[wongarsu]

One issue might be rotor span. Ingenuity has pretty big rotors to counter the thin atmosphere (about 4 feet top-to-tip).

On earth rotor sizes are limited by the speed at the wing-tip. Once you make the rotor too long the tips start approaching supersonic speeds, giving you all kinds of weird mach effects. To make matters worse, the speed of sound is about 30% lower on Mars compared to near earth's surface.

[jessriedel]

Yea good point. Apparently the blade tip speed on Ingenuity is Mach 0.6-0.7!

[skovati]

Interesting note about this: the speed of sound on Mars is only ~70% of that on Earth, due to less atmospheric density. Might change your Mach numbers!

[Metacelsus]

It's not due to less density, but rather a different gas composition (CO2 vs. N2+O2).

[algas]

Let's run the numbers!

The speed of sound in an ideal (calorically perfect) gas is given by

  a = sqrt( gamma * R * T )

where gamma is the ratio of specific heats (thermodynamic property of a gas, which may vary with temperature), R is the individual gas constant, and T the temperature of the fluid. All of these are going to be different on Mars versus on Earth:

  Earth:
  R = R_atm = 287 J / (kg * K)
  gamma = 1.44
  T = 293 K (taking room temperature as an average temperature)

  Mars:
  R = R_CO2 = 188 J / (kg * K)
  gamma = 1.37
  T = 210 K (from a quick google, about -60 deg C)

If the Martian and Earth atmospheres were at the same temperature, then the speed of sound on Mars would be 80% that of the speed of sound on Earth. Given the temperature difference, the speeds of sound are

  a_mars = 232 m/s
  a_earth = 347 m/s

So yes, much of the difference is due to the composition: the Martian atmosphere has a higher atomic weight, which leads to a lower individual gas constant, and decreases the speed of sound. However, a substantial amount of the difference is simply due to the different temperatures on the surfaces of the two planets.

[jessriedel]

I included it

[mmbop]

Oh interesting! I can see how that would be a harder problem (although not insurmountable since some planes on earth have supersonic propellers).

Just a quick edit - wow, u didn’t realize the span was already 4ft! Anything much larger could definitely be hard to pack inside a fairing!

[AlexAndScripts]

What about multiple smaller rotors? Or would that cause weird turbulence effects? Could we use some kind of jet engine?