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Building a celestial body Michael Mol 08-01-2007
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Posted by Warm Worm on August 1, 2007, 6:15 pm
>
>
>
>
>
> > Dear Michael Mol:
> > ...
> >> The question becomes, what would one want to build
> >> such a thing out of?
>
> > *Want* to build it out of wimmun.
> > *Have* build it out of unobtanium.
>
> >> I don't have a background in mechanical or
> >> architectural engineering, which is why I come to
> >> you folks. I imagine one would want concrete, as I
> >> picture the stresses forcing the shell to compress
> >> laterally across its surface. But how much concrete
> >> would one need?
>
> > Concrete won't work, not enough strength for a shell that will support a
> > human.
>
> >> How much water (A scarce resource in space,
> >> though Jupiter happens to have a large ice moon)
> >> would be needed?
>
> > Enough to drink.
>
> >> Another engineering-related line of thought I
> >> can't follow up on is the effect of tidal forces...
> >> Jupiter has several large moons. How much of
> >> an effect would they have on the shell?
>
> > Our Moon swings the Earth around a little circle, and there is a "three
> > meter high" lump that follows the Moon around. Jupiter is massive enough
> > that it swings the Sun around a point just outside the surface of the Sun.
> > You'd have real difficulties keeping the shell in place.
>
> >> Incidentally, this is part of a sci-fi world-building
> >> project I'm working on called Grokked Universe.
> >> Here's the relevant thread:
> >>http://guforum.shortcircuit.us/index.php?topic=38.0
>
> > Dyson sphere. Ringworld.
>
> Second reference to Dyson on the group in a month!

Alright Michael! High-five!


Posted by Michael Bulatovich on August 1, 2007, 6:56 pm

>>
>>
>>
>>
>>
>> > Dear Michael Mol:
>> > ...
>> >> The question becomes, what would one want to build
>> >> such a thing out of?
>>
>> > *Want* to build it out of wimmun.
>> > *Have* build it out of unobtanium.
>>
>> >> I don't have a background in mechanical or
>> >> architectural engineering, which is why I come to
>> >> you folks. I imagine one would want concrete, as I
>> >> picture the stresses forcing the shell to compress
>> >> laterally across its surface. But how much concrete
>> >> would one need?
>>
>> > Concrete won't work, not enough strength for a shell that will support
>> > a
>> > human.
>>
>> >> How much water (A scarce resource in space,
>> >> though Jupiter happens to have a large ice moon)
>> >> would be needed?
>>
>> > Enough to drink.
>>
>> >> Another engineering-related line of thought I
>> >> can't follow up on is the effect of tidal forces...
>> >> Jupiter has several large moons. How much of
>> >> an effect would they have on the shell?
>>
>> > Our Moon swings the Earth around a little circle, and there is a "three
>> > meter high" lump that follows the Moon around. Jupiter is massive
>> > enough
>> > that it swings the Sun around a point just outside the surface of the
>> > Sun.
>> > You'd have real difficulties keeping the shell in place.
>>
>> >> Incidentally, this is part of a sci-fi world-building
>> >> project I'm working on called Grokked Universe.
>> >> Here's the relevant thread:
>> >>http://guforum.shortcircuit.us/index.php?topic=38.0
>>
>> > Dyson sphere. Ringworld.
>>
>> Second reference to Dyson on the group in a month!
>
> Alright Michael! High-five!

Hi-5!

In both cases I seem to have made similar references to Dyson going steady
with Mary Jane...



Posted by Michael Mol on August 1, 2007, 2:47 pm

> > I don't have a background in mechanical or
> > architectural engineering, which is why I come to
> > you folks. I imagine one would want concrete, as I
> > picture the stresses forcing the shell to compress
> > laterally across its surface. But how much concrete
> > would one need?
>
> Concrete won't work, not enough strength for a shell that will
> support a human.

Wouldn't that depend on how thick the shell was?


> > Another engineering-related line of thought I
> > can't follow up on is the effect of tidal forces...
> > Jupiter has several large moons. How much of
> > an effect would they have on the shell?
>
> Our Moon swings the Earth around a little circle, and there is a
> "three meter high" lump that follows the Moon around. Jupiter is
> massive enough that it swings the Sun around a point just outside
> the surface of the Sun. You'd have real difficulties keeping the
> shell in place

I'm not worried about the shell staying in place. (See my other
comment about gravitational centering.) I'm more concerned about
stresses induced in the rigid structure of the shell.

>
> > Incidentally, this is part of a sci-fi world-building
> > project I'm working on called Grokked Universe.
> > Here's the relevant thread:
> >http://guforum.shortcircuit.us/index.php?topic=38.0
>
> Dyson sphere. Ringworld.

I read Dyson's original article, I own all the Ringworld books, and I
just finished reading Niven's "Playground of the Mind" collection. (I
checked it out at the library specifically for the "Bigger than
Worlds" article it contains.)

