How far to beam power up a Beanstalk?

(Henry Spencer) :

In article ,
Fred B. McGalliard wrote:
...I can imagine a narrow beam radar providing power, through
most weather conditions anyway...

Unfortunately, the comparatively long wavelength of radar means you need a
huge antenna system to form a narrow beam. It's very poorly suited to
providing power to small vehicles.

However the BeanStalk is fixed in position mostly. Can not the beam just be
treated as a fixed beam also running beside the beanstalk? Does not that
mean all you would only need a fixed antenna on the ground with no moving
parts nor electronic steering. If possible that should be relativly cheap to
make.

but the only way to get power out of a light
pulse is to run a heat engine, or to run a solar cell, the efficiency of
which is low enough that power dissipation is a big deal.

Note that simple solar cells are 50%+ efficient when illuminated by laser
light at a well-chosen wavelength. Their inefficiency in sunlight is
because so much of the incoming energy is at wavelengths poorly matched
to the cell characteristics.

Another question, at 4000 miles up the gravity drag only a quarter that is on
the surface, at 8000 miles only a ninth. Instead of trying to make a system
that can hit a target for about 40,000 miles, why not switch to just sunlight
after 8000 miles up, the lower power from the solar cells is countered by the
lower power needed to over come gravity for each fix unit of lenght you climb.

Earl Colby Pottinger
--
I make public email sent to me! Hydrogen Peroxide Rockets, OpenBeos,
SerialTransfer 3.0, RAMDISK, BoatBuilding, DIY TabletPC. What happened to
the time? http://webhome.idirect.com/~earlcp

Earl Colby Pottinger wrote:
(Henry Spencer) :

In article ,
Fred B. McGalliard wrote:
...I can imagine a narrow beam radar providing power, through
most weather conditions anyway...

Unfortunately, the comparatively long wavelength of radar means you need a
huge antenna system to form a narrow beam. It's very poorly suited to
providing power to small vehicles.

However the BeanStalk is fixed in position mostly. Can not the beam just be
treated as a fixed beam also running beside the beanstalk? Does not that
mean all you would only need a fixed antenna on the ground with no moving
parts nor electronic steering. If possible that should be relativly cheap to
make.

The stalk will move a bit.
The problem is that the ground antennas need to be damn huge.
To get a target spot of 500m at 50000Km, you need a dish of several
kilometers diameter.

but the only way to get power out of a light
pulse is to run a heat engine, or to run a solar cell, the efficiency of
which is low enough that power dissipation is a big deal.

Note that simple solar cells are 50%+ efficient when illuminated by laser
light at a well-chosen wavelength. Their inefficiency in sunlight is
because so much of the incoming energy is at wavelengths poorly matched
to the cell characteristics.

Another question, at 4000 miles up the gravity drag only a quarter that is on
the surface, at 8000 miles only a ninth. Instead of trying to make a system
that can hit a target for about 40,000 miles, why not switch to just sunlight
after 8000 miles up, the lower power from the solar cells is countered by the
lower power needed to over come gravity for each fix unit of lenght you climb.

There are a few issues with eclipses early on.

In article ,
Ian Stirling wrote:

The stalk will move a bit.
The problem is that the ground antennas need to be damn huge.
To get a target spot of 500m at 50000Km, you need a dish of several
kilometers diameter.

How about a phased array, rather than one big dish? You should be able
to adjust the beam a little to compensate for the deflection without
physically moving the antennas. And how about only using ground power up
to 5000-10000km, or whatever height solar becomes more useful at?


There are a few issues with eclipses early on.

Just let it stop? No reason it couldn't...

--
Christopher James Huff
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG:
http://tag.povray.org/

Christopher James Huff wrote:

In article ,
Ian Stirling wrote:

The stalk will move a bit.
The problem is that the ground antennas need to be damn huge.
To get a target spot of 500m at 50000Km, you need a dish of several
kilometers diameter.


How about a phased array, rather than one big dish?

Doesn't help much if any. The problem is that you need emitters closer
together than the wavelength- otherwise the gaps act as a diffraction
grating.

So the array ends up with the same total area.

You also have the problem that each emitter has to emit pretty
accurately towards the destination anyway- otherwise you're wasting
energy- you can't just spray the photons around in random directions and
assume the phasing will magically kill off the ones that aren't heading
towards the elevator car. Unless I'm missing some strange quantum effect
that doesn't happen.

This is a different problem to normal uses of phased array- in those
cases you are trying to receive faint signals and you are using the
phasing to attenuate a signal based on its direction. Here attenuation
results in loss of expensive power.

You should be able
to adjust the beam a little to compensate for the deflection without
physically moving the antennas. And how about only using ground power up
to 5000-10000km, or whatever height solar becomes more useful at?

Carrying two receivers might work, rectennas are likely to be quite
lightweight, but the ground transmitter still needs to be about 5 orders
of magnitude bigger than an optical system.

There are a few issues with eclipses early on.

Just let it stop? No reason it couldn't...

Costs lots of money, because you are reducing the usage of the elevator
proportionately.

Ian Woollard wrote:
Christopher James Huff wrote:

In article ,
Ian Stirling wrote:

The stalk will move a bit.
The problem is that the ground antennas need to be damn huge.
To get a target spot of 500m at 50000Km, you need a dish of several
kilometers diameter.

snip
You also have the problem that each emitter has to emit pretty
accurately towards the destination anyway- otherwise you're wasting
energy- you can't just spray the photons around in random directions and
assume the phasing will magically kill off the ones that aren't heading
towards the elevator car. Unless I'm missing some strange quantum effect
that doesn't happen.

