# Primitive spacecraft would benefit from being large



## Onyx (Jun 9, 2018)

I was just reading up on the radiation problem in space, and any sort of passive shielding from cosmic rays works largely on thickness. But once you achieve the desired thickness of shielding, you're golden. 

Let's say you want to use 2m of water to protect the crew. On a smallish capsule, adding a 2 meter thick jacket of water would quadruple the volume of the ship and make the vast majority of the mass of the life support module water - which is extremely heavy:

A capsule that is 6m in diameter has a volume of 113 cubic meters. Put a 2m jacket of water around it and you've increased the volume to 524 cubic meters. Assuming the internal volume weighed nothing, your crew capsule density would weight about 78% that of a solid ball of water.

But if your ship is quite large, the same 2m of water gives exactly the same amount of shielding but the internal volume it protects is relatively enormous. with a 20m diameter internal volume, your density is 42% that of water including the shielding. At 100m your the weight is down to only 11% that of water, and if you can make a 1km sphere of a ship, the water jacket is only 1.1% of your total weight.

So while it doesn't make sense to carry enough fuel to push a ball of mostly water to Jupiter, building a very large ship to send a whole bunch of people and stuff starts to become much more efficient because the shielding drops to such a small amount of the total mass.


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## schonovic (Jun 27, 2018)

I don't know so much about water for protection. battlestar galactica used armor as well as star wars. Not to say armor is that great either. Armor can take 60 gigaojoules/m^3. Battlestar Galactica armor had approximately 1.6 Km x .39Km x .01Km of armor plate when completely covered. This is about 624,000m^3. This could take 3.744x10^17 joules before evaporation,probably less.
A50 kiloton nuke 100m away will put out (50/10000)x2.6^27=1.3x10^25 joules in hard x-rays which will vaporize 38 galacticas in a perfect universe but realistically about 6 since they don't huddle.
Star Wars on the otherwise has a form of energy deflection shield but not mass deflectors therefore had to use disintegrators to go through an asteroid field. That same 50 megaton nuke will put out a neutron fluency of 9x10^20 neutrons. 624,000 star destroyer crew members would receive a lethal dose of neutrons. So I'm wondering if it would be safe to drink the water after a nuk strike. And these are nukes we've been able to make for over 50 years. Just wondering.


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## Brian G Turner (Jun 27, 2018)

Onyx said:


> I was just reading up on the radiation problem in space, and any sort of passive shielding from cosmic rays works largely on thickness.



Something I've thought about is using electromagnetic shielding. If you're generating enough energy to power then it shouldn't be too hard to distribute some of this for shielding. Actually, using a conductive fluid such as water as a medium for this may work, but I suspect there may be more effective transmission methods.


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## -K2- (Jun 27, 2018)

Brian G Turner said:


> Something I've thought about is using electromagnetic shielding.



Think "magnetic bottle" (a real world thing) now reverse it, same-same as what you're suggesting I believe.



schonovic said:


> So I'm wondering if it would be safe to drink the water after a nuk strike. And these are nukes we've been able to make for over 50 years. Just wondering.



If you are speaking of his suggestion of using water as shielding, know that when something is irradiated (given that doesn't inspire a self perpetuating reaction of the media), it does *not* become radioactive.  The radiation may damage it, it might even alter it.  However, it will not then become radioactive itself.  Something only becomes radioactive when the radioactive material contaminates it... Even still, it is then in and of itself not radioactive.  It is simply that it now carries the radioactive material on or within it.



Onyx said:


> ......So while it doesn't make sense to carry enough fuel to push a ball of mostly water to Jupiter, building a very large ship to send a whole bunch of people and stuff starts to become much more efficient because the shielding drops to such a small amount of the total mass.



There is no reason why you couldn't use water as a _'supplemental'_ shielding, however water in and of itself is by a vast margin not a very good insulator regarding radiation, and then there are the issues of temperature extremes of which water is a very good conductor of (so expansion and contraction become an issue).

