Ingenuity- Mars 2020 Helicopter, Deployment & Operations Options

[PaulH51]

GIF - Helicopter 'Launch Lock' unlatched

Rotated / Cropped SHERLOC frames from Sols 35 & 36

[rlorenz]

Great info there. One thing that caught my eye was the future use of a helicopter as "Fetcher":

Unlikely, IMHO. Helicopters do not scale up well.

Beyond the obvious thrust challenge in the thin Martian atmosphere (i.e. classic momentum theory, and the Mach/Reynolds aerodynamic issues common to all aeronautics), there are some other rotorcraft-specific issues that actually are rather challenging for Ingenuity that one only confronts when one gets into the real details of design and test.

First is heat transfer. The thin atmosphere gives almost no cooling. The Ingenuity motors have parts made of beryllium to act as a heatsink, but even then I think overheating is actually the limiting factor on fliight duration, not battery energy.

Second is aeroelasticity. There's a similarity parameter called the Lock Number (that I hadnt heard of until I started working with rotor people on Dragonfly) that is important in assessing the structural damping of blade flexing. Again, the thin atmosphere is the problem, it provides no damping so blade oscillations can build up.

Both of these issues get worse as you scale up. So at the NIAC / Powerpoint / student-final-year-project level, yes you can mock out neat-looking hexacopters and stuff in the 10-20 kg range and they look like they should fly, but once you really start poking into the thermal and mechanical design, I bet even those would not work out.

[Phil Stooke]

https://mars.nasa.gov/mars2020-raw-images/p...00LUJ01_800.jpg
Ingenuity drops down a bit, getting ready...
Phil

[fredk]

The thin atmosphere gives almost no cooling. The Ingenuity motors have parts made of beryllium to act as a heatsink, but even then I think overheating is actually the limiting factor on fliight duration, not battery energy.

That's interesting, and surprizing. Since the rotors must produce comparable downward thrust on Mars as on Earth (to within an order of magnitude, anyway, considering the lower gravity), via a much greater rotor velocity, I might've guessed that the cooling effect of that air would be comparable too. I guess that means that thrust doesn't scale the same as conductive cooling with air density.

[Mogster]

I remember cooling being a serious issue for the Apollo LRV. Everything was always ridiculously cold and at risk but then as soon as it started to operate it’d get so hot it’d be close to melting...

Interesting issues to overcome when more performance is needed from possibly human carrying vehicles on the Moon or Mars.

[mcaplinger]

I guess that means that thrust doesn't scale the same as conductive cooling with air density.

Can't speak to exactly how they're modeling it, but my own experience with R/C helicopters of similar scale suggests that rotor downwash is a pretty ineffective way of cooling the motors on Earth, and nothing about the Mars helicopter suggests anything different. Take a look at figure 6 in https://trs.jpl.nasa.gov/bitstream/handle/2...L%2318-4405.pdf which shows the propulsion motors getting up to something like 85C worst-case. Of course, the range is extremely wide, suggesting that the thermal analysis is very conservative, as they usually are.

[PaulH51]

Another step closer (sol 38)

[Sean]

Almost there...

[Marvin]

It's getting exciting!

Here's what Flights 1 - 3 might look like:



https://www.hou.usra.edu/meetings/lpsc2020/pdf/2096.pdf

[neo56]

Animation of Ingenuity deployment from sols 35 to 38.

[PaulH51]

Looking good with all 4 legs deployed on sol 39 (processed & cropped)

[rlorenz]

That's interesting, and surprizing. Since the rotors must produce comparable downward thrust on Mars as on Earth (to within an order of magnitude, anyway, considering the lower gravity), via a much greater rotor velocity, I might've guessed that the cooling effect of that air would be comparable too. I guess that means that thrust doesn't scale the same as conductive cooling with air density.

Thrust goes as rho * V squared. So for a given thrust and disk area, Martian density 50x less means the downwash has to be 7x faster

Power goes as rho * V cubed. So to generate that thrust we have to put 7x more power through the motor

But the heat transport goes as something like rho * V, so we have 7x less heat removal for a given temperature difference.


Now of course the disk loading of Ingenuity is a factor of a few less than typical small terrestrial drones, to make these effects a bit more manageable (and keep the rotor tips subsonic, etc.) but you can see the scaling really points to overheating being an issue for sustained operation at low density

[djellison]

So what you're saying is...Dragonfly has it easy

[Bill Harris]

Thrust goes as rho * V squared. So for a given thrust and disk area, Martian density 50x less means the downwash has to be 7x faster

Power goes as rho * V cubed. So to generate that thrust we have to put 7x more power through the motor

But the heat transport goes as something like rho * V, so we have 7x less heat removal for a given temperature difference.


Now of course the disk loading of Ingenuity is a factor of a few less than typical small terrestrial drones, to make these effects a bit more manageable (and keep the rotor tips subsonic, etc.) but you can see the scaling really points to overheating being an issue for sustained operation at low density

One could assume that "technology will find a way", but this may have hit a wall in this case. R/C helicopters use brushless motors as standard, which have the windings on the stationary periphery of the motor and the rotating core of the motor containing a magnet array. The windings could be cooled via a circulating fluid or by coupling to a large heatsink, but the problem will be cooling that core magnet array. Typically that is force-air cooled, but that wi'll be impossibly difficult with the thin Mars atmosphere.

[htspace]

Wow!