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 3 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.