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Floating on a liquid surface is markedly different from floating within a fluid (liquid or gaseous). To float on a liquid, one merely needs to maintain a lower

Floating on a liquid surface is markedly different from floating within a fluid (liquid or gaseous). To float on a liquid, one merely needs to maintain a lower average density within the vessel than the surrounding liquid. Assuming a largely hollow vessel (as with a ship or barge), it's possible to add or remove considerable payload without losing stable flotation characteristics as the draft of the vessel automatically compensates for the variation, displacing more or less liquid, and maintaining equilibrium. To float in a fluid, one must maintain precise neutral buoyancy, which is an entirely different animal. As pressure varies with depth or altitude, the tendency is for a vessel to contract as it sinks and expand as it rises, leading to a runaway buoyancy shift (increasingly negative with depth, increasingly positive with height). Many military submarines operate at comparatively shallow depths, often only slightly more than their overall length* (for larger submarines), given both the immense pressures of even modest ocean depths (a few hundred metres), and the compounding nature and risks of runaway buoyancy loss. Plans for cargo airships face corresponding problems in that when offloading cargo or passengers it is necessary to vent or otherwise scavenge lifting gas (the expense and/or challenges of either venting or compressing helium are great), or to onboard a corresponding mass of ballast. Where suitable water is plentiful the latter is fairly viable, but there are many applications proposed for cargo airships which suggest transport of heavy cargos sites with limited capabilities for same (no facilities, deserts, salt- or otherwise-contaminated water which might play poorly with buoyancy-compensation systems aboard the airship). Rocket launch from an inhabited floating atmospheric platform would require accumulation of large stores of fuel (the Tsiolkovsky rocket equation also works against you), as well as presenting various risks associated with enormous barely-contained explosions (should you be lucky). The risks are immense, and hand-waving them away is disingenuous to the extreme.

JumpCrisscross 1y ago
> To float in a fluid, one must maintain precise neutral buoyancy, which is an entirely different animal You're right, I was oversimplifying. An aerial or submerged launch platform, then. > the tendency is for a vessel to contract as it sinks and expand as it rises, leading to a runaway buoyancy shift (increasingly negative with depth, increasingly positive with height) This is inherent to the cloud city design. Rockets would be a subclass of buoyancy risks, eclipsed entirely by atmospherics. > Rocket launch from an inhabited floating atmospheric platform would require accumulation of large stores of fuel (the Tsiolkovsky rocket equation also works against you), as well as presenting various risks associated with enormous barely-contained explosions This is a fair criticism. It's also solved by having offboard propellant storage and launch platforms. > risks are immense, and hand-waving them away is disingenuous to the extreme Didn't mean to suggest it isn't risky. Just that the risks from the rocket launch component are dwarfed by many, many others, and to the extent there are risks here, they are ones we've already solved on Earth in analogous contexts. (Maintaining buoyancy isn't remotely the main problem with launching rockets from high-altitude blimps.)
- dredmorbius 1y ago
  How do those offboard propellent storage and/or launch platforms keep from plummeting to the planetary surface? What is their buoyancy-management system? You're offloading the problem, not solving it.
  - potato3732842 1y ago
    >What is their buoyancy-management system? Some of y'all have never seen a marina with floating docks and it shows. More of the same. This entire problem is basically ye-olde spaceX barge only with different factors in the equation and running in both directions (instead of just landing). Yes, without a hard cut in buoyancy like you get with something that's way denser than air floating in something way denser than it all the math gets a little wonky but it's all still fundamentally the same. When you load a few million pounds of shit you sink a few thousand feet instead of a few inches like a barge in water would, and when that weight turns out to be a rocket that yeets itself you move around thousands of feet or miles instead of feet like a barge, but when you're floating in the air with nothing to crash into who cares.
    - dredmorbius 1y ago
      The dock/barge case is addressed here: <https://news.ycombinator.com/item?id=45330638 https://news.ycombinator.com/item?id=45330638> An aerostat doesn't float on a liquid at stable equilibrium through draft displacement, it is suspended in a fluid, with the problems noted previously. Docks and barges (along with general watercraft) may be constructed arbitrarily robustly from strong and resilient materials. Aerostats somewhat less so.
      - potato3732842 1y ago
        >The dock/barge case is addressed here: <https://news.ycombinator.com/item?id=45330638 https://news.ycombinator.com/item?id=45330638> Addressed naively and wrongly hence the ongoing discussion >An aerostat doesn't float on a liquid at stable equilibrium through draft displacement, it is suspended in a fluid, with the problems noted previously. If you let a baloon go will it reach space? No, because the atmosphere is not constant density. Balloon type objects have the nice side effect of expanding and contracting to reach buoyancy/weight/structural equilibrium. It's not like a submarine "flying" though the water. It's more like a fish expanding/contracting to ascend/descend. More literally, it's like a weather balloon that rides at different attitudes depending on what the weight of your payload is. If you really need to change altitude quickly (or perhaps in response to taking on or losing mass) it wouldn't be all that difficult to inflate/deflate (i.e. change displacement) a subset of whatever device provides buoyancy. Think of it like a heavy lift ship flooding itself (reducing displacement) to change draft. Like I said, the lack of a "hard cut" between atmosphere and ocean makes the math wonky compared to what we're used to, but the physics DGAF. >Docks and barges (along with general watercraft) may be constructed arbitrarily robustly from strong and resilient materials. Aerostats somewhat less so. You could say the same thing about boats vs port facilities. Yeah, it's an engineering problem but it's a fundamentally well understood one. The way your hand gets forced in terms of material choices might make cost go through the roof, but it the design side of things shouldn't be all that terrible.

