Re: Solar powered lasers in space

On 14 Sep, 02:44, Alan Anderson <arand...@xxxxxxxxxxxxx> wrote:

These systems are easily adapted to controlling beams from space and
having the beams avoid unwanted targets while at the same time
illuminating desireable targets.

From LEO, the time delay is significant.

How high are the space-based beam emitters? Let's call it 300 km, so
the round-trip light travel time is a nice round 2 milliseconds. How
fast would you need to move your hand through the beam in order to
outpace its ability to avoid illuminating your thumb? Let's call the
width of your thumb a nice round 2 centimeters.

Two centimeters in two milliseconds. That's a nice round ten meters per
second. And it's an easily achievable speed. Wave your hand quickly
through the beam and the system won't be able to avoid your thumb. A
bird flying into the beam at a sedate ten miles per hour would have the
leading half inch of its head and wings exposed to the full strength of
the beam. A small aircraft flying at 200 knots would travel more than
20 cm in the time it takes for the active holographic technique to
respond.

And that's the last I'm going to say about it.

40% seems a very high efficiency for a laser, but even if this figure
is accepted there is one overiding problem. At noon in a desert you
have about 1kw/m^2 coming in through solar power anyway. The obvious
question to me is why not have solar power in the desrt and be done
with it. Lets generate power and split water into hydrogen and oxygen
in the desert using the energy that comes from the Sun anyway. Why
have a space laser as an intermediate stage?

Is it to get power 24/7? Well you have to be above LEO to effectively
extend the desert day.

The usual version of a space energy system is a microwave system. This
has the advantage that it could be beamed fairly close (close enough
for high voltage transmission lines to be practical) to where it is
consumed. Would be at MEO and operating 24/7. Phased arrays would
ensure the correct location and the correct amount at each location.

There again the system would have to be economical with regard to
alternative concepts. My pet concept is in fact a biological system
for generating hydrogen.

In fact the niche a laser system would occupy is in fact that of
providing intense energy for space launches and possibly for other
types of transportation.

Let us think for a moment about optical systems, phase arrays etc.
Some time ago I contributed to a thread on a space telescope

http://groups.google.co.uk/group/sci.space.policy/browse_frm/thread/89ccc3292c970893/5068e6a9a0d213b3?lnk=st&q=&rnum=5&hl=en#5068e6a9a0d213b3

and came to the conclusion that in space diffraction limited optics
could be made extremely large. Suppose our laser system (it is
important to have a system not just a sinle laser) is 1km across, we
wish to focus on an area 1.22m in diameter, we are 10,000km away, and
our wavelength is 1 micron. How large? 1.22 lambda/d = 1,000,000 and
we are 10^7 m away. We need a mirror approx 10m across. 100m would be
quite easy to build.

The talk in another posting was about velocities and reaction times.
In fact the criteria is unanticipated acceleration. The velocity can
be anticipated. Lasers are capable of giving hundreds of MW/m^2 if
focussed. That is their forte. Not in power generation systems.

Lasers could help with deep space probes and also with supersonic and
hypersonic aviation. I understand that NASA has in fact performed
tests using a ground based system.
http://members.fcac.org/~sol/station/planetar.htm

http://www.esa.int/SPECIALS/GSP/SEM2YD6DIAE_2.html Powering spacecraft
on surface of Mars. Even more useful in deep space.

http://personal.ee.surrey.ac.uk/Personal/L.Wood/DARPA-fractionated-spacecraft/darpa-fractionated-spacecraft-agenda.pdf
DARPA conference. Shows work is going on.

http://www.spacedaily.com/news/laser-97b.html This is quite an old
reference

http://groups.google.co.uk/group/sci.space.policy/browse_frm/thread/89ccc3292c970893/5068e6a9a0d213b3?lnk=st&q=&rnum=5&hl=en#5068e6a9a0d213b3


- Ian Parker

.

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