Starpower?

(william mook) wrote in message . com...
1.56 GW of light when reflected fully produces 1 kgf of thrust. So,
500,000 metric tons of thrust requires 780 TW. Call it one
quadrillion watts. The sun produces 386 billion Q-Watts. So, by
capturing all of the sun's output and delivering 12% of it reliably
anywhere across 100 ly would be sufficient to accelerate 40 billion
ships each 500,000 tons in mass per year.

This is sufficient to power self-propelled space colonies near light
speed for every family or individual on the planet.

By structuring materials on the nanometer scale it is possible to make
highly reflective films as described here;

http://www.3m.com/about3M/technologi...olutions.jhtml

http://www.photonics.com/spectra/tec...77/QX/read.htm

These films are 99.9% reflective and are made of plastic. Films made
of ceramics, carbon layers, or vapor deposited layers of metal like
tungsten, can be equally reflective and thin and withstand high
temperatures.

A square meter of film that absorbs 0.1% of the light that falls on it
will need to dissapate heat at 1/1000th the rate of a film that
absorbs all the light that falls on it.

The materials of the shell that I described earlier in this thread
could absorb and re-radiate energy at about 10 MW per square meter.
Multiply this by 2,000 - which involves radiation from both sides of a
hot film and a factor of 1,000 because 99.9% of the energy is
reflected, and you end up with 20 GW per square meter of reflected
radiation to sustain this level of radiation into the vacuum. At
these power levels 'thrust films' would produce around 10 kgf per
square meter. At 10 grams per square meter a film by itself if
illuminated at 20 GW per square meter would undergo accelerations of
1,000 gees! This is sufficient to accelerate to nearly light speed in
8 hours.

Of course as speeds increase doppler shifts reduce thrust levels.

Practical speed limits are about 1/3rd light speed. So, at 1,000 gees
a film by itself would take less than 3 hours to accelerate to light
speed.

Of course, a structured film could be fired at another structured film
for a new sort of accelerator. One involving significant mass as well
as significant energy.

Back to the more mundane uses of such a high powered film, consider a
500,000 ton spacecraft would require 50 square kilometers of laser
light sail to accelerate at 1 gee. This is a sail about 8 km in
diameter. A 100 ton spacecraft - something the size of the space
shuttle - would require only 10,000 square meters of film to
accelerate at 1 gee. A disk 120 meters across.

It takes about a year at 1 gee to get to light speed. About 4 months
to get to 1/3rd light speed at 1 gee.

It is the nature of dichroic film to become more reflective the more
layers one uses. This has diminishing returns though as things get
thicker. Even so, it may be feasible to make films that are 1,000
times more reflective than considered here. If such super reflective
films become possible this increases the energy one may handle with a
film by 1,000 times. That's 10 tons per square meter, and 20 TW per
square meter. At these super illumination levels it would take a film
only 50,000 square meters to lift a super tanker and only 10 square
meters to lift the space shuttle.

What this means is that the films can pretty much be the outer skin of
the spacecraft - avoiding the need to handle sails of the stuff to
produce thrust. A disk shaped craft with super reflective skin, with
perhaps deployable super reflective fins for guidance and boosted
acceleration.

The super-tanker sized vehicles would enjoy constant acceleration
during boost phases of flight and might be spun to produce gravity
forces during coast phases of flight.

This is the second time I've responded to this. I responded on the
7th - and my comments have yet to see the light of day. Which is
unacceptable in my book.


(Alex Terrell) wrote in message . com...
(william mook) wrote in message . com...
(Alex Terrell) wrote in message . com...
You would also heat up the sun, which would increase the rate of the
fusion, which would heat up the sun, which could go Nova.

Really?

Consider, two spherical surfaces one nested inside the other sharing a
common center. One is 800,000 kilometers across (the surface of the
sun) another 2,000,000 kilometers across (the surface of the power
shell).

Now, if the temperatures of each of the surfaces are such that the
amount of energy radiated from a sphere 2 million kilometers across is
equal to the amount of energy radiated from a sphere 800,000
kilometers across - there is no opportunity for energy to accumulate
in the sun, increasing rate of fusion presumably.