JumpCrisscross 1y ago

> To float in a fluid, one must maintain precise neutral buoyancy, which is an entirely different animal You're right, I was oversimplifying. An aerial or submerged launch platform, then. > the tendency is for a vessel to contract as it sinks and expand as it rises, leading to a runaway buoyancy shift (increasingly negative with depth, increasingly positive with height) This is inherent to the cloud city design. Rockets would be a subclass of buoyancy risks, eclipsed entirely by atmospherics. > Rocket launch from an inhabited floating atmospheric platform would require accumulation of large stores of fuel (the Tsiolkovsky rocket equation also works against you), as well as presenting various risks associated with enormous barely-contained explosions This is a fair criticism. It's also solved by having offboard propellant storage and launch platforms. > risks are immense, and hand-waving them away is disingenuous to the extreme Didn't mean to suggest it isn't risky. Just that the risks from the rocket launch component are dwarfed by many, many others, and to the extent there are risks here, they are ones we've already solved on Earth in analogous contexts. (Maintaining buoyancy isn't remotely the main problem with launching rockets from high-altitude blimps.)

dredmorbius 1y ago
How do those offboard propellent storage and/or launch platforms keep from plummeting to the planetary surface? What is their buoyancy-management system? You're offloading the problem, not solving it.
- potato3732842 1y ago
  >What is their buoyancy-management system? Some of y'all have never seen a marina with floating docks and it shows. More of the same. This entire problem is basically ye-olde spaceX barge only with different factors in the equation and running in both directions (instead of just landing). Yes, without a hard cut in buoyancy like you get with something that's way denser than air floating in something way denser than it all the math gets a little wonky but it's all still fundamentally the same. When you load a few million pounds of shit you sink a few thousand feet instead of a few inches like a barge in water would, and when that weight turns out to be a rocket that yeets itself you move around thousands of feet or miles instead of feet like a barge, but when you're floating in the air with nothing to crash into who cares.
  - dredmorbius 1y ago
    The dock/barge case is addressed here: <https://news.ycombinator.com/item?id=45330638 https://news.ycombinator.com/item?id=45330638> An aerostat doesn't float on a liquid at stable equilibrium through draft displacement, it is suspended in a fluid, with the problems noted previously. Docks and barges (along with general watercraft) may be constructed arbitrarily robustly from strong and resilient materials. Aerostats somewhat less so.
    - potato3732842 1y ago
      >The dock/barge case is addressed here: <https://news.ycombinator.com/item?id=45330638 https://news.ycombinator.com/item?id=45330638> Addressed naively and wrongly hence the ongoing discussion >An aerostat doesn't float on a liquid at stable equilibrium through draft displacement, it is suspended in a fluid, with the problems noted previously. If you let a baloon go will it reach space? No, because the atmosphere is not constant density. Balloon type objects have the nice side effect of expanding and contracting to reach buoyancy/weight/structural equilibrium. It's not like a submarine "flying" though the water. It's more like a fish expanding/contracting to ascend/descend. More literally, it's like a weather balloon that rides at different attitudes depending on what the weight of your payload is. If you really need to change altitude quickly (or perhaps in response to taking on or losing mass) it wouldn't be all that difficult to inflate/deflate (i.e. change displacement) a subset of whatever device provides buoyancy. Think of it like a heavy lift ship flooding itself (reducing displacement) to change draft. Like I said, the lack of a "hard cut" between atmosphere and ocean makes the math wonky compared to what we're used to, but the physics DGAF. >Docks and barges (along with general watercraft) may be constructed arbitrarily robustly from strong and resilient materials. Aerostats somewhat less so. You could say the same thing about boats vs port facilities. Yeah, it's an engineering problem but it's a fundamentally well understood one. The way your hand gets forced in terms of material choices might make cost go through the roof, but it the design side of things shouldn't be all that terrible.