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My AEROSTAT CITY is under siege by space pirates!

Picture Dubai on air pillows. I imagine a chain of a few dozen aerostat "city blocks" linked together like a pontoon bridge to form a long flexible wing, each are an industrial "pillow" with luxury residential towers on top. During the attack they will break apart or deliberately separate into disconnected islands. See my placeholder art, but (like an iceberg) there is more volume beneath the cloud deck (and thanks to Worldbuilding, I now know the towers will not poke out of the clouds like this).

There's no chance the pirates will try to occupy the city, there are not enough of them and they aren't well organized. Their strategy is to "shake the tree" until the wealthy and powerful evacuate in private yachts. The pirates then pick and choose which targets they will follow into deep space where they will be isolated, easy pickings. They might keep this up for weeks or months before a naval force arrives from another system.

They will be aided by colluders from within the city performing acts of sabotage, so the strategy is to overwhelm the city's rescue sevices and induce a panic. Normally pirates would not be this bold, and the city is slow to understand the strategy.

enter image description here (my placeholder artwork)

Since the city is an aerostat it's no fortress. It can rise and lower in the atmosphere and surf the winds to maneuver. It has a squadron of jet fighters and short-range energy weapons. It's defense relies on gunships patrolling in orbit, so the pirates are staying out of range.

Can the pirates use torpedoes and depth charges to attack the city?

Here's my strategy:

  1. Pirate ships draw away the gunships with distraction attacks.
  2. Torpedo ships sneak into the planet's lower atmosphere.
  3. They launch torpedoes laterally under the city.
  4. Torpedoes disperse a swarm of (barrel bomb? chemical?) explosives that rise like reverse depth charges.

What I don't know:

  • How does traveling lower in the atmosphere effect the torpedo boats? Can I use common submarine tropes and say the boats are designed for deep atmosphere? I am assuming the city must eventually find and destroy the boats to stop the attack, but I don't know how far I can carry a submarine metaphor. Can a deep atmosphere torpedo boat "hide" from the city above – maybe in the shadow of the explosive swarm?
  • What are the explosives and how nearby do the explosions need to be? I am assuming they can't simply detonate an atomic bomb deep under the aerostat – or can they?
  • The pirates want wealthy targets who are fleeing the city with their valuables intact and radiation free. They are not trying to destroy the city outright, but they don't necessarily care about the welfare of any refugees – however for the sake of pirate morale, how might the attack be modulated to maximize panic and confusion, without simply nuking everyone or sinking the city?
  • I'm looking for a "death by tiny cuts" strategy that would also make economical sense for pirates (no exotic weapons, no suicide missions, just ad hoc ships being sneaky).
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  • $\begingroup$ Is the idea that the atmosphere is really thick? That it has a really high pressure that your people can somehow survive? So everything is floating in this thick atmosphere? Otherwise the depth charge analogy doesn't make much sense. $\endgroup$ – Schwern Mar 24 '17 at 23:01
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The atmosphere of Venus is a hellscape. Forget the temperature and pressure, the whole thing is intensely acidic (sulphuric acid, if I remember correctly). Your pirate's deep atmosphere ships (while possible) will need to be made of some seriously tough material and have engines that can deal with the wind.

Oh, yeah, the wind. It's like a hurricane. A really strong hurricane. All the time. The heat is enough to drive some seriously serious weather. Which raises an issue for your plan:

'Dropping' your depth charges will require you to have pretty good knowledge of the weather conditions between your ship and the aerostat. Getting that information may (and I say may because I'm not a Venusian meteorologist) be incredibly difficult, and accurately plotting a 'drop' course will be hard, if not downright impossible.

However this gives us an option for an alternate avenue of attack. If the aerostat is anchored then it will be designed to stand up to those winds on a pretty much constant basis. If it isn't anchored then it will be designed to stand up to some insane wind shear anyway, since it won't be large enough to streamline properly. What it probably won't be designed to stand up to (since they're pretty rare in the high atmosphere) is a series of hard objects being hurled through the air by 100 m/s winds. If your pirates can fill the air on the windward side of the city with such projectiles they can cause quite a lot of damage. If we make the projectiles the right shape then they can get up to truly horrendous amounts of kinetic energy.