You are correct in the steady state. However, you are effectively
increasing the insulation of the sun. Therefore, to emit the same
amount of radiation, it needs to be hotter.

How much hotter?

This is easy to compute. First figure out how much energy gets back
to the solar surface. Then figure what the new stable temperature of
the solar surface must be in order to handle this increased heat load.

The first is a simple matter of geometry. Examine a point on the
surface of the larger shell and sum over all points on the shell.
Find that about 4% of the energy in the outer shell will indeed find
its way back to the smaller sphere of the sun - worst case. Clever
engineering can get around this. But let's not argue that point.
Lets look at worst case and see if your concerns are valid.

The second is a simple application of Stephan's law. Since energy
radiated from a black body grows at a rate equal to the fourth power
of the temperature, we merely take the quartic root of 1.04 to see
that the temperature of a jacketed solar surface is less than 1%
higher than the unjacketed solar surface. So, the temperature would
rise from 5,770K to 5,827.7K

However, if it's hotter,
it will emit more radiation.

It will rise to a temperature that allows it to shed the extra 4% of
energy the jacket is radiating back to it. There is a feedback. It
will actually be the infinite sum;

1 + (1.04 - 1) + (1.04^2 - 1) * (1.04^3 - 1) + ... + (1.04^n - 1)

Which is pretty darn close to 1.04

So, let's say the new temperature of the solar surface is stabilized
at 5,828K


This will either stabilise at a hotter
sun, or be unstable and go nova.

Well, temperature and pressure determine the reaction rate deep within
the core of the sun. This is absolutely true. It is also true that
increased temperature will result in increased reaction rate by
increasing the kinetic energy of the hydrogen atoms flying around
inside the sun. The Lawson relations give us reaction rate
temperature and pressure curves for fusion of hydrogen.

Please note that the sun's brightness is already increasing because of
increasing levels of helium at the solar core. Helium fuses hotter
than hydrogen. Hydrogen fusion makes helium. So, the sun is about
twice as bright today as it was when the Earth first formed. In about
900 million years the sun will be too bright to sustain life as we
know it on Earth. These changes are slow, and this is not the cause
of the recent rise in Earth temperatures during the rise of industry
on Earth. That's due to increased levels of CO2.

Changes in temperature on the sun's surface on the order of tens of
degrees may increase solar wind or increase convection rate - both
very slightly - and not be communicated deep within the sun. If so,
reaction rate won't change.

Any changes that do communicate deeply to the core of the sun will
take a long time to occur. It takes thousands of years for energy to
pour out of the sun from the center. It will likely take at least as
long for any effects of a poorly designed jacket to communicate
temperature changes deep to the core of the sun.

But, for fun, lets look at worst case. Within 1 million years of
applying a poorly designed jacket -one that radiates energy back to
the solar surface- the core temperature of the sun will indeed rise by
58C because of this.

The core temperature is measured in tens of millions of degrees. So,
this increase in temperature will be around 1 part per million
increase. Given the relation of temperature to fusion rates this
worst case would cause a 1 part per trillion increase in fusion rates.
A one part per trillion increase in reaction rate would cause a
brightening of the sun equal to 1 part per trillion, increasing energy
flow by the same amount.

Application of Stephan's law again says that the quartic root of
1.000000000001 times the original energy flow is equal to one quarter
part per trillion increase in temperature. Summing all these very low
numbers over time yeilds less than one quarter part per trillion
increased brightness of the sun.


Another way of looking at it: The spehere will reflect / reemit a
proportion (50%) of the energy it receives in the direction of the
sun. That has the effect of increasing the heat generated by the sun
by more than 50%.

Your geometry is wrong. Only 8% of the inward moving radiation will
actually make it back to the sun. 92% of the inward moving radiation
will miss the sun and hit other parts of the shell that surrounds the
sun. This is 4% of the total energy since half the energy is radiated
outward and has no chance of hitting the sun at all.

Getting the geometry right makes it simple. Points on the surface of
a shell that emits as a black body will radiate in all directions
equally. All points at a radius of 1,000,000 km will see a 400,000 km
radius sphere subtend 41 degrees of the entire sky. A ball that
subtends 41 degrees covers about 4% of the entire sky viewed from that
point.

