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I would like to have a zero-g spaceship factory in orbit, and would like to know if this is a viable way to work:

I imagine melting metal into a big blob, then inserting a tube and pumping in gas to inflate it. Perhaps external struts and molds can push and pull on the form to shape it as it expands and cools.

Some questions.. would this actually work and be controllable? how long would it take for the metal to cool? How could a quenching technique to rapidly cool the metal be implemented? A good answer would provide details on how this whole forging process could work.

Assume a near-future level of technology.

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    $\begingroup$ "How this whole forging process could work": what forging process? The question does not describe a forging process, it describes glass blowing. Metals do not really behave like glass. (And why would you want the metal to cool "rapidly"? Most of the time this is undesirable.) (And why do you believe that making the outer shell of a ship is the difficult part? It isn't.) $\endgroup$ Commented Jul 11, 2020 at 14:00
  • $\begingroup$ Probably not. What you're describing is essentially the same as glassblowing, but glass is a viscous liquid, which allows control & shaping. Most (perhaps all) metals have an abrupt transition between solid and fully liquid states. $\endgroup$ Commented Jul 11, 2020 at 15:30
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    $\begingroup$ This has been proposed as a method of rapidly turning asteroids (which tend to have lots of cracks in them) into air-tight vessels in which to build habitats. $\endgroup$ Commented Jul 11, 2020 at 17:11
  • $\begingroup$ @alexp rapid cooling makes the metal harder, andeless ductile. I'm assuming this would be desirable, do you disagree? $\endgroup$ Commented Jul 11, 2020 at 18:42
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    $\begingroup$ Why would it be desirable to make the metal brittle? Most definitely you don't want to do that for any structural elements. For blades and such, maybe, sometimes. Look up heat treatment for a taste of how complicated this whole issue is. $\endgroup$ Commented Jul 11, 2020 at 19:03

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Vacuum deposition

This is ideal for space. Micro-gravity will ensure an even deposition and space is already a vacuum!

You would use a carefully crafted spherical mould of light but rigid materials (it doesn't have to stand up to gravity and you could even use an actual inflated balloon as the mould).

Use the process from the inside of the mould. The slow atomic/molecular deposition will make the resulting sphere incredibly pure, strong, and the thickness will be exact to nano-meters. You can even build up layers of different metals for extra strength. Hull openings are simple: they are just built into the mould - no drilling or cutting.

Vacuum deposition (or vacuum evaporation) is a PVD process in which the atoms or molecules from a thermal vaporization source reach the substrate without collisions with residual gas molecules in the deposition chamber.

https://www.sciencedirect.com/topics/chemical-engineering/vacuum-deposition

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  • $\begingroup$ You can't use an inflated balloon as a mold for vacuum deposition because as soon as you inflate it, there isn't a vacuum inside... $\endgroup$ Commented Jul 12, 2020 at 2:55
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    $\begingroup$ @Cadence: There is plenty of vacuum on the outside, though. You would apply an electric charge to your balloon, and the opposite charge to the material you intend to deposit. $\endgroup$ Commented Jul 12, 2020 at 5:06
  • $\begingroup$ The one thing which I really liked about inflating was that it can easily explain slightly wobbly shapes. This works very well for my style. I assume vacuum deposition is ultra precise to the shape of the mould. Any idea how I could introduce more of a dirty, homemade feel to it? $\endgroup$ Commented Jul 12, 2020 at 14:28
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    $\begingroup$ If people buy balloon moulds and use them as suggested by @jamesqf, then deflate and store them away, they will start to get baggy in certain areas. They will also get fold lines and various stains etc from being stored flat in someone's locker together with their trainers and gym kit etc. $\endgroup$ Commented Jul 12, 2020 at 14:49
  • $\begingroup$ What is the benefit of this over just inflating the metal in a mold? Also you can't use an inflated balloon as a mold unless it is rigid, otherwise the pressure holding the balloon open will prevent deposition. $\endgroup$ Commented Jul 12, 2020 at 15:51
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While, as others have already pointed out, using glass-blowing techniques on steel is impractical to say the least (not to mention the fact that it'll radiate nearly 300 kilowatts per square meter in infrared radiation when molten) and that bulk metallic glasses are somewhat of an exotic medium.

However, that's not to say that there is no application for inflatable metals, may I introduce you to the main intake fan blades of the Rolls Royce Trent 1000 jet engine. I'll let Rolls Royce explain...

Rolls Royce hollow blade cross-section

Rolls Royce hollow blade process

It's a kind of middle ground between blow molding and stamping, titanium panels come in as a stack of flat sheets with a special bonding pattern which are them inflated like a balloon inside a mold. Probably as close as you're realistically going to get to an inflatable metal ship.

You could make the outer shell of your space ship really strong by using large pre-fab panels which both frees up the inside (less internal bracing needed) and as a bonus, the finished structure closely resembles a whipple shield which gets you some extra protection from asteroids and hyper-velocity balistic projectiles.

whipple shield

Whipple shields work by having multiple layers to break up and spread out impactors. After all, a cloud of sand is easier to stop than a bullet with the same total mass and speed.

