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*To*: KellySt@aol.com, stevev@efn.org, jim@bogie2.bio.purdue.edu, zkulpa@zmit1.ippt.gov.pl, hous0042@maroon.tc.umn.edu, rddesign@wolfenet.com, David@InterWorld.com, lparker@destin.gulfnet.com, bmansur@oc.edu*Subject*: Re: MARS*From*: T.L.G.vanderLinden@student.utwente.nl (Timothy van der Linden)*Date*: Fri, 01 Mar 1996 23:05:30 +0100

> >>>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. 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. Timothy