Re: HKP and the Kra'Vak

On Fri, 4 Jun 1999, djwj wrote:

> Oerjan Ohlson wrote:
> 
> >> We had a discussion about the Railgun earlier. Let's review:
> >> The basic construction of the railgun (at its simplest this is a
> >>scientific
> >> device for acclelerating particles) consists of a LARGE capacitor,
> >>two
> >> stainless steel rails, and a small thin wire. The capacitor is
> >>connected
> >> with each lead connected to one rail, forming an electrical circut
of
> >> rail-capacitor-rail. The rails are mounted paralell to each other.
To
> >>fire
> >> the basic railgun you drop the thin wire across the rails
completing
> >>the
> >> circut.
> 
> >With you so far, except that the "small thin wire" is basically an
DSAP
> >rod penetrator.
> 
> No you aren't with me. The small thin wire is the Railgun equavilent
of
> gunpowder. the DSAP penetrator is placed in front of the point you
drop the
> wire (the specifics of not dropping the rod on the rails I don't know
> about), the wire vaporises to a plasma state, is propelled by magnetic
> accelleration, which in turn pushes the penetrator rod down the
barrell.

this was clearly a misunderstanding, then: djwj and oerjan are
describing
essentially the same system. the key characteristic is the use of the
Lorentz effect (current flow along the rails sets up a transverse
magnetic
field which interacts with the flow through the round to generate a
propelling force) to accelerate the round. in oerjan's description, the
conducting part of the round is the projectile itself, a "fairly
standard
rod penetrator [in a sabot]", in djwj's, the conductor is a plasma
generated by the electrical vapourisation of a metal wire.

i must say that i'm a little troubled by oerjan's description - firstly,
the round needs to be a good conductor, which i would imagine precludes
making it from depleted uranium or tungsten, although i suppose you
could
put a copper coating around it, and secondly, if the accelerating force
is
coming from the current flow throught the round itself, why is a sabot
needed? unless that sabot is the bit with the conductor, in which case
it
all makes perfect sense.

djwj's description matches what i was told by the television a while ago
(must be true, then ...), which is that the round used was a block of
kevlar with an aluminium foil backing: the aluminium was vapourised to
generate the conducting plasma. of course, that was a while ago, and
highly experimental, there is no reason why both these approaches aren't
being used, and oerjan is, after all, the professional weapons engineer
here (i'm assuming djwj isn't, but i actually have no idea ...).

some back-of-the-envelope scribblings (well,
back-of-someone-else's-chemistry-notes, anyway) indicate that railguns
would be troubled by the Hall effect - accelerating a plasma though a
magnetic field will generate a back-voltage which opposes the current
needed to drive the round. i would guess this is a limiting factor in
railguns, but frankly i have no real idea.

> > No, they don't. Current military *railguns* - only experimental so
far
> > - - use the Lorenz force (the one caused by the electrical current
> > flowing in the closed loop) to propel a fairly standard rod
penetrator
> > - - "fairly" standard, since the sabots have to be different from
the
> > ones you use in a normal DSAP tank round.
> 
> This sounds more like a Mass Driver than a Railgun.

massdrivers also use magnetic acceleration, but not the lorentz effect:
rather, they use a sort of magnetic travelling wave thing (i don't know
what this is formally known as), like linear particle accelerators and,
i
think, maglev trains. i think that this requires that the round be
diamagnetic (like aluminium - it tries to escape from a magnetic field).
anyway, it's more complicated than a railgun, but requires no plasma and
no physical contact between the round and the barrel, as opposed to a
railgun, which does: the conductor has to touch the rails (leading to
heavy wear) and in the plasma type, there has to be a seal between round
and barrel for the plasma to be effective.

> >This is... wrong. Each action creates a reaction of equal size in the
> >opposite direction; Newton's laws don't magically disappear just
> >because you use magnetism instead of explosives as a propellant.
> 
> You've never left heavy electrical construction equipment ungrounded
have
> you? I don't know how it works, I think it has something to do with
the
> action-reaction affecting the magnetic "event" rather than the weapon
> directly, but asking a dedicated physisist (or Geo-Magnetist) about
this
> would be a better idea.

newton's third law is not violated. full stop.

> The railgun, originally, was a device for small particle accelleration
> experiments,

i'm pretty sure you're thinking of a massdriver here. railguns require a
conducting object to accelerate, and i'm fairly sure electrons et al
don't
conduct electricity :). and i'm not sure about the 'small' - there are
some damned big particle accelerators around.

> but like most scientific devices someone figured how it could
> be used for war.

i've yet to see a bench centrifuge or a spectrophotometer used for war.
they're pretty heavy, though, so i suppose you could hit someone with
one.

anyway, those future tech kinetic weapons in full:

railgun - uses the lorentz effect to accelerate a round between two
electrically charged rails. may or may not involve plasma.

massdriver - uses magnetic peristalsis to accelerate a round, like a
particle accelerator. doesn't involve plasma.

electro-thermal gun - uses electrical heating to drive expansion of a
gas
or plasma to push a round down a barrel. involves plasma, or hot gas at
any rate.

tom