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19 hours ago, ARS said:

If a tank is too heavy to cross a bridge, does widening the track to distribute the load over larger area helps to mitigate it? (Doesn't have to be moving fast, just cross the bridge, when deep fording isn't an option)

 

55 minutes ago, DDE said:

I doubt it, dispersing the weight over a mrginally greater area would only help preserve the surface of the bridge - it won't prevent the legs from snapping or the span of the bridge from collapsing. A tank long enough to distribute weight across several spans is not really an option.

Also partly relevant: it's a trap!

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So...  I know you specified a bridge - but let me comment for a moment about tank tracks and wheels. 

 

We call the wheels of a tank 'road wheels', and what the tracks actually do is create an impromptu road over terrain for the tank to drive on.  If you ever see one, or a good video, you will notice that the track is laid down in sections as the tank moves, and then picked back up as the tank passes.  In most cases, the treads and track sections are lying still on the ground while the road wheels travel along the track. In other words, the track is at rest WRT the ground.  This is important because it is less likely to dig into the earth like a spinning tire might.  The drive wheels or sprockets are actually putting tension on the track to propel the tank along the 'road' rather than directly to the ground like a wheeled vehicle does. 

The width of the track (and length, but see next) and the arrangement and spacing of the road wheels on the track contribute to the Ground Pressure of the tank. 

https://www.mathscinotes.com/2016/06/tank-track-ground-pressure-examples

https://en.m.wikipedia.org/wiki/Ground_pressure

 

Lower ground pressure is important when you have sloppy terrain that vehicles can sink into. 

So - on to the bridge thing... 

 

Yes, to a degree, having the weight distributed does make it less likely to experience a bridge or road failure - but bridges are built to sustain a load over time.  Put too much load on the bridge at one time and it's going to fail.  Repeat traverse of the max load can also shorten the life of the bridge (I.e. The first three tanks might make it across one at a time , but the fourth might collapse the span). 

 

So - I'm not a bridge engineer - but I can tell you that there are a lot of modern bridges that can support tanks during combat. The bridge weight limit / rating may not be recommended for the tank's weight - but quite often can be used by tankers - but it might be permanently weakened and need to be rebuilt once folks start waging peace again. 

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Piling on an anecdote here. I was recently inspecting an augering job with a rig that looked  a lot like this, maybe 80 feet tall, except it was in a very muddy hole. My boots sunk about 4 inches into the mud. On tracks, the crane only sunk about 2 inches. So the pressure is quite low. The total load on the other hand...

Bridge limits are usually specified by weight

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On 11/6/2020 at 1:14 AM, Nightside said:

Piling on an anecdote here. I was recently inspecting an augering job with a rig that looked  a lot like this, maybe 80 feet tall, except it was in a very muddy hole. My boots sunk about 4 inches into the mud. On tracks, the crane only sunk about 2 inches. So the pressure is quite low.

And that's before we get to the vehicles designed specifically with reduced ground pressure in mind.

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Quote

Slowly they moved off, and were soon toiling heavily. In places the snow was breast-high, and often Boromir seemed to be swimming or burrowing with his great arms rather than walking.

Legolas watched them for a while with a smile upon his lips, and then he turned to the others.

`The strongest must seek a way, say you? But I say: let a ploughman plough, but choose an otter for swimming, and for running light over grass and leaf or over snow-an Elf.'

With that he sprang forth nimbly, and then Frodo noticed as if for the first time, though he had long known it, that the Elf had no boots, but wore only light shoes, as he always did, and his feet made little imprint in the snow.

 

 

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4 hours ago, DDE said:

And that's before we get to the vehicles designed specifically with reduced ground pressure in mind.

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The Russians have a LOT of very interesting stuff to deal with Tundra.  Although, tbh, I've never seen the 'gun' variant of that particular one... I have seen it in School Bus form!

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16 hours ago, JoeSchmuckatelli said:

The Russians have a LOT of very interesting stuff to deal with Tundra.  Although, tbh, I've never seen the 'gun' variant of that particular one...

Magnolia is a very recent 120 mm prototype that seems to be from Project Nabrosok, which also gave us the 82 mm Drok and the 120 mm Flox:

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4 hours ago, JoeSchmuckatelli said:

300 million dirty wet planets lurking around the galaxy. 

Assemble the boarding parties! 

But did you forget about Titan already?

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I find Titan a bit chilly.  I prefer sub-tropical planets in the habitable zones of G2V /Va stars. 

