From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Fri Jul 30 1999 - 11:17:01 MDT
Well, Eugene raises some interesting points. I believe there
are solutions for his objections. Discussion follows --
> Eugene Leitl <eugene.leitl@lrz.uni-muenchen.de> wrote:
>
> It is just that Mercury already receives quite a
> lot of radiation. Even if you increase the amount if incoming power by
> one order of magnitude (doubtful the more delicate structures can
> survive it), does it really allow the dismanting process to complete
> in just 11 d? You must be hiding at least a few big envelopes back
> there.
The problem revolves around how "efficiently" you can convert
the incoming power to a useful form (avoiding the production of
heat). If you beamed the power back with diode lasers at say
520 nm, this is a particularly efficient absorption band for
CCDs (or equivalent photovoltaic) structures. I would think
that you get a minimum of 70% perhaps up to 90% conversion
efficiency (light to electricity). Traditional PV conversion
efficiencies are taken for the entire black-body spectrum of
radiation (which is why they are so low). You could also beam the
energy back in a wide-band form and heat "boilers" to very
high temperatures (1500-3000 C probably depending on materials).
The gas vapor pressure could be converted directly into mechanical
energy for moving materials around or powering turbines. The
thermodynamic efficiency of boilers operating at these high
temperatures is quite high.
I suspect that if you had the energy coming in at a very specific
wavelength, you could construct receivers with just the right
thickness and bandgap so that "virtually" all of the photons
are absorbed and generate electrons. What you would really
want is a structure that allowed the electrons to be immediately
moved from the PV conversion material into a superconductor.
You still have a problem of where to sink the waste heat. There
are two solutions to this -- the planet and space. The planet
turns out to be a good starting point. As you put more heat
into the planet it becomes much easier to dismantle. In my
calculations for dismantlement I assume a very conservative value
that requires breaking every atomic bond in the planet. If you
pump a lot of heat into the planet, it becomes soft enough to
"flow", then you might centrifuge it to seperate the elements
(or as RF points out in Nanomedicine, you could "weigh" each
molecule). Then you "pump" the desired elements into containers
on the mass drivers and launch it into space. This saves a lot
of energy over breaking all of the atomic bonds.
The other possibility is radiating the excess heat into space.
Diamond makes a great conductor and carefully engineered surfaces
can be > 99% efficient at radiating the heat (see also below).
It is worth remembering that only the sun-side of Mercury is hot.
The non-sun side is pretty cold and it is thought that ice may
exist in sheltered craters at the poles (just like on the moon).
>
> > You could vaporize the planet completely, but I favor a strategy
>
> Vaporization is sure easy, but I do not see a way to recapture the
> material quantitatively. The point is not destruction, the point is
> retaining the bulk of the material (especially volatiles) for
> constructive purposes.
You start with vaporization points on the planet, surrounded by
a much larger (thin) condensation surface (that radiates the
heat into space. The vapor coming off the planet condenses
onto the cooler surfaces and is then manipulated into ever larger
condesation surfaces. You could envision successive layers of condensation
meshs/grids that are heated by the gas mixture but are cool enough to
condense specific elements. As the distance from the planet increases,
elements condense at lower and lower temperatures. This is not
too much different from how the solar system actually formed.
> I grant you nanotechnology, but this does not address the maximum
> tolerable energy flux on planetary body's surface.
The thing is that using nanotechnology, you can "grow" the surface
and therefore the radiator power. You put the radiator panels
on top of diamondoid beanstalks. You grow the diamonoid beanstalks
from the bottom, circulating cooling fluid up through the stalks and
and through the radiators. The surface, keeps expanding outward as
the heat dissipation requirements increase. There are holes in the
radiators that allow the high density energy to be beamed through the
radiators down to the surface. [Or a really clever design merges
the MW/PV energy absorbers/converters with the radiators].
> You could carry off some energy as hot cargo, but how much can you
> carry off with mass launchers? These coils get awful hot after a
> while.
Yep, you could essentially "liquify" the planet, then "flash-freeze"
the surfaces of the mass-pods long enough to launch them. The do
the final heat radiation step in space. Alternatively you could
launch hot stuff in something like sapphire or tungsten carbide "crucibles".
Since there are relatively abundent poor heat conductors, you should
be able to easily construct a big vessel, pour in some liquid planet,
lanuch it and repeat the process. You don't really care if the thing
melts "in transit" since wherever you collect it is going to have
a large capacity to radiate the heat away. If you have energy
to burn, you could ionize the molecules and simply run them through
particle accelerators.
Since you have the problem of cooling the energy collectors (even if
they are 99.99999% efficient at converting the incoming energy), you
have the problem of cooling the mass drivers as well. You dump the
heat first into the planet, then into space (either in the payload
or through radiators, as pointed out above). Your comments
do point out the need to do some more in depth calculations regarding
how you partition the waste heat as the dismantlement occurs. My gut
feel is that it goes: (1) Planet, (2) Departing Mass, (3) Radiators.
Re: collector areal density & times for collector migration
>
> I don't think these are relevant questions. The bottleneck is
> obviously elsewhere.
Without real designs, it is difficult to say exactly where the
bottlenecks might be. I identified these because these have
the greatest effect in the simulations I have run. As you point
out the heat radiation problem is a significant concern, but with
3-4 possible solutions it it appears to be resolvable.
>
> Yes, and it is a very stringent constraint indeed. Either you have
> numbers I am unaware of, or you're glossing over the issue.
Well, I did "gloss over it" in my discussion, because I viewed
"vaporization" as the final (ultimate) approach to the problem.
As your comments point out, it may come down to the question of
what is faster -- vaporization & condensation or mechanically
growing the planet. The first is energy constrained, while the
second is most likely element abundance constrained.
> It was most assuredly intentional. It is difficult to envision more
> efficient launch mechanisms than launching from top of beanstalks (and
> a large beanstalk sees mostly 4 K space, not Mercury surface, and,
> well, some of Sun's), Mercury is probably small enough for them (and
> doesn't have an atmosphere).
Good point, so it looks like you have beanstalk mass-driver launchers
that circulate cooling fluid to thin radiators spanning the area between
the tops of the beanstalks, possibly with some holes to allow highly
focused energy beams back to the surface.
> How do you intercept a ~Mt/s stream of evaporated planet? (Or Gt/s,
> don't know how much mass Mercury has divided by 10^6 s).
You simply get far enough away. It turns out that you need to
handle about 10^17 kg/sec (average). That works out to a sphere radius
of ~10^5 km (less than the radius of the sun) from Mercury if you want to
handle 1 kg/m^2. Since that seems rather conservative, I think the sphere
could be much smaller. For comparison, average radius of Mercury's
orbit is 5x10^7 km. You should be able to use the momentum of the
launched mass (if you give it a little extra) to push the mass
collectors away from the planet at the rate that matches the
increase in mass volume (due to the increased power being beamed
down to the planet).
>
> > > I would like to see an easy way of translating power into such a
> > > coordinated activity as planet dismantlement.
> > The easiest approach is to vaporize the planet and condense it.
>
> How do you duct it? How do you trap it?
No ducting required, condense it at the distances allowed by
natural radiation (as the solar system did). Since you can
can construct condensation radiators that radiate "outward"
and are much more efficient than "natural" dust this should
be easy to control. You could either orbit the condensers
or "grow" them from the surface (more beanstalks). To radiate
the heat effectively they would have to be thin, and therefore
lightweight.
Diamondoid radiators (lower temp than say Al2O3, SiC or Fe2O3),
require a sphere that is *smaller* than Mercury's orbit
(2 x 10^7 km vs. 5 x 10^7 km). And since to minimize the
radiation/heat damage, you want to construct your collector/radiator
sphere *outside* of Mercury's orbit, everything seems to hang
together. Of course, moving the mass from a Mercury encompassing
sphere to a Sun encompassing sphere is going to be an orbital
nightmare, but it doesn't seem impossible.
> Energy is cheap, very cheap. A 0.1 um metal foil would reflect ~95% of
> incoming radiation. One needs to tranform the planet into a fleet of
> solar sailers which try to keep the original body in focus. The higher
> the initial insolation the faster the initial growth. Difficult to do
> that trick with Pluto.
Agreed. I thought about doing the asteroids, because you can do
more in parallel, but the problem is the insolation is an order
of magnitude lower when starting out. For exponential growth
you win big with large up-front energy contributions.
>
> Once again, energy is not a problem. The Sun produces sufficient
> amounts of power that it is difficult to get enough material to make
> the computers -- and then there is lots of fusable hydrogen out there
> in the Oort.
No, the material in Mercury or the Moon provide sufficient Mass
(whether the element abundances are correct is an unresolved
question) to build sufficient 1 cm^3 nanocomputers to use the
entire power output of the sun. The *killer* for MBrains is
the radiator mass, not the nanocomputer mass. The further
out you build the radiators (for the outer MBrain layers) the
more mass is required (heat radiation scales with T^4, so
cool radiators suck mass bigtime). The denser you try to
make your nanocomputers the more power you consume circulating
the cooling fluid and the larger the radiator mass (due to the
higher coolant pressures). There are clearly limits to how
dense you can do the computing given the mass and energy
available. What the exact tradeoffs are probably depends
highly on the computer architecture.
When I have time (ROTFL), I've got the CAD/modeling program to do a
real design of one of Eric's nanocomputers and a "real" radiator.
With that in hand we should have at least one valid point for
possible MBrain subcomponents. In current calculations, I
assume collector/radiator masses of ~1 kg/m^2 which seems "reasonable".
> Asteroids are fundamentally much easier, because the volume/surface
> ratio and absolute gravitation are not an issue. There is even no need
> to launch, just transfer material to the rim, which flattens things
> and increases surface along with launching mirrors/building rectenna
> arrays/microwave beamers. The only minus is that most of them reside
> beyond Mars' orbit, but that just delays the matters somewhat if you
> have to start from a microgram seed.
Yep. In theory, you could do both Mercury & the Asteroids
simultaneously. You could also do part of Mercury (say until it gets
to the point where you have melted the planet and the nanomachines,
then use that energy to do the asteroids one by one. No matter
what strategy you choose, once you are up in the TW power range
the whole thing goes pretty quickly. Asteroids do suffer from
the requirement that you have to either make the collectors
thinner or use more mass (interestingly enough, the estimates
I have show Mercury as having more mass than the asteroid
belt by at least an order of magnitude). You also have
longer travel times for optimal positioning from Asteroidal
orbits.
> I am not sure how you can integrate the multiple factors of surface
> increase into one simulation. Lot's of orbiting mirrors will sure
> produce lots of evaporating planet smoke....
True, perhaps better to ionize the smoke and put it into carefully
focused particle beams. Energy directed down, Mass directed up,
so long as nobody changes lanes it should work.
Robert
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