INGENUITY’S SUCCESS BLAZES TRAIL FOR FUTURE HELICOPTER USE IN SPACE

With blades spinning five to 10 times faster than a chopper on Earth and performance nearly three times better than designed, NASA’s Mars Ingenuity helicopter has spearheaded greater use of rotorcraft to explore the Red Planet and other neighbors in our solar system.

Now the Ingenuity team must confront an issue familiar to those who operate aircraft down home: how to keep component wear from curtailing their aging bird’s service life. The helicopter was designed and built to fly five times on Mars as a technology demonstration. At press time, it had completed 13 flights and a 14th was being planned. By its third flight, on April 25, Ingenuity had achieved the tech demo’s third and final objective when it climbed to 16 feet (5 meters) altitude, flew downrange about 164 feet (50 meters) and back at a top speed of 6.6 feet per second (2 meters per second).

“From millions of miles away, Ingenuity checked all the technical boxes we had at NASA about the possibility of powered, controlled flight at the Red Planet,” said the director of NASA’s Planetary Science Division, Lori Glaze. “Future Mars exploration missions can now confidently consider the added capability an aerial exploration may bring to a science mission.”

Design work already has begun on the feasibility of a larger Mars helicopter, and Ingenuity’s success may well have NASA assessing the value of rotorcraft exploration for missions to Venus planned for 2028-2030.

Engineers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. and AeroVironment, the contractor that built the flight vehicle for JPL, gained some time to work on extending Ingenuity’s life further. In early October, the helicopter and its “mother ship,” the Perseverance rover, went into an operational standdown as Mars’ and Earth’s orbits put the Sun between them. Known as a conjunction, this makes communications with spacecraft on Mars unreliable when that planet is within 2 degrees of the Sun.

Ingenuity was programmed to send basic system health information through the conjunction (Oct. 2 to Oct. 14) to Perseverance. In addition to its primary science and exploration duties, the rover is the helicopter’s base station, relaying communications to and from Earth. The rover was to transmit that health data to JPL after the conjunction.

In mid-September, the helicopter team was preparing for Ingenuity’s 14th flight. Ingenuity’s two, 4-foot-long coaxial rotors would run at 2,700 rpm to compensate for decreasing atmospheric density as the operating area in the Jezero Crater, just north of Mars’ equator, moves into summer. Mars seasons change like those on Earth, but they vary in duration. A Mars year lasts 687 Earth days.

Thirty Sols

The Ingenuity team had planned for five flights over 30 Sols at atmospheric density’s 1.2 to 1.5 percent that of Earth’s density at sea level. (A Sol is one Martian day; it’s 2.7% longer than an Earth day.) With Ingenuity in its sixth month of operation, densities were dropping to 1 percent of Earth’s density. (At 95 percent carbon dioxide, the Martian atmosphere is also lighter than Earth’s.) Even at the higher densities, Ingenuity was flying at the equivalent of about 100,000 feet on Earth.

Earlier flights had run at 2,537 rpm. By comparison, the light, two-person Guimbal Cabri G2 helicopter’s main rotor turns at 540 rpm and the Sikorsky Aircraft 19-passenger S-92 main rotor spins at 258.

The flight preparations included a high-speed rotor spin test at 2,800 rpm on the ground. Ingenuity passed that test on Sept. 15 at 23:29 PDT (or 11:11 Local Mean Solar, or Mars, Time) in the Jezero Crater. The rotors were spun up to 2,800 rpm, briefly held there and then spun down to a stop, per the test plan. The helicopter’s other systems performed flawlessly, according to NASA.

A key test objective was to see if the higher rpm caused resonant vibrations in Ingenuity’s structure, a common challenge in rotorcraft that can cause problems with sensing and control and lead to mechanical damage. No resonances were detected at the higher rpm, NASA said. That cleared Ingenuity to proceed with the 2,700-rpm test flight to a brief hover at about 16 feet (5 meters) altitude.

