation. Without Earth’s thick atmosphere to damp blade flapping, a traditional rotor design would likely tear itself to pieces on Mars. So the blades for Ingenuity were designed to be extremely stiff while at the same time being thin, lightweight, and very broad across the blade root, to provide the extra lift needed in the thin Martian atmosphere. The blades are built from a composite of carbon fiber skins over a molded foam core, and each of the two coaxial rotors is just over 1.2 meters in diameter.
Another way in which Ingenuity differs from terrestrial multicopters is in the flight control systems. Where most quads only have fixed-pitch rotor blades and use differential motor speed to achieve pitch, yaw, and roll control, Ingenuity uses a pair of swashplates to control each rotor’s collective and cyclic pitch. Each titanium swashplate is controlled by a trio of tiny servos anchored to the rotor mast, and is connected to the rotor using connecting rods machined from polyetheretherketone (PEEK) plastic.
Unsurprisingly, the motors for Ingenuity are pretty special too. Like most terrestrial multicopters, Ingenuity’s two motors are of the brushless DC design, but the similarities stop there. The 46-pole stator was hand-wound using copper wire with a rectangular cross-section, to allow for better packing than would be possible with round copper wire. The high coil count and the exotic lightweight materials used for the housing make the motors very efficient, very light, and very compact. They’re also hard to spot in the photograph below; one is barely peeking out from between the top rotor and the solar panel, while the other is nestled between the bottom rotor and the swashplate for the upper rotor.
As configured for flight, Ingenuity is just as gangly and unwieldy as every other helicopter. Finding a way to stow any helicopter in a compact, shippable package is a complex task, and only more so when it needs to survive a rocket launch, a six-month interplanetary journey, and a high-energy autonomous landing on Mars.
Ingenuity’s design makes at least the job of stowing the craft straightforward, thanks to its thin profile when the rotor blades are aligned with each other. The landing legs are another thing, though. The hinges for the four legs need to allow the legs to bend up toward the rotors to make the package as small as possible, while still providing shock absorption for landings. To accomplish both goals, each leg is attached to the body by a deployment hinge, which snaps the leg down to their deployed position under spring pressure. Just after the deployment hinge is a titanium and aluminum flexure, a compliant mechanism that absorbs the shock of up to 2 meters per second landings.
Deploying Ingenuity to the Martian surface will be a complex, highly orchestrated process. The helicopter is stowed in the underbelly of Perseverance, behind the bay holding the Adaptive Caching Assembly. Despite the presence of a robot arm in the ACA bay, Ingenuity will not be plucked from storage and placed gingerly on the surface. Rather, as the video below (source: NASA/JPL) demonstrates, after a debris shield is jettisoned and the rover moves away to a clear zone, the helicopter will swing down from its horizontal storage position to a vertical orientation. The legs will be released to spring into position, and the helicopter will be dropped a few inches to the Martian surface. It’s an operation that will no doubt take days to complete, as each stage of deployment will be preceded and followed by multiple systems checks and surveys of the area with the many cameras sprouting from the rover and the helicopter itself.
Once free from the plutonium-warmed belly of Perseverance, Ingenuity will have to deal with the extreme cold of Martian nights all by itself. The electronics needed to run the helicopter, including the already-charged batteries, the flight controller, and the cameras and other sensors needed for the look-down navigation system and altimeters, are all housed within a thermal management enclosure called the Helicopter Warm Electronics Box, or HWEB. This box dangles between the helicopter landing legs and uses a combination of thin-film heaters and a layer of polyamide insulation to both absorb as much solar thermal energy as possible while reducing heat loss from the inside of the HWEB to the Martian atmosphere.
Having an autonomous helicopter at your disposal might seem like a dream come true for mission planners, allowing them to quickly get eyes on some interesting feature to see if it’s worth slowing driving the rover over for a closer examination. And indeed, that’s something that NASA very much has in mind for future missions. But Ingenuity has far more modest goals, mainly as a proof-of-concept of non-terrestrial flight.
After proving that it can survive the ride to Mars, descent and landing, and being successfully deployed to the surface, Ingenuity is scheduled for only a few short flights. None of the fully autonomous test flights will last no more than 90 seconds, and the helicopter will stay within 50 meters of Perseverance and never get more than 5 meters above the surface.
Assuming Ingenuity is able to get off the surface and land safely again, a total of up to four additional flights will take place. But once the test period is over, Perseverance will move on, hopefully leaving the spidery helicopter upright on the Martian surface, having fulfilled its mission of proving that aircraft can operate on other worlds.
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