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This is an article from the November/December 1990 issue of Aerospace &
Defense Science.  When I tried to subscribe to this magazine, in the beginning
of 1991, I was told it was no longer being published.  It may have been
reincarnated at a later date.  If anyone knows anything about this please let
me know.
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              From Dreamworld to Realworld: ElectroMagnetic Guns

                              by Terry L. Metzgar

Think 'railgun' and the mind conjures up visions of orbiting space weaponry.
A neat idea, yet unrealistic in today's budgetary and political climate.  But
while glasnost may have deflated the space-battle fantasy, pulsed-power
electromagnetic development has taken a more practical, earthy approach,
demonstrating such dramatic performance that even skeptics are forced to admit
there's more to pulsed-power electromagnetic technology than meets the eye.

Three years ago, as scientists built the latest generation of ElectroMagnetic
Guns (EMGs), critics viewed such developments with curiosity and amusement.

Curious because the electrically driven railguns posted unprecedented
velocities, and amusement because their 'little finger' sized plastic
projectiles seemed hardly a threat to anyone.

But even the skeptics acknowledged that, for raw velocity, railguns are
unsurpassed.  At least three different labs posted velocities in the 8-
kilometer-per-second (kmps) class.  There remains some question as to the size
and integrity of projectiles, but no doubt the speed was far in excess of any
conventional gun.

We had several laboratory EMGs with proven hypervelocity capability using 'pop
gun' size projectiles.  Today, thanks to strong R&D funding, pulsed-power
electromagnetic devices have grown from laboratory 'toys' into developmental
weapons.

In the space of three years, bore size grew from 20mm to 90mm.  Projectile
mass increased from 1 gram to over 2,000 grams.  Power sources have expanded
from around 4 megajoules to 60 megajoules, with plans for gigajoule-region
power supplies.

Common Sense Takes Beating

It's the energy delivered on target that matters.  The DARPA/Army/DNA
sponsored 'Miramar Gun B' at Maxwell Labs, makes people sit up and take
notice.

Photos offer graphic evidence that Gun B rips through a 5-inch steel witness
plate like a chainsaw through paper mache, at 2.6 kmps.  Once velocity reaches
3.4 kmps, they've found no witness plate left to photograph.  It simply
disintegrates, leaving only tiny steel shards.  Projectiles look like an
overgrown shotgun shell shotcup inverted backward.  Seems harmless enough
until you see what they do to steel plate.

A little piece of plastic punches a ragged 90mm hole that makes you wonder
what it could do with a kinetic penetrator.  Common sense takes a beating
here, because any fool knows plastic can't penetrate steel plate.  But funny
things happen at railgun velocities.  It's a different world.

Both projectile and targets behave like liquids, requiring scientists to use
hydrodynamic equations to describe the impact interaction.

The Miramar Gun isn't exactly a stranger to kinetic penetrator either. They've
launched a dart-shaped 2.05-kilogram tungsten kinetic penetrator at 2.4 kmps,
with armor penetration that remains highly classified.  Considering what it
does with plastic, the secrecy is quite understandable.

More Powder Won't Do It

ElectroMagnetic Guns, function like a linear electric motor.  The 'barrel'
consists of two highly conductive “rails” with the bullet resting between the
rails and within the bore.  When a current is applied to one rail, it flows
down that rail, where it shorts across a path provided by either a mechanical
armature, metallic fuse link, or foil backing.  The magnetic field formed by
the arc across the rails pushes the projectile down the barrel, and, if
everything goes as planned there is a railgun launch.

Seems like a lot of effort to accomplish the same thing as your garden variety
artillery piece.  But there's a good reason for the EMG approach.  It's speed.
If you're after the kind of velocity need to perforate next-generation armor,
or from a space science perspective, to 'shoot the moon,' gunpowder just won't
make the grade.

The speed at which chemical propellants can push a projectile is limited by
the speed at which gas expands.  For all practical purposes, conventional-gun-
launched projectiles can't go much above 1.8 kmps.  Dump in all the powder you
want, the bullet won't go any faster.

Thus chemical propellants or gunpowder faces and apparently impassable gun-
launched velocity barrier based on the physics of gas expansion.  It follows
that if you wish to exceed the speed at which a gas expands, find something
else to push the bullet.

That's where the EMG fits into the picture.  Since an electromagnetic pulse
travels at near light speed (186,000 miles per second) it provides a
propulsion source immune to the limits of gas expansion.

EMGs Sound Simple...

Essentially, EMGs are based on a simple concept.  Build a near square barrel,
2 feet wide on each side and 9 meters long with a 90mm circular bore.  Place
two conductive rails on each side of the bore with the projectile nestled
between them.  Dump 32 megajoules of energy to the rail, and blast projectiles
at hypervelocity or, if you wish, right off the planet.

EMGs have enough raw velocity to loft projectiles beyond the Earth's
atmosphere and into space.  A little known actuality that may have
ramifications well beyond the anti-armor role.  But that's another story.

Suffice to say that ElectroMagnetic Launchers (EMLs) in general and railguns
in particular off a new dimension in hypervelocity.  Sounds simple enough, but
it's not that easy.

