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What would, roughly, a scientific research station look like on a gas giant? Use Saturn as a baseline. The station would float because of a "balloon" carrying nothing. Vacuum is the lifting "gas", as the atmosphere of most gas giants is already hydrogen/helium.

The pressure at the average elevation of the station would be 1 bar, or just a tiny bit less than 1 atm. The average temperature would be around -139 C.

Power is generated by fusion of atmospheric hydrogen and helium, as Saturn is too far away from the sun for solar power to be meaningful.

The stations will be floating along in the wind, so they will experience a stiff breeze at most, because they aren't actually going that fast relative to the wind.

Materials come from regular shipments from drop bots delivered by mass drivers in the asteroid belt/moons. (I haven't decided yet)

Drop bots are large machines that are mainly cargo pods built to withstand atmospheric entry, with helicopter blades that extend when they slow down enough.

23rd century tech, use your best judgement.

Some ideas I had, might not be the most practical: Would it have a huge central sphere with the station built on a strip around its center? Would it be a number of small balloons around the edges of the base? I feel like that'd be sacrificing buoyancy for redundancy.

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    $\begingroup$ If you would explain the downvote I could fix the problem? $\endgroup$ – Desolationgame Apr 11 '16 at 22:30
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    $\begingroup$ A huge central sphere with the station built on a strip around its center sounds cool, but I think it would tip over easily. Better to go with a tried-and-true dirigible / blimp design, with the "station" underneath the "balloon". No way it would tip over then since all the weight is on the bottom. $\endgroup$ – BrettFromLA Apr 11 '16 at 22:36
  • $\begingroup$ I'm guessing the downvote is because the question is too specific (is option A or option B better?) and doesn't do a good job of explaining the overarching issue. Are you asking about the feasibility of having a vacuum chamber for buoyancy (it seems rather implausible to me)? Are you worried about balance and stability? $\endgroup$ – Rob Watts Apr 11 '16 at 22:36
  • $\begingroup$ @RobWatts I'm not asking about feasibility or anything, I'm asking how such a station would be designed. The two examples I gave weren't options, but just ideas. I'll edit the question to be more clear. $\endgroup$ – Desolationgame Apr 11 '16 at 22:40
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    $\begingroup$ The downvote is mine. Your question lacks basic details like "Where in Saturn's tropsosphere will the research station be situated?", "What kind of power sources are available and what do they run on?", "how should the station deal with the insane wind speeds on Saturn, if they need to deal with them at all?"...basically, there's no detail about the context of this station, its occupants, their roles or goals or the technologies available. $\endgroup$ – Green Apr 12 '16 at 12:52
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Maintain altitude using a heated gas envelope, much like a hot air balloon on Earth

While it's true that a vacuum is much less dense than atmospheric hydrogen at 1 bar, the pressure vessel/flight envelope required to maintain buoyancy adds unnecassary weight. The primary problem with a "lifting vacuum" is that there's 1 bar of pressure pushing inwards on the envelope but nothing pushing back out. Thus the envelope alone must withstand a uniform 1 bar of compression plus fluctuations in atmospheric pressure plus wind loads plus safety margin for unplanned ascents or descents. While doing this is probably possible, I believe there are easier ways.

Heat the Hydrogen

Instead of creating a hard vacuum, use a gas envelope to contain heated hydrogen. Getting heat is both easy and essential. Since fusion power is available, heated hydrogen is abundantly available. As opposed to a hard vacuum in the flight envelope, filling it with heated hydrogen does a couple of beneficial things.

  • It gives shape to the envelope as well as structural integrity.
  • It prevents ammonia ice build up. Just as water ice build up on airplanes causes significant problems, so would the build up of ammonia ice cause problems on the surface of the space station. If the envelope is heated above the melting point of ammonia ice then no ice can accumulate.
  • If the shape of the envelope is variable then inflating or underinflating parts of the envelope will allow the research station some degree of mobility.
  • An envelope that must only maintain tension loads can be made much much lighter than a structure dealing with compression loads. This is the same difference in "lightness" observed between suspension bridges vs older stone bridges. We want this envelope and the associated research station to be as light as possible.
  • In the event of reactor failure, the residual heat in the envelope will give the scientists/engineers on the research platform time to solve the problem before they lose buoyancy.

Envelope Constraints

The envelope should have the following characteristics:

  • Volume: Sufficient to maintain station buoyancy at any altitude between 0.1 bar and 10 bar. This gives an altitude range of just above the water ice clouds at the bottom of the troposphere all the way to the top of the troposphere. This should include a 50% safety margin on envelope volume. Being able to fly to 0.1 bar means that rendezvous with incoming supply ships can be done above the clouds and using visual flight rules instead of instruments. Being able to see where you're going is always preferable.
  • Abrasion resistance: Even thought the relative winds experienced by the station should be relatively small, ammonia ice and ammonium hydrosulfide ice may be abrasive. Over a long enough period this could cause the envelope to lose pressure and fail. While upwellings of water ice air are uncommon, the envelope should be able to handle exposure to water ice crystals as well.
  • Chemical resistance: I'm unfamiliar with the chemical properties of the three ices in Saturn's atmosphere but the envelope should be able to resist these effects as well.
  • Redundancy: Should the envelope fail causing an unplanned descent into Saturn's atmosphere, a back up envelope will be very handy to have. This should be inflatable in fairly rapid order.

Station Constraints

The station will be positioned at the bottom of the envelope for stability's sake. Aside from the envelope, the research station has the same concerns as a space station with the reduced requirements for cooling. However, there are a couple of things to mention.

  • Redundant Power Supplies: With this design, heat equals life, both for the envelope and for the human occupants. It is therefore essential that the fusion powerplants be online at all times.
  • Hydrogen Processing: These will need to run all the time, preferably with multiple units running at the same time. Sulfur can be discarded back into the atmosphere. Excess hydrogen can be stored in the envelope or low pressure tanks on the station. Nitrogen can be used to supplement the station's atmosphere.
  • Oxygen Processing: The station must be able to position itself low enough in the atmosphere to gather water ice. This provides infinite oxygen supplies. All other metals will need to be delivered from off-world.
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    $\begingroup$ Fun fact: the "surface gravity" in Saturn's troposphere is approximately 0.3g. $\endgroup$ – Green Apr 12 '16 at 17:46
  • $\begingroup$ Great answer! I wish I could mark two answers, but I'll prioritize yours for detail. $\endgroup$ – Desolationgame Apr 12 '16 at 17:49
  • $\begingroup$ I was basically starting to write this answer, but got bogged down in the (fun, actually) details of just how hard it would be to get a vacuum balloon to work. $\endgroup$ – Rob Watts Apr 12 '16 at 17:53
  • $\begingroup$ @RobWatts, I made it as far as "pressure vessels aren't buoyant at 1 bar." then quit. I appreciate the work you put in your answer. $\endgroup$ – Green Apr 12 '16 at 17:55
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If I were designing it, I would have a large rectangular block (think two or three Borg Cubes squeezed together) as the main body of the station. This block is supported by several "balloons" around the upper rim. The weight is below the buoyancy source, so it will be more or less stable. To add some extra stability, underneath the block have a large pool of water with a massive floating weight in it. This weight will act as a counter-balance; should the station start to tilt, the buoyancy of the water will cause it to move "up" the tilt, adding weight to bring the station back to level. Note that this will add lots of weight, so you will need extra balloons to compensate.

But if there's one thing you learn in engineering, it's that redundancy is better than failure.

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Let's look at some of the difficulties your gas giant research station will need to deal with. To start with, let's take a look at the idea of using a vacuum balloon.

1 bar is equal to 0.1MPa. Using the formulas supplied by an answer on physics.SE about pressure in a sphere, the formula for the required thickness of the sphere is $t = p r / (2 \sigma)$, where $\sigma$ is the compressive strength in MPa, $p$ is the pressure (0.1MPa), and $r$ is the radius of the sphere. The mass of the shell is roughly $t\times 4\pi r^2\times\rho$ where $\rho$ is the density of the material. Using 0.19kg/m^3 as the density of Saturn's atmosphere at that point, we get $0.19\times \frac{4}{3}\pi r^3$ kilograms displaced by the sphere.

Combining these formulas, we can get the mass of the shell as a function of the compressive strength and density of the shell material and the radius of the sphere - $0.4\pi r^3\rho /(2\sigma)$. Now let's look at the ratio of the mass of the sphere to the mass of the atmosphere displaced - $\frac{0.4\pi r^3\rho/(2\sigma)}{0.19\frac{4}{3}\pi r^3}\approx \frac{3\rho}{4\sigma}$. Fortunately for us the formula has managed to simplify quite nicely. In order to float, this ratio needs to be less than one - it needs to displace more mass than it weighs. This could be reached by a hypothetical material with a compressive strength of 1MPa and a density of 1kg/m^3.

Unfortunately, this isn't good news for the vacuum balloons. Steel has a compressive strength somewhere around 300MPa, but has a density of about 8000kg/m^3. That leaves the ratio at about 20, nowhere near being able to support even itself much less a research station. The shell could be thinner by using internal supports, but even then you wouldn't be able to get an over 95% reduction in the total amount of steel being used.

Also, you'll notice that all radius contributions cancelled out in the final equation. This means that making the sphere larger or smaller makes no difference.

A hot-air balloon really is the way to go. Unlike needing something that will resist compression, like steel, you can use a more flexible material with a high tensile strength, like kevlar or graphene. Graphene has a ridiculously high tensile strength, but by itself it's not airtight. It's quite reasonable that by the 23rd century we will have developed either a graphene-like compound or something that we can coat graphene with to have a strong, lightweight airtight material.

From there, just follow @Green's answer. It's pretty much what I was going to say.

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If it was on Saturn Rockets, blimps, spaceships, hell even one of Davinci's flying machines. Since the air is mostly comprised of Hydrogen the air wouldn't combust, but it all depends on what you would like specifically, and what type of future you are thinking of (steam punk, cyber punk, etc).

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