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New idea Laser launcher/scoop systems

Kelly Starks writes:
 > 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.

Actually, now that I think about it there is a dangerous likely flaw in
this fuel launcher idea.

Remember a while back when I ran the numbers on fuel-to-payload ratios
for different fuel types?  Remember that hydrogen came out at _minimum_
to need 1,000,000 units of hydrogen to one unit of payload to reach even
low relativistic speeds.

So, suppose we do prelaunch fuel along the ship's track, on the
assumption the ship will catch up to the fuel and can then pick it up
and use it to decelerate.  For each chunk of fuel, how much ship mass
can that chunk effectively decelerate?  Not bloody much if the ship is
travelling at relativistic speeds.  The fuel packet may not be able to
decelerate its own canister down from 1/3 c, let alone any useful
fraction of the ship.  At 1/3 c, you'd have to put more energy into
accelerating the fuel than you could get out of the fuel.

If we could launch chunks of antimatter, it would be no problem.  But
the fundamental problem is that hydrogen fusion is nowhere near
efficient enough to reach high relativistic speeds; even with 1000:1
fuel-to-payload ratios you can't get near 1/10 c.