 

18 Sco is only 45 LY away - although I don't think we've found planets there yet 

I keep hoping someone will throw Webb at the sky so we can get a peek at it 

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1 hour ago, JoeSchmuckatelli said:

18 Sco is only 45 LY away - although I don't think we've found planets there yet 

We happen to be in a fairly sparse portion of the galaxy, but even here the cube law works in our favor. While there is nothing worth visiting in the immediate vicinity, once you go out to a few light decades, which means you're doing generation ships anyways, you might as well go out a few light decades more, and now we are talking about hundreds of candidate star systems, many of them with F, G, and K stars of the main sequence that ought to have planetary systems. We're bound to find something colonizable among these.

Whether any habitable planet is currently worth building a generation ship for is another question. Personally, I would focus on better exploitation of the resources in Sol first. If we can start mining asteroids for all of the construction materials, building habitats in space will become rather affordable. I can see an appeal of an Earth-like planet where you can just go outside without a protective suit and with minimal, if any, breathing apparatus, but we can make do with space stations and planetary colonies in Sol for now.

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13 minutes ago, K^2 said:

Whether any habitable planet is currently worth building a generation ship for is another question. Personally, I would focus on better exploitation of the resources in Sol first. If we can start mining asteroids for all of the construction materials, building habitats in space will become rather affordable. I can see an appeal of an Earth-like planet where you can just go outside without a protective suit and with minimal, if any, breathing apparatus, but we can make do with space stations and planetary colonies in Sol for now.

Once we have a few decades of experience with long-term space habitats, it should be fairly trivial to make one with lots of spare parts/resources, some high efficiency engines(probably something ion if nothing better by then),, and an etra-large fuel tank, and send them off to the best candidate systems.

Especially if parasite craft would let it harvest any essential consumables from gas giants and small moons, even if the destination system does not have anything you might want to live on. 

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3 minutes ago, Terwin said:

Once we have a few decades of experience with long-term space habitats, it should be fairly trivial to make one with lots of spare parts/resources, some high efficiency engines(probably something ion if nothing better by then),, and an etra-large fuel tank, and send them off to the best candidate systems.

Once we have experience building space habitats that try to be as self-sufficient as possible, I suspect we'll learn a lot of reasons why it's easier said than done. So I don't know about that "trivial" bit, but eventually, yes, I imagine we ought to be able to get to a point where it's just extra mass to be expended along the voyage on leaks, ablation, etc.

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On 11/5/2020 at 11:14 PM, Nightside said:

Piling on an anecdote here. I was recently inspecting an augering job with a rig that looked  a lot like this, maybe 80 feet tall, except it was in a very muddy hole. My boots sunk about 4 inches into the mud. On tracks, the crane only sunk about 2 inches. So the pressure is quite low. The total load on the other hand...

Bridge limits are usually specified by weight

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To counter this next to my office some set up large mobile crane for some reason, they did not know the parking lot was on top of an underground garage, then they used the hydraulic jacks to elevate and level the crane and then move the crane arm one of the jack broke trough the roof of the garage and the crane fell over, I say it was luck none was killed and they missed the buildings with a few meters. 
It would been easy avoided simply walking over the the stairs at the end would show the door to the underground garage, and most of the parking lots are on top of garages in the first place. 

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I know there is no real answer to how, or even if atmosphere relates to the gravity /size of a planet...

But if we found a slightly larger terrestrial planet (say 1.2g) and atmosphere scaled linearly, would we be able to breathe and function at sea level in an atmosphere with identical proportion of gasses as earth?   (or would we risk something like the bends and need to be in the mountains to be safe?) 

4 hours ago, magnemoe said:

To counter this next to my office some set up large mobile crane for some reason, they did not know the parking lot was on top of an underground garage, then they used the hydraulic jacks to elevate and level the crane and then move the crane arm one of the jack broke trough the roof of the garage and the crane fell over, I say it was luck none was killed and they missed the buildings with a few meters. 
It would been easy avoided simply walking over the the stairs at the end would show the door to the underground garage, and most of the parking lots are on top of garages in the first place. 

That can happen with sewer pipes as well.  Kind of the opposite of using wide tracks to reduce the ground pressure - the leveling plates put the weight onto a small point.  Once something long and heavy starts moving... 

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8 hours ago, JoeSchmuckatelli said:

I know there is no real answer to how, or even if atmosphere relates to the gravity /size of a planet...

But if we found a slightly larger terrestrial planet (say 1.2g) and atmosphere scaled linearly, would we be able to breathe and function at sea level in an atmosphere with identical proportion of gasses as earth?   (or would we risk something like the bends and need to be in the mountains to be safe?) 

It don't looks like higher pressure has an negative impact as long as its moderate 1.5 bar is moderate here, you might run an risk of bends if you moved up on an mountain where it was 0.7 bar however if used to living at 1.5 bar. 
They has done month long stays at 2 bar and probably also much higher pressure but not sure if that has been month long, people also live up at 5 km attitude. 