...But There's The Old Tupperware Number

For one thing, 32 megajoules applied all at once has a tendency to transform
rails the way your oven transforms tupperware.  Only more and better.  No
hypervelocity launch, just a twisted mass of melted components.  The same
pressure plays havoc with mechanical parts, too.  Repulsive force between the
rails can cause barrels to 'blossom.'  Only a massive prestressed bolting
arrangement prevents rail flexing that could otherwise prove catastrophic.

Program manager for DNA on the Thunderbolt Project, Air Force Maj. Pedro
'Pete' Rustan puts it this way, 'The greatest challenge is trying to find the
correct rails and insulator material.  Everytime you find one with low
ablation, it has weak mechanical strength.'  Such is the difficulty of
ElectroMagnetic Launcher designer.

Railgun engineers came face to face with this dilemma.  Which is a tough
assignment given the limitations of material technology and the fact that no
one had ever launched anything that big that fast without doing the old
'tupperware number.'

They had several options, none of which were a sure thing.  Their reasoning
went something like this: If existing material can't withstand the heat and
stress, try exotic material.  Some teams tested this approach, with mixed
results and considerable expense.

A second choice: Avoid the heat and stress of initial projectile dwell time by
giving it a gas-injected running start before entering the railgun breach.

Just as an electric motor has difficulty overcoming inertia, so does a railgun
have difficulty with initial acceleration.  Giving a little push at the start,
helps avoid 'breech-end constipation.'  This technique works so well that it's
become a standard fixture on most designs.  A helium gas preinjector squirts
the projectile up to around 1 kmps, where the railgun takes over for a 'top-
end' acceleration.

In other words, high-speed gas injection avoids the material-related
'tupperware phenomenon' by sidestepping the issue.  Compress helium at a
pressure high enough to spit the projectile at a 1-kmps running start and
everything is 'just cricket.'  But when you try to boost a bullet with
compressed gas, the old bugaboo of gas expansion rears its ugly head.

The physical limit of gas expansion, as a function of Mach number and the
specific heat within the gas injector, places a limit of 1.3 Mach.  For all
practical purposes it restricts compressed gas injector velocity to around 3
kmps.

GT Devices and FMC are reported working on solutions to this problem.  The
technique attempts to raise the gas Mach barrier by heating the plasma.
Whether such electrothermal guns prove practical, remains to be seen.

The people at Miramar opted for a different solution.  Embracing gas
preinjection still left them with unsolved material erosion problems.  Heat
and pressure are so high that a certain amount of bore material ablates, mixes
with plasma, and leaks out in front of the projectile.

If this ablation is unavoidable no matter how exotic the material, just make
the bore and projectile size big enough that a little leakage doesn't hurt.
That's part of the reason for the big 90mm bore.  Results to date seem to
corroborate the large-bore-size theory, prompting some to speculate that
plasma containment and energy conversion efficiency improves as the bore
diameter increases.

Should this prove true, and indications are that it will, BIGBORE railguns may
be the wave of the future.  Look for monster 'fatboy' intercontinental
artillery.  Therein perhaps lies the secret of success: Some measure of
progress in critical material issues, along with larger bore diameters.

Of course, the $50.00 question is, 'Where does superconductor technology fit
into the EML equation?'  Program spokesmen will respond only with a cryptic
'No comment,' but rest assured, they're well aware of potential benefits
surrounding superconductor EML components.  Whether this translates into
existing hardware remains open to speculation.

Finding Right Projectile

Although today's railguns display a remarkable virtuosity in ventilating armor
plate, the technology isn't without some warts.  Several thorny issue's remain
unsolved, not the least of which is finding a projectile tough enough to
withstand the friction generated by traveling 6 to 8 kmps through the
atmosphere.  At that speed, conventional material will simply vaporize.

Classified studies are addressing projectile thermodynamics.  One option being
a 'cool nose cone' with a 'sacrificial coating' that burns off under friction-
generated heat, like a space capsule re-entry shield.

Another 'glitch' involves atmospheric attenuation or the velocity loss
associated with simple 'wind resistance.'  Scientists at NASA's Jet Propulsion
Laboratory estimate that 'if it's properly shaped, the loss need not be
significant...that shape is long and thin like a pencil...with a velocity loss
of around 10 percent.'  Other sources believe the loss would be greater than
10 percent.

Either way, EML surface launch requires a streamlined projectile with high
heat tolerance.  Such items are, at least officially, still 'conceptual.'

Until then, atmospheric attenuation and friction will probably limit practical
railgun anti-armor rounds to under 6 kmps, which is still roughly 3.5 times
faster than any conventional cannon.  (The NATO 120mm smoothbore tops out at
about 1.7 kmps.)

Feeding the Glutton

Then there is the power supply question.  Railguns, by their nature, are
energy gluttons.  Feeding their appetite is a nontrivial task.  Until high-
energy density superconducting inductors become available, EML engineers must
choose homopolar generators (HPGs), compulsators, plain old batteries, or
capacitors.  Each approach has certain advantages and disadvantages, which are
briefly characterized below.

