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>>>4. Has anyone figured out just how long the accelerator needs to be since 
>>>has to be linear?  More to the point, can we keep the linear accelerator
>>>short enough and, therefore, light enough to produce relativistic exhaust
>>Some long time ago I figured out that it would probably be too long, the
>>formulas are not so easy to integrate so at that time I used some repeating
>>summation. I assumed that we would not have a constant acceleration of the
>>mass, but a constant power input. This means that initially it accelerates
>>fast but at the end much slower. Relativistic effects do make this
>>difference worse.
>>But as Kevin corrected me a week ago, why not use a torus instead a lineac?
>>You seem to have a reason for not liking a torus, could you tell me what
>>that reason is?
>A few nights ago I read in an old LIT newsletter about someone noting that a 
>torus would cancel the acceleration gained by pushing against the ions.  My 
>admitantly limited understanding of vectors made me think about how you 
>can't push against your sides and expect to go forward.

What happens is that the particles are pushed at their sides but also at
their backs.

>I hope that I am very wrong because a torus design makes for nice stacking 
>of a potentially flimsy accelerator.   What I mean is has anyone thought 
>about what it would be like, structurally, to push what amounts to a 10 km 
>long acceleration tower at 10 m/s^2?  For that matter, speaking of other 
>starship structures, what would it be like to push a ram scoop (a really 
>tall wire mesh cone) at the same rate?

It's quite certain that the one who wrote that was wrong, because there are
many torroidal accelerators in the world. Of course a torroidal accelerator
does need much more energy than a linear accelerator, because besides
pushing the particles forward it continously has to push the particles aside.

>>I think we should not worry about that too much, for me this is just a
>>problem for the gigantic-energy stack (i.e. problems involving creation and
>>containment of gigangtic energies).
>Unfortunately, the hardware involved in accomplishing energy containment for 
>our accelerator will up our ship dry mass.  A 10km long ion accelerator is 
>not going to be terribly light as it is.  I originally was under the 
>impression that we could keep the ship dry weight at 100,000 t o 250.000 
>tones.  Sadly, it seems that we are putting more and more mass into the 
>engine structure which exponentially increases our fuel/RM problems.  

Say one metre weighs 100 kg (including magnets and all) then 10 km=1000 tons.
That may be acceptable, but then again 100 kg/metre is a very wild guess.

But how much will a 10 by 10 km sail weigh? Say 1 square metre=100 grams, then 
1E8 m^2=10,000 tons.

You may think 100 grams is too much, but the sail has to drag the whole
ship, so it should be quite strong.