8
$\begingroup$

I am envisioning a weapons team that can carry around a coilgun about the same size as a recoilless rifle. SPG-9 RCL crew, http://www.military-today.com/artillery/spg_9.htm

The L55/12.M Crew Served Coilgun from Kaelo Engineering is the newest advancement in electromagnetic weaponry. Using advanced switching technology and high quality superconductors this long range, 12-stage anti-armour device can be carried by a team of 4 and rapidly assembled in a position of your choosing. Capable of firing a 2kg tungsten projectile in a ferromagnetic sabot at up to 4.7 km/s. These small and very high velocity projectiles will bypass shields and all but the most advanced point defense systems to neutralize almost any ground based target or even low flying aircraft in a pinch! Just dont forget to anchor the tripod and properly calibrate the recoil dampening system! Kaelo Engineering, bringing a secure future to you at hypersonic velocity.

Kaelo Engineering is not responsible for injuries resulting from spontaneous destructive quench events or field containment failure. No ferromagnetic objects, other than Kaelo approved ammunition, should be located within 5 meters of the L55/12.M during operation, discharge all coils promptly after use. Read L55/12.M operators manual FOM2187 in full prior to use, included with each gun and tripod package. Ammunition supplied seperately.

So that's the general idea, one issue. I'm trying to figure out a compact method of powering a 22MJ system like this. Compact fusion reactors exist in this universe but even the smallest models are still too heavy for a 4 man team to carry in the field. What are some probable near near-future (within 50 years or so) power generation/storage systems that can make this a practical system in both energy output and weight?

As mentioned before: Needs to be relatively light weight, enough for a single man to practically carry. 60 pounds absolute maximum or easily disassembled into pieces and reassembled.

Use known or theoretically feasible technology given proper advancement.

Not be unreasonably dangerous to operate or carry over rough terrain.

Edit: Optional: Quiet enough to maintain a level of stealth given proper concealment and preparation time.

$\endgroup$
3
  • 4
    $\begingroup$ 2 kg @ 4.7km/s out of a 60lb launcher means that the launcher would try to recoil backwards at about mach 1 (345m/s under the overapproximation that the 4.7km/s figure is relative to the ground). I don't think a tripod would cut it. $\endgroup$
    – TLW
    Feb 1 at 1:46
  • 1
    $\begingroup$ Ditto, under the same 1.7ms assumption as the answer below, the recoil apparatus would have to tolerate over half a million kg of force. (I know, some people don't like talking about force this way. The force exerted by half a million kg in one earth gravity. ~5.5 meganewtons.) Now, this is only for a short time. Still! $\endgroup$
    – TLW
    Feb 1 at 1:51
  • 4
    $\begingroup$ There is deep contradiction between launching a multi km/s projectile and stealth. It will leave an ionization trail pointing back at the launch site that a blind man could follow with his cane. $\endgroup$
    – Dan
    Feb 1 at 14:09

5 Answers 5

14
$\begingroup$

The problem isn't quite the energy needed, but the power. 22MJ is what you'll get out of half a kilo of diesel (that's a volume of a bit over half a litre).

No, the issue is accelerating the projectile to 4.7km/s in a short distance. If your barrel is a conservative 4m long, you need to accelerate your projectile at over a quarter of a million gravities, and it'll clear the barrel in about 1.7 milliseconds. That requires a peak power of 13 gigawatts, and that's a fearsome thing to try and switch in man-portable equipment. Power is inversely proportional to barrel length, so a 2m barrel requires twice the power as the 4m model

(incidentally, I'm not going to run the numbers now, but I think that a ferromagnetic sabot will simply explode in the barrel and you might do well to not give too many details on the precise mechanisms and materials involved)

You've got basically two choices here. One is to use any old electrical charging system you can put your hands one (such as trioxidane's fuel cells, though I'd seriously recommend something that's liquid at room temperature for your fuel... I'm a fan of methanol, FWIW) and an energy storage system. Maybe some kind of ultracapacitor or superconducting magnetic energy storage might do, though you'd need some breakthroughs in technology to reach the power levels you want in a human-portable size and weight.

For something more interesting though, I'll steal an idea from the old Colonial Marines Technical Handbook and suggest a superconducting homopolar generator, which the author suggested as a power supply for a kind of plasma weapon. This kind of power source is very old and conceptually quite simple... Faraday himself made one. Here's an example project from the late 70s which was capable of generating ~52MW peak power over 3.5ms in a pair of 30cm wide rotors: FDX--A Fast Discharge Homopolar Generator.

Your coilgun uses a descendant of that design, using much more technologically advanced materials in the form of high temperature superconductors and carbon nanofibres to hold the rotors together and compact lightweight high quality vacuum chambers, etc. An external power source (like those fuel cells) is required to drive the motors that spin the rotors up, and once spinning you don't want to be moving the thing about and it will go bang in a very exciting way if anyone shoots it. You can discharge the generator directly into the switchgear of your coilgun, which brakes the rotor to a halt in the time it takes for the projectile to clear the barrel. Spin it up again to repeat.

The system is implicitly stealthy (unless you're using a combustion engine to spin up the rotor, as FDX did with gas turbines) because if your generator is so inefficient as to vibrate and make a noise it'll probably toast itself in very short order given the amount of energy it has built up. The motors that spin the rotors up may or may not be stealthy, depending on the technology you have available to you.

$\endgroup$
1
  • 1
    $\begingroup$ That works very well for what I'm trying to do actually. There is a limited amount of handwavium in this universe so having advanced metamaterials like that is consistent. I'm thinking the tripod and barrel mount are split up between 2-3 people, the homopolar or equivalent generator and fuel is carried by another like a backpack, and the ammunition by yet another person(okay so maybe 5 people in a team). Of course the generator carrier would not be wearing it while it's in use, it would be secured to the ground, preferably dug into a small trench for protection. How does that sound? $\endgroup$
    – Boo Radley
    Jan 31 at 21:17
5
$\begingroup$

Hydrogen

A kilo of hydrogen can release about 100MJ in a hydrogen fuel cell. That is more than enough to power your gun. Advancements in fuel cell technology is required to have enough hydrogen converted into power all at once. If liquid you can store 71kg/m³, which means a kg would fit in 10x10x14cm (+/- 4x4x5,5 inch) on the inside container. This is at high pressure and close to 0° Kelvin though. Hydrogen is also notorious for how easy it can escape containment. It requires special tanks to keep it stored for a long time without practical loss. The advantage is relatively low power loss when putting it through the fuel cell (the above 100MJ is already with the loss included). Diesel and such already have a lower energy density and using it in a combustion generator loses most to heat.

When setting up you would depressurise the liquid, causing it to heat up, expand and transform to gas. This can be used by many tiny fuel cells that produce electricity. This is then converted to the right Voltage and Amperes and put into the coils.

At worst it'll be a separate box that'll house the whole electrical unit, which then is connected by a single wire to the gun.

Most difficult parts are the containers for the hydrogen and fuel cells that can rapidly transform the hydrogen to electricity without adverse effects. Cooling will also be difficult. Pressurising can be done before they go into the field.

However, you have access to compact fusion generators. These require a lot better cooling, so cooling doesn't seem a problem. With such advances it'll be easy to imagine fuel cells being tiny, light and even more efficient than they already are.

The waste is water. If the container breaks it'll release a jet of hydrogen that'll combine into water, unlikely to explode or catch fire. If it does catch fire it'll be a single quick jet of fire upwards.

It should be safe to use on rough terrain. The 60 pound or 27kg limit is difficult to know, but as we're talking about future technologies we can say that hydrogen fuel cells can be made to be so small and loght that the whole electricity box is light enough.