This 4% increase if allowed to occur will cause a 1% rise in suface
temperature. This 1% rise in temperature if not disposed of by the
sun in any other way (convection and solar wind changes) will increase
the core temperature by 1 part per million. This rise in core
temperature will increase the reaction rate by 1 part per trillion.
This rise in reaction rate is trivially small compared to the original
increase in temperature. The periods of time involved and the changes
in brightness involved - worst case - are swamped by natural increases
in brightness that are occuring right now due to accumulating helium
at the solar core.


But this begs an interesting question. Could one increase the rate of
fusion? (Nova's are something quite different see
http://observe.arc.nasa.gov/nasa/spa...rdeath_4a.html
)

This would brighten the star and increase total output of the sun.
Which could be interesting if more energy is needed. Could this be
done? I don't know. It does seem that if we have a sphere made of
reflectors that return energy to the sun, then the sun would heat up.
This is likely to result in an increase in the solar wind. Which
could be interesting. The solar wind might be harvested for raw
materials like hydrogen, helium, and other elements it contains.

But since it takes on the order of 10,000 years for energy to move
from deep inside the sun to the surface, it is likely to take equally
long for surface changes to affect deep processes like rate of fusion
(if it affects it at all!)

But it would be cool to be able to control illumination levels on each
of the planets while independently controlling stellar output over
some range. But, I'm not smart enough to figure out right now if this
will indeed happen. Maybe a solar astronomer can give us a clue.

One thing is for certain. Theconditions are not right for a nova.

It would shorten the life of the sun, but that life span is
fantastically long to start out with.


Novas occur because of compositional changes that occur in the core of
stars over very long times. They do not occur because of changes in
opacity of shells that surround a star. Stars entering dusty regions
of the galaxy for example do not suddenly go nova. Their solar winds
change to process the change in surface condition. This suggests that
there may be a very small change in the nature of the solar wind worst
case. But, stars caught in very opaque clouds don't have much of a
change. The sun encased in a power shell is likely to have no
observable changes in its physical condition - since natural changes
will overwhelm any man made changes.

(william mook) writes:

(Alex Terrell) wrote in message
. com...
(william mook) wrote in message
. com...
(Alex Terrell) wrote in message
. com...
You would also heat up the sun, which would increase the rate of the
fusion, which would heat up the sun, which could go Nova.

Really?

Consider, two spherical surfaces one nested inside the other sharing a
common center. One is 800,000 kilometers across (the surface of the
sun) another 2,000,000 kilometers across (the surface of the power
shell).

Now, if the temperatures of each of the surfaces are such that the
amount of energy radiated from a sphere 2 million kilometers across is
equal to the amount of energy radiated from a sphere 800,000
kilometers across - there is no opportunity for energy to accumulate
in the sun, increasing rate of fusion presumably.

You are correct in the steady state. However, you are effectively
increasing the insulation of the sun. Therefore, to emit the same
amount of radiation, it needs to be hotter.

How much hotter?

This is easy to compute. First figure out how much energy gets back
to the solar surface. Then figure what the new stable temperature of
the solar surface must be in order to handle this increased heat load.

The first is a simple matter of geometry. Examine a point on the
surface of the larger shell and sum over all points on the shell.
Find that about 4% of the energy in the outer shell will indeed find
its way back to the smaller sphere of the sun - worst case. Clever
engineering can get around this. But let's not argue that point.
Lets look at worst case and see if your concerns are valid.

The second is a simple application of Stephan's law. Since energy
radiated from a black body grows at a rate equal to the fourth power
of the temperature, we merely take the quartic root of 1.04 to see
that the temperature of a jacketed solar surface is less than 1%
higher than the unjacketed solar surface. So, the temperature would
rise from 5,770K to 5,827.7K

I'm sorry, but stars don't work that way. Gravitationally bound bodies such
as stars exhibit the rather peculiar property of "negative heat capacity"
--- see http://www.arxiv.org/abs/cond-mat/9812172. Slow down the rate at
which heat escapes from a star (effectively increasing the opacity of its
atmosphere), and it _EXPANDS AND GETS BRIGHTER AND REDDER_. (The same thing
happens if one compares high-metallicity stars to low-metallicity stars
of the same mass: The higher opacity resulting from the higher fraction
of "metals" causes the high-metallicity star to be larger, redder, and more
luminous. The same thing is happening to the Sun: As it gets older and more
hydrogen gets converted to helium, it gets brighter, fatter, and redder.)

