MARS COPTER CONFIRMED FOR 2020!

[Wjolcz]

16 hours ago, Corona688 said:

Mars has a thinner atmosphere, which requires faster motors, which require bigger batteries.  That adds mass, requiring even FASTER motors and BIGGER batteries.  Rinse, repeat until you break even.  Does this cycle of diminishing returns sound familiar to a KSP player?

That's interesting. I was not aware of that actually. I always thought simply making the blades longer/with more surface area would fix the problem of thin atmosphere.

[VaPaL]

A question that rises (at least for me) is: how it will handle storms? I mean, of course it won't fly during one, but being this light and with this amount of surface it probably will be dragged by the wind, won't it? There's some grabbing mechanism? What if it hit the rover while being thrown away?

[sevenperforce]

1 hour ago, VaPaL said:

A question that rises (at least for me) is: how it will handle storms? I mean, of course it won't fly during one, but being this light and with this amount of surface it probably will be dragged by the wind, won't it? There's some grabbing mechanism? What if it hit the rover while being thrown away?

Nah, the dynamic pressure from wind storms isn't nearly high enough to throw it around. 

[Ultimate Steve]

2 hours ago, sevenperforce said:

Nah, the dynamic pressure from wind storms isn't nearly high enough to throw it around. 

Yes, I don't see this much in arguments anywhere. Wind on Mars would have a really hard time blowing anything over (sorry, The Martian!) and likewise, on Venus, assuming you managed to survive the pressure and temperature, the slightest breeze will be tossing you around quite a bit.

[sh1pman]

37 minutes ago, Ultimate Steve said:

Yes, I don't see this much in arguments anywhere. Wind on Mars would have a really hard time blowing anything over (sorry, The Martian!) and likewise, on Venus, assuming you managed to survive the pressure and temperature, the slightest breeze will be tossing you around quite a bit.

The atmospheric density at surface level on Venus is 50 times higher than on Earth. Wind is more like ocean waves there. If there’s a 10 m/s wind, you’re going to travel at 10 m/s with it.

Edited May 15, 2018 by sh1pman

[Cunjo Carl]

On 5/12/2018 at 8:35 AM, kerbiloid said:

It has gravity ratio ~0.38, but density ration ~0.016  (0.02 / 1.225), so atmo/gravity  is ~25 times less.
So, It needs either 5 times longer blades, or rotate them 5 times faster, doesn't it?

 

So I was curious where we're going to get the factor of 25 from, so I poked around for a bit to figure it out.

Large terrestrial RC helicopters are often operated at around 2krpm, but can be altered to go as low as 1krpm by increasing the blade pitch, so the motor can impart more useful torque at a given speed. Put relative to an 'average' RC helicopter in a normal running mode , the mars helicopter would be able to gain this factor of ~3 from similar high blade pitch, a factor of ~2 from high rpm (3krpm vs 2krpm), and a factor of ~4 from the large blade span for size (1.1m vs .55m typical for size). As a small bonus, the coaxial blade design apparently provides a small boost to lift as well, often estimated to be 1.1x or 1.2x. Also, it's JPL, I'm sure they can work magic with the blade cross-section to get a healthy dose of free extra as well.

Bottom line, when compared to an average RC helicopter, it's able to get its advantages from a mix of places, which makes sense in a way.

A quick comparison of the rotor diameters of terrestrial helicopters (black) to martian helicopters (red):

Spoiler

 

Vertical axis is Rotor Diameter in meters. These values are driven by engineering constraints on the craft, and it's very interesting how the RC and the full sized helicopters follow different trends. Perhaps unsurprisingly, long blade spans are more problematic as scale increases, likely because for uniform load, blade deflection increase with the 4th power of the length. I wrote "Weight" on the independent axis, but it's "Mass" Tut tut... The RC copters I found were a handful of popular ones from enthusiast forums and amazon.

Hmm... This chart needs more martian helicopters.

 

 

I'm still gunning for the dust being a tricky design factor. After all the coaxial rotor has a pair of tiny, concentric, 6krpm contra-rotating concentric shafts, and some high speed low temp bearings that'll be dusted and temp cycled on a daily basis. I've worn out bearings for way less than that! There's not a ton known about martian dust, but apparently it's 3-30um particles on the topsoil, which is the same as extremely high mesh sand blasting fines, which really get everywhere. This said, at only 90 seconds of flight per day though, the wear and tear might come slow enough to last a while. Nothing's going to seize on 3um particles after all, just wear out after a few hours of running. I'm Interested to see what solutions they go for, if they decide to.

The image within shows the mechanical linkages involved for a coaxial rotor design, it looks pretty darn cool. 
 

Spoiler

 

 

 

For testing back in the lab, I have a suggestion

 

Edited May 16, 2018 by Cunjo Carl

[kerbiloid]

Spoiler

2 hours ago, Cunjo Carl said:

So I was curious where we're going to get the factor of 25 from

0.38/0.016 ~= 24

 

[Cunjo Carl]

13 hours ago, kerbiloid said:

  Hide contents

0.38/0.016 ~= 24

 

Well, yes! I guess I meant to say I'm curious how they're going to get a factor of ~25 more thrust out of the rotor than a normal RC helicopter would. It's a big factor! So that's what I went to find out. Pleasantly the available factors they're improving (or are able to easily work in) came out to about (roughly) 25x on paper. Of course, it was all back of the envelope, so to be taken with a bag of salt, but still interesting food for thought, I think.

Edited May 16, 2018 by Cunjo Carl

[Corona688]

On 5/15/2018 at 3:45 AM, Wjolcz said:

That's interesting. I was not aware of that actually. I always thought simply making the blades longer/with more surface area would fix the problem of thin atmosphere.

It becomes the same problem, as bigger rotors are heavier, requiring larger motors, requiring larger batteries, etc, etc.  Plus they're also constrained for weight and size.

Edited May 16, 2018 by Corona688