Light Gas Bullet

[MrSamosa]

Alright, this is my first attempt at Improvised Weaponry - be gentle <img border="0" title="" alt="[Wink]" src="wink.gif" /> .

<img src="http://MrTea.50megs.com/LGB.GIF" alt=" - " />

This is an attempt to make the Light Gas Gun concept, that Anthony brought up in the "Penetrating Thick-Skinned Targets" thread, more practical and usable. From the picture, you can probably see how this is supposed to work: Primer goes off, sets off the main charge, expanding gasses push the piston forward, piston compresses the gas, breaks the weakened metal, and the projectile is fired. The Stopper's purpose is to stop the piston from flying out of the cartridge, thus making the cartridge's ejection much cleaner.

The Explosive that is used could probably be anything - High order or low order. While most light gas guns use gunpowder or a hydrocarbon, I was thinking more along the lines of a High Explosive such as RDX or PETN...but maybe I'm being a bit adventurous.

For the piston, perhaps lead would be a good choice of material due to its high density.

The actual gas that is used would probably be Nitrogen, due to the way that I imagine the projectile being built: Primer is loaded, followed by the Explosive, then the piston on top, and the Liquid "Gas" poured in, projectile is placed on top, projectile is attatched to the cartridge. Nitrogen would be ideal due to the fact that Liquid Nitrogen is readily available.

The purpose of the Weakened Metal is to serve as the rapture point on the bullet; the point that gives in to the high pressure. Therefore, it breaks in a predictable way, releasing the bullet down the barrel.

Unfortunately, this will not include the portion of the Light Gas Gun that creates a vacuum in front of the projectile...unless you are firing it in Space :) .

What kinds of muzzle velocities can be expected from this idea? I'm not expecting the 7km/sec mark, but hopefully something higher than the usual speed of bullets. Ideally, this could serve as a cheaper replacement to the high-caliber rounds... Anyhow, do you see this as having much potential?

[Zach]

It sounds/looks like it would work pretty well, to me. Although I don't know anything about it would actually work, the idea looks good. I think a couple of problems you would run into would be:
+ The weaker metal behind the projectile - how would you ensure the metal would lose integrity (that looks nerdy) at the same place, every time? I don't know if where it gives out matters on the trejectory of the projectile, due to the fact that its in a barrel to guide it.
+ Piston - I think the piston would deform if made from lead; maybe hardened lead? (Antimony added I think) Steel might be your best bet.
Qs I've got - Does the shell/casing extend over the projectile?
how does the vaccume infront of the projectile affect it?
Would the barrel of a gun be able to contain that much pressure?

<small>[ November 09, 2002, 01:15 AM: Message edited by: Zach ]</small>

[pyromaniac_guy]

unless I'm missing something here, your projectile is only going to be accelerated while the propellent is pushing the piston towards the stopper, ie not a very short distane, and you will end up with a very high pressure resivour of proppelant gas that isnt accelerating your projectile... not very efficent for making a round go fast :(
in a conventional firearm the round is being pushed out the barrel throughout the entire trip out of the barrel.. your design doesnt do this...

[Anthony]

I've got to agree there. I think you might just have found a less efficient way of transfering the energy from the propellant to the round. Also, I may be wrong here, but I think the use of a light gas (e.g. hydrogen) is important. IIRC due to the fact that you can pack more molecules of a light gas into a given volume at a given pressure, so it stores more energy.

The purpose of evacuating the barrel is to eleminate air-resistance in the barrel, which is a lot more of a problem in a barrel than it is when the projectile is flying through the air. In a barrel, the projectile has to push all of the air in the barrel, along its length and out of the muzzle.

[MrSamosa]

The real problem that I see with the design is the high explosive or the extremely high-pressure gas causing the cartridge to explode, thus damaging the whole weapon.

The casing would not extend over the projectile, rather it would be attatched to the projectile by a strip of weakened metal. The picture I provided above is not counting in any specific measurements, it is more of a case of me putting an idea into a picture. The stoppers would probably be closer to the back of the projectile, so that the piston has more room to travel and build the gas up to a higher pressure. The reason I thought that a High Explosive would be best suited for this purpose is because of how quickly it releases gasses and, applied to this situation, would push the piston forward.