Yep, you are.
You can consider it properly in quantum terms, but it gets rapidly
complex, but it works just as well considering it classically as
waves.

As I understand it, I'm sure someone will post if I've got
it drastically wrong.

Considering a transmitter.
Imagine a 1Km parabolic dish, fed from one feedhorn 2Km away.
The whole dish is evenly illuminated by the feedhorn, and creates
a parallel beam.

The key to understanding this is that each position on the dish
acts as a little transmitter.

If you replace the dish with an array of onmidirectional transmitters fed by
signals with equivalient delay to the distance from the feedhorn,
then there is no difference to the signal in the far field.
(you can also add extra delays to reshape the dish so that it's "flat")
(the spacing needs to be well under a wavelength to closely approach
the performance of a dish)

As the number of transmitters falls, if the overall power is kept
constant, and the overall shape and size of the array does not change,
then the size of the focal point does not change.
However, as you decrease the number of transmitters, the amount of
constructive interference that happens outside the beam increases, and
this wastes energy.

This constructive interference happens in all directions for omnidirectional
antennas.
If you replace the omnidirectional antennas with more directional ones,
then all that happens is that the constructive interference that
would have happened in the directions that the new tighter beams miss
does not happen.

There is probably some very complicated formula for determining the
minimum number of transmitters to achieve a certain percentage
of power into a given angle, but if I ever knew it I've forgotten it.

In article ,
Ian Woollard wrote:

Doesn't help much if any. The problem is that you need emitters closer
together than the wavelength- otherwise the gaps act as a diffraction
grating.

Diffraction works with single photons. Yeah, it's weird.
Anyway, the idea would be that you can make very small adjustments to
the beam direction and focus without having to have extremely precise
machinery, which is likely to be less reliable.


This is a different problem to normal uses of phased array- in those
cases you are trying to receive faint signals and you are using the
phasing to attenuate a signal based on its direction. Here attenuation
results in loss of expensive power.

It'll be lossier than an ideal dish, but I'm assuming power won't be
extremely expensive, and it may actually be better than any giant dish
we could build for a reasonable cost.


Carrying two receivers might work, rectennas are likely to be quite
lightweight, but the ground transmitter still needs to be about 5 orders
of magnitude bigger than an optical system.

Or have a two-stage elevator...the lower stage carries little more than
rectennas and a power link, the upper stage carries the payload and
solar panels. When it reaches the point where solar is more useful, the
lower stage unhooks itself and descends back to Earth.

Perhaps 3 stage...with the middle stage being mainly fuel cells to carry
the payload through the eclipse zone.


There are a few issues with eclipses early on.

Just let it stop? No reason it couldn't...

Costs lots of money, because you are reducing the usage of the elevator
proportionately.

But it may be an acceptable compromise early on.

--
Christopher James Huff
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG:
http://tag.povray.org/

In article ,
Earl Colby Pottinger wrote:
Another question, at 4000 miles up the gravity drag only a quarter that is on
the surface, at 8000 miles only a ninth. Instead of trying to make a system
that can hit a target for about 40,000 miles, why not switch to just sunlight
after 8000 miles up, the lower power from the solar cells is countered by the
lower power needed to over come gravity for each fix unit of lenght you climb.

Whether that works out depends on the numbers. If the laser system is
putting several times as much power into the array, it may not be worth
switching, especially since it will probably add complexity (e.g. the need
to track the Sun -- a laser-receiver array can probably be fixed).

It's just not all that hard to put essentially all of a laser's output
power into a few square meters of solar array at 40,000km. It's not worth
accepting major compromises just to avoid it.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

In article ,
Christopher James Huff wrote:
The problem is that the ground antennas need to be damn huge.
To get a target spot of 500m at 50000Km, you need a dish of several
kilometers diameter.

How about a phased array, rather than one big dish?

Oh, you'd probably use a phased array, just because it's easier to build
than a dish that size. But it still has to be huge and densely packed
with emitters.

Despite the annoying issues with weather, lasers work a lot better for
this application.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

(Henry Spencer) :

It's just not all that hard to put essentially all of a laser's output
power into a few square meters of solar array at 40,000km. It's not worth
accepting major compromises just to avoid it.

How hard/expensive? Basicly how much diffirence in aiming 8000 miles vs
40000 miles is there in terms of costs.

Earl Colby Pottinger

--
I make public email sent to me! Hydrogen Peroxide Rockets, OpenBeos,
SerialTransfer 3.0, RAMDISK, BoatBuilding, DIY TabletPC. What happened to
the time? http://webhome.idirect.com/~earlcp

In article ,
Earl Colby Pottinger wrote:
It's just not all that hard to put essentially all of a laser's output
power into a few square meters of solar array at 40,000km. It's not worth
accepting major compromises just to avoid it.

How hard/expensive? Basicly how much diffirence in aiming 8000 miles vs
40000 miles is there in terms of costs.

The only real difference is bigger optics. Not *simpler* optics, mind
you, because even for much shorter distances you'll need active optics to
compensate for atmospheric turbulence. But even in quite large sizes,
this is very nearly off-the-shelf hardware now; the Keck telescope has
everything you'd need except for power-handling capability. The cost
would be higher but not a lot higher.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |