# Re: New idea Laser launcher/scoop systems

```At 11:40 AM 3/6/96, Steve VanDevender wrote:
>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.

I ran some numbers for fuel to ship mass ratios to get to (or down from)
various speeds, depending on the spec impulse.  (I think that was one of
the docs I sent around for review.  1/10th was fairly easy, 3/10 ths became
obsurd without well over 2,000,000 spec imp fuel.  Higher then .3c and you
get a geometric escalation of fuel to weight ratios.  (Which is why I never
consider them for internally fueled operations.)

The fuel launched by the launcher, will take more energy to launch then it
will return, but it will return it on the ship.  Look at it this way, the
microwave beam systems will probably need to beam hundreds of times (maybe
thousands of times) the power that the the ship will catch.  Other wise
there's to much risk the sail will drift part way out of the beam, or into
a weak area of the beam.

We are not going to be able to do this without a fantastic amount of power.
Which means a huge support infastructure.

Lifes tough with those kind of speeds.

Kelly

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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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