Almost no empty seats remained in the large auditorium for opening day of the 2024 Transformative Vertical Flight conference devoted to helicopter research. Håvard Grip, a chief engineer at NASA, stood before the waiting crowd of aeromechanical engineers to deliver his presentation on the many triumphs—and the downfall—of Ingenuity, the record-setting helicopter that flew scores of missions on Mars before ultimately crashing in mid-January 2024.
“We learned a ton,” said Grip of the process the agency used to design Ingenuity, which had opened an entirely new frontier in helicopter research with its flights through Mars’s thin, otherworldly air. Everything from the design to the testing protocol had been developed from scratch. He described the groundbreaking helicopter’s testing lab as a “poor man’s” wind tunnel: to measure wind movements on their prototype, the researchers used a giant arm that latched onto the copter and swung it around inside a 25-foot room that replicated Mars’s atmospheric conditions.
From a mechanical engineering perspective, Ingenuity was a decisive victory, completing 71 successful flights. But the helicopter also had an Achilles’ heel: the autonomous navigation software that was so crucial to the mission’s success also struggled to orient the craft in bleak, featureless terrain because it relied too much on small-scale features such as rocks as waypoints. On what would become the helicopter’s final flight, when NASA directed Ingenuity to land on a bland, sandy flat, the craft lost its bearings, tilted over sharply, and drove a rotor into the sand, snapping off the blade’s tip.
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“In retrospect, we can see how that terrain is different from other kinds of terrains that we’ve flown in,” said Grip, who had previously acted as Ingenuity’s chief pilot in the helicopter’s early days. “And it turns out that it was just a little bit too challenging for Ingenuity to handle, and so that is a lesson, right?”
Ingenuity has flown its last mission, but within a decade, at least one of its technological children may embark on a voyage to distant planets. Several speakers at the conference presented new or updated designs for helicopters that benefited from the knowledge gained fromtheir pioneering parent’s off-world flights.
At the time, the conference was galvanized by the overall sentiment that NASA saw helicopters as an integral part of planetary exploration. Now, however, after a recent mix of good and bad news, the future is murkier.
An Airborne Renaissance
There are compelling reasons to include helicopters in interplanetary missions. Rovers tend to be slow and simply can’t navigate some of the more formidable terrains. On the other hand, NASA has designed helicopters capable of reaching peak speeds of almost 70 miles per hour on Mars, ascending to the peaks of its mountains and then descending into the depths of its craters left over from ancient oceans. The maneuvering prowess of one design should even give it the ability to explore the insides of caves.
Ingenuity’s achievements have sparked something of a renaissance in helicopter science, and NASA has developed lists of specifications for several more planetary helicopters, most of which will never be built. But each new configuration helps NASA to learn and iterate, with the hope of identifying the best possible designs for a range of challenging conditions.
The most developed plans include Dragonfly, which recently received the green light to head for Saturn’s largest moon, and the Sample Recovery Helicopter (SRH), which faces an unclear future as a potential part of the space agency’s troubled Mars Sample Return (MSR) program. Other possibilities include a few concept helicopters—for instance, the Mars Science Helicopter, which wouldn’t need a rover or lander as a “mothership,” and the Planetary Telemetric Helicopter for Investigation and Analysis (PYTHIA), designed to travel down the Red Planet’s giant lava tubes.
Ingenuity’s Lessons
During his talk, Grip joked that Ingenuity’s biggest achievement was simply showing that flying on an alien world could be done. “It’s been an incredible ride,” he said. “I’ve worried a few times that we made it seem too easy. Flying on Mars is actually hard.”
Many unearthly challenges face helicopters on Mars. The key physics concepts that affect copters’ aerodynamics are air density, the speed of sound and something called the Reynolds number, which tells you how turbulent the airflow is around the vehicle.
For Mars, NASA’s engineers initially only focused on compensating for the unique air density, which is often quoted as a mere 1 percent of Earth’s. Such “weak” air makes it significantly harder for a helicopter to generate lift. William Warmbrodt, chief of aeromechanics at NASA’s Ames Research Center, says the situation is also a bit more nuanced. The geologically lower areas of Mars have higher air densities, making helicopter flight possible. But higher elevations have an air density of less than 1 percent, requiring more sophisticated flight designs.
Initially Ingenuity’s design was based solely on overcoming the challenge of low-elevation air densities. But when the team tested its first prototype in free flight, it was unstable, wobbling and jittering during tests. Grip says the team didn’t dare try again until it had mastered how the speed of sound and Reynolds numbers affected flight. This culminated in the researchers’ second prototype, which “was indeed successful.”
To help plan future missions, scientists used flight-log data to learn about handling the atmospheric conditions on Mars. By looking at the minute adjustments in thrust that Ingenuity autonomously used to stabilize itself during flight, they could infer detailed information on the wind velocity for different altitudes—something impossible to measure via earthly telescopes or instrumentation in Mars’s orbit or on its surface.
“The thrust vector has to point into the wind because it has to overcome fuselage drag due to the winds,” Warmbrodt explains.