That said, I see the point you're getting at, yet keep in mind the more mass you're trying to get moving and then stop becomes an issue.  You might save yourself on shielding to _some_ degree (of whatever type you choose), but then you have to consider that it will require more energy/fuel to get the craft moving and then stop it.  So, you now have to accommodate for more fuel, it then gets larger... more fuel... larger again... etc..

Also keep in mind that a sphere increases in surface area rather dramatically as it increases in diameter.  That surface area being what you need to shield (based upon your suggestion that you just fill it), then increases significantly compared to what it can contain.  As an example:

A 200' diameter sphere has a surface area of: r = 100 ft, V = 4188790 ft3, A = 125664 ft2, C = 628.319 ft
A 220' diameter sphere has a surface area of: r = 110 ft, V = 5575280 ft3, A = 152053 ft2, C = 691.15 ft

That minor 10% 20' diameter (10' radius) changes our surface area by  26,389 square feet.  That's a massive change.  Keep in mind, though your volume increases even more dramatically, you have to support the structure just that much more (once again adding more mass/weight).  One that might behoove you into considering having shielded living areas, and other areas (such as controls) that might be radiation sensitive, and then not worrying about the rest.

K2


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## Onyx (Jun 28, 2018)

-K2- said:


> There is no reason why you couldn't use water as a _'supplemental'_ shielding, however water in and of itself is by a vast margin not a very good insulator regarding radiation, and then there are the issues of temperature extremes of which water is a very good conductor of (so expansion and contraction become an issue).


Not sure what you mean. The best shielding for cosmic rays are substances with large amounts of hydrogen, like polyethylene and water. Water is also necessary for life support, easy to liquify and harden and widely available from sources in space. We use it in nuclear reactors and rod storage pools, and it is so effective divers go into those pools.

Are you talking about a different sort of radiation? Is there another substance that is much better aside from liquid hydrogen for cosmic rays? Using metals just increases the secondary radiation from nuclear fission in larger atoms.



-K2- said:


> Also keep in mind that a sphere increases in surface area rather dramatically as it increases in diameter.


That's exactly what all my examples were to illustrate - the internal volume increases dramatically compared to the 2m thick surface jacket of water because volume is cubic and surface area is a square.


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## -K2- (Jun 28, 2018)

First off as to the size of your sphere, I guess I didn't make my point clearly.  Your suggestion is "larger is better."  I'm saying that besides the surface area increasing dramatically due to using a sphere, the internal area increases even more so.  Now I get what you want to have happen, but what I'm saying is that then you need to support that significantly larger area as well.

Sure, build it in space and the supporting structure is not so much of an issue.  Where it becomes an issue is when you go to move the thing.  That supporting structure (for floors, walls and so on), still has to be moved as well.  That means they have to be strong enough to endure the forces of stopping and starting.  Perhaps even more than that, the sphere has to remain pressurized.  So, that internal structure also helps pull the outer skin all together so it doesn't blow out.

So, as you increase the surface area (adding weight), then have to support the internal structure which now encompasses much more area, that adds more weight.  More weight equals more fuel to stop and start it, and therefore you need more space (and containment for the fuel...) etc. etc..

That said "cosmic rays" as a generic term doesn't cover the number of different types of radiation that are encountered.  Add to that heat (and though the same cold), minor impacts (remember, if you poke a pinhole into that outer-skin containing the water, it will then vent working like an engine), etc..

In any case, "I" am not an expert... Yet, cooling ponds are exactly that, not absolute shielding.  But, I'd suggest not taking my word for it, and checking instead with the experts.  Here are just a few links of thousands that might be of interest to you, one even mentions 'water shielding':

How to Protect Astronauts from Space Radiation on Mars

http://www.dartmouth.edu/~sshepherd/research/Shielding/docs/Parker.pdf

Spaceflight Radiation Health Program

Space Radiation Analysis Group - NASA, JSC

K2


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## Onyx (Jun 28, 2018)

I'm not sure I understand your objections.

The gross increase in volume for a modest increase in surface area (as measured by the mass of the 2m thick water shield) was the whole point of the OP. You get a substantial return in volume for a minor investment in surface water shield.