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8 hours ago, JoeSchmuckatelli said:

But if we found a slightly larger terrestrial planet (say 1.2g) and atmosphere scaled linearly, would we be able to breathe and function at sea level in an atmosphere with identical proportion of gasses as earth?   (or would we risk something like the bends and need to be in the mountains to be safe?)

A lot of otherwise harmless gases become toxic at higher pressures. Most crucially, oxygen is toxic at just a few atmospheres with the same gas fraction. Of course, it's not a problem if there is simply less oxygen in the atmosphere. Unfortunately, nitrogen also becomes toxic at similar pressure. The gas mixtures used for deepest dives consist of helium and oxygen precisely to avoid these problems. Atmosphere consisting primarily of helium with just enough oxygen to prevent hypoxia can theoretically be breathable at very high pressure. But it's not the sort of thing that's likely to form naturally anywhere.

I don't know if there are any lower limits on atmospheric pressure for a super-Earth. Obviously, we can have smaller planets with much higher pressure, like Venus, but whether something with Earth-like pressure is possible on much heavier world, I don't know. In either case, the atmospheric pressure cannot be a lot higher if it is to be breathable.

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20 minutes ago, K^2 said:

A lot of otherwise harmless gases become toxic at higher pressures. Most crucially, oxygen is toxic at just a few atmospheres with the same gas fraction. Of course, it's not a problem if there is simply less oxygen in the atmosphere. Unfortunately, nitrogen also becomes toxic at similar pressure. The gas mixtures used for deepest dives consist of helium and oxygen precisely to avoid these problems. Atmosphere consisting primarily of helium with just enough oxygen to prevent hypoxia can theoretically be breathable at very high pressure. But it's not the sort of thing that's likely to form naturally anywhere.

I don't know if there are any lower limits on atmospheric pressure for a super-Earth. Obviously, we can have smaller planets with much higher pressure, like Venus, but whether something with Earth-like pressure is possible on much heavier world, I don't know. In either case, the atmospheric pressure cannot be a lot higher if it is to be breathable.

Yes, but this require multiple bars, less than 2 bar should not be much of an problem unless very oxygen rich. 3 bar is probably ok too but might be bad long term. Much higher and you get problems, kind of doubt you can have too much oxygen as all will burn too easy. 
 

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5 hours ago, kerbiloid said:

On the planet with 1.2 atm at sea level wise hillbillies live on the hills at 1.0 atm.

In the science fiction RPG campaign I'm running right now, that's exactly what one of the planets is like. Thick atmosphere, unhealthy at sea level, but with several large shield volcanoes. So the settlements ring the volcanoes at the habitable altitude. Real estate is at a premium.

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2 hours ago, TheSaint said:

In the science fiction RPG campaign I'm running right now, that's exactly what one of the planets is like. Thick atmosphere, unhealthy at sea level, but with several large shield volcanoes. So the settlements ring the volcanoes at the habitable altitude. Real estate is at a premium.

Hmm. In Larry Niven’s “Known Space” literary universe, there is an egg-shaped planet called Jinx. Seen from space, it is banded like an Easter egg: the poles are in vacuum and the equatorial region is a high-pressure hell, inhabited by bandersnatchi.  The human-habitable zones are in between, of course. 

IIRC that world is described in World of Ptaavs 

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6 minutes ago, StrandedonEarth said:

Hmm. In Larry Niven’s “Known Space” literary universe, there is an egg-shaped planet called Jinx. Seen from space, it is banded like an Easter egg: the poles are in vacuum and the equatorial region is a high-pressure hell, inhabited by bandersnatchi.  The human-habitable zones are in between, of course. 

IIRC that world is described in World of Ptaavs 

There was also Plateau (aka Mount Lookitthat), from A Gift From Earth, which also served as an inspiration.

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In LBA/LBA2 Little Big Adventure there is a layer of dense gas separating two habitable zones, above and below the gas, each with its own biological species (including humanoid ones).

Airships are floating across the gas layer, connecting the above-gas "islands" (actually, mountain tops).

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2 hours ago, StrandedonEarth said:

Hmm. In Larry Niven’s “Known Space” literary universe, there is an egg-shaped planet called Jinx. Seen from space, it is banded like an Easter egg: the poles are in vacuum and the equatorial region is a high-pressure hell, inhabited by bandersnatchi.  The human-habitable zones are in between, of course. 

IIRC that world is described in World of Ptaavs 

And the poles are pretty much in vacuum, problem with this is that the planet would turn into an sphere because of hydrodynamic. 
Plateau would work a bit better but see issue with high pressure oxygen and nitrogen atmosphere who is stable and breathable on an very high mountain. 

Now as the depth get warmer you could have an high temperature ecosystem, say 50-60 centigrade and 2 bar and one bar terrestrial, you can not have lots deep oceans here. Say you drop sea level 3 km and added atmosphere to keep modern sea level at one bar as an teaser. 

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