When a helicopter’s rotor blades approach the speed of sound, things can get a little dicey.
Above about Mach 0.8, shock waves, drag, unstable flow, and turbulence can create significant stresses that don’t occur at lower speeds.
Here on Earth, that’s a problem that can be solved with extensive testing and clever engineering. Designing a craft that can operate in the alien aerodynamics of Mars is a different matter.
Perseverance’s Ingenuity helicopter operated entirely within the subsonic regime, below about Mach 0.7 – a choice made to avoid potentially mission-ending aerodynamic surprises on humanity’s first powered aircraft on another planet.
The next-gen Mars helicopter, currently under development as part of the SkyFall project, is going to go harder. In a simulated Mars atmosphere, NASA engineers have pushed the rotors to Mach 1.08, a speed that significantly expands the capabilities of the next helicopter.
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“The successful testing of these rotors was a major step toward proving the feasibility of flight in more demanding environments, which is key for next-gen vehicles,” says aerodynamicist Shannah Withrow-Maser of NASA’s Ames Research Center in Silicon Valley.
“We thought we’d be lucky to hit Mach 1.05, and we reached Mach 1.08 on our last runs. We’re still digging into the data, and there may be even more thrust on the table. These next-gen helicopters are going to be amazing.”
Ingenuity in its heyday. (NASA/JPL-Caltech)
Although Earth and Mars have a lot in common, even small differences in atmospheric properties can change the way an aircraft operates. And the differences are not small: Mars’ atmosphere is extremely thin, just 1 to 2 percent as dense as Earth’s.
Ingenuity, which arrived on Mars with the Perseverance rover in January 2021, was a bold experiment to test whether rotor-powered flight was even possible on the red planet, and also collect data on flight performance that could inform future helicopter design.
It was originally planned to take just five flights. It flew 72 times before it crashed in 2024, not because of its flight apparatus but because it was unable to gauge its distance from the ground while descending.
An engineer in the testing chamber with the two-blade design. (NASA/JPL-Caltech)
The relatively conservative mission design was crucial, giving Earth’s engineers a detailed understanding of Mars aerodynamics to incorporate into new aircraft. Ingenuity walked so SkyFall can run.
“If Chuck Yeager were here, he’d tell you things can get squirrely around Mach 1,” says engineer Jaakko Karras of NASA’s Jet Propulsion Laboratory (JPL).
“With that in mind, we planned Ingenuity’s flights to keep the rotor blade tips at Mach 0.7 with no wind so that if we encountered a Martian headwind while in flight, the rotor tips wouldn’t go supersonic.
“But we want more performance from our next-gen Mars aircraft. We needed to know that our rotors could go faster safely.”
Because the atmosphere of Mars is so much thinner than Earth’s, the sound barrier, known as Mach 1, sits at a much lower speed – around 869 kilometers per hour (540 miles per hour), compared to 1,225 kilometers per hour (761 miles per hour) at sea level on Earth.
JPL engineer Jaako Karras with the two-blade rotor. (NASA/JPL-Caltech)
To test their rotor designs, JPL has a special chamber in which air pressure is lowered to simulate Mars’s atmospheric conditions. It’s also reinforced so that if a blade snaps, it won’t yeet itself across the lab and lodge in something important.
In this chamber, the team tested two rotor designs – one with three blades and one with two – while they watched from a nearby control room.
The rotors on the three-blade design spun at up to 3,750 rpm, meaning the tips reached speeds of Mach 0.98.
The two-bladed rotor, with longer blades, only needed to reach a rate of about 3,570 rpm to achieve the same Mach 0.98 speed. For context, Ingenuity’s rotor blades never exceeded 2,700 rpm.
A fan inside the chamber was then switched on, pelting the rotors with headwinds. Ultimately, the rotor tips reached a top speed of Mach 1.08.
Related: NASA Reveals New Mars Helicopter Design, Inspired by Ingenuity’s Success
This dramatically increases lift by about 30 percent, which will allow the next helicopter to carry heavier payloads than Ingenuity. This means more science instruments can be packed onboard.
If all goes according to plan, the SkyFall mission will launch towards the end of 2028, carrying three helicopters on board, NASA says.
These helicopters will help scout for human landing sites and map water ice under the surface of Mars.