I don't see how a large sphere is a much greater engineering problem then a small one. We can boxes here on earth that are a single story tall or hundreds. A space ship that we could build anytime soon would have a maximum thrust well below 1 g, so the internal structure of the large ship doesn't need to be particularly strong.

The water jacket could be liquid, ice, or layers of both. It could even be all ice. Both of which provide decent impact resistance to micrometeorites, can be utilized for heat handling issues along with a system that taps off energy through temp differentials and provides a supply of crew water and nuclear drive fuel.


The main point of the OP is that if you want to send ships to Mars, Jupiter, etc, it would be much more efficient in terms of propellant mass to build one large ship with a crew and supplies for longer, more involved missions than sending several small crews in small ships that have a huge percentage of mass tied up in shielding. Relatively speaking, your greatest mass expenses are shielding and fuel, so decreasing the net amount of shielding per crew member by using a larger ship and crew means less fuel per crew member as well and the whole thing becomes easier to.


So instead of building a poorly shielded series of primitive rockets, we should build an almost as primitive but huge Orion and equip it for years of use.


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## -K2- (Jun 28, 2018)

I understand your points.

K2


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## -K2- (Jun 28, 2018)

JFTR, using the two spheres I listed which are not very big (60.96m & 67.056m), using your 2m thick water shield, that comes out to this:

r = 30.48 m O.D. water, (A = 11674.5 m2 x 1000 kg) x 2 m = 23,349,000 kg water
r = 33.528 m O.D. water, (A = 14126.2 m2 x 1000 kg) x 2 m = 28,252,400 kg water

Though I haven't looked, find the calculated amount of fuel to stop and start the weight of each (and remember, that is just the water, not the weight of the ship to contain it).

HTH!

K2


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## Onyx (Jun 28, 2018)

-K2- said:


> JFTR, using the two spheres I listed which are not very big (60.96m & 67.056m), using your 2m thick water shield, that comes out to this:
> 
> r = 30.48 m O.D. water, (A = 11674.5 m2 x 1000 kg) x 2 m = 23,349,000 kg water
> r = 33.528 m O.D. water, (A = 14126.2 m2 x 1000 kg) x 2 m = 28,252,400 kg water
> ...


Sure. Now do the calculation for how much fuel it will take to get the 1728 5m inner diameter ships that would be required to move the same crew volume as one 60m sphere.

Comes out to 316,250 kg x 1728 = 546.480,000 kg water, or 23.4 times the one 60m sphere.


(BTW, your numbers are close by not actually right. You take the larger volume and subtract the smaller volume, then multiply by 1000. You can't just multiple by 2m. A 30.48m radius volume - 28.48 volume is 22,237,170 kg.)


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## Brian G Turner (Jun 28, 2018)

schonovic said:


> I'm wondering if it would be safe to drink the water



Actually, Onyx, using water as a shield could be a very clever solution to a number of issues, not least water supply for humans on board, even biomes, plus waste heat management.

That's if the physics holds up - figuring all that out sounds like a dedicated job for someone more dedicated than me.


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## Onyx (Jun 28, 2018)

Brian G Turner said:


> Actually, Onyx, using water as a shield could be a very clever solution to a number of issues, not least water supply for humans on board, even biomes, plus waste heat management.
> 
> That's if the physics holds up - figuring all that out sounds like a dedicated job for someone more dedicated than me.


Hey, if the peak acceleration is low enough, the actual structure of the vessel could be just reinforced ice. If there is water in the moon, you could throw it into orbit on free solar power and melt cast it with mirrors in between two mylar bubbles surrounding the completed core volume of the ship. 

Layers of ice vs liquid water could be isolated between mylar and vacuum to keep the ship's heat from melting the outer ice jacket, and use circulating water to produce electricity from waste heat using an ammonia condenser generator to help power the ship. The drive could be embedded in an extra thick section of reinforced ice at the back end, and the entire internal structure could be tensioned fabric and a carousel deck that is supported like a bicycle rim on tensioned cables running to a hub at the core pylon running from the front of the sphere to the drive. You'd end up with a hollow ice ball with a tent city on the inside.

NASA would hate it.


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## Brian G Turner (Jun 28, 2018)

Sounds like a good concept for a hard SF story. Would be interesting if that's part of what you're already doing.


