"Carey Sublette" wrote in message nk.net...
"John Thingstad" wrote in message
news
On 31 Aug 2004 03:05:11 -0700, Abdul Ahad
wrote:
..
Robert Forward suggest not bringing it allong but focusing huge
lasers on a sun sail. (probaly the most plausible suggestion I
have seen so far)

Once you've got a stationary phased laser array to deliver power to a
distant vessel, the question becomes:
"What is the most effective way to use this energy?"

A reflector sail is a terribly inefficient way to use it (unless you are
already travelling at high relativistic speeds).

What makes the solar sail attractive is that it is a simple inexpensive way
to harness energy that is free, and which is also within the reach of
current technology.

Once one contemplates building a vast space laser energy projector the
energy is far from free, and you are working in the realm of future
technology anyway.

Roughly speaking, energy in a rocket propulsion system is used most
efficiently when the exhaust velocity is approximately equal to the burn-out
velocity. In this realm the mass ratios are not extravagant, and the
fraction of energy ending up in payload velocity is pretty good.

So capturing the beamed energy, converting it to electricity, and using it
to power what is essentially a low energy (but extremely high current)
particle accelerator would provide efficient use.

I envisage a modular "sail" of independent units that collect light, convert
it to electricity and power arrays of accelerators that are harnessed
together to pull the spacecraft.





Larry Niven prefers the Buzzard ramscoop, a fusion jet that uses a
huge magnetic field in front of the ship to scoop up interstellar
hydrogen.
(Probaly won't work..)

From what I can see we are nowhere near solving the problem.
Even when fusing hydrogen, say, the energy to weight ratio is not
favorable.

--
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

People have proposed solar sails driven by laser or maser close to the
sun. Having the power source stay home has many advantages. However,
even a laser has some beam divergence so that as the sail gets VERY
far away the sail intercepts very little of the beam. You can imagine
concentrating the beam with some sort of lens but there is no material
that could withstand the required power densities. Furthermore, to
get good angular resolution of the beam with such a lens, you need
very large apertures, many tens of miles. People have proposed large
mirror systems consisting of very thin polymer but as these get very
large, they might be difficult to fabricate or control.
I propose an alternative to such a lens or mirror.

The so-called M2P2 drive has been proposed where a plasma is
"inflated" by a magnetic field that also traps the plasma. Int eh
M2P2 drive, the solar wind interacts with the plasma to produce
thrust. The M2P2 mag-sail can be VERY large, tens of Km or more.
Instead of using the M2P2 mag field/plasma sail to provide thrust, I
propose to use it as either a lens or collector of radiation.
As a lens, a plasma has an index of refraction that depends ont he
wavelength of the radiation and the plasma density. Such a lens would
have small index and probably very long focal length but could have
extremely large aperture. Such a lens could be placed in the outer
reaches of the solar system to direct more radiation from the power
source located near the sun onto the distant spacecraft. Such a lens
might also be used as a telescope collecting radiation from a distant
object for pure science. The lens could also be used to collect the
weak signal from the distant spacecraft. Of course, the lens needs a
power source and plasma media to replenish that that is lost as the
plasma leaks into space.
As a collector of radiation, teh trapped electrons in the plasma
interact with the incoming wave and gain energy and are collected and
the signal extracted from them. Obviously, the efficiency is
dependent on the plasma density.
Such lens systems (or even mirror systems) would have severe optical
aberrations and I am assuming this problem could be solved.


Any thoughts on this?