From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Bubbleworld (was Ringworld)
Date: Sep 07 1995
Newsgroups: sci.space.tech

Since we know that Ringworlds are unstable, and require 'unobtainium',
the real question is what is the largest constructible habitation?

The best answer I know of is a rotating bubbleworld, which would be
several times the diameter of Jupiter.

A bubbleworld is a shell of living space held up by gas filling the
interior.  Eventually, as you make them larger, the gas in the middle
self-gravitates.  If you try to make a larger bubbleworld by pumping
in more gas, it will contract under it's own gravity.  So there is
a largest size for any set of initial conditions.

If we assume that the bubbleworld is non-rotating, and the living
space is at one atmosphere at the gas interface, the gas used is
hydrogen, and the gas is at the same temperature as the living
space i.e. 300K, then the answer is a sphere about 240,000 km in
radius.

If we assume that the living space has an average density of 10
kg/m^3 (air is 1.2 kg/m^3, the balance is people, houses, trees, etc.),
then the living space has a limited thickness based on breathability.
After 2400 km of thickness, the air will be at the equivalent of
3000 m above sea level on Earth, which is about the limit of ordinary
breathing with no problems.

The bubbleworld is too diffuse to hold the atmosphere in by gravity,
so an outer shell (steel is handy) is used to keep it in.  It works
out to 500 m in thickness.  The total mass of such a world is 3x the
Earth's, so there should be enough raw material to build one out of
a typical solar system.

The bubbleworld has about 1400 times the surface area of the Earth,
and about a million times the useable living volume.  For example,
you could divide it into 700 'floors', each with a 'sky' 3.5 km
high.

You can build somewhat larger if you rotate the bubbleworld, to
counteract the self-gravity of the gas in the middle, and also if
you heat the gas in the middle to lower it's density (but then you
would have to insulate and cool the living space).  The rotating
case is beyond my ability to analyze.

The limit on how many people can live there is driven by waste
heat radiation from the surface.  If we allow 10 kW per person
for power use of all kinds, we need 20 m^2 per person, so the total
population would be limited to 36,000 trillion, or about 6 million
times that of Earth.  Given the factor of a million in living
volume, that works out to 6x the living density of Earth.  Since
a lot of the Earth's living space is not useable (ocean, desert,
polar regions), the effective living density is not very different
from what we have today.

Dani Eder

From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Bubbleworld (was Ringworld)
Date: Sep 13 1995
Newsgroups: sci.space.tech

acase@Glue.umd.edu (Andrew Case) writes:

>Dani Eder <ederd@bcstec.ca.boeing.com> wrote:
>[cool stuff deleted]
>>The bubbleworld is too diffuse to hold the atmosphere in by gravity,
>>so an outer shell (steel is handy) is used to keep it in.  It works
>>out to 500 m in thickness.  

>Why 500m? What are the design considerations used to arrive at this figure?
>Is there a safety margin built in to allow for the fact that the weather
>in a bubbleworld of the size you describe is likely to be quite interesting,
>perhaps extremely violent? I haven't yet seen a careful analysis of the
>effects of weather on a space habitat. Would be interesting to do, though.

>.....Andrew

>P.S. Neat idea. Do you have cites for further info?

500 m of steel at 0.02 m/s^2 local acceleration of gravity produces
a force of 78,000 N/m^2 down, which equals the air pressure up.  Thus
the atmosphere is held down by the weight of the steel, and the steel
shell floats on top of the atmosphere.

If the habitable volume is divided into 'floors' a couple of km thick, 
and partitioned every 10,000 km horizontally, you would not expect
weather patterns any worse than on Earth.  I have yet to see a storm
on earth that could do much to 500 m of steel.  Since you have almost
complete freedom to configure the habitable volume, think of it more
like being inside a building - where you control the weather.

As far as I know, I invented the bubbleworld idea in response to 
a dinner discussion at one of the Princeton Space manufacturing
conferences.  The question was what was the largest space colony 
that could be built.  By removing the constraint that the whole
colony had to be rotated to get artificial gravity, I got the answer
I did rather than 100 km or so in diameter that you get for the
all-spinning case.

Dani Eder

From: ederd@bcstec.ca.boeing.com (Dani Eder)
Subject: Re: Bubbleworld (was Ringworld)
Date: Sep 11 1995
Newsgroups: sci.space.tech

gherbert@crl.com (George Herbert) writes:

>Dani Eder <ederd@bcstec.ca.boeing.com> wrote:
>>If we assume that the living space has an average density of 10
>>kg/m^3 (air is 1.2 kg/m^3, the balance is people, houses, trees, etc.),
>>then the living space has a limited thickness based on breathability.
>>After 2400 km of thickness, the air will be at the equivalent of
>>3000 m above sea level on Earth, which is about the limit of ordinary

>Umm, I don't get it.  2400 km of thickness in which direction?
>You said people are living on the inside of the sphere, shouldn't
>pressure go up as you get closer to the center (weak self-gravitation)?
>Speaking of which, there will be weak self-gravitation pulling people
>down towards the center... do you hang off the outer shell you describe
>with gravitational down towards the center, or what?

The 2400 km figure is from the inner hydrogen core/living volume
boundary to the outer steel shell.  By analogy to a soap bubble, it
represents the thickness of the soap film.  Gravitation points to
the center of the Bubbleworld, and is on the order of 1/1000 of a
gravity.  Thus over the 2400 km of height, the ambient air pressure
will drop as much as it does on Earth over 2.4 km.  Gravity is low
enough that essentially arbitrary structural arrangements are
possible.  For example, if you want to hang your city from the
outer shell and spin it to get 1 gee locally, that can be done
with real materials.

>>The bubbleworld is too diffuse to hold the atmosphere in by gravity,
>>so an outer shell (steel is handy) is used to keep it in.  It works
>>out to 500 m in thickness.  The total mass of such a world is 3x the
>>Earth's, so there should be enough raw material to build one out of
>>a typical solar system.

>500 meters?  You should be able to reduce that to nearly zero, by
>placing the outer shell where outward atmospheric pressure and
>inwards gravity balance...

That is a design choice.  The particular design I chose had the
shell divided into ten 50-meter thick shells separated by a km or
two each, as a multi-layer asteroid/comet impact shield, with the
spaces in-between filled with air that drops in pressure 10% per
layer.  The pressure in a bubbleworld drops slowly with altitude, 
so moving the shell outwards would entail supplying a lot more
atmosphere.  On the other hand, several thousand km of air might
be more effective as a asteroid/comet shield.  This needs more study.

>>The limit on how many people can live there is driven by waste
>>heat radiation from the surface.  If we allow 10 kW per person
>>for power use of all kinds, we need 20 m^2 per person, so the total
>>population would be limited to 36,000 trillion, or about 6 million
>>times that of Earth.  Given the factor of a million in living
>>volume, that works out to 6x the living density of Earth.  Since
>>a lot of the Earth's living space is not useable (ocean, desert,
>>polar regions), the effective living density is not very different
>>from what we have today.
>>Dani Eder

>This sounds great, though I don't fully understand it yet...
>definitely worth further investegation.  Sounds much more
>practical than inventing unobtanium to make a ringworld 8-)

Another feature of a bubbleworld is that it can be built incrementally.
Start with a pressurized sphere of moderate size, with multi-layer
outer shell as described above.  Add one more outer shell, increase
the pressure in all the layers.  Then the innermost layer can be
dismantled and recycled into the next outer layer (plus a little
bit for the fact that the outer shell is bigger than the inner
shell).  Meantime, in the center you are expanding the central
bubble filled with hydrogen.  Your cities or whatever will move
outwards as the shells expand, thus moving apart from each other
as more 'land' gets built - sort of like the galaxies move away
from each other in the Big Bang.

Now, I don't really propose that a big bubbleworld is the 'best'
answer to anything.  The concept was developed to answer the
question 'what is the largest space colony that can be built',
with the answer 'several times the diameter of Jupiter'.

Personally, I would be much more comfortable with a large number
of 'bubbles' a few thousand km in diameter each, for 'all your
eggs in one basket' reasons.

Dani Eder