Railguns designs for hand held rifles and spacecraft

Question

I'm writing a book and I want to use rail-guns for advanced space age factions in warfare.

I would like to make them as realistic as possible so I did a bit of digging myself, but most of the information is on our world level of technology, so my question is:

• What designs should the rail-guns be? I will put them into three categories size, material, and use

About size: I don't think a pistol would be practical at all, so what about a rifle? I know about the overheating problem, so what size should I use?

About material: I was thinking the shells should be Tungsten, though I know that they can shoot just about anything if encased in metals, but I'm looking for more cost effective ammunition (note: can't be any materials just found on earth like fossil fuels or gunpowder?) but the main thing is stopping the gun from blowing up so is there any elements I can use it can be as advance as you want like "nano-carbon"?

I am thinking of using a configurable power setting. Like a level one setting that would just be used against infantry and level two armoured targets, level three walls and so on; the power source is not an issue at this point.

Answer 1

1. Railguns can shoot anything conductive. The energy of a projectile is mv^2. Tungsten is heavy and so the mass goes up. That is good if you want to maximize the kinetic energy because you are shooting at something hundreds of miles away in the atmosphere. Handheld weapons are not that accurate unless you build in computer supersniper guidance (which I like for a space battle). A projectile will not shed energy to friction in space. You projectiles can be less massive and will still pack a wallop because of the large velocity. You can make them out of carbon fiber. Your space marines (who have to lug them around) will thank you.

2. Real railguns ablate their rails in a big way. The rails must be prepped before firing. You could make the rails carbon fiber also and the space marines sand them down between shots, or have a bore cleaner on a stick they push in and out of the muzzle to clean it.

Alternatively you could have each railgun deliver one shot. The space marines carry many barrels. Each one telescopes out and is attached for its single use, then discarded.

3. A single use discardable railgun offers the possibility of a single use capacitor for each barrel. It would discharge with the single shot then be discarded.

4. A lightweight alternative to metal or carbon fiber rails could be plasma rails. My scheme for this: http://www.halfbakery.com/idea/Plasma_20Rail_20Railgun#1186761424 The rails are made of a thin wire. A charge ablates them into plasma and then the plasma will carry the charge. You need new wires with each shot. You can unroll them from a spool on the gun.

5. Longer rails (longer barrel) = more acceleration. Higher current = more acceleration. You will run into trouble with ohmic heating in your projectile and rails. You could make your projectile and rail superconductors for a partial fix to this. Another scheme: http://www.halfbakery.com/idea/Superconducting_20railgun_20projectile#1137552946

Ah the halcyon days of 2006...

Answer 2

There are several answers to questions about railguns/coilguns right here on worldbuilding stack exchange:

Is there any advantage in a railgun/coilgun having multiple barrels?

What's more viable as futuristic infantry weapons, rail/coilguns or laser rifles?

Feasibility of coilgun system for sub-luminar interplanetary transport

What are the advantages of a coilgun v.s. a railgun?

I can also point you to the ever useful Atomic Rockets site, where you can find the equations needed to exactly answer your questions.

The shorter answer is you will need to define rather carefully what it is you want to do. An infantry weapon used as a long range "battle rifle" (semi automatic, relatively large calibre [7mm+]) will have different requirements from something resembling a current military assault rifle (selective fire, 6mm - calibre projectiles). An anti material rifle capable of damaging buildings or vehicles will be a much different beast.

The requirements are then going to drive your power system and then you can look at things like heat rejection, power coupling (a lose connector will really spoil any grunt's day....) and so on. Coilguns are electromagnetic weapons as well, and you can compare the efficiencies between the two types of weapons:

Look at Atomic rockets and see what happens when you try to fire a dime sized projectile at 100G out of a 100km long barrel...

Here's a quick method to estimate what kind of performance you can get out of a coilgun. Some folks here might find it interesting.

First, decide on the efficiency of your coilgun. Coilguns are linear brushless electric motors, and brushless electric motors have demonstrated efficiencies of 90% to 95%. Superconductive electric motors might have efficiencies of 98% to 99%. Denote this as a decimal, and call it e; that is e = 0.9 to e = 0.95.

Next, decide on the length and radius of your projectile. Decide on what your projectile is made of and find its mass

mass = density * length * radius2 * &pi (and remember to use consistent units).