Pyromaniac- I'm not sure if you caught this or not, but as the piston moves forward, the gas reaches extraordinary pressures. I believe in some light gas guns, they've turned Hydrogen pressurized Hydrogen to the point that it becomes solid (anyone confirm this?). Seeing as the cartridge is not suited to handle these sorts of pressures, it is going to burst. And with the structure weakened at specific points (i.e.: the points attatching the cartridge to the projectile), we can predict which portions will give in to the pressure first. As the weaker metal raptures, it separates the projectile from the cartridge and the High-Pressured gas should propel it down the barrel.

Another advantage I see to such a bullet now is little to no Muzzle Flash: High Explosives are not known for their pretty fireballs, and the true propellant in this bullet is mere Nitrogen Gas.

EDIT: I made this post while Anthony was, so I did not get to see his <img border="0" title="" alt="[Wink]" src="wink.gif" /> . Isn't Nitrogen a light gas? I believe it was listed in the list of potential gasses to be used in a Light Gas Gun. Although it was not the best choice (that goes to Hydrogen, but I have not heard much of Liquid Hydrogen), it still worked pretty well.

In this design, the real work horse is not so much the explosive, but the gas... How much do you expect the air resistance to slow the projectile? Would it reach a point that, upon exiting the barrel, it has ordinary muzzle velocities? (Sorry, I do not know much about Physics =\)

<small>[ November 09, 2002, 01:26 PM: Message edited by: MrSamosa ]</small>

[Mr Cool]

You couldn't use a HE, it'd crack/shatter or otherwise destroy your gun.
A light gas is needed, that's why they're called "light gas guns" and not "plain-old ordinary air guns". Well, they would work with nitrogen, but wouldn't be anything spectacular. Helium is normally used since hot hydrogen can make steel brittle.
I don't think light gas guns could make solid hydrogen, since the compression will raise the gas to thousands of degrees C, well above the critical temperature of hydrogen (which is a very cold emperature), so it couldn't even be liquefied no matter how much you compress it.

"In this design, the real work horse is not so much the explosive, but the gas..." Wrong (although I kinda see what you're trying to say I think). All the energy comes from the explosive, through the gas, with resulting losses due to heating etc. You'd be able to get more energy just by using the explosive, but for a higher velocity the light gas gun (or two-stage light gas gun, as here) concept is used because normal propellants aren't suitable for high velocities.

Basically, with practical round lengths you'd be unable to achieve the compression needed for a weapon that is better than conventional ones. You need the gas to be able to fill the barrel and still be at at least several atmospheres of pressure.

[vulture]

</font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica"> IIRC due to the fact that you can pack more molecules of a light gas into a given volume at a given pressure, so it stores more energy</font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">Not true. One mole of hydrogen gas takes up as much space as one mole of Radon gas at the same temperature and pressure.

pV = nRT &lt;=&gt; pV/RT = n where n is the number of moles of any given gas.

[firebreether]

The reason they use hydrogen is because light gases have higher speeds of sound. The expansion of the gas will only really be able to make it up to the speed of sound, after that some funky shit goes on :p also, higher temp gases also raise the speed of sound. So, H₂ mixed with high temp = the possibility of some insanely high muzzle velocities which is why they are being researched/used

[spydamonkee]

why not use a far more simpler aproach to this idea
take a 30-06 for an example.load one round into the breach then fill the barrel with your light gas then force another projectile half way down the barrel

--------------------------------------------------
: :D %%%%%%%%%%%%%D
--------------------------------------------------

Bullet : :D , light gas %%%%, projectile

you could also some how vacume and seal the last half of the barrel to rid it of air resistance. barrel might just stand up to the extra pressure, of course testing on this would be done remotly i assume.

just my 2 cents

<small>[ November 09, 2002, 09:55 PM: Message edited by: spydamonkee ]</small>

[pyromaniac_guy]

Mr samosa,

I do know a tiny bit about light gas guns, and my issues with your design are two fold.

First I thin you are rgeatly overestimating the amount of work that can be done by the gas resivour. All of the research with light gas guns i have read about used huge volumes of gas to do the work. unless you plan on having a round for a 30-06 sized projectile that is 10ft long and 100mm in diameter, i dont think the gas will do anything for you.

Secondly, all you are doing is conveying kinetic energy from the rapidly expanding combustion gasses of a propellent to a projectile, via the gas. if you have the ability to release the energy stored in the gas resivour much more quickly than burning a propellent, then you ahve the chance to accelerate the projectile faster than the propellant would. but remeber the conservation of mass energy.. since the system is far from ideal it's not going to be very efficent, therefore even though you might have the ability to propel rounds fast, it's going to come at the expense of requiring a lighter round...