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## chrispenycate (Jun 28, 2018)

I'm in agreement with Brian's first comment - use magnetic fields. Look at the Earth - the several kilometres of atmosphere do absorb a tiny part of the particle storm, but the bulk of the particle flux is diverted into the Van Allen radiation belts by the Earth's magnetic field. Jupiter's moon system, an excellent source of raw materials has an even worse particle storm.

Assuming we can keep temperatures low enough for superconductors current will flow forever, unless it's deviating charged particles. We have the choice of one very big field, like a bar magnet, or multiple smaller zones, concentrating the flux onto numerous independent targets.

But do we need to shield the entire ship, or just living quarters, delicate electronics and algae tanks? Lots of other things you might want to transport that don't mind too much about mild irradiation.

In my solar system series I weave the superconducting magnetic coils into skin-tight inner spacesuits so protection continues outside the hull - perhaps you wouldn't need the outer ones at all, just local. Unless you wanted to take a shower.


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## -K2- (Jun 28, 2018)

chrispenycate said:


> ...In my solar system series I weave the superconducting magnetic coils into skin-tight inner spacesuits so protection continues outside the hull - perhaps you wouldn't need the outer ones at all, just local. Unless you wanted to take a shower.



It always seemed reasonable to me that 'one day' those very sources of radiation that we need to shield 'ourselves' from, might very well be used to generate or at least supplement energy sources... perhaps even to power that shielding themselves.

K2


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## Onyx (Jun 28, 2018)

chrispenycate said:


> I'm in agreement with Brian's first comment - use magnetic fields. Look at the Earth - the several kilometres of atmosphere do absorb a tiny part of the particle storm, but the bulk of the particle flux is diverted into the Van Allen radiation belts by the Earth's magnetic field. Jupiter's moon system, an excellent source of raw materials has an even worse particle storm.
> 
> Assuming we can keep temperatures low enough for superconductors current will flow forever, unless it's deviating charged particles. We have the choice of one very big field, like a bar magnet, or multiple smaller zones, concentrating the flux onto numerous independent targets.
> 
> ...


Do we know how to build that kind magnetic field today, do we know how to operate electronics within such a field and does it take an enormous power source to build it?

I was just thinking about how we're going to get to Mars in the next decade or so using technology we have. Is this something we can do, or is duplicating the kind of field the earth's spinning core makes a little beyond us right now?


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## chrispenycate (Jun 28, 2018)

Can I have high temperature superconductors? I should be able to build the simple version  from my spares kit apart from the superconducting wire (well, not to scale, but yes, present day tech. Since it's superconductor I could probably power it from a condensor, and only recharge when solar wind or a flare damps out the current.

Mind, if you're using an ion drive, the shielding and the acceleration fields might be contradictory - with loudspeakers I'm only doing one thing with a magnetic field at a time and only using one geometry. But if you're going into Jupiter's moon system you'll need a decent size powerpack anyway (please can I not be restricted to solar? Even a fuel cell, or a fixed.base megawatt laser?) 

Need some experiments, it's bound to have bugs in it - still, the theory is not complicated, and won't disturb electronics - no violent pulses. Anyway, you'd screen all the sensitive gear - there's enough odd stuff in space you don't want getting into your control circuits. Grins.


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## Onyx (Jun 28, 2018)

chrispenycate said:


> Can I have high temperature superconductors? I should be able to build the simple version  from my spares kit apart from the superconducting wire (well, not to scale, but yes, present day tech. Since it's superconductor I could probably power it from a condensor, and only recharge when solar wind or a flare damps out the current.
> 
> Mind, if you're using an ion drive, the shielding and the acceleration fields might be contradictory - with loudspeakers I'm only doing one thing with a magnetic field at a time and only using one geometry. But if you're going into Jupiter's moon system you'll need a decent size powerpack anyway (please can I not be restricted to solar? Even a fuel cell, or a fixed.base megawatt laser?)
> 
> Need some experiments, it's bound to have bugs in it - still, the theory is not complicated, and won't disturb electronics - no violent pulses. Anyway, you'd screen all the sensitive gear - there's enough odd stuff in space you don't want getting into your control circuits. Grins.


This article implies that a pure system would take 10 GWs and you'd have to create a layered system so those inside the field don't get mega-MRI headaches, which no one is sure if that kind of field cancelation is do-able.

I think a truly advanced spacecraft would have magnetic shielding. I just don't know how long it will take to get right compared to more primitive methods.


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## chrispenycate (Jun 29, 2018)

Perhaps 10 GW, but it wouldn't have to be charged up all at once. Avoid wearing a mechanical watch while travelling round, but if you're hitting the same intensity as the Earth's magnetic fields - I have magnetised loudspeaker drivers several hundred times over that, and while I may have had screwdrivers leaping in the air, and cables jumping off the floor I've never had physiological effects - you need to go a couple of orders of magnitude higher before worrying the nervous system. Are you sure your field generator wasn't for a Buzzard ramjet?


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## Onyx (Jun 29, 2018)

chrispenycate said:


> Perhaps 10 GW, but it wouldn't have to be charged up all at once. Avoid wearing a mechanical watch while travelling round, but if you're hitting the same intensity as the Earth's magnetic fields - I have magnetised loudspeaker drivers several hundred times over that, and while I may have had screwdrivers leaping in the air, and cables jumping off the floor I've never had physiological effects - you need to go a couple of orders of magnitude higher before worrying the nervous system. Are you sure your field generator wasn't for a Buzzard ramjet?


"Bussard"? Actually, such an engine would provide protection because it gathers stuff away from the crew and toward the drive, and could be directed to protect against other angles.

But magnetic effects on people are real - they've been studied in MRI's, used in certain types of brain stimulation and have even permanently mitigated the effects of autism. The field required is much more intense than an MRI to get a proton moving at a large fraction of C to change direction in just a few meters. The only likely way that's going to work out is for there to be a cancellation field inside the ship, leaving the active field only outside, and that's a tricky proposition.


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## Brian G Turner (Jun 29, 2018)

So far as I understand it, the Earth's magnetic field is relatively weak and doesn't seem to have destroyed us humans - but does a great job of redirecting harmful charged-particle radiation. 

I wouldn't imagine an EM field for a starship having to be particularly powerful either - but I would also imagine a transfer of momentum from charged particles in the solar wind allowing the EM field to also effectively act like a sail.

However, I'm much more interested in the idea of a casing of ice for shielding.


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## SilentRoamer (Jun 29, 2018)

When we do start zipping around the Solar System we need to watch out for the biggest (and invisible) thing in the Solar System - Jupiters magnetic field! 

I like the idea of a hybridised version of this - low energy magneto response fields coupled with a H2O circulation and purification system - at the end of the day any sufficiently large ship (crewed) is going to need sufficient amounts of liquid water - they are necessarily in a closed environment where most water will be recycled, so why would you NOT use this in an effective and complimentary way of radiation shielding - it could even act as a physical failsafe in case of Ai/computer/mechanical failure. 

Just my thoughts on an interesting thread.


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## Onyx (Jun 29, 2018)

Brian G Turner said:


> So far as I understand it, the Earth's magnetic field is relatively weak and doesn't seem to have destroyed us humans - but does a great job of redirecting harmful charged-particle radiation.
> 
> I wouldn't imagine an EM field for a starship having to be particularly powerful either - but I would also imagine a transfer of momentum from charged particles in the solar wind allowing the EM field to also effectively act like a sail.
> 
> However, I'm much more interested in the idea of a casing of ice for shielding.


The earth has a huge, low intensity field. I don't think the proposals for ship fields have them extending 65,000 km toward the sun like the earth's field does. And the earth also has mechanical shielding from the atmosphere, making it a hybrid system. If you go a third of the way up into the atmosphere to 35,000 feet the radiation is 100 times what it is at sea level, and I don't think that's because of the magnetosphere but the atmosphere.

The magnetosphere also has two holes at the poles. Cosmic radiation hitting the upper atmosphere is what we call the Northern Lights. Transpolar airliner crews really get fried.


