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Source for high energy pulse

E

Ethan

Jan 1, 1970
0
A couple years ago a buddy and I built a rail gun in the garage. The
goal was simply to see how fast we can make something move.

For anybody not familiar with this, it is a type of linear motor that
uses two parallel conducting rails and a conductor that slides along
them. When a large current is put through the rails the resulting
magnetic field creates a force that pushes the conductor along the
rails. In theory, extreemly high velicities can be achieved this way.

It worked reasonably well, however we finally ran into the limitation
of not having a good enough way to store the energy to be dumped into
the projectile. We were using a capacitor bank made up of the
capacitors from scrapped motor controllers. Bolting togethor 90 motor
controller cap banks gave us a 0.18F, 180V capacitor, so about 3kJ.
When you short it across the rails you get one hell of a lot of
current and the projectile conductor really goes. The best velocity
was between 200 and 300 mph.

The limitation was mostly due to not enough energy. I read somewhere
that a high power rifle slug can have up to about 6kJ of kinetic
energy, and we were only getting 30-40J of kinetic energy. (Our
efficiency was pretty low)

I have been meaning to dust the thing off and try again, but have not
been able to come up with a better energy source. I would like to be
able to dump better than 100kJ of electrical energy in about 1mS.
Does anybody have any ideas about how to do this? The most compact
way to store energy is chemically, so mabye there is some way to get
the energy out of a battery really quickly, or use an explosive to
push a magnet, like a homopolar motor, or some such thing.

If anyone has any ideas for an energy source I would love to hear
them.

Ethan
 
J

John Larkin

Jan 1, 1970
0
A couple years ago a buddy and I built a rail gun in the garage. The
goal was simply to see how fast we can make something move.

For anybody not familiar with this, it is a type of linear motor that
uses two parallel conducting rails and a conductor that slides along
them. When a large current is put through the rails the resulting
magnetic field creates a force that pushes the conductor along the
rails. In theory, extreemly high velicities can be achieved this way.

It worked reasonably well, however we finally ran into the limitation
of not having a good enough way to store the energy to be dumped into
the projectile. We were using a capacitor bank made up of the
capacitors from scrapped motor controllers. Bolting togethor 90 motor
controller cap banks gave us a 0.18F, 180V capacitor, so about 3kJ.
When you short it across the rails you get one hell of a lot of
current and the projectile conductor really goes. The best velocity
was between 200 and 300 mph.

The limitation was mostly due to not enough energy. I read somewhere
that a high power rifle slug can have up to about 6kJ of kinetic
energy, and we were only getting 30-40J of kinetic energy. (Our
efficiency was pretty low)

I have been meaning to dust the thing off and try again, but have not
been able to come up with a better energy source. I would like to be
able to dump better than 100kJ of electrical energy in about 1mS.
Does anybody have any ideas about how to do this? The most compact
way to store energy is chemically, so mabye there is some way to get
the energy out of a battery really quickly, or use an explosive to
push a magnet, like a homopolar motor, or some such thing.

If anyone has any ideas for an energy source I would love to hear
them.

Ethan

Somewhere in the distant past I recall a high-field (megagauss+)
electromagnet that was powered by a mechanical energy storage thingie.
There was a metal disk, I'm guessing a foot or two in diameter and
maybe an inch thick, on a shaft, spinning very fast. There was a
magnetic field across the disk (electromagnet? can't remember) and a
contact brush. So spin it up, apply the transverse field, and it
becomes a single-turn generator that powered the actual electromagnet
coil. I recall that the thing stopped in half a revolution or
something from maybe 30,000 RPM, dumping all its stored energy.

Generators are sometimes used as pulse power energy storage devices. I
wonder if a car starter on a flywheel might be useful here; they're
pretty low impedance devices. Hook the brushes up to your rails, spin
it as fast as you dare, energize the field hard, and push your
projectile onto the rails. Parallel many for more oomph.

Something like that.

John
 
S

Scott Stephens

Jan 1, 1970
0
Ethan said:
I would like to be
able to dump better than 100kJ of electrical energy in about 1mS.

