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Re: RE: starship-design: Plasma Engine




In a message dated 8/31/98 8:44:11 PM, lparker@cacaphony.net wrote:

>Steve,
>
>
>
>> The analysis I did a long time ago, which was in terms of
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>> mass-energy conversion and fuel-to-payload ratios, implied that
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>> fusion had pretty a exorbitant fuel-to-payload ratio for getting
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>> to high relativistic speeds (0.8 - 0.9 c), on the order of 10^6:1
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>> or worse.  To get a fuel-to-payload ratio of less than 10:1 we
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>> pretty much have to have an antimatter photon rocket.
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>>
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>> Timothy did indicate that you can get pretty good thrust with
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>> using lots of low-velocity reaction mass, but unfortunately if
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>> you make the reasonable definition that "fuel" is really "energy
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>> source plus reaction mass", this implies an even worse
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>> fuel-to-payload ratio, especially if you're trying to go for long
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>> continuous acceleration.  Sure, you can get theoretically good
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>> results if you make a reaction that pushes a lot of mass at a low
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>> speed in one direction and a little mass to very high speed in
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>> the other, but that only works if the reaction is essentially
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>> instantaneous.
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>>
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>> The ultimate result is that even for really fast interplanetary
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>> transportation you don't need to have any major amount of
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>> mass-energy conversion, but for relativistic interstellar travel
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>> you do; you have to convert a mass larger than the payload to
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>> energy to get a payload to speeds of 0.8c or higher, and this
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>> requires either an incredible amount of low-efficiency fuel or
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>> even a pretty amazing amount of antimatter.
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>
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>All true. To put it simplistically, an interstellar drive needs to expel
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>LOTS of reaction mass at HIGH velocity. Nothing we currently have can do
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>both at the same time. These newer plasma engine concepts come close and the
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>various concepts for "hybrid" antimatter catalyzed fusion may do even
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>better, but they all are still far short of what is really needed for a true
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>interstellar drive.
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>
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>Being a science fiction author is SO much easier...
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>
>
>Lee

Gives you an idea why ultra conservative NASA is geting desperate enough to
play withthe "breakthrough physics group."  ;)

Kelly