For several months, NASA has been flying the Ingenuity Mars Helicopter on Mars. Mission operators have learned a lot about flying a helicopter on another planet and have learned more about the Ingenuity platform’s strengths and weaknesses. Significant effort has gone into maximizing the strengths of the helicopter’s design while minimizing its flaws.
As NASA has learned more about Ingenuity’s strengths and weaknesses, piloting the helicopter has become easier, but one aspect is becoming more difficult. The changing seasons on Mars, according to NASA, are resulting in a decrease in atmospheric density.
Because the atmosphere on Mars is always low, one of the reasons NASA wasn’t sure if they’d be able to fly Ingenuity at all was because of the low density of the atmosphere.
The atmospheric density on Mars is decreasing even more as the seasons change. NASA planned flights with atmospheric densities of 1.2 to 1.5 percent of the density of the atmosphere on Earth at sea level. This translates to a density of 0.0145 to 0.0185 kg/m3.
With the changing seasons, NASA expects to see densities as low as 0.012 kg/m, or about 1.0 percent of the density of the earth’s atmosphere.
While those percentages are very close, the lower densities have a significant impact on Ingenuity’s ability to take to the Martian sky.
The lower design limit for atmospheric density was 0.0145 kg/m3, but NASA claims the helicopter has thrust that is at least 30% greater than what is required to fly at that density. With the changing atmospheric density in the coming months, a reduction to 0.012 kg/m3 would mean an extra thrust margin of as little as 8%. That would imply that the helicopter would operate very close to stall conditions.
The solution is for NASA to increase the rotor rotational speed to 2800 RPM, which is more than ten percent faster than it is now. Faster rotor rotation could cause vibrations, which could cause other issues as well as increased wear on the helicopter. In the coming days, NASA plans to test new approaches to keep the helicopter flying as long as possible.
A significant increase in speed has a number of drawbacks. One of them is related to aerodynamics: In combination with wind and helicopter motion, a rotor speed of 2,800 rpm could cause the tips of the rotor blades to collide with the air at nearly 0.8 Mach – or 80 percent of the speed of sound on Mars.
The speed of sound on Mars is about 34% of that on Earth, which is different from what we’re used to. If the blade tips approach the speed of sound, they will experience a significant increase in aerodynamic drag, which will make flight impossible.
This phenomenon is not expected to occur in Ingenuity’s rotor until even higher Mach numbers, but this has never been confirmed in Earth-based testing.
Unknown resonances in the helicopter structure are another potential issue. Ingenuity, like all mechanical systems, has resonances that, when excited at specific frequencies, can cause large vibrations. It’s critical to make sure there are no significant resonances at the rotor speed used for flight, as this could damage hardware and cause the flight control system’s sensor readings to deteriorate.
Several components of Ingenuity’s design will face increased demands: the motors will have to spin faster, the electrical system will have to deliver more power, and the entire rotor system will have to withstand the increased loads that come with increased rotor speeds. All of this adds up to a significant challenge, but by approaching the problem slowly and methodically, NASA hopes to be able to fully test the system at higher rotor speeds and keep Ingenuity flying in the months to come.