Re: MARS

[Brian Mansur <bmansur@oc.edu>]

>>4. Has anyone figured out just how long the accelerator needs to be since 
it
>>has to be linear?  More to the point, can we keep the linear accelerator
>>short enough and, therefore, light enough to produce relativistic exhaust
>>velocities?

>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.

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?

>>5. Can we even produce the magnetic fields in an accelerator necessary to
>>get an exhaust velocity of .9996c for .62kg/sec. or even a .75c exhaust
>>velocity using say a 1km long accelerator?  My understanding is that field 

>>generators that confine magnetics fields have a tendency to blow up.  I 
hope
>>I'm wrong but I thought you might know if this concern applies to the
>>designs we've discussed.

>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.