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Thread: Could this starship's/robot's legs stand the pressure/forces of walking on a planet?

  1. #1 Could this starship's/robot's legs stand the pressure/forces of walking on a planet? 
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    In video game series ''Mass Effect'' there are these sentient starships known as ''Reapers''. They are 2 km tall. I've done some estimates on their mass and the main body would have a mass of 30 million metric tons. Their legs are 100 meters at the thickest and around 20 meters at the thinnest. At average they seem 70 meters thick. Their legs are around 500 meters tall. So my question is that could these 5 legs bear the 20 million metric load with real life physics and material science? Like could these legs even if they were made of graphene and other strong and lightweight materials bear the 20 million metric ton load given by the main body? Or would the legs likely snap when walking on a planet's surface? And in the game they land on a planet with 1.14g so could these legs stand that gravitational force? And if you need the mass for the legs I've done estimations that a leg should have a mass of 2 million tons. I know there is not a lot of parameters for this question but given the instinct your experience brings and the information I've provided, could these legs stand the forces without snapping?


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    IF, this hypothetically existed, the likelyhood that the ship, as described, would function as you say is pretty high. I'm not sure how you arrived at your mass approximation, but materially speaking the whole of the ship would likely be made out of materials that are very lightweight, and the density of the entire thing would likely be around the density of water, if not lighter, given the function of the ship. I'm thinking it'd be made of graphite, titanium, aluminum, and other lightweight composites that likely don't exist. In order for the thing to be able to "walk" it would have to be able to support it's entire mass on 4 of the 5 legs, for a realistic walk more like 2 of the 5 legs. Assuming it has a very well formed skeletal structure, akin to an animals, the "hip" joints and legs would need to be able to support roughly 15 Teragrams each, or under the pressure of the proposed planet, about 165 Giganewtons of pressure. Now, for a skeletal structure, I'd use carbon nanotubes. Multi-walled carbon nanotubes are some of the strongest materials yet created in a lab. with a 63GPa yield point, to support this ship you'd need a mass of these nanotubes approximately 1 meter in diameter. This is all assuming that the civilization that creates these ships has a much better handle on material science than we do, and are able to coax out these lab properties we've found on an industrial scale.


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    Thanks for the answer! I've been wondering this for some time and it's nice to have an answer from an expert to this question.
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    Just one final question about this. Harbinger (the leader of these starships) has only four legs and doesn't possess the middle leg at all. So would a 4 legged version of this starship function as well?
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    Genius Duck Moderator Dywyddyr's Avatar
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    Regardless of whether or not the legs are strong enough I doubt there's any surface that they could walk on.
    With that mass and such a small footprint they'd just punch holes in the ground and sink...
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    Can you estimate how much smaller they'd have to be to be able to walk on a surface? Could a 1 million ton version that is 650m tall walk on a surface without sinking to ground?
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    Compressive strength of soils will be only in the hundreds of psi, if that. Bedrock could vary from a few thousand psi up to 50,000 psi, or more. I'll leave you to work out the required surface area of the 'foot' that would be needed. (Excuse the non SI units. I work with rock strengths in psi.)
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    So for the 1 million ton version I measured the total footprint of 5 legs to be 1800000cm^2 and got got 555kg/cm^2 pressure caused by the legs. So it should do fine if my calculations are correct.
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  10. #9  
    Genius Duck Moderator Dywyddyr's Avatar
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    Quote Originally Posted by Tyzuris Coronati View Post
    Can you estimate how much smaller they'd have to be to be able to walk on a surface? Could a 1 million ton version that is 650m tall walk on a surface without sinking to ground?
    It's not the height it's the area of the foot on the ground.
    I.e. total weight divided by contact area will give you the ground pressure.

    A quick measurement says that the thing is 88mm tall (on the screen) and the foot is 0.5 mm diameter.
    So that means that the foot is (0.5/88) x 2000 = ~11.4m dia = 102 m2.
    Can't decide if there's 5 or 6 legs, but say 6 = 612 m2.
    That's ~49,000 tonnes/ m2 = 480,526,000 N/m2 = 480 MPa.
    That's enough to seriously damage steel.