But during final automatic checkout on Sept. 18, Ingenuity detected an anomaly in two of its small flight-control servo motors. Ingenuity canceled the flight per its programming, JPL’s Ingenuity Mars Helicopter deputy operations lead, Jaakko Karras, said Sept. 28.

Swashplate, Cyclic, Collective

Like most helicopters, Ingenuity is controlled in flight by manipulating a swashplate connected by cyclic and collective links to each pair of rotor blades (upper and lower) as they spin about the rotor mast. Six maxon precision motors modified DCX 6M brushed DC servos move the swashplates, three for the upper swashplate and three for the lower one. Ingenuity’s six servos, at 0.4-inch (10-millimeter) diameter, are much smaller than the motors that power the rotors, but they are critical to stable, controlled flight. So Ingenuity performs an automated check before every flight, Karras said, driving each servo through its range of motion and verifying that it reaches each commanded position. This is like the pre-flight controls check every pilot is expected to do before takeoff. The Ingenuity team refer to this check as the “servo wiggle.”

Data from the failed check showed that two of the upper swashplate servos — Numbers 1 and 2 — began to oscillate with an amplitude of about 1 degree about their commanded positions just after the second step of the check, Karras said. This triggered the cancellation.

The team on Sept. 21 and Sept. 23 had Ingenuity do additional servo wiggle checks, just as mechanics on earth would do. Mechanics here could sympathize with the results. The servos passed; the checks failed to repeat the discrepancy.

One theory for the oscillations is that the servo gearboxes and swashplate linkages are showing wear now that Ingenuity had flown eight more flights than originally planned, Karras said. “Wear in these moving parts would cause increased clearances and increased looseness and could explain servo oscillation.”

Another is that the high-speed spin test left the upper rotor’s servos 1 and 2 loaded in an oscillation-inducing way not encountered before. The team is working through the anomaly, Karras said. “We’re optimistic that we’ll get past it and back to flying again” after the conjunction.

The conjunction carries its own set of risks, Karras has said. Dust storms could cover the SolAero light, efficient inverted metamorphic multi-junction solar panels mounted atop Ingenuity, leaving them unable to charge the bird’s six Sony VTC4 lithium-ion batteries. Each is about the size of an AA battery. Also, coarse dust could penetrate electronics or machinery.

Perseverance and Ingenuity were launched as NASA’s Mars 2020 mission July 30, 2020 on an 860,000-pound-thrust United Launch Alliance Atlas 5 401 (with a 23,000-pound-thrust Centaur upper stage) from Cape Canaveral Air Force Station, Florida. After a curving cruise flight of about 300 million miles and 203 days, Perseverance was winched down to the Martian surface from a “Sky Crane” descent stage. The folded Ingenuity was tucked up under the rover’s belly, in the Lockheed Martin-built Mars Helicopter Delivery System, which also protected the helicopter from debris during landing. On April 3, that system lowered the unfurled bird to the ground.

Ingenuity stands 19.3 inches (49 centimeters) inches tall and weighs about 1.5 pounds (0.68 kilograms) on Mars, which has about 38% of Earth’s gravity. Here, it weighed about 4.0 pounds (1.8 kilograms) on Earth. That mass is split about 55/45 percent between structure and systems designed and made by AeroVironment and subcontractors and guidance, navigation, control and power systems made by JPL with a mix of custom and commercial-off-the-shelf units.

Perseverance is the size of a car — 10 feet (3 meters) long, 9 feet (2.7 meters) wide and 7 feet (2.13 meters) tall, with a mass of 2,260 pounds (1,025 kilograms), or about 859 pounds (389.5 kilograms) on Mars. Built by JPL, it is considered NASA’s most advanced planetary rover.

The duo’s mission is to search for signs of past microbial life, collects and return rock samples and demonstrate technologies addressing challenges of human expeditions to Mars.

