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From: ISW@cup.portal.com (Isaac S Wingfield)
Newsgroups: sci.electronics,sci.physics
Subject: Re: Effectiviness of rail vs. coil guns
Message-ID: <28795@cup.portal.com>
Date: 11 Apr 90 06:34:26 GMT
References: <12336@etana.tut.fi>
Distribution: usa
Organization: The Portal System (TM)
Lines: 57

Karri Palovuori writes:

>How efficient is a rail gun? I mean in per cent, how much kinetic energy
>can be achieved per joule electrical energy consumed?
>
>I was just calculating the impulse, that a rail gun can give to the projectile
.
>The force is I*l*B, if B  (magnetic flux density) is perpendicular to the
>current I. l is the width of the projectile, say, 10 mm. On the other hand
>the current equals dQ/dt, Q is electrical charge. The impulse is Fdt, F is the
>force. The impulse can also be written m*dv, v is velocity. From these you can
>get:
>
>m*dv = l*B*dQ <=> dv = l*B*dQ/m
>                                                                              

>Since the mass, flux density and width of the projectile are constant, the    

>equation can be integrated to get
>
>delta-v = l*B*delta-Q
>
>The magnetic flux can be 2 tesla if it is generated with separate iron-cored
>coils.                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^

Back in the early '80's I had a conversation with a Scientist who was actually
working on rail guns. According to his description of the technology at that
time, no magnetic field other than that generated by the current was involved.

He said the force was proportional to I^2 / inductance per unit length (I
think; at any rate, it was current squared divided by some function of L). In
order to get interesting forces you need (as I've said before), interesting
currents -say 1/2 to 1 MegAmp.

In order to get currents like this to flow, it's necessary to absolutely
minimize the inductance. Regular capacitors, no matter the voltage or
capacitance, will not discharge fast enough due to internal L.

$pecial low inductance capacitor$ are u$ed for thi$ purpo$e.

Note also that a large charged capacitor can produce substantial internal
*mechanical* forces. They're usually balanced by opposing charges on the next
plate over, but under conditions of instantaneous discharge, large imbalances
can exist, which can destroy the capacitor (read "EXPLODE"). If the cap is not
designed for fast discharge, don't use it for this kind of application.

Connections external to the cap also must be low-L. "For best results, these
wires should be wider than they are long".

He mentioned that if "muzzle" velocity could be increased by between one and
two orders of magnitude, two pellets of deuterium fired at each other would
undergo fusion due to sheer mechanical impact.

Isaac (don't point that thing at me, buster)
isw@cup.portal.com