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## Brian G Turner (Jun 30, 2018)

Onyx said:


> I don't think the proposals for ship fields have them extending 65,000 km toward the sun like the earth's field does.



I didn't propose that a starship would need an EM field the size of a planet - simply that the principle might be useful.


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## Onyx (Jun 30, 2018)

Brian G Turner said:


> I didn't propose that a starship would need an EM field the size of a planet - simply that the principle might be useful.


I was just trying to illustrate how the earth's large, diffuse magnetic field is essentially different than a tiny, high intensity one on a spaceship. You can cut stone with a jack hammer or a river, but they do things on different scales and intensities.


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## Brian G Turner (Jul 3, 2018)

Onyx said:


> The water jacket could be liquid, ice, or layers of both. It could even be all ice.



Thinking about this, rather than build starships from scratch, wouldn't it potentially be more plausible to hollow out icy bodies already present in the solar system? I suspect launching mining equipment for that task would be far cheaper than launching all the materials required for building a superstructure.

I presume there must already be some SF that follows this??


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## SilentRoamer (Jul 3, 2018)

Brian G Turner said:


> Thinking about this, rather than build starships from scratch, wouldn't it potentially be more plausible to hollow out icy bodies already present in the solar system? I suspect launching mining equipment for that task would be far cheaper than launching all the materials required for building a superstructure.
> 
> I presume there must already be some SF that follows this??




In a lot of SF shipyards are normally outside of a gravity well, which makes total sense, you ship in/strip mine from much smaller gravity bodies than on a planet. I expect it always likely to be non trivial to escape a planetary gravity well. 

In fact once you have sufficient industry already present in space it would be much cheaper to do everything from space. It would also have other benefits like Zero G manufacturing.


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## Onyx (Jul 3, 2018)

Brian G Turner said:


> Thinking about this, rather than build starships from scratch, wouldn't it potentially be more plausible to hollow out icy bodies already present in the solar system? I suspect launching mining equipment for that task would be far cheaper than launching all the materials required for building a superstructure.
> 
> I presume there must already be some SF that follows this??


I'm not sure it would be:

1. You'd need some sort of ship to get to a comet to capture it and take it to wherever you want to built the ship. Since you don't have one safe for crews, you'd need to send a robot, and it would have to have a powerful drive to get to the comet, which means leaving earth orbit with lots of fuel (assuming it could use comet water as return fuel).

2. You'd have to have some sort of equipment in orbit to carefully hollow it out. A non-trivial engineering project, and not without dangers because the internal structure of the comet might not remain stable once you start to burn the inside out.

3. The final ship, if made from a natural comet, will be heavier than something molded from water, and less structurally sound.


By contrast, the moon is nearby, has lava tubes that would offer protection from cosmic rays and water. The moon has abundant sunlight to power a mass driver, and no atmosphere to interfere with launching things from ground level, so no propellant needed. So if there is trapped water on the moon, it would take less infrastructure to get to it and send it into earth orbit, the project could be one aspect of a larger moon project and you don't need to put any fuel in orbit to do so.

Once you throw the water mass off the moon to the protection of low earth orbit, mylar reflector arrays could use solar again to melt the water. The shape and location of the inflatable structures would be controlled by air pressure - which could be relatively low in a vacuum and still be effective. After that forming shells of water or ice comes down to temperature control . The finished product contains the minimum amount of water to provide everything needed and the strongest, most homogenous structural ice.

I guess I would contrast the difference between transporting a huge rock to then hollow out into a house, or making the house out of poured concrete.


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## Vertigo (Jul 3, 2018)

Onyx said:


> I'm not sure it would be:
> 
> 1. You'd need some sort of ship to get to a comet to capture it and take it to wherever you want to built the ship. Since you don't have one safe for crews, you'd need to send a robot, and it would have to have a powerful drive to get to the comet, which means leaving earth orbit with lots of fuel (assuming it could use comet water as return fuel).
> 
> ...


It has long been hypothesised but not yet confirmed that there may be considerable quantities of water shaded from the sun (which would over long periods of time tend to strip the hydrogen away for it to be subsequently lost to space) in craters at the poles.


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