Re: Mass Drivers (was GEV and Grav Vehicles)

A lot of interesting discussions have been going here about mass drivers
and 
how they work.	This has always been a big interest for me.  Some 
clarification of what a mass driver is might help.

A mass driver (AKA a "Rail Gun") uses magnetic attraction between 
electromagnets to propel a payload.  The electromagnets are made up of
coils 
conducting current which produces magetic fields.  The payload is stored
in 
a bucket coil which makes up one electromagnet.  The other
electromagnets 
are made up of a series of drive coils which form a tube.  The 
electromagnetic fields are used to repel or attract each other.  AFAIK
the 
most common mass driver in use now is the pull type using attraction.

Most people seem to think there is just one big force involved that 
accelerates the payload when it actually uses several smaller pushes or 
pulls over the length of the tube.  As the bucket accelerates down the
tube 
each drive coil turns on followed immediately by one turning off.  Over
time 
the bucket gains more and more speed from the interaction of magnetic
fields 
converting electrical energy into physical energy.

I think the first mass driver which came out in the 70's propelled
objects 
at about 33 g's.  The last one that I am familiar with could propel
objects 
as high as 1,800 g's.  They were used to study potential surface to
space 
tranportation as well as SDI.

In videos of tests that I have seen, the only noticeable transfer of
energy 
back to the tube itself was when the controls for the driver coils were
not 
in sync.  The coils would cause a combination of push and pull and the
tube 
would appear to shake back and forth.  This was only done to demonstrate

what could happen if the control was not precisem, otherwise the tube
was 
typically mounted to a secure platform.  The video greatly illustrated
how 
the mass driver remains still during firing by showing it fire with a 
something like a fish tank attached to the tube mount.	The water in the

fish tank never moved when the controls were functioning correctly. 
When 
they did not, the water shook back and forth.  I read somewhere where
they 
are using mass drivers now for research on earthquake preventitive
measures 
for buildings.

Mike

Michael Miserendino
Senior Software Engineer
Lincoln Re
mtmiserendino@lnc.com

>>> owner-gzg-l@CSUA.Berkeley.EDU at internet 12/01/99 02:19AM >>>
>Remember, energy is mv**2, but momentum is mv. In other words, if you
have 
a
>very small projectile and accelerate it relatively slowly, you will
build
up a
>lot of energy without much reverse momentum. The "v" component would be

more
>important than the mass component. The launcher and vehicle would have
a 
lot
>of mass compared to the projectile. I think it was in the end of
Gibson's
Mona
>Lisa Overdrive that a mass driver fired from a blimp. Recoil wasn't
much of 
a
>problem.

Heh.  Keeping in mind, of course, that Gibson himself is not exactly a
techno-genius...  He's a guy with a vivid imagination, but wrote all
those
books on a TYPEWRITER because he didn't own a computer...

Someone more physics-literate than me correct me here if I'm wrong, but
how
do you get around the Newtonian "for every action there is an equal and
opposite reaction" thing...?  If a 1 kilo projectile is accellerated to
1000m/s in a combat-useful length of time (ie relatively immediate) then
there's going to be a recoil whether it was fired out of a present day
cannon or accelerated by a mass driver.  Recoil doesn't go away just
'cause
you're using electro-magnetism rather than chemical explosives, does it?
(leaving aside such stuff as recoil compensators, inertial dampers, etc
etc).  If a tank (65 tonnes) now gets rocked back on its tracks by a
shot
from the main cannon, then a gev is going to suffer the same effect
achieving the same result, no?	If you're using a mass-driver on the
gev,
it will still have to deal with substantial recoil, won't it?  If the
GEV
is of the same mass as the tank (65 tonnes), it will have to have some
kind
of counter-force to the recoil, or it will be shoved around...

>
>If the vehicle can hover ala a helicopter

...then it isn't a GEV...

, it would need to generate more lift
>than you get just from ground effect.

remember to keep in mind the difference between "ground effect" a-la GEV
(pressurized cushion of air contained inside a flexible skirt which
supports the mass of the GEV) and "ground effect" a-la aerodynamic-lift
aircraft (such as helocopters and airplanes).  Quite different...

You don't really get "lift" per-se from hovercraft.  The active
aerodynamic
force "lift" keeping the craft up off the ground is minimal (you get a
bit
off the lift fans, but not much... certainly not enough to keep the
hovercraft up.	Helocopters derive their "staying-up" power from
aerodynamic lift (ie lower-pressure air on top of their wings, in this
case
rotor blades, and higher pressure air below the wings... the high
pressure
pushes into the low pressure, thereby keeping the wing up... but the
pressure differences are achieved by the speed of the wing moving
through
the air and the shape of the aerofoil)	whereas hovercraft are just
creating an enclosed cushion of air to ride on.  Same as how an
avalanche
can travel so far, so fast - riding on a cushion of compressed air which
hugely reduces friction.

 I think then, by definition, you don't
>have a GEV but a rotary wing tank. Or, perhaps, a hybrid
GEV/helicopter. 
But
>the lift required would be enormous for a helicopter. I'm not sure you 
could
>lift a tank with a bottom mounted propeller. I'd imagine you'd either
need
>very long blades or an incredible amount of speed to generate the
necessary
>lift. I'm not sure that's feasible. I'd love to see a calculation on
the
speed
>of the blades and the temperature they'd build up from spinning fast
enough.

>

I'd say impossible.  Or at least WAY unlikely. You'd need rotors so long
as
to be impractical for the bottom of an armoured vehicle - spinning them
fast doesn't do you too much good 'cause then they go supersonic and it
really starts to screw things up...  That's the main limiting factor for
top speeds of helicopters today - they have to avoid getting their
rotors
over the speed of sound or nasty things happen - especially since it
happens to the advancing rotor on one side of the aircraft and not to
the
retreating rotor on the other side.

>Oh, and the skirt... You'd need SOMETHING around blades to protect it,
I'd
>think. 
>
>On the other hand, with a fusion plant maybe you could do it. I'm still
not
>sure it's feasible. I have my doubts, but they are kind of fun. I
always 
did
>like the Slammers stories (even if Drake does have a problem with 2-D
>characters...).

I like 'em too.  But you aren't going to have flying tanks of the
Slammers'
size by using bottom mounted rotors, that's for sure...  

>
>Personally, I'm a big follower of strong AI (I know... I've been down
this
>road before on this list! *L*). I personally think the way to go is
turretless
>tanks. Essentially self propelled anti tank weapons mounted on a
rising,
>rotating mount. Just a chassis with an autoloading gun that can rise
from a
>hull down position. Then, do away with the human driving it, by
replacing 
the
>driver with an AI. There's a limit to how small they could be, as they 
would
>have to hold ammunition. Couple it to a small calibre projectile fired
from 
a
>mass driver, and you have something very hard to hit and very
powerful...
>

But boring to game with.... it's the human element that makes the games
fun
at all :)

Adrian Johnson
ajohnson@idirect.com