So we launch the depth charges some way off the city and use a small amount of explosive to distribute the projectiles rather than cause direct damage. We then let the wind take the projectiles to pierce the city.

Ah, yes. I said pierce. That's because my ideal projectile would essentially be a polymer coated, sharpened tube with a parachute (or soft plastic bell) attached to the sharp end. This would give your pirates a hail of hollow harpoons peppering the city (with the added benefit of it being a scattershot attack, hitting all the segments of city and any nearby defenders).

Remember that sulphuric acid atmosphere? Imagine that slowly flooding your home. Wouldn't you want to get away from that?

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  • $\begingroup$ Sprinkling the windward sky with tiny projectiles is a wonderful improvement! Rather than a slow submarine "cat and mouse" the pirates would send small fast ships flying ahead of the aerostat, and it lends an excuse to use the city's jets and electro weapons to try to shoot them down. It definitely seems more thrilling while keeping my pirates at a distance, and even using cheaper weapons.... Thank you! Nice way to turn the problem on it's side. $\endgroup$ – wetcircuit Mar 25 '17 at 17:15
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    $\begingroup$ @wetcircuit: Literally! $\endgroup$ – Joe Bloggs Mar 26 '17 at 8:33
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One problem with the "depth charge" analogy is that depth charges rely on a property of liquid water not shared by a gaseous atmosphere: water cannot be compressed. They use this to greatly amplify their range of effect.

Second problem is modern ground-based skyscrapers are already engineered to withstand shocks, and buildings on floating platforms in a thick atmosphere would have to be be engineered against even stronger shocks by their very nature.


Shock Wave In Water

Unlike a gaseous atmosphere, the density of water does not change with depth or pressure. It's always going to be about 1000 kg/m3. It's incompressable. The increased pressure with depth comes from your vessel having to hold up an increasingly large column of water above it.

When a depth charge goes off underwater, the volume of the formerly solid bomb is rapidly turned into very high pressure gas. This expands against the surrounding pressure of the water. Since water is incompressible, the expanding gas shoves it out of the way in an expanding shock wave until the pressure of the gas equals the pressure of the water, then water rushes back in creating another shock wave. This goes on until the pressure equalizes, or the bubble reaches the surface.

The first shock is damaging, but the following cycle of inward and outward shocks can break a submarine's back as it is first bent one way, then the other.

The effect is devastating, but because the pressure underwater is so high, and water is so dense compared to air, the range is rather short. 100 kg of TNT needs to be within 3 to 10 meters to disable a submarine.


Shock Wave In Air

A shock wave in air is different. Air can be compressed. Again, the solid bomb becomes a ball of gas expanding at supersonic speed pushing against the surrounding air. The air cannot get out of the way fast enough, so it compresses into a pressure wave in a sphere around the explosion. The explosion is like a plow going through the earth building up a bigger and bigger pile of dirt in front of it.

That pile of air, the pressure wave, smacks into things. Since it's so much less dense than water the wave can travel further, but it has a much diminished effect from a depth charge.

There's also no cyclical rebound effect to cause the flexing that is so damaging to ships. Instead, as the explosion expands there will be a brief low pressure zone in the center and you'll get a second, much weaker, pressure wave going back towards the center of the explosion.

You can see this in old atomic bomb test footage. First the target starts to smolder, that's the light and heat hitting it first. Then the shock wave hits and typically blows everything apart, followed by a second rush a air backwards.


Shock Waves Vs Buildings

If your buildings are engineered to withstand the rigors of flying through a turbulent atmosphere, they're already built to withstand shock waves.

Your buildings have to withstand the shifting of their bases as they float and maneuver through the atmosphere. This would be very similar to modern earthquake engineering designed so the building will dampen the effect of the ground shifting under it. The shock of a bomb going off well under the city would have a similar effect and the building would be designed to weather it.