    Even worse is that to shear through steel is dependent on the edge length of the feet (circumference of 1 foot is ~35.8 m so six feet gives 214.8 m).
    From memory (and it's definitely a long-term memory since it's an Imperial value!) [mild] steel requires 22 tons/ sq. inch to punch holes in it.
    So, with a bit of juggling we determine that if this thing walked on steel it's capable of punching holes straight through a "surface" (i.e. plate) that's 12773.6 inches thick.
    That's 0.2 of a MILE!
    Last edited by Dywyddyr; February 10th, 2014 at 02:46 AM.
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    Quote Originally Posted by Dywyddyr View Post
    Quote Originally Posted by Tyzuris Coronati View Post
    Can you estimate how much smaller they'd have to be to be able to walk on a surface? Could a 1 million ton version that is 650m tall walk on a surface without sinking to ground?
    It's not the height it's the area of the foot on the ground.
    I.e. total weight divided by contact area will give you the ground pressure.

    A quick measurement says that the thing is 88mm tall (on the screen) and the foot is 0.5 mm diameter.
    So that means that the foot is (0.5/88) x 2000 = ~11.4m dia = 102 m2.
    Can't decide if there's 5 or 6 legs, but say 6 = 612 m2.
    That's ~49,000 tonnes/ m2 = 480,526,000 N/m2 = 480 MPa.
    That's enough to seriously damage steel.

    Even worse is that to shear through steel is dependent on the edge length of the feet (circumference of 1 foot is ~35.8 m so six feet gives 214.8 m).
    From memory (and it's definitely a long-term memory since it's an Imperial value!) [mild] steel requires 22 tons/ sq. inch to punch holes in it.
    So, with a bit of juggling we determine that if this thing walked on steel it's capable of punching holes straight through a "surface" (i.e. plate) that's 12773.6 inches thick.
    That's 0.2 of a MILE!
    Okay I have seen a fan made picture of a non canonical Reaper that would be cruiser class instead of 2km dreadnought and would be 650m tall and one of its legs would have 6x8m surface area so it'd be 48x5 which would mean its legs have the surface area of 240m^2. And using my knowledge of the game I've come up with 1 million metric tons mass for the fan made Reaper cruiser. So 4166.67 tons/square meters pressure for the ground. How would this sound for walking on the surface?
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    Genius Duck Moderator Dywyddyr's Avatar
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    Try here.
    Category Types of rocks and soils Presumed bearing value
    Non-cohesive soils Dense gravel or dense sand and gravel >600 kN/mē
    Medium dense gravel,
    or medium dense sand and gravel
    <200 to 600 kN/mē
    Loose gravel, or loose sand and gravel <200 kN/mē
    Compact sand >300 kN/mē
    Medium dense sand 100 to 300 kN/mē
    Loose sand <100 kN/mē depends on
    degree of looseness
    Cohesive soils Very stiff bolder clays & hard clays 300 to 600 kN/mē
    Stiff clays 150 to 300 kN/mē
    Firm clay 75 to 150 kN/mē
    Soft clays and silts < 75 kN/mē


    On top of that list there's also a maximum load (independent of "ground pressure") that any surface will bear.
    Your given figure equates to ~41000 kN/ m2: in short the thing is f*cked from the word "go".
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    So even the smaller one sinks into the ground but bedrock could take it given the figure of a few thousand psi up to 50000 psi but bedrock is hundreds of meters deep so even bedrock won't benefit at all.
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  14. #13  
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    How much would you guys estimate this 2km starship's mass to be if it was made from carbon nanotubes which have the density of 1,3-1,4 grams/cm^3?
    This is how a Reaper looks inside so they're quite hollow:
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  15. #14  
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    They look like insects so I'm guesing, no, they will not work. Massive walking things need straight collumnular legs and big flat feet like an elephant's or a brontosaurus's. Angeled legs put an additional stresson the hip joint.
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