Oh my, we are ambitious! But 100kJ isn't for real - you will warp yer
rails. There is an interesting website on hyperplastic magnetic metal
forming that explains.
Does anybody have any ideas about how to do this? The most compact
way to store energy is chemically, so mabye there is some way to get
the energy out of a battery really quickly, or use an explosive to
push a magnet, like a homopolar motor, or some such thing.

Every so often I begin contemplating how to turn a bullet into a flux
compression generator. Sometimes I think making the cartridge from
ferrite-glass, fast-quenched to shatter, might save me the trouble of a
pulse compressor.
If anyone has any ideas for an energy source I would love to hear
them.

I found some 10J kilovolt capacitors on the Herbach & Rademan site
today, was tempted to buy a few for snicks 'n grins. Not being rated for
pulse duty, they will soon die. Expensive Maxwell pulse caps can often
be found on E-bay.

Something that might prove interesting (and practical) would be a
mechanical wakefield-like gun. Tiny bullet in magnetic sabot gets fired
from conventional barrel. Magnetic sabot looses energy to coils around
barrel, which is transfered to a pulse-compressor. The bullet leaves the
conventional (gas-accelerating) barrel and enters the magnetic rail
accelerator, where the energy pulse from the magnetic sabot accelerates
the tiny slug to hyper-velocities.

Preferred uses would include shooting needle-sized projectiles through
tank armor and at long range, aircraft. Perhaps even satellites =)

No doubt you'll get better answers from the Railgunners mailing list, or
perhaps alt.high-voltage, et.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************
 
M

Mac

Jan 1, 1970
0
A couple years ago a buddy and I built a rail gun in the garage. The
goal was simply to see how fast we can make something move.

For anybody not familiar with this, it is a type of linear motor that
uses two parallel conducting rails and a conductor that slides along
them. When a large current is put through the rails the resulting
magnetic field creates a force that pushes the conductor along the
rails. In theory, extreemly high velicities can be achieved this way.

It worked reasonably well, however we finally ran into the limitation of
not having a good enough way to store the energy to be dumped into the
projectile. We were using a capacitor bank made up of the capacitors
from scrapped motor controllers. Bolting togethor 90 motor controller
cap banks gave us a 0.18F, 180V capacitor, so about 3kJ. When you short
it across the rails you get one hell of a lot of current and the
projectile conductor really goes. The best velocity was between 200 and
300 mph.

The limitation was mostly due to not enough energy. I read somewhere
that a high power rifle slug can have up to about 6kJ of kinetic energy,
and we were only getting 30-40J of kinetic energy. (Our efficiency was
pretty low)

I have been meaning to dust the thing off and try again, but have not
been able to come up with a better energy source. I would like to be
able to dump better than 100kJ of electrical energy in about 1mS. Does
anybody have any ideas about how to do this? The most compact way to
store energy is chemically, so mabye there is some way to get the energy
out of a battery really quickly, or use an explosive to push a magnet,
like a homopolar motor, or some such thing.

If anyone has any ideas for an energy source I would love to hear them.

Ethan


I don't know, but it seems like a bank of car batteries would do the
trick. You might need parallel series combinations, depending. You would
have to do some experimentation to find the equivalent series resistance
of the batteries, then work out what arrangement would deliver the most
current to your load. I guess the basic rule is that if the rail pulls the
battery voltage close to zero (when the projectile is running), add more
batteries in parallel. When the voltage starts to get near 7 or 8 Volts,
you are probably better off adding another bank of parallel batteries in
series with the first bank, as you have reached the point of diminishing
returns for parallelism.

If you then add another bank in series, you may want to adjust the number
of parallel batteries in each bank upwards. The goal should be to have at
least 6 Volts across each parallel pack during discharge. I have no idea
how many batteries this will end up using. It may be totally infeasible.

Anyway, it sounds dangerous and fun. Be careful!

Oh, and you will need very thick cable, with very short runs.

Mac
--
 
B

Bert Hickman

Jan 1, 1970
0
Ethan said:
A couple years ago a buddy and I built a rail gun in the garage. The
goal was simply to see how fast we can make something move.

For anybody not familiar with this, it is a type of linear motor that
uses two parallel conducting rails and a conductor that slides along
them. When a large current is put through the rails the resulting
magnetic field creates a force that pushes the conductor along the
rails. In theory, extreemly high velicities can be achieved this way.