To Go Where No Man Has Gone Before

Jezero Crater — at 28 miles (45-kilometer) in diameter — was the site of a lake more than 3.5 billion years ago, according to scientists. They say its inner rim contains deposits of carbonates, minerals that on Earth helped form fossils billions of years old. These include seashells, coral and some stromatolites — Earth rocks formed by microbial life along ancient shorelines with plenty of sunlight and water. NASA says the carbonates along Jezero’s rim makes it a prime scientific hunting ground.

Ingenuity is enabling a more thorough exploration there, scouting safe paths for Perseverance and flying over and photographing sites beyond the rover’s safe reach. For example, Flight Nine on July 5 scouted an area called Séítah. Team scientists consider it geologically interesting. But its rock- and boulder-strewn terrain were difficult for Perseverance to traverse initially.

“Flight Nine was explicitly designed to have science value by providing the first close view of major science targets that the rover will not reach for quite some time,” the Perseverance deputy project scientist, Ken Williford, said. It and subsequent flights over Séítah enabled the Perseverance team to chart a path into the region and focus in on potential targets of geologic and astrobiological interest. In late September, the rover was driving deeper into Séítah.

Beyond aiding the rover team, Flight Nine broke records for duration and cruise speed and nearly quadrupled the distance flown between two sites. Ingenuity flew for two minutes 46 seconds and covered about 2,051 feet (625 meters) at about 11 mph (5 meters/second) at altitudes up to about 33 feet (10 meters).

Achieving the performance Ingenuity has demonstrated on Mars required the AeroVironment design and production team to overcome many challenges.

Planetary protection was the top one. NASA goes to Mars looking for signs of life. It cannot afford to have spacecraft contaminating Mars with Earth organisms. During assembly and before launch, spacecraft surfaces are frequently wiped down with alcohol. They undergo biological cleanliness tests. Electronics compartments are sealed and vented through high-efficiency filters. Components that can take it are heated to 230 degrees Fahrenheit (110 degrees Celsius) or hotter to kill microbes.

The bake-outs also reduce volatile outgassing when a vehicle flies into space’s vacuum. Ben Pipenberg, AeroVironment’s engineering lead on the Ingenuity program, explained why. “Any outgassing of the materials ends up accumulating on the coldest items on the spacecraft. That’s typically things like camera lenses.”

Light but Strong

Ingenuity also had to be light yet strong enough to withstand launch and orbital insertion g forces and vibrations. Every additional pound put in orbit requires an extra pound of thrust from the launchpad. But Ingenuity’s constraint wasn’t a launch one. It was how much the bird could lift on Mars.

“We talk about how hard it is to fly a helicopter with that really thin atmosphere,” Pipenberg said. But “Ingenuity’s primary constraint flying on Mars is getting the weight low enough, not necessarily the aerodynamic power required.”

“It’s a very carefully balanced problem between being light enough to actually fly on Mars and being strong enough withstand launch loads,” Sara Langberg, an AeroVironment aeromechanical engineer on the project, said.

Mars’ atmosphere posed a big problem, however.

A Mars helicopter is not a new idea. A 1993 scientific paper proposed one. JPL and AeroVironment together toyed with concepts in the late 1990s. In 2000, spurred by Sikorsky, the American Helicopter Society’s annual competition for aerospace engineering students called for such a design. In 2013, JPL tagged AeroVironment for a tech demo project that would become Ingenuity. But when engineers in December 2014 loaded a small-scale demonstrator for flight tests in JPL’s 25-foot-diameter Space Simulator, which allowed them to replicate Mars’ carbon dioxide-laden atmosphere, it proved uncontrollable.

The demonstrator had been built like an Earth helicopter. But the “heavy” atmosphere here dampens a fast-spinning rotor’s inertial forces. Rotor control systems here benefit. With no atmospheric dampening on Mars, inertial loads would dominate unless the team came up with solutions, said Jeremy Tyler, an AeroVironment senior aeromechanical engineer on the Ingenuity project.