Dyson doesn't posit a rigid structure. His theory accepts any
structure (or arrangement of structures) capable of capturing and
harnessing all of the energy output of a star. Using it as a
habitable surface poses difficulties in terms of gravity and retaining
an atmosphere. (Though I once calculated that, given all the mass in
our solar system, one could build a a shell around our sun with the
density of the International Space Station, an inner radius of 1AU,
and thickness of 2km. So it's not impossible to inhabit such a
structure.)


Posted by dlzc on August 1, 2007, 3:32 pm
>
> > > I don't have a background in mechanical or
> > > architectural engineering, which is why I come to
> > > you folks. I imagine one would want concrete, as I
> > > picture the stresses forcing the shell to compress
> > > laterally across its surface. But how much concrete
> > > would one need?
>
> > Concrete won't work, not enough strength for a shell
> > that will support a human.
>
> Wouldn't that depend on how thick the shell was?

Think for a minute. The shell has to support itself against 1g with a
very tiny curvature, and it will have to support a point mass of a few
hundred pounds. And do all this with 5000 psi maximum stress (or
less). Not in this Universe.

> > > Another engineering-related line of thought I
> > > can't follow up on is the effect of tidal forces...
> > > Jupiter has several large moons. How much of
> > > an effect would they have on the shell?
>
> > Our Moon swings the Earth around a little circle,
> > and there is a "three meter high" lump that follows
> > the Moon around. Jupiter is massive enough that
> > it swings the Sun around a point just outside
> > the surface of the Sun. You'd have real difficulties
> > keeping the shell in place
>
> I'm not worried about the shell staying in place.
> (See my other comment about gravitational
> centering.)

It didn't make sense there either. Displace the centers of mass off
center, and they *will not* recenter without work being applied.
Gravitation is not a force.

> I'm more concerned about stresses induced in
> the rigid structure of the shell.

Something this size is not rigid. Something the size of a building is
not rigid. And you are right to worry about stresses. I think they
are insurmountable with any matter.

> > > Incidentally, this is part of a sci-fi world-building
> > > project I'm working on called Grokked Universe.
> > > Here's the relevant thread:
> > >http://guforum.shortcircuit.us/index.php?topic=38.0
>
> > Dyson sphere. Ringworld.
>
> I read Dyson's original article, I own all the Ringworld
> books, and I just finished reading Niven's "Playground
> of the Mind" collection. (I checked it out at the library
> specifically for the "Bigger than Worlds" article it
> contains.)
>
> Dyson doesn't posit a rigid structure. His theory
> accepts any structure (or arrangement of structures)
> capable of capturing and harnessing all of the energy
> output of a star. Using it as a habitable surface
> poses difficulties in terms of gravity and retaining
> an atmosphere. (Though I once calculated that, given
> all the mass in our solar system, one could build a
>shell around our sun with the density of the
> International Space Station, an inner radius of 1AU,
> and thickness of 2km.

Hard to build anything out of hydrogen ice.

> So it's not impossible to inhabit such a structure.)

David A. Smith


Posted by Warm Worm on August 1, 2007, 10:42 am
> Here's an interesting thought problem for you folks.
>
> The gravitational acceleration a person experiences is derived from
> the mass of the body we're relating to, and the distance of the person
> from that body's center of gravity. On Earth's surface, this is
> nominally about 9.8 m/s^2, or what we call "one gravity." On
> Jupiter's "surface", the experienced acceleration is much greater.
>
> However, if one could hold a position at 110,000 km from Jupiter's
> center of gravity, one would experience an acceleration of about one
> gravity, making the planetary experience, on the whole, much more
> comfortable. (Radiation belts aside.) If one were to build a shell
> around Jupiter with an outer radius of 110,000 km, one would also get
> a surface area of 19.6 billion square miles, or over 99 times the
> surface area of Earth. *Lots* of living space, and, unlike some other
> celestial megastructures, your atmosphere is stuck to your shell by
> gravity.
>
> The question becomes, what would one want to build such a thing out
> of? I don't have a background in mechanical or architectural
> engineering, which is why I come to you folks. I imagine one would
> want concrete, as I picture the stresses forcing the shell to compress
> laterally across its surface. But how much concrete would one need?
> How much water (A scarce resource in space, though Jupiter happens to
> have a large ice moon) would be needed?
>
> Another engineering-related line of thought I can't follow up on is
> the effect of tidal forces...Jupiter has several large moons. How
> much of an effect would they have on the shell?
>
> Incidentally, this is part of a sci-fi world-building project I'm
> working on called Grokked Universe. Here's the relevant
> thread:http://guforum.shortcircuit.us/index.php?topic=38.0

I fear that's a question that only RebarGuy can answer.

I, however, specialize in terraforming, and think it would be far
easier to just terraform Mars, or maybe even Venus or Europa than
build a concrete sphere around Jupiter.
As Pat inspired; if you have the knowledge to build a concrete shell,
your knowledge would likely find it impractical.


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