It worked reasonably well, however we finally ran into the limitation
of not having a good enough way to store the energy to be dumped into
the projectile. We were using a capacitor bank made up of the
capacitors from scrapped motor controllers. Bolting togethor 90 motor
controller cap banks gave us a 0.18F, 180V capacitor, so about 3kJ.
When you short it across the rails you get one hell of a lot of
current and the projectile conductor really goes. The best velocity
was between 200 and 300 mph.

The limitation was mostly due to not enough energy. I read somewhere
that a high power rifle slug can have up to about 6kJ of kinetic
energy, and we were only getting 30-40J of kinetic energy. (Our
efficiency was pretty low)

I have been meaning to dust the thing off and try again, but have not
been able to come up with a better energy source. I would like to be
able to dump better than 100kJ of electrical energy in about 1mS.
Does anybody have any ideas about how to do this? The most compact
way to store energy is chemically, so mabye there is some way to get
the energy out of a battery really quickly, or use an explosive to
push a magnet, like a homopolar motor, or some such thing.

If anyone has any ideas for an energy source I would love to hear
them.

Ethan

Ethan,

For the amateur experimenter, the affordable choices will pretty much
come down to a secondary cell (lead acid) battery bank or a high voltage
capacitor bank. Preaccellerate the projectile before it enters the rail
gun using compressed air. BTW, if you use a battery bank, you'll need a
beefy fusible link in series with the gun in case the projectile gets
"stuck"... :^)

-- Bert --
--
 
S

Sir Charles W. Shults III

Jan 1, 1970
0
Real rail guns sometimes use flux compression generators- where
capacitors are discharged into a winding that is then "squeezed" by a shaped
explosive charge. As the compression wave travels up the windings, they are
compressing the windings and also the magnetic field they contain.
The result is a sort of one-shot transformer shaped like a long copper
tube surrounded by the windings. Of course, you want multiple shots, so
that is not very practical. But the first thing to do is raise your
capacitor bank voltage a lot. Just doubling it will have a huge effect on
your output power.
Second, a typical rail gun will use a projectile that has a foil
backing- the foil is vaporized and becomes a conductive plasma, which is
expelled from the gun through Lenz law. Whatever is on the front of that
vaporized foil gets the benefit of the acceleration. But also note that in
serious rail guns, you must provide some serious support for the rails, as
they will tend to expand outward, splaying apart. I suggest solid phenolic
(cloth filled) as the material to mill your rail supports out of. It is
very inert and non-conductive, can be backed up by anti-magnetic stainless
steel banding, and is easy to machine.
Note that magnetically permeable materials in your rail path are energy
stealers. They will trap some flux lines and heat themselves. Go to the
least reactive materials you can get. Also, your projectile should be
something that will not trap lots of flux as well, as you will not want to
induce a lot of heating. Projectile expansion can jam the rails. I suggest
that you wrap a thin film of Teflon around your projectile because then you
don't put lubricants on your rails. Lubricating compounds tend to disrupt
the arc if you use the foil method.
What sort of stepdown transformer are you using? You might get good
results if you make a mechanically compressed transformer powered by a
compressed air shot.

Cheers!

Chip Shults
 
P

Paul Burridge

Jan 1, 1970
0
Something that might prove interesting (and practical) would be a
mechanical wakefield-like gun. Tiny bullet in magnetic sabot gets fired
from conventional barrel. Magnetic sabot looses energy to coils around
barrel, which is transfered to a pulse-compressor. The bullet leaves the
conventional (gas-accelerating) barrel and enters the magnetic rail
accelerator, where the energy pulse from the magnetic sabot accelerates
the tiny slug to hyper-velocities.

I like the idea of pre-accelerating the projectile by other means
first. However, I'd suggest using CO2 gas as a propellent. There are
readily available recievers, regulators and pressure lines available
for this purpose which could be easily actuated by an electronic
trigger and the forces available from CO2 alone when used at the
higher pressures one can contain with commercially available fittings
are quite staggering. I'd then use the rail gun for final boost. Might
easily exceed the Earth's escape velocity.
 
S

Scott Stephens

Jan 1, 1970
0
Sir said:
Real rail guns sometimes use flux compression generators- ....
The result is a sort of one-shot transformer shaped like a long copper
tube surrounded by the windings.

How about a brass ammo cartridge, with the primer replaced by a spark plug?
But also note that in
serious rail guns, you must provide some serious support for the rails, as
they will tend to expand outward, splaying apart.

It recently occurred to me that a projectile might be placed at the
center of a seyfert-shaped flat pancake coil spiral. When energized, the
flat coil unwinds under magnetic pressure. The coil flies backwards, the
projectile in the center forwards.

I suggest solid phenolic
(cloth filled) as the material to mill your rail supports out of.

Carbon fiber is stronger, but perhaps too conductive though? Keep short
& laminate with insulating glass filaments?
Note that magnetically permeable materials in your rail path are energy
stealers. They will trap some flux lines and heat themselves. Go to the
least reactive materials you can get.

I was thinking that for initial low velocities at the barrel root,
(where hysteresis losses would be lower) ferrites could shorten the
barrel length.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************
 
S

Scott Stephens

Jan 1, 1970
0
I like the idea of pre-accelerating the projectile by other means
first. However, I'd suggest using CO2 gas as a propellent.

I suppose you can get several kilobars pressure that way. But cartridge
propellants are so nice, compact, portable, cheap.
I'd then use the rail gun for final boost. Might
easily exceed the Earth's escape velocity.

11 Km/S? 12 Megajoules to do a 200 gram slug? IIRC I've heard of 30 Km/s
velocities attained. It the trick of converting energy forms. You don't
want a warehouse full of caps or batteries, do you? No. We want a nice,
compact, even portable energy source.

Something with the form-factor of the ubiquitous Soviet/Chi-com RPG-7
we've all come to know and love so well, that would fire a mega-joule
worth of 10 gram tungsten needle at long-range or armored targets.

Somehow to integrate the magnetic pulse compressor into the propellant
system.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************
 
R

Rene Tschaggelar

Jan 1, 1970
0
Paul said:
I like the idea of pre-accelerating the projectile by other means
first. However, I'd suggest using CO2 gas as a propellent. There are
readily available recievers, regulators and pressure lines available
for this purpose which could be easily actuated by an electronic
trigger and the forces available from CO2 alone when used at the
higher pressures one can contain with commercially available fittings
are quite staggering. I'd then use the rail gun for final boost. Might
easily exceed the Earth's escape velocity.


CO2 has it end at the tripplepoint : 30 bar, 30 degC

Above is it liquid.

Rene
 
J

John Larkin

Jan 1, 1970
0
I like the idea of pre-accelerating the projectile by other means
first. However, I'd suggest using CO2 gas as a propellent. There are
readily available recievers, regulators and pressure lines available
for this purpose which could be easily actuated by an electronic
trigger and the forces available from CO2 alone when used at the
higher pressures one can contain with commercially available fittings
are quite staggering. I'd then use the rail gun for final boost. Might
easily exceed the Earth's escape velocity.

Current hypervelocity launchers tend to use an explosive charge to
drive a metal piston that compresses hydrogen or helium, which in turn
pushes the real payload. Rupture disks act as sort of gaseous schmitt
triggers. No projectile can be pushed by gas faster than the speed of
sound in the gas, so hot, high pressure H2 or He work best. Most of
these guns have to be rebuilt every shot, as the piston slams into the
end of its bore and extrudes into the smaller final barrel. Yucch.

There's a paper in this month's Review of Scientific Instruments about
a gun that uses fast valves instead of rupture disks and manages to
brake the piston before it wrecks the gun. They get something like 5
km/sec, moderate for this sort of thing, but can do many shots a day.

Big guns, from dreadnaughts to em launchers, are high-maintenance
items.

Rail guns seem to have gone out of fashion. There's a lot of energy in
a little gunpowder. Same tradeoff in cars: a little tank of gasoline
or a ton of batteries or a thousand tons of capacitors.

John
 
E

Ethan

Jan 1, 1970
0
I like the flux compression generator concept. I will have to look
into it more. What I saw on various web sites from a google search,
it looked interesting, though I don't really understand how/why the
thing works. I don't see how compressing the gap between coaxial
transformer windings adds energy, so it must have something to do with
the coils getting shorter. Looks like a trip to the library is called
for.

