The Physics of Rail Guns & B5



I. The concept of the electric gun.

Essentially electric guns come in two flavors, rail guns and coil guns. There is technically such a thing as a maglev gun but it is closely related to the coil gun concept. As I've explained in previous posts the rail gun is a very simple device consisting of two metal slats, a conductive projectile,
and a series of magnets (used to provide a field). Coil guns are essentially a series of hollow electromagnets (or air core solenoids if you prefer) that suck a ferrous projectile throught themselves when turned on and off by a timing cicuit.

II. The physics behind the guns.

A) Rail Guns

Vector mechanics is the heart and soul of what makes rail guns work. Without going to in depth let us say that all of the most important equations for a rail gun can be derived the definition of the magnetic field (B). The final equation to calculate the force exerted upon the rail guns
projectile is F=i(LxB). Where i is the current running through the rails, L is the width of the projectile (or more properly it is the length of the path that the electric current takes through the projectile), and B is the strength of the magnetic field. Because the size of the projectile is easily controlled it is the current and magnetic field strength that become the greatest limiting factors.

On the positive side a rail gun can easily be designed to also take atvantage of the principles of gas expansion used in modern firearms. This is accomplished by designing the rails as part of a sealed barrel. The projectile is traded for a highly resistive material while a very thin fuse (typically gold wire) is glued to its back. When the current runs through it the fuse is vaporized directly into its charged gas state (also known as plasma). Since charged gases still conduct electricity the gas behaves exactly as the fuse would have allowing current to pass through and permitting the rail gun's projectile to be propelled by both the expansion of the gas from the vaporized fuse and the magnetic force acting upon the gas pushing out of the barrel.

B) Coil Guns

I think I may have said this before but coil guns are no where near as simple as rail guns. The force can that will act upon your projectile is not easily calculated. Suffice to say that in depth knowledge of quantum physics is a prerequisite if you want to discuss the ways in which magnetism
operates on ferromagnetic materials. Perhaps the best way to design a coil gun that I know of is to experimentally determine how particular materials behave when exposed to a magnetic field. Through experimentation one can define a relationship between magnetic field intensity and the force that acts upon a ferromagnetic material. You will of course need to know the permeability and hysteresis curve for the material in question.

Once a relationship between magnetic field intensity and the force acting upon your chosen projectile has been established it is only a matter of designing a timing ciruit to fire your magnets in a sequence that will use as much of the barrels length as possible. After the timer the chief design concern will be how much magnetic energy the coil can deliver to the projectile. This can be determined through the use of several magnetic equations and the hysteresis curve. I'll provide the basic magnetic energy equation, it is derived from the other relevant equations and so can be used to derive those equations should anyone be interested in the excercise. W=1/2(MMF)(Flux) MMF stands for magnetive motive force, don't be confused by the nomenclature it is not the actual force acting upon the projectile. The hysteresis curve is important for calculating the maximum flux that can be usefully forced through your projectile material. At a certain point the field strength affecting the projectile will saturarate. At this point no noticable return can be seen in the projectile's performance no matter how much more field strength is created by your magnet.

III. Drawbacks

Well, now that I've bored you all with an ton of fairly vague and compicated math let us turn to the obvious drawbacks of these systems.

A) Rail Guns

A rail guns most glaring problem is the amount of space required for all of its components. The barrel alone has two very important and quite seperate electrical systems operating, electromagnets and the rails. Because you can not generate any force on a current unless you have a pre-existing magnetic field you simply must have magnets set perpendicularly. Magnets require their own set of controls and power sources which consumes space. The rails require a substantial and very hardy power source capable of delivering enormous currents on command. These also take up space. For
example, an 80 mm prototype at the Green Farm facility required a capacitor bank that was as big as a room, of course that was about ten years ago. We should keep in mind that even if the capacitor bank had shrunk to half a room it's still to big to put on a tank. The rail gun's other serious drawback is barrel wear. This is what happens when you repeatedly send killiamps of current down aluminum rails to vaporise a tiny bit of gold that links them. Between the heat from the high current, the grinding action of the projectile moving down the barrel and additional heat from exposure to plasma, tiny pits are eaten in the surface of the rails. This is the exact same process that occurs to the rotors of your car's brakes, just exascerbated by additional heat sources. Eventually these pits become big enough to permit the plasma conducting gas to flash past the projectile.