Also find the projectile cross-sectional area

area = radius2 * π

Decide how fast you want your projectile to be going and find its final kinetic energy

kinetic energy = 0.5 * mass * velocity2 (again remember to use consistent units).

Given the efficiency of your coilgun, you can find out how much your projectile heats up. You might figure that half of the wasted energy goes into the projectile, and thus your projectile will gain a heat energy of

heat energy = 0.5 * (1/e - 1) * (kinetic energy)

Look up the specific heat of the material your projectile is made of, commonly called C. Then your projectile reaches a temperature of

projectile temperature = (heat energy) / (C * mass) (again make sure your units are consistent).

If you are using a synchronous coilgun with a permanent magnet in the projectile, this temperature needs to be less than the Curie point or the projectile will become non-magnetic. If your coilgun projectile is made of superconductors and you are using Meissner effect repulsion, this temperature will need to be less than the critical temperature of the superconductor or your superconductor will become non-superconducting. If you are using an asynchronous coilgun which uses inductive forces on conductive loops, this temperature will need to be less than the melting temperature of your projectile. If the temperature is too high, you will either need to use a material that can handle higher temperatures, make the coilgun more efficient, or accept a lower velocity for the projectile.

Decide the maximum magnetic field your coilgun can handle. If you are using a synchronous coilgun with permanent magnets (probably in the projectile, with the field coils along the barrel) you are limited by a saturation field of around 0.2 to 2 tesla beyond which your efficiency falls off rapidly. If you are using superconductors, your field is limited by the critical field of the superconductor. For conventional BCS-type superconductors this limits you to fields of several tens of tesla or less, for high Tc superconductors you might be able to get to 100 to 200 tesla. If using an asynchronous coilgun that uses induction to launch normally conductive projectiles there is no obvious physical upper limit to the magnetic field strength, although high field strengths will require massive bracing to keep the barrel from exploding.

Now assume that the barrel is filled with field, and that the projectile sweeps the field out of the barrel, turning the field energy into kinetic energy (this is not actually how coilguns work, but it gives the physical upper limit based on energy conservation). The energy density is about 400 kJ/m3/T2 times the square of the magnetic field strength (398,098 J/m3/T2 to six significant figures). Call this value K

K = 400 kJ/m3/T2

You now know the volume needed in the barrel based on how much energy the projectile ends up with

volume = kinetic energy / (K * (magnetic field)2)

Since you know the cross-sectional area of the projectile and thus of the barrel, you know how long the barrel needs to be

length = volume / area

If the barrel is unacceptably long, you will either need to figure out how to get a stronger field in the barrel, make the projectile shorter (if you do the math, you can see that the barrel length will be a multiple of the projectile length for a given field, material, efficiency, and final velocity) or make due with a lower velocity of the projectile.

As an example, suppose we have a synchronous coilgun, and that the coilgun can generate 1 tesla fields (a good number that will not saturate the ferromagnet). Our presumed ferromagnet is probably mostly iron, with about 8000 kg/m3. To reach 100 km/s, you will need 40 TJ per cubic meter of projectile. Since this is 100 million times the energy density of the field, you will need the projectile to sweep out 100 million times its volume in order to accelerate up to the desired speed. This means you need an accelerating track 100 million times the length of your projectile. If the projectile is the size of a dime, with 1mm thickness, you will need a 100 km long track. If 2.5% of the energy goes into the projectile as heat as a result of inefficiencies, you get 100 GJ of heat per cubic meter of projectile, or 12 MJ/kg. This is three times the specific energy liberated by detonating high explosives, so you can expect your projectile to explode like a bomb inside your coilgun barrel. Consequently, this appears to be an unworkable design.

Answer 3

For me, as a gun nerd, the best idea would be to use as many firearm analogues as possible.

The hardest question to answer in my perspective would be how to power such a weapon. Without getting too much into physical details, railguns would probably have detachable magazines that contain both the projectile and a cheap, disposable battery. These magazines would be engineered so that the battery would use the last of its energy when the last shot is fired, and then the magazine would be swapped out just as in a modern rifle. Your power adjustment suggestion could be implemented through magazines of varying voltages.

For bullets, I would say either lead or steel would be sufficient. Against unarmored targets you actually want a softer projectile so that it deforms more and dumps more of its energy into the target. Steel bullets would be a better choice against armored targets for the opposite reason.