I had been thinking about something more like a switched reluctance
motor. If you take a transformer and put a large primer current into
one winding, then pull apart the core so the reluctance of the
magnetic circuit gets much larger, the energy from pulling apart the
core would be dumped into one of the windings. It would probably take
an explosive to pull one end off the transformer device, but then I
guess that is the idea.

In the rail gun we built there were many other limitations besides the
energy source, however most of them had reasonable solutions. First
of all we had the current passing through the projectile itself. This
had problems with welding to the rails if we tried to use a current
larger than 8kA, and the shape of the front of the projectile was
important to keep an arc from forming in front of it, and then the arc
gets blown out the end of the gun and the projectile just sits there.
Using an insulator for a projectile and pushing it with a plasma I
expect to work better.

We were using a steel core to hold the rails togethor, which I expect
to be more than strong enough to keep them from bowing out. The steel
core was used to increase the magnetic field strength with an external
field winding. The core was pushed far into saturation before the
shot, so loosing energy to to the magnetic flux of the core was not an
issue, however I expect eddy current in the core were a problem, but
we never did try to measure this loss. Howfully in the next one we
can use a laminated core or use a non-conductive material such as
powdered iron.

Initaily accelerating the projectile with an explosive would be a good
idea, especially if the projectile is going to be pushed by a plasma.
Using the rail gun to push hot gasses from an explosive should be able
to push the velocity beyond the speed of sound of the gas, which is
the normal limitation of a conventional gun.

Another big problem was instumentation. It is not easy to measure
current around 8kA, we did it by measureing dv/dt on an oscilloscope.
Any ideas on how to do this with a big pulse generator? Hopefully we
will be getting currents much larger than 8kA. We also need to find
an easy way to measure projectile velocities, but this shouldn't be
too difficult.

All this is getting dangerous and more than a little scary, so I don't
know how much can be done in my garage. I'll have to be very cautious
and maybe do it in somebody else's garage. We'll see.

Ethan


P.S. Thanks for the reasonable responses. Usually if you mention
‘rail gun' on the internet you attract a lot of crazies trying to blow
up or destroy things, and not sugesting anything usefull.
 
S

Scott Stephens

Jan 1, 1970
0
Ethan said:
I will have to look into it more. ....
I don't see how compressing the gap between coaxial
transformer windings adds energy, so it must have something to do with
the coils getting shorter.

It works like a transformer. Energy is stored in a magnetic field of a
magnetizing current loop. If the field energy stays constant, reducing
turns must increase amps.

The conceptual difficulty is thinking that shorting a metal coil
requires no force, therefore no work can be done to transform
explosive-kinetic to magnetic and current energy. But you must consider
the metallic conductors to be solid-state plasma, that is magnetized
according to the current flowing through it. It is much the same as the
voltage increasing across plates of a capacitor that are being forced apart.

I discussed it once upon a time:
http://groups.google.com/groups?hl=...seekm=Y0XM9.446137$NH2.30798@sccrnsc01#link11

I referenced a paper: Checkout J. Phys. D: AAppl. Phys. 28 1995 807-823
"Design, construction and testing of explosive-driven helical
generators" Novac, Smith, Stewarson, et.

Their FCG is primed with 40kJ from 50kA capacitor bank, uses 15kg of HE
and delivers 1MJ at 7 MA to the load in around 50uS for some 20
Gigawatts peak output, and it looks to me like a pipe bomb with a fluxed
solenoid wrapped around it =)

Converting energy from explosive to magnetic field then back to kinetic
energy seems inefficient. I'll bet there are better methods to make
hyper-velocity guns.

For instance, a cylindrical stack of chevron-shaped disks of high
explosives that are detonated in-phase from a timing manifold of
det-cord. A projectile fired into such a detonating cylinder could surf
the detonation wakefield to hypervelocities. Of course, that's probably
how modern formed or shape charges work, to an extent.

Don't need anything too fancy. Just a cylinder of HE just outside the
muzzle of a 50 caliber sniper rifle, and a method to trigger it when the
slug is at the right point.

--
Scott

**********************************

DIY Piezo-Gyro, PCB Drill Bot & More Soon!

http://home.comcast.net/~scottxs/

**********************************
 
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