B) Coil Guns

The main problem that coil guns have is inefficiency. This problem is caused by the electromagnet. Each loop in the electromagnet can be imagined as its own little magnet. As the projectile travels through the core of the magnet more and more of these loops end up behind it, slowing it more and more. The trick is to time it so that the next magnet in line turns on, the current magent in use turns off, and the projectile keeps speeding down the barrel. At best a single magnet can only exert positive acceleration upon the projectile for half of its length. After the mid point it acts like a brake. Now the next magnet in line can be used to overcome this effect but, the last magent in the sequence always loses half of its effective acceleration length.This can lead to a number of problems not the least of which is unsteady projectile acceleration and even braking of the projectile while inside the barrel. Maglev devices also suffer from this problem.

The second drawback to the coil gun is similar to the rail gun. Namely it requires a very hardy power source that can nearly instantly supply vast amounts of current. These systems consume space. Coil guns have an additional drawback in that they require complex timing circuits to drive the magnets. These may not consume much space but poor calibration can have serious negative effects on the gun's performance.

IV) The Benefits

Lest you think all is gloom and doom I will now tout the benefits of these devices.

A) Rail Guns

The primary benefit from the rail gun is that it is simply much more powerful than a conventional gun. I'll use an example I've used in the past. The main gun (I forget this weapon's calibre) on a U.S. Battleship can deliver a whopping 125kJ of energy to its target. By comparison an 80mm prototype operating at Green Farm was able to deliver 5MJ (almost 50 times as much energy) to its target. Not only did this much smaller gun pack a bigger punch but rail guns also have vastly increased ranges over conventional artillery. At such high rates of velocity and large amounts of
energy rail guns are closing in on the ability to turn any solid slug into a high explosive (like shooting your opponent with a meteor).

B) Coil Guns

The primary advantage of coil guns is also increased power. Although to a somewhat lesser extent than a rail gun. This is accomplished by converting the acceleration curve from an exponential curve that quickly goes negative to a sinusoidal that spends virtually all of its time in the positive quadrant. While I have no real life examples just imagine that your projectile gets a fresh kick down the barrel as each magnet in the coil gun turns on.

V. Game Terms

Getting more "on topic" as it were. I can definatly see where many primitive races including the EA used or continue to use rail gun technology. However, the damage for these weapons would more appropriately be piercing. This is because, unlike a hill, mountain, or other planetary surface feature, space going vessels simply lack the mass and density to prevent a high speed projectile from going right through them. However, we are stuck with a game mechanic. So to explain it away we could simple say that these weapons fire a high explosive round with a contact fuse. Their rate of travel is so high that they affectively ignore the armor on the location where the impact and within nanoseconds detonate causing great destruction to the local area of the impact. These explosives are not themselves capable of penetrating any armor which explains why excess damage is lost.

Moving on we see that the EA abandoned rail gun technology for the most part after the Earth-Minbari War. I suggest that barrel wear was a contributing factor to this. However, the new Warlock Destroyer employs rail guns. We can simply explain this away by stating that new space saving power equipment allowed multibarrelled rail guns to be employed. Cycling barrels would dramatically increase a weapon's useful lifespan without actually improving it's overall characteristics. Although, a faster rate of fire might be appropriate.

The use of coil guns seems logical as an explantion for the Centauri matter cannon. The smaller damage is actually likely a smaller explosive charge and has nothing to do with the weapon's muzzle velocity. On the whole it requires greater technological finesse to employ these weapons which is in keeping with the Centauri being a more advanced culture than Earth. It would be interesting and appropriate to see variations of these weapons that had piercing mode as an option. I do not believe that these weapons would be useful on tanks although their value as artillery either on the ground or from orbit would be very great indeed. Just something to keep in mind for Gropos.