$\endgroup$
5
  • $\begingroup$ You've got your units of volume a bit mixed up there. At the given density, 1kg of LH would have a volume of ~14000cc or 14l. Maintaining cryogenic temperatures in the field sounds like a nightmare. Compressing the gas to ~800bar might be easier, but to be honest I feel like a different choice of fuel is probably a lot more sane. Also, I doubt you'll be able to drive a coilgun directly from some little fuel cells as the power-density would be too low. You'll need a nice ultracapacitor and some switching hardware, and charge it all up before you can shoot. $\endgroup$ Jan 31 at 20:33
  • $\begingroup$ @StarfishPrime So it sounds like Hydrogen fuel cells are out for a method of man portable power in this application. Do you think enough power could be stored in some superconducting ring batteries or would a different source of power generation be more practical? $\endgroup$
    – Boo Radley
    Jan 31 at 22:14
  • 2
    $\begingroup$ I love this idea and I hope @BradleyKnauer sees why. It's boring to haul around a "power source." It doesn't create limitations that are useful for story telling or games. But take Triox's idea and convert it to a single-use magazine! Now we're cooking with gas! Your Marines can only haul around so many magazines just as they can only haul around so much ammo! One shot, eject the mag, run like crazy! Manufacturers are vying to create the best-yield-lowest-cost commodity magazine (a cheap one might blow up!) Yeah! I love the Hydrogen cell idea. $\endgroup$
    – JBH
    Feb 1 at 1:46
  • $\begingroup$ If we want to use hydrogen & we're okay w/ a little handwavium, how about a form of metallic hydrogen that's metastable below ~0.1-5 Kelvins? If it can exist, it would be extremely energy dense & probably could be made to decompose into super heated H2 & launch things at or above light-gas-gun speeds w/ a single stage propellant. But it would explode catastrophically in case of heat or impact, which adds tension & balance for the story $\endgroup$ Feb 1 at 5:44
  • 1
    $\begingroup$ @StarfishPrime I did make a mistake in the volume. Will fix that, thanks. The thing is that this is a future where compact fusion reactors exist. A leap in hydrogen fuel cell technology as well as cooling (definitely less good cooling than the compact fusion reactors require) doesn't seem a stretch to me. $\endgroup$
    – Trioxidane
    Feb 1 at 10:15
2
$\begingroup$

Lithium ion batteries

60lb would be doable for a single shot weapon using today's technology. Lithium Ion batteries have a power density of about 1 MJ/kg. Which means that you'd need to drain a 22kg(48lb) battery to fire the thing.

... but a Light Gas Gun may be better

The speed of sound for light gasses like hydrogen and Helium is much faster than for the heavier gasses that make up the air. Compressed Helium (not as efficient but way safer then hydrogen) can expand outwards at a speed of 1007 m/s. That said, if you funnel the gas down a conical , you can make it speed up as the conical narrows achieving speeds closer to 7000m/s (well above your goal.) The way light gas guns work is to use a traditional explosive to compress the gas using a piston until it ruptures a disc. The explosion takes much more time to expand than the light gas does; so, between the explosive to light gas transfer and the conical funneling you get a couple of stages of a mechanical advantages: trading off power for speed until your reach your 4.7km/s

Modern Gun powder releases about 3 MJ per kilogram, meaning you'd need 7.3 kg of explosives to reach your 22 MJ goal. Using today's technology: the thing that makes light gas guns so heavy is all the thick materials it takes to handle all the extreme pressures involved, but in 50 years or so, material technology could make the casings and barrels needed for such a weapon far thinner and more light weight. So, when you factor in the weight of the sabot, the piston, the casing, and the explosive, the whole shell could be 9-15ish kg meaning 1 team member could possibly carry 2-3 shells

$\endgroup$
2
  • $\begingroup$ Interesting, I had considered light gas guns a long time ago but I moved away from them for reasons that I cant remember at the moment. On the subject of using gunpowder to get it all started it did get me thinking. The groups that would be using these weapons have access to a more energetic form of 'gunpowder'(it's not really gunpowder but is effectively used in the same way). Handheld rifles can easily achieve muzzle velocities around 1200-1500 m/s with fairly large bullets (>100 grains). And cannons can get up to 2500 m/s. Would this reduce the weight of the shells used in an LGG? $\endgroup$
    – Boo Radley
    Jan 31 at 21:33
  • 1
    $\begingroup$ @BradleyKnauer It is less about reducing the weight of the shell and more about achieving higher mussel velocity. LGGs allow a slow expanding explosive force to be converted into a fast expanding explosive force. In general, everything about a LGG will be proportionally bigger and heavier than a conventional fire arm to deal with the higher pressures involved and extra space for the LG piston; however, since this is a near future scenario, it is very reasonable to assume that those limits can be mostly negated through better materials. $\endgroup$
    – Nosajimiki
    Jan 31 at 22:49
1
$\begingroup$

Explosives!

Specifically an explosively pumped flux compression generator. https://en.wikipedia.org/wiki/Explosively_pumped_flux_compression_generator

flux generator

There is mature technology for storing lots of energy as stable chemicals: explosives. The explosively pumped flux compression generator generates a single powerful pulse.

An explosively pumped flux compression generator (EPFCG) is a device used to generate a high-power electromagnetic pulse by compressing magnetic flux using high explosive.

An EPFCG only ever generates a single pulse as the device is physically destroyed during operation. An EPFCG package that could be easily carried by a person can produce pulses in the millions of amperes and tens of terawatts.[citation needed] They require a starting current pulse to operate, usually supplied by capacitors.

They would work great for your coilgun. One per shot. I ran across these looking for how to power an EMP device and if it is that kind of fiction your soldiers could use their EPFCGs for a homebrew EMP. Or use them as explosives.

Don't neglect the scene where the coilgun unit runs out of ammo and so loads their coilgun with pieces of scrap metal and uses it as a blunderbuss.

$\endgroup$
3
  • $\begingroup$ So if I am understanding this correctly you could potentially have traditional looking shells but the casing contains one of these devices instead of propellant? $\endgroup$
    – Boo Radley
    Feb 1 at 20:49
  • $\begingroup$ @BradleyKnauer - that would work. Although it might make for better story telling to split them up and have the EPs and the projectiles (maybe various kinds?) separate. If it looks like a shell and acts like a shell but it is just real fancy that does not make much of a story. $\endgroup$
    – Willk
    Feb 1 at 21:27
  • $\begingroup$ I will keep that in mind. I’m thinking of a few designs right now. One works by carrying charges and rounds separate. A charge is loaded into a shielded compartment while the round is loaded from the breech like a traditional recoilless rifle or AT gun. The shotgun idea sounds good as well. I imagine canister rounds filled with steel ball bearings would wreck havoc on infantry and slow moving aircraft. $\endgroup$
    – Boo Radley
    Feb 1 at 22:26
1
$\begingroup$

This obviously is a non-answer since a 'best answer' has already been selected, but here is is nonetheless.

One of the closest things we have to a perpetual motion machine is a super-cooled super-conductive LC tank circuit at resonance. The collapsing coil field discharges into charging the capacitor which then discharges into the coil field which then collapses when the capacitor is fully discharged and discharges into the capacitor which then charges until the coil field is fully collapsed. It's complicated until it isn't. The graphs of current and voltage at the resonant frequency show it clearly. Capacitors and coils operate exactly opposite.

A pure LC tank circuit (no R, or resistance) can build up to enormous voltages and enormous currents. Then, send the enormous current at an enormous voltage into the rail gun.

Some old-fashioned camera flash units used such a system, for building up huge voltages and currents from a small battery. They had a particular whine that increased in frequency as the tank circuit charged up. They are also used in radio frequency tuning circuits, to amplify the very weak radio wave, at a selected resonant frequency.

At the dead-zero-center of the waveforms, a small 'kick' from a battery or such sends the oscillating voltage and current higher, like a small push on someone on a swing sends them higher and higher

Use super-conductive material, with almost zero resistance, and the equations produce almost infinite voltage and stored current at the resonant frequency. Except for the resistance of the wiring, there is almost no power loss. At peak voltage there is no current, and at peak current, there is no voltage, so the equation for P=EI is zero.

So a power 'energy storage' system based on a portable RC tank circuit (no generator needed) would provide the required power, and then recharge over time form some electrical generator or source over time, the same way the camera flash units recharged over time and then discharged all of the electrons (current) in the tank at once. These flash units were the size of a walnut, including the battery.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .