5 ms·

They know exactly what the air density and gravity is and tested for the exact conditions. It it makes to the surface in one piece it will fly.

macksd 6y ago
I realize they're very smart and very thorough, but this sounds an awful lot like "I ran the test suite - if you can apply the patch in production it'll work".
- mhh__ 6y ago
  Luckily physics works pretty well. Unfortunately, everything is trivial in theory.
pengaru 6y ago
Isn't Mars known for extreme winds and dust storms, which seem like quite the challenge for an ultralight drone resting on the surface, let alone flying autonomously?
- MrFlynn 6y ago
  Since the atmosphere of Mars is 1% the pressure of Earth's, even very high velocity wind would have little energy behind it.
- jojobas 6y ago
  Wind forces are defined by dynamic pressure q, which is proportional to speed squared divided by density. At 1% the Earth's air density, a 100m/s wind on Mars (half the speed of sound!) exerts the same force as 10m/s breeze on Earth. So far the fastest winds observed on Mars are about 30m/s. So, the thin air that makes it hard to fly on Mars makes it safe to stay on the ground.
  - pengaru 6y ago
    Doesn't the suspended dust carried in these wind storms need to be taken into consideration? I presume they contribute significantly to the density.
    - jojobas 6y ago
      It's said that during dust storms Martian air carries some 100-1000 dust particles of 1-10um per cm^3. Running the worst case numbers, 1000 10um particles per cm^3 adds up to about 1/3 the air density. Realistically it's maybe 10% of density increase.
      - tobmlt 6y ago
        I remember from my flying days that: “The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear counter-intuitive. This occurs because the molar mass of water (18 g/mol) is less than the molar mass of dry air.” (Not sure I like the language above but that’s how Wikipedia says it) Is this potential effect accounted for in your numbers? I.e. What happens to the molar mass of Martian atmosphere when Martian dust is suspended in it? At any rate, dust is bad for the moving parts of flying machines.
        jojobas 6y ago
        This is about gaseous water, not water droplets. Water droplets (fog) increase density (again, not by much). Dust doesn't have a meaningful vapour pressure and can't influence air density this way.
  - codeduck 6y ago
    10m/s is a pretty fresh breeze.
wongarsu 6y ago
Simulating Martian air density is trivial, just build a vacuum chamber, remove the air, add some CO2 up to the right pressure. But simulating less gravity is incredibly hard: Tethers are cheap but don't really allow free flight. Parabolic flights only give you a few seconds in fairly cramped space, and getting the low air density inside a plane sounds tough. A large spinning laboratory in space that can be spun up to simulate the desired gravity would be a solution, but we don't have that.
- op03 6y ago
  Why would it be complicated though? Is gravity changing a lot? I thought it was just a constant you plug into equations. Air density on the other hand I would think can change for hundred different reasons from one second to the next.
  - simonh 6y ago
    In context the comment is about simulating the conditions physically, hence discussion of vacuum chambers and aircraft.
- onethought 6y ago
  For the moon practice lander they used a rocket that always pointed down to cancel out enough of Earths gravity to match the Moons gravity. Neil Armstrong seemed to like it :D They could do something like that.
  - ggm 6y ago
    Neil Armstrong seemed to like it :D It nearly killed him. I don't think anyone actually liked it, and I bet a lot of people in risk assessment went hard on the "nixon needs to prepare the death letter" story.
  - KMag 6y ago
    > used a rocket that always pointed down Nit: the Lunar Lander Research Vehicle actually used a CF700 turbofan[0], not a rocket, to partially counter-act gravity. [0] https://en.wikipedia.org/wiki/General_Electric_CF700 https://en.wikipedia.org/wiki/General_Electric_CF700

macksd 6y ago

I realize they're very smart and very thorough, but this sounds an awful lot like "I ran the test suite - if you can apply the patch in production it'll work".

mhh__ 6y ago
Luckily physics works pretty well. Unfortunately, everything is trivial in theory.

pengaru 6y ago

Isn't Mars known for extreme winds and dust storms, which seem like quite the challenge for an ultralight drone resting on the surface, let alone flying autonomously?

MrFlynn 6y ago
Since the atmosphere of Mars is 1% the pressure of Earth's, even very high velocity wind would have little energy behind it.
jojobas 6y ago
Wind forces are defined by dynamic pressure q, which is proportional to speed squared divided by density. At 1% the Earth's air density, a 100m/s wind on Mars (half the speed of sound!) exerts the same force as 10m/s breeze on Earth. So far the fastest winds observed on Mars are about 30m/s. So, the thin air that makes it hard to fly on Mars makes it safe to stay on the ground.
- pengaru 6y ago
  Doesn't the suspended dust carried in these wind storms need to be taken into consideration? I presume they contribute significantly to the density.
  - jojobas 6y ago
    It's said that during dust storms Martian air carries some 100-1000 dust particles of 1-10um per cm^3. Running the worst case numbers, 1000 10um particles per cm^3 adds up to about 1/3 the air density. Realistically it's maybe 10% of density increase.
    - tobmlt 6y ago
      I remember from my flying days that: “The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear counter-intuitive. This occurs because the molar mass of water (18 g/mol) is less than the molar mass of dry air.” (Not sure I like the language above but that’s how Wikipedia says it) Is this potential effect accounted for in your numbers? I.e. What happens to the molar mass of Martian atmosphere when Martian dust is suspended in it? At any rate, dust is bad for the moving parts of flying machines.
      - jojobas 6y ago
        This is about gaseous water, not water droplets. Water droplets (fog) increase density (again, not by much). Dust doesn't have a meaningful vapour pressure and can't influence air density this way.
- codeduck 6y ago
  10m/s is a pretty fresh breeze.

wongarsu 6y ago

Simulating Martian air density is trivial, just build a vacuum chamber, remove the air, add some CO2 up to the right pressure. But simulating less gravity is incredibly hard: Tethers are cheap but don't really allow free flight. Parabolic flights only give you a few seconds in fairly cramped space, and getting the low air density inside a plane sounds tough. A large spinning laboratory in space that can be spun up to simulate the desired gravity would be a solution, but we don't have that.

op03 6y ago
Why would it be complicated though? Is gravity changing a lot? I thought it was just a constant you plug into equations. Air density on the other hand I would think can change for hundred different reasons from one second to the next.
- simonh 6y ago
  In context the comment is about simulating the conditions physically, hence discussion of vacuum chambers and aircraft.
onethought 6y ago
For the moon practice lander they used a rocket that always pointed down to cancel out enough of Earths gravity to match the Moons gravity. Neil Armstrong seemed to like it :D They could do something like that.
- ggm 6y ago
  Neil Armstrong seemed to like it :D It nearly killed him. I don't think anyone actually liked it, and I bet a lot of people in risk assessment went hard on the "nixon needs to prepare the death letter" story.
- KMag 6y ago
  > used a rocket that always pointed down Nit: the Lunar Lander Research Vehicle actually used a CF700 turbofan[0], not a rocket, to partially counter-act gravity. [0] https://en.wikipedia.org/wiki/General_Electric_CF700 https://en.wikipedia.org/wiki/General_Electric_CF700