Tall buildings also have to be engineered to withstand the wind pushing against their sides, the lateral wind load. A flat slab skyscraper is basically a big sail. Too rigid and it will snap. Too flexible, and it can oscillate; if the wind gusts just right, it will tear itself apart as what happened in the Tacoma Narrows Bridge.

Modern skyscrapers are engineered to be flexible, but also have buffers to prevent oscillation. They're also engineered to be aerodynamic to cut through the wind.

All these engineering necessities of a ground-based skyscraper are even more necessary for a skyscraper on a moving platform in a thick atmosphere. They'd be greatly buffed up and even more able to withstand the shock wave from bombs bursting nearby.


How To Induce Panic In An Aerostat

Since this city is a lighter-than-air vehicle, there's two possibilities: the atmosphere is so thick the city is naturally buoyant, or the city requires vast chambers of low pressure to be buoyant.

If it's the former, a very dense atmosphere, the residents presumably cannot survive in the atmosphere. The buildings are presumably sealed. All the pirates have to do is break that seal, either from outside or inside. Some will run to sheltered internal areas, but others will make a break for it. Probably the richer residents with private vehicles and somewhere else to go.

If it's the latter, the city is held up by buoyant chambers, then they have to threaten to puncture those chambers; to sink the city. They don't have to actually do it, just cause enough damage for the richer residents to make a break for it in private vehicles. Again, this could happen from the inside or the outside.

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  • $\begingroup$ I was afraid of this, a conventional explosion in air is just a loud bang… While I don't want to destroy the buildings I want to force the city to break apart into sections. Disrupting the "air bag" containment with maybe something more high-tech, magnetic mines or "implosion" bombs might be more exotic anyway... $\endgroup$ – wetcircuit Mar 25 '17 at 17:00
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When predators attack a group of large prey animals (like wildebeest) they try to separate a smaller or weaker individual from the herd. I propose this strategy would be good for the pirates too.

/ During the attack they will break apart or deliberately separate into disconnected islands. /

Once this happens the pirates attach hooks to a smaller city component, cut whatever moorings it has and make off with the entire thing. One little piece of city will exhaust its defensive capabilities pretty quick. They just hide it in the clouds elsewhere on the planet and hold it and its inhabitants for ransom. This is sort of like what the Somali pirates do now.

I can envision a very cool story where something like this happens and then a twist: within this stolen citylet is something unexpected. The pirates wind up with more than they bargained for.

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  • $\begingroup$ :D was thinking along similar lines, but my sympathy sorta leaning towards the pirates so was going to have the super luxury towers be top-heavy liabilities…. I love stealing a whole neighborhood and dragging it to pirate land! $\endgroup$ – wetcircuit Mar 25 '17 at 15:47
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    $\begingroup$ Thinking about this scenario - I think a city under attack would not break apart on purpose and no-one would leave. Basically they are under siege. The break apart would have to be the doing of the inside saboteur. $\endgroup$ – Willk Mar 26 '17 at 0:56
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If pirates can afford some sort of craft that can run for long periods of time deep within the atmosphere of Venus or a gas giant, why wouldn't the colony be built there themselves? The biggest hurtle for both is purely the durability of the structure in extreme gravity. Likewise, the people in the ships and city would be subject to those extreme forces unless you have some sort of anti-gravity technology (which would be even more difficult to produce than artificial gravity). The most effective strategy would likely be to SLOWLY sink the city so that the inhabitants evacuate to avoid the extreme pressures below. It's almost like Venice (on Venus) sinking into the ocean. Eventually the structure will flood (or compress) and the previous inhabitants have to leave. Queue pirates waiting in ambush.

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  • $\begingroup$ The gravity on Venus is near Earth's, so no need for artificial gravity. The pressure at the aerostat is around 1 bar and the temperature is like Miami, this is the whole point of a Venusian aerostat. The pressure and temperature at lower depths is much higher, and there are different economies of maintaining a small DSRV craft vs an entire city of luxury apartment buildings in that environment. $\endgroup$ – wetcircuit Mar 24 '17 at 21:15

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