On a more mundane but equally important note, Warmbrodt says the Ingenuity team was elated to confirm that consumer electronics could handle interplanetary exploration. Ingenuity’s brains came from the equivalent of a simple mobile phone processor, which was able to withstand the extremes of a rocket launch and a Mars landing, as well the low pressure and variable temperatures of the alien world’s surface—a fact that came up a few times during talks at the conference.Previously, NASA had only used custom hardware which required costly testing to prove spaceworthiness. The ability to use commercially available components, Warmbrodt says, would provide a huge cost benefit in future designs.
Course Correction
Post-Ingenuity, a pair of SRHs were supposed to be the first flying machines to return to Mars. Appearance-wise, an SRH closely resembles Ingenuity, with a similar stacked rotor blade configuration on top of its frame. But an SRH has an additional trick beyond what Ingenuity was able to handle because it’s meant to pick up specimens for a return to Earth.
Since 2021, as part of NASA’s MSR plan, the space agency’s Perseverance rover has been collecting and stashing small sample tubes around Mars’s Jezero Crater in an area dubbed the Three Forks depot. Athena Chan, a NASA mechanical engineer working on the SRH project, says that to grab those tubes, the design for SRH includes an arm and gripper, as well as wheels, enabling it to drive short distances if needed.
Chan was in the middle of designing SRH test equipment for a new wind tunnel when NASA held a press conference on Monday, April 15, to announce that it had “descoped” the program’s helicopters. A report from an independent review board had found that the overall MSR effort could cost upwards of $11 billion, far more than the originally proposed $6 billion. The review board also expressed consternation for the mission’s timeline, which estimates pegged at bringing Mars rocks back to Earth circa 2040. At a press conference, NASA administrator Bill Nelson told reporters that this will be the same decade in which the agency is “gonna be landing astronauts on Mars,” ostensibly making the sample returns a moot point.
At the moment, NASA’s Science Mission Directorate has updated the MSR design specifications in a way that “does not allow accommodation of helicopters.” It also included a back door, however, that would allow NASA to potentially squeeze in a helicopter “for added risk reduction,” albeit with a tighter budget. The answer to the question of when helicopters will once again fly through Mars’s not-so-friendly skies is up in the air.
Next Stop: Titan
The only helicopter with a guaranteed launch—just confirmed on April 16—is Dragonfly, which will sample a world even farther afield: the surface of Saturn’s moon Titan. The samples will be analyzed on the spot instead of returned to Earth.
Unlike Mars, Titan has an atmosphere akin to a thick soup—60 percent denser than Earth’s. Such a dense atmosphere favors helicopters because the propellers will be able to push against the thick air to generate an enormous amount of thrust.
“The density of the atmosphere means a human might be able to fly by ‘swimming’” in Titan’s air, Warmbrodt says.
Such a favorable aerodynamic environment allowed engineers to design a much heavier craft, one that more closely resembles something from science fiction. While Ingenuity and Dragonfly share a stacked coaxial rotor design, that’s where the comparison ends. Roughly the size of a car and weighing more than 900 pounds, Dragonfly is large enough to make Ingenuity look like a pip-squeak. And it has four sets of stacked rotors ringing its perimeter instead of just one sprouting from its center, Ingenuity-style.
Engineers have also equipped Dragonfly with a nuclear power source, foregoing the solar cells used by Ingenuity and its ilk. Plutonium 238 will power Dragonfly’s radioisotope thermoelectric generator in order to charge up the craft’s batteries for flights. NASA engineer Jason Cornelius says that the nuclear power source was needed because the sun’s light is too faint to charge solar panels on Titan. That faintness arises from two things—the moon’s great distance from the sun as well as its thick, light-attenuating atmosphere.
“We also use the heat from the nuclear power plant to keep the vehicle warm,” he adds—a useful benefit because the surface of Titan is around –290 degrees Fahrenheit (–180 degrees Celsius).
Returning to the Red Planet
In late 2023 Grip left the Ingenuity program to become chief engineer for the Mars Science Helicopter (MSH). He said the design for the MSH hasn’t been completely settled yet, but it is likely to be the first Mars-bound mission to abandon the stacked rotor blade configuration and instead use a hexacopter design that will space six rotor blades around the craft’s body. Standing a little more than four feet tall and weighing just five pounds, it will also be larger and heavier than Ingenuity and able to carry a gross weight of around 68 pounds, including about 18 pounds of payload.
“The best way to think about it is that it’s one concept that has received a fair bit of attention and study over several years,” Grip says, “but it’s not an approved mission.”
Another promising concept is PYTHIA, designed to navigate Mars’s caves. In the past, active volcanoes on Mars pushed hot lava to the surface via tunnels. After the volcanoes cooled, the tunnels hardened into large lava tubes.
At the conference, a NASA team presented PYTHIA’s proposed design of a smaller drone body with four rotors and eight blades. A prospective landing site has already been selected: Arsia Mons, Mars’s third-highest point of elevation, at around 38,000 feet. Thanks to Ingenuity’s flight data, engineers know the extremely low air pressure that PYTHIA will face and have modeled how to compensate so it can navigate such heights.
As helicopters line up to travel to distant, inhospitable worlds, they are opening up new possibilities for the exploration of not just extraterrestrial science but also the science of flight. Such insights could one day help helicopters here on Earth. “It’s not a direct one-to-one comparison, yet flying on Mars can help with designing new rotorcraft flying at very high altitudes on Earth,” Chan says.