"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/

Carey:

Instead of having the starship carry the fuel and use on-board energy
to accelerate it, why not have the ship capture fuel packets that have
been previously been sent out in streams so that when the ship is
accelerating, it reaches a packet at very small relative velocity.
There is no net savings in energy but this does enable you to
investmost of your capital in the fuel mass driver close to home where
it is able to serve several ships. Of course, this does not solve the
problem of slowing down but does help in the acceleration phase. For
a "Mini-Mag Orion", I once did the calculation and found that if the
fuel was sent as a stream of fuel from a neutral particle beam, the
fuel current was reasonable as was the energy/atom in MeV of the fuel
kinetic energy (I dont remember the specifics). A fuel mass driver
close to home could use an energy source of large size that doesnt
need to be acclerated. Even a ion drive scaled up from the existing
Deep Space craft maybe becomes reasonable if you do not have to carry
the fuel but capture it in a stream sent to the craft which uses a
nuke reactor to provide electricity for the ion engine. I am sure
this idea isnt original but have never seen it elsewhere.

David OHara