What you need to compute is instead how much more _area_ the Sun will need
to dispose of the increased heat flux at the lower equilibrium temperature
induced by the increase in the effective opacity of its atmosphere.


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

An impressive use of numbers. Thank you for your time and brain.

You appear to be broadly correct, though I think I see one small error

(william mook) wrote in message . com...
Another way of looking at it: The spehere will reflect / reemit a
proportion (50%) of the energy it receives in the direction of the
sun.

Not quite. A point on a shell operating as a black body radiator will
emit energy in all directions. It is true that slightly more than
half its energy outward and slightly less than half its energy inward.

The half that is emitted outward will not heat the sun. The half that
is emitted inward will be radiated in all directions from that point.
Only a small range of angles will intercept the solar surface and send
energy back to the sun.

Yes, but the rest will go the shell, 50% to be emitted again towards
the inside.

A point at a radius of 1 million kilometers from the center of the sun
will see the solar surface subtend about 43.6 degrees. This is about
4% of the sky that the point at 1 million kilometers sees. So, about
4% of the isotropic radiation radiated from each point will find its
way back to the sun. Assuming of course that the radiators are not
engineered to shadow the sun.

But 96% goes to the shell, and 48% is reemitted. Assuming your 4% is
correct, the radiation back to sun would be 4% / (1-i) where i is the
proportion reflected/reemitted back inwards. For a black body shell,
i=50%, so 8% ends up on the sun's surface. (A perfect, mirror would
give i=1, and would give some interesting effects).

That has the effect of increasing the heat generated by the sun
by more than 50%.

The surface temperature of the sun inside such a shell may rise in
temperature sufficient to dump this extra 4% of energy into space -
assuming each point radiates in all directions. This temperature to
achieve this may be computed by Stephan's Law;

P = sigma * T^4 = 5.67e-8 W/m2/K4 * T^4

where T is in kelvins. The sun's surface temperature is 5,770 K. An
increase of 4% of power output from the sun would require the surface
temperature rise by

1.04^(1/4) = 1.00985

or a little less than 1%.

1.08^(1/4) = a little less than 2%

That is, the surface temperature of the sun worst case would increase
by less than 58 C - from 5,770 K to 5,828 K. The core temperature
which operates in the tens of millions of degrees would, once steady
state was achieved in 10,000 years - would rise by the same 58 C -
from say twenty million to twenty million and 58. Kinetic energy
scales as the square of temperature so this one part in a million rise
would cause a one part in a trillion rise in kinetic energy and result
in one part in a trillion rise in reaction rates - worst case. This
is not sufficient energy to maintain the higher temperature so it is
not sufficient to cause a runaway heating effect as you suggest.
Therefore there is no runaway rise.

seems the sun is pretty stable.



But this begs an interesting question. Could one increase the rate of
fusion? (Nova's are something quite different see
http://observe.arc.nasa.gov/nasa/spa...rdeath_4a.html
)

This would brighten the star and increase total output of the sun.
Which could be interesting if more energy is needed. Could this be
done? I don't know. It does seem that if we have a sphere made of
reflectors that return energy to the sun, then the sun would heat up.
This is likely to result in an increase in the solar wind. Which
could be interesting. The solar wind might be harvested for raw
materials like hydrogen, helium, and other elements it contains.

But since it takes on the order of 10,000 years for energy to move
from deep inside the sun to the surface, it is likely to take equally
long for surface changes to affect deep processes like rate of fusion
(if it affects it at all!)

But it would be cool to be able to control illumination levels on each
of the planets while independently controlling stellar output over
some range. But, I'm not smart enough to figure out right now if this
will indeed happen. Maybe a solar astronomer can give us a clue.

One thing is for certain. Theconditions are not right for a nova.

What proportion would you need to reflect back to do so?


It would shorten the life of the sun, but that life span is
fantastically long to start out with.