3 ms·

Does it need to be more extensive than "make the very bottom ridge of the dome wider than the dome itself (some kind of "lip") and place it into the groove carv

by rimliu 7y ago

Does it need to be more extensive than "make the very bottom ridge of the dome wider than the dome itself (some kind of "lip") and place it into the groove carved in the rock?

pbhjpbhj 7y ago
I'd have thought you'd use pegs (pitons, or piles) as for tents on Earth?
FeepingCreature 7y ago
Yes... Remember that there's 1 bar of atmosphere in there. 150 feet (45m) makes for a circle of 140m circumference but 1600m^2 area. Since 1 bar ~= 100_000Pa, each of that square meters has 100KN of upwards force on it, for a sum of 160MN distributed over 140m of circumference or ~1MN per meter. So assuming the mass of the dome is negligible, we're talking about the equivalent strength of holding on to a mass of about 100 metric tons (compared to Earth gravity) per meter of dome. Ie. the amount of support you need per meter is the amount that you would need on Earth to hold up 100 metric tons against gravity. That's about a Boeing 737. Per meter.
- rimliu 7y ago
  Steel cable with a diameter of 1inch has a breaking strength of 47 tonnes. Give it a safety margin and assume it can hold 10 tonnes. So you need 10 cables per meter. So I assume in case the "lip" being just a sheet of steel it can be way thinner than 25mm. https://imgur.com/a/h92bLIp https://imgur.com/a/h92bLIp
  - FeepingCreature 7y ago
    Sure but you need to be sure that the thing you're attaching it to can hold the weight. Ie. if the martian floor was the ceiling, you'd have to be able to hang a plane off that attachment point without it ripping out. In other words, your lip needs to be distributing force onto a sufficient volume of ground to make up 100_000 metric tons of martian soil, per meter of circumference.
    - rimliu 7y ago
      Actually I think just making the underground part cone-shaped would be much better and simpler solution than that lip.
- kortex 7y ago
  That's why the author suggests peridiodic anchors, like an air mattress. As other commenters have pointed out, you don't want the structure to deflate in the event of decomp, so a strut which is useful secondarily in compression is useful here. Basal fiber is about 3GPa tensile, call it a MoS of 6 for a neat 500MPa working strength. If you plug int wolfram alpha `1 atmosphere * 1 square meter / 500 MPa` you get 2 cm^2 of support (tethering) per sq m of roof. That's reasonable. But here's the rub: compressive strength lags tensile strength. Even granite is a mere 130 MPa. So even a strut which has some compression resiliance is not gonna be enough to fully abate a collapse. So it's very likely a Martian structure would have some arc to it, as it both benefits tension under nominal operation and some compression in failure mode, at least long enough to keep pressure from dropping too low. I imagine rip-stop at all sorts of scales will be implemented. It's not gonna be like an inflatable golf dome. https://www.engineeringtoolbox.com/compression-tension-strength-d_1352.html https://www.engineeringtoolbox.com/compression-tension-stren... https://en.m.wikipedia.org/wiki/Ultimate_tensile_strength https://en.m.wikipedia.org/wiki/Ultimate_tensile_strength

by rimliu 7y ago

Does it need to be more extensive than "make the very bottom ridge of the dome wider than the dome itself (some kind of "lip") and place it into the groove carved in the rock?

pbhjpbhj 7y ago

I'd have thought you'd use pegs (pitons, or piles) as for tents on Earth?

FeepingCreature 7y ago

Yes... Remember that there's 1 bar of atmosphere in there. 150 feet (45m) makes for a circle of 140m circumference but 1600m^2 area. Since 1 bar ~= 100_000Pa, each of that square meters has 100KN of upwards force on it, for a sum of 160MN distributed over 140m of circumference or ~1MN per meter. So assuming the mass of the dome is negligible, we're talking about the equivalent strength of holding on to a mass of about 100 metric tons (compared to Earth gravity) per meter of dome. Ie. the amount of support you need per meter is the amount that you would need on Earth to hold up 100 metric tons against gravity. That's about a Boeing 737. Per meter.

rimliu 7y ago
Steel cable with a diameter of 1inch has a breaking strength of 47 tonnes. Give it a safety margin and assume it can hold 10 tonnes. So you need 10 cables per meter. So I assume in case the "lip" being just a sheet of steel it can be way thinner than 25mm. https://imgur.com/a/h92bLIp https://imgur.com/a/h92bLIp
- FeepingCreature 7y ago
  Sure but you need to be sure that the thing you're attaching it to can hold the weight. Ie. if the martian floor was the ceiling, you'd have to be able to hang a plane off that attachment point without it ripping out. In other words, your lip needs to be distributing force onto a sufficient volume of ground to make up 100_000 metric tons of martian soil, per meter of circumference.
  - rimliu 7y ago
    Actually I think just making the underground part cone-shaped would be much better and simpler solution than that lip.
kortex 7y ago
That's why the author suggests peridiodic anchors, like an air mattress. As other commenters have pointed out, you don't want the structure to deflate in the event of decomp, so a strut which is useful secondarily in compression is useful here. Basal fiber is about 3GPa tensile, call it a MoS of 6 for a neat 500MPa working strength. If you plug int wolfram alpha `1 atmosphere * 1 square meter / 500 MPa` you get 2 cm^2 of support (tethering) per sq m of roof. That's reasonable. But here's the rub: compressive strength lags tensile strength. Even granite is a mere 130 MPa. So even a strut which has some compression resiliance is not gonna be enough to fully abate a collapse. So it's very likely a Martian structure would have some arc to it, as it both benefits tension under nominal operation and some compression in failure mode, at least long enough to keep pressure from dropping too low. I imagine rip-stop at all sorts of scales will be implemented. It's not gonna be like an inflatable golf dome. https://www.engineeringtoolbox.com/compression-tension-strength-d_1352.html https://www.engineeringtoolbox.com/compression-tension-stren... https://en.m.wikipedia.org/wiki/Ultimate_tensile_strength https://en.m.wikipedia.org/wiki/Ultimate_tensile_strength