One solution was to use a unique swashplate design. Most swashplates are symmetrical, with the cyclic and collective links pivoting about a single point in a single plane. That makes some links longer, but “there’s no lateral motion,” Tyler said. He decided Ingenuity would use offset links, which shortened them. This gave the helicopter faster cyclic response (and made the swashplates smaller and lighter). Engineers weren’t concerned about collective response, which moves the helicopter up and down. But Ingenuity would fly on Mars with winds gusting to more than 13 mph (6 meters/second). Faster cyclic response would allow it to maintain pitch and roll in those winds, Tyler said.

Another solution was to add inertial counterweights. Without atmospheric dampening, Ingenuity would experience the “tennis racquet” effect that flattens rotor blades at high speed. Ingenuity’s performance is derived from specifically twisted blades with a camber, or longitudinal curve, unusual for helicopters. NASA couldn’t afford to lose those. Langberg designed the counterweights – hollow horns, each containing a titanium ball, near the blade hubs – to offset that effect. This also allowed the team to use smaller servos, which now would counteract lower loads.

Environmental Challenges

Other challenges the team overcame included building a helicopter that can survive Mars temperatures of minus 148 degrees F (minus 100 C) to 68 degrees F (20 C) and developing a flight control system that would allow the bird to operate autonomously. Controlling it from Earth was never an option. Signals from JPL take from 3 to 22 minutes to reach Mars.

They clearly succeeded. Ingenuity’s 230-Sol-and-counting track record is largely unblemished. That Sept. 18 checkout failure was one blemish. Also, when it first prepared to fly in April, Ingenuity had trouble switching into flight mode, which prevented its rotors from spinning at full speed.

Another occurred on May 22, on the first, 492-foot (150-meter) leg of the sixth flight — the first flight of the helo’s new operational demonstration phase.

With Ingenuity at 33 feet (10 meters) altitude, it adjusted speed and began rocking back forth. The rocking persisted throughout the flight’s remaining 213 feet (65 meters). Sensors indicated roll and pitch excursions of 20 degrees plus, large control inputs and power consumption spikes. Ingenuity landed safely.

To navigate on Mars, Ingenuity pairs a Bosch inertial measurement unit (IMU) with its navigation camera, which points down and shoots at 30 images a second most of the time it is airborne. The IMU measures accelerations and rotational rates, integrating that information over time to estimate position, velocity and attitude. An onboard control system reacts to the estimated motions, adjusting control inputs 500 times a second. The navigation system, powered by Qualcomm’s Robotics Flight 801 platform, processes each camera image, determining when it was taken. A navigation algorithm then predicts what the camera should be seeing at that time based on recognizable surface features from preceding images. The algorithm looks at where those features are in the image and uses the difference between their predicted and actual locations to correct its position, velocity and attitude estimates.

About 54 seconds into the flight, a glitch occurred in the image stream from the navigation camera, JPL’s Ingenuity Mars Helicopter chief pilot, Håvard Grip, said May 27. A single image was lost. This left later images delivered with inaccurate timestamps. Subsequent algorithm corrections were based on wrong timestamps, causing the system to continually “correct” for phantom errors. Large oscillations ensued.

Ingenuity was able to maintain flight and land safely within about 16 feet (5 meters) of the intended location because considerable effort went into “ensuring that the helicopter’s flight control system has ample stability margin,” Grip said. “This built-in margin was not fully needed in Ingenuity’s previous flights, because the vehicle’s behavior was in-family with our expectations, but this margin came to the rescue in Flight Six.”

Grip added, “Flight Six ended with Ingenuity safely on the ground because a number of subsystems — the rotor system, the actuators, and the power system — responded to increased demands to keep the helicopter flying.” The flight uncovered a vulnerability that required fixing, he said, but it also confirmed the system’s robustness in multiple ways.

“NASA now has flight data probing the outer reaches of the helicopter’s performance envelope,” Grip said. “That data will be carefully analyzed in the time ahead, expanding our reservoir of knowledge about flying helicopters on Mars.”