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.

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JumpCrisscross 3 days ago | parent [-]

> 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.)

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dredmorbius 3 days ago | parent [-]

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.

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potato3732842 3 days ago | parent [-]

>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.

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dredmorbius 3 days ago | parent [-]

The dock/barge case is addressed here: <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.

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potato3732842 3 days ago | parent [-]

>The dock/barge case is addressed here: <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.

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dredmorbius 3 days ago | parent [-]

The analogy isn't letting go of a balloon, it's of dumping a large mass of payload (rocket + fuel) from an aerostat quickly.

The aerostat will rise. It will float higher in the atmosphere, with decreased pressure around it. It will expand. It will then rise still further.

And there's no ready supply of solid or liquid ballast (as would be available on a near-ground cargo drop) to compensate for the lost mass.

This is untenable for any manned / habitable module, and you'd all but certainly want any of same well outside the danger zone of a rocket malfunction.

One likely consequence is that any launch aerostats would be at best highly unstable in their altitude and station-keeping characteristics. It's quite possible that a disposable, single-use design might be required. Given that materials would likely have to be shipped from Earth, or possibly from near-Venus asteroids via space-mining, this considerably increases cost and complexity of any such missions.

Aerostats, as lighter-than-air craft, have vastly more-tightly constrained mass budgets than any water-based floating structures. Ignoring and/or waving that away is obtuse in the extreme. Particularly given the additional concerns and considerations of launch-capable structures. Existing aerostats and rockets operate at the outer limits of engineering design capabilities, and still go boom with some regularity, often due to exceeded structural tolerances.

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potato3732842 2 days ago | parent [-]

>The aerostat will rise. It will float higher in the atmosphere, with decreased pressure around it. It will expand. It will then rise still further.

Now explain weather balloons. Why don't they rise to infinite altitude?

Like I said, the numbers are all wonky, but the principals are the same.

If there is too little mass for the amount of bouncy just compress your gas and hold some reserve buoyancy/balloons to inflate if you expect to be able to deal with rapidly increasing mass.

	▲	dredmorbius 2 days ago \| parent [-]
		Weather balloons reach a stable equilibrium altitude because 1) they're designed to expand as they rise (at quite an impressive ratio) and 2) they're not suddenly gaining or losing 100s of tonnes of mass. At least one if not both those prereqs is missing from the observed case. Though discussing the matter further has lost virtually all appeal.