Homopolar Generators.  Massive rotating machinery that, although more compact
       than the early generation of capacitors, remains too large for mobile
       field applications.  Based in principle on a Faraday rotating disk,
       they need a large inductor for pulse shaping.

Compulsators.  Primarily a development of The University of Texas Center for
       Electromechanics, they're intended to address the shortcomings of HPGs.
       The devices function something like an alternator with extra windings
       for sharp-pulse shapes.

Batteries.  Probably the least expensive route to EML hypervelocity.  Eglin
       Air Force Base is building a power supply for their family of EMLs that
       consists of 14,000 interconnected car batteries.  The battery method
       saved an estimated $20 million over comparable capacity alternatives.
       It works fine for stationary facilities but might be a bit much to lug
       about the battlefield.

Capacitors.  The most popular power supply for several good reasons.  Power
       delivery is more precise than HPG’s.  Although energy-to-weight ration
       is less than desirable, the development curve shows considerable
       promise.  Several years ago, the Miramar 32-megajoule capacitor bank
       would have weighed a crushing 320,000 kilograms.  Thanks to some
       intensive development efforts at DNA, current technology shrinks it
       down to around 10 tons.  Project Mile Run will try to condense that 10
       tons into a 1.2-ton package.

The goal of course, is to place the railgun with its 32-to-50-megajoule power
supply into a tracked vehicle chassis -- something like the existing MLRS
chassis.  If Project Mile Run can pull it off, the improved copolymer-resin-
based capacitor technology could conceivably move rail guns into the small
arms arena.

Brilliant Rocks, Grins, and a Thunderbolt

Scientists at the University of Texas Center for Electromechanics recently
completed a DARPA-sponsored 9-megajoule railgun that employs HPG- and
compulsator-powered railguns emphasizing inherent compactness when compared
with early-generation capacitors.

Best performance to date is 1.13 kg at 1.98 kmps.  They are also on the
leading edge of compulsator research, developing a repetitive power source
firing up to 60 times a second.  Think 'brilliant hypervelocity rocks' at
3,600 rounds a minute and it's enough to give you a case of 'the grins.'

Part of their effort includes a small caliber, crew-served weapon, with a
rapid fire compulsator power source.  They hope to mount it on a vehicle in
the Hummer class.  The performance goal is 10 to 30 grams at 2 kmps.

Picture a little launcher roving around the battlefield spewing sheets of
aspirin-size bullets at 3,600 rpm, and you get an idea of EML's small arms
potential.

From a sheer velocity standpoint, UT CEM has a coaxial railgun that achieved
plasma speeds of 40 kmps.  This is roughly 89,000 miles per hour.  Under SDIO
sponsorship, the Westinghouse Research and Development Center in Sunnyvale,
Calif., is constructing a 56mm/60-megajoule 'Thunderbolt' railgun.  This
device incorporates lessons learned on the SUVAC2 subscale prototype.

SUVAC2 encountered problems related to segmented barrel construction and
improper firing sequential firing of the distributed energy source.

Thunderbolt is one of the few surviving railgun programs directed toward
lethality demonstrations on the top end of the hypervelocity scale.  They hope
to achieve velocities around 15 kmps.

Eglin has a family of 16 railguns, ranging from HPG’s to capacitors to the
previously mentioned battery-powered devices.  They've converted a former
surface-to-air missile site into the first open air railgun target range.

Overlooking the Gulf of Mexico on Okaloosa Island, the test facility provides
the first opportunity to 'open the throttle.'  Blasting railgun projectiles
high out over the Atlantic allows research into guidance and control and
surface-to-air capabilities.

To Have and Could Have

With a clear distinction between 'what we now have' and 'what we could have,'
possibilities are as follows:

Anti-Armor.  Readers are well aware of the urgency surrounding Soviet reactive
       armor deployment.  It has left us scrambling to find a reliable method
       of ventilating existing and next-generation Warsaw Pact armor should
       that ever become necessary.  Given the railgun's inherent velocity
       advantage over conventional guns, look for someone to mount one in a
       tracked chassis soon.

Surface-to-Air and Anti-Ballistic Missile Defense.  The raw velocity of
       railguns offers a dual-role capability.  The same hypervelocity
       potential that makes railguns ideal for perforating armor can deliver a
       launch-to-target speed optimal for the air defense arena.

Anti-Satellite Employment.  Political considerations make discussions of this
       sort 'uncomfortable' for those suffering from testosterone deficiency,
       but it does not alter the reality that, from a pure launch-potency
       viewpoint, today’s railguns are a genuine threat to low-earth-orbit
       satellites.

Electromagnetic Armor.  Although not a railgun, electromagnetic armor is a
       direct spinoff from EML capacitor programs.  It remains highly
       classified, but initial reports indicate EMA application increases
       survivability by a factor of 10.  The technique uses an outer steel
       ground plate that, upon contact, energizes, forming a magnetic field
       that deflects shaped-charge warheads.

All things considered, EMGs have come a long way.  They’ve advanced from
toddler size laboratory toys into lethal, BIGBORE, next-generation anti-armor
candidates.

Author Terry L. Metzgar is a freelance writer specializing in communications
and defense technology.
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