Hi,
A couple days back, Tim ran threw the numbers to show what acceleration a
'fuel packet' would need to get it up to 1/3rd light speed. Assuming a 100
meter long launcher, the numbers came out at E14 m/s^2 I.E. 10 trillions
G's. I was obviously upset to hear this. However that information and a
flip comment I made about the size of a fuel packet ("it could be as big as
a freight car if you wanted") combined.
Laser launcher
Assume the average fuel canister is the size of a 6 meter in diameter
cylinder about 5 meters long. I think that should hold about a hundred
tons of fusion fuel (6Li?) but of course that would vary. This canister is
heavily reinforced (you'll see why), and the ends are covered in a thick
plug of reaction mass (could be anything from fiber reinforced ice, to
solid kevlar). A floating laser 'tug' fires a laser at this plug of
reaction mass. One quick pulse to vaporise a layer off the bottom. Then a
heavy pulse to turn the vapor to super heated plasma. I'E. a pulsed
rocket. Keep this cycle up a couple hundred times a secound. You now have
a laser rocket.
Note the canister has no internal systems. Steering is handeled by aiming
the beam off to one side of the base. The uneven thrust will turn the
canister, and subsequent pulses will thrust along the new vector. Range is
limited by the optical precision of the laser. Given what a Hubble
telescop can do over interstellar ranges. I'll assume the system can do
this out to 100,000 miles.
So, if you station a laser tug every 100,000 miles or so. They can take
turns boosting a string of canisters. Given orbital mechanics. They will
have to be continuously boosting themselves around to stay acceptably close
to the 'Launcher' track. (No stable orbits.) Note that the exact
possition of the tugs isn't important, but they must know exactly where
they and the canisters are. Given this system the launcher can be as long
as you need at the moment. If you space them out every 60,000 miles for
100,000,000 kilometers (about a 1,000 tugs spaced from here to Mars.) The
average G load on a canister exiting at a speed of 1/3rd C, is E5 m/s^2, or
about 100,000 gs. Which seems reasonably possible for a solid block of
reinforced metal and whatever.
Laser scoop
The ship uses a variation of this to catch the fuel canisters. The ship
has bow lasers that fire on the back of the canister to boost it forward
and stear it onto the ships course vector. Assuming the laser booster can
function hiting the base at over 60 degrees from the center line, and can
hit the target at 100,000 km. The ship can 'catch' a fuel canister over
80,000 km off to the side. Far better than the scoop systems we were
considering. Which could increase the the range at which the ship can be
externally fueled. (Hopefully out to about 3,000 au's, since it would take
that far for the ship to boost to 1/3rd c.)
If we assume a fusion fuel with a specific impulse of 2 million (we might
even get to 2,400,000 if we push it) the ship would need to receave 5
times its weight in fuel. To get to 1/3rd C.
I.e.
100,000,000 m/s delta V
--------------- = 10,000,000 secounds of thrust
10m/s^s Accell.
Spec imp of 2,000,000 pounds of thrust/(pound of fuel consumed per secound)
= 5 times total ship mass.
?? Thats a lot better than I was expecting.
Actually we could do better than that, since I wasn't considering the extra
boost you could get using the canisters as extra reaction mass.
Please sanity check this everyone!! I am feverish!
Kelly
Oh,
When the ships returning to Sol you don't need the fuel accelerated to high
speeds. So you launch softer canisters largly composed of reaction mass,
probably Ice. The ship stears them into its path and dethonates them with
its lasers and dust shield. The ship then rams into the resulting high
temperature plasma with a highly charged bow plate. The power for the bow
plate comes from leting a fraction of the plasma blow through the
accelerator core.
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Kelly Starks Internet: kgstar@most.fw.hac.com
Sr. Systems Engineer
Magnavox Electronic Systems Company
(Magnavox URL: http://www.fw.hac.com/external.html)
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