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Your ships are made of metallic glass.

https://en.wikipedia.org/wiki/Amorphous_metal

An amorphous metal (also known as metallic glass or glassy metal) is a solid metallic material, usually an alloy, with disordered atomic-scale structure. Most metals are crystalline in their solid state, which means they have a highly ordered arrangement of atoms. Amorphous metals are non-crystalline, and have a glass-like structure

These alloys are unusual hybrids, with some properties of metal and some properties of glass. You can blow mold metallic glass as though it were plastic.

Thermoplastic blow molding of metals

Blow molding of bulk metallic glass Even though fast cooling and forming are decoupled during TPF of BMGs, thin sections with a high aspect ratio remain challenging to create when using techniques where the BMG is in physical contact with the mold. This is due to stick conditions between the BMG and the mold and the resulting parabolic flow patterns16. In order to eliminate such stick conditions, physical contact between the BMG and the mold must be avoided, at least while significant tangential strain is generated. We will show that this can be achieved by TPF-based blow molding.

image

There is a lot of seriously non-fictional materials science behind this. I could imagine there might be reasons you want a thin bubble of conductive metallic glass for your spacecraft. Less mass to move, for one. It might be possible to imbue a charge to this thin metal skin which will deflect charged cosmic rays. Or have a series of these bubbles one within the next Russian doll style, with charged vacuum between.

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  • $\begingroup$ I'd prefer to just work with simple materials instead of exotics, but it's an interesting idea! $\endgroup$ Commented Jul 11, 2020 at 18:40
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Your biggest challenge would be keeping it warm, not cooling it down. I'm assuming that you are seeking large bubbles. Think about how fast you would have to blow to get it to size in a minute or two.

What you might actually want to do is inflated it with a low pressure high temperature has to keep it warm while inflating.

The biggest challenge would be that there is a natural instability in blowing. Once a bubble gets thin, it wants to stretch more, making it thinner. Glass blowers have to bring a lot of skill to the table to keep things in check. They use a lot of gravity as a tool. You would need a new tool

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  • $\begingroup$ Maybe some kind of spinning? $\endgroup$ Commented Jul 12, 2020 at 14:21
  • $\begingroup$ Use a mold, thinning will be far less of an issue. $\endgroup$ Commented Jul 12, 2020 at 15:52
  • $\begingroup$ @John How would that help? $\endgroup$ Commented Jul 12, 2020 at 17:00
  • $\begingroup$ the material will stop thinning once it meets a surface, that is why glassblowers use molds for certain shapes. $\endgroup$ Commented Jul 13, 2020 at 2:34
  • $\begingroup$ @John ahh, so as long as the material hasn't thinned too much by the time it hits the mold, it will help? That's a good technique to keep in one's pocket when blowing interstellar glass bubbles! $\endgroup$ Commented Jul 13, 2020 at 4:44
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You need a mold, and rubble may be the answer. No matter what you do you need a mold of some kind,otherwise with a hard vacuum on the outside you will never get a regular shape. The best idea would be to use something that you will leave on it, as it will be nearly impossible to remove a mold from metal deposited in a vacuum without damaging it. Ideally something that will act as a radiation shield since a thin layer of metal will not be very a good radiation shield.

Take a hollowed out asteroid, which are naturally full of cracks and micro-holes and inflate a balloon of molten metal on the inside let pressure differential drive metal into all those openings and seal the inside. Rock makes for a very good radiation shield.

Alternatively you don't even have to start with a whole asteroid, you can fill a net/mesh shell or a second balloon with stony rubble (mining leftovers) and use the metal balloon on the inside to seal and bind it into a solid. This also means you are not limited to spheres as you can get any shape you can build the net in. basically you are using the rubble like a mold to blow the balloon up into.

You will need two things you may not have considered, a very hot gas to fill it to keep it hot as it expands as the gas will cool drastically as it expands, and computer controlled pressure to regulate the internal pressure as it inflates, you need to be able to back off the pressure when you reach the desired shape.

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  • $\begingroup$ Thanks, but thats not really what I'm asking about $\endgroup$ Commented Jul 12, 2020 at 14:21
  • $\begingroup$ @Innovine I think it is I am just not explaining it well, I will try more detail. $\endgroup$ Commented Jul 12, 2020 at 15:41
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This is a variation of a technique Dandrige M Cole thought about in the 1960's, although he was speaking in terms of heating an entire asteroid and "blowing" the molten asteroid into a hollow bubble as a colony.

The problem with "blowing" is that there is very little means of controlling the process. Glassblowers use centrifigal force and gravity to control the size, shape and thickness of the end product, a zero G bubble has a very distinct possibility of bursting and tumbling out of control.

Many of the other posters have the right idea, use the heat energy to use vapour deposition on a form to make a seamless pressure vessel or hull.

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