[Anthony]

firebreether, I'd be rather surprised if expanding gases were limited to the speed of sound, otherwise compressed air guns wouldn't be able to accelerate projectiles &gt;mach1 - which they do.

Vulture, I don't doubt that you're right on this. My reference was that a compressed air gun, which releases a metered volume of gas at a set pressure, will impart considerably more energy to a projectile when filled with helium, than air, for the same fill pressure.

[Snapy]

I dont mean to flame on my first post,but as spydaamonkey said, the strait powder/projectile will give the projectile a fair bit more energy.I supose the folowing could be a good tradeof:

------------------
ccccccccccccc$$pppp
------------------

c = powder
$= gas
p= projectile

I just thought of this but this might add some punch.Fill the top of the powder with a heavy smoking making powder(pardon me,enlisgh is not my main language).Leave a small vacuum between that and the projectile.This would generate some gas and give the extra thrust you'r looking for.

[john_smith]

This won't work. First of, the sole reason for using the light gas is it's greater escape velocity. After about 8.000fps(?) common propellant gas can't even get out of it's own way, let alone accelerate a bullet, so propellant is used for compressing the light gas instead, and this in turn for pushing the bullet. Because the compressing piston can't move faster than the escape velocity of propellant gas (actually less), it has to be of much greater diameter than the bullet, thus necessitating a bottlenecked type chamber (In actual light gas guns, the "bottleneck also acts as piston stopper). As most of the energy for this thing is still provided by the propellant, you'd need a huge powder charge. Also, if I remember correctly, the minimal recommended barrel length for this was 200x the calibre(bullet).

[nbk2000]

I found this little animated GIF that shows the operating principle behind the light gas gun.

<img src="http://www.emi.fhg.de/Abteilungen/Impaktphysik/LGG_20_Bilder.gif" alt=" - " />

[vulture]

Am I correct if I assume this thing has forward "recoil" because of the piston being stopped in the barrel?

[Jacks Complete]

To correct a few things in this thread:

Normal nitro-based propellant runs out at velocities far below those you can get from a light gas gun. The gasses are just too heavy.

The easy way to think of this is like electronics. The huge BP charge is like a lead acid battery, which pumps up a capacitor (the light gas), which then discharges really fast.

The pressure and heat builds right up, then the disc ruptures, letting the hot light gas (hydrogen is 7km/s and helium is 4 km/s) rush up the barrel, pushing the projectile ahead of it.

It is kind of like an air rifle.

btw, air rifles cannot fire projectiles faster than the speed of sound without dieselling. The reason for a few systems being apparently able to, is that they are using hotter air, from whatever source. This means it travels faster than the air outside the barrel, and hence you get the crack when the pellet hits the slower moving air!

There is a way to get even higher velocities. Most boats now tack across the wind, rather than just letting the wind push them forwards. This is because the aerofoil shape of the "wing" (sail) causes lift, and this will pull/push the boat faster than the wind is actually travelling. The same trick can be used in LGGs, and velocities up to 15km/s have been reported.

As for fielding one, well, they take a while to get set up, and the one I saw (honestly) was about 30 ft long, two feet thick, and was being used to shoot things to see what happened. I can't say what or where, though. Trust me, it was huge, and bolted to lots of concrete.

The barrel was 15ft long, but thinner. The burst discs were precision cut things about 2 mm thick, and the projectile did 2800 metres per second. My flippant response was that my 7.62 could do that. Then I realised I had just said something really dumb... 2800 ft/s is my rifle. This gun was three times faster. It was also firing a projectile ten times heavier, iirc. The guy told me how they got the army to stuff a naval 15" gun with propellant to see what they could do, but it blew itself to bits before it got near 2800m/s. This gun was running Helium, btw, and was dialed right down.

The support equipment was extreme, and so was the four foot thick blast wall. It seems that should the disc ever hold, the piston would come back, or else the gun would burst. Which would be bad!

[ossassin]

I just suggest that you actually insert the bullet into the casing, which is the customary method. Otherwise, it looks feasible. I still don't understand why you'd do this when you can use a conventional round, though. The bullet creates most of the sonic boom, not the gases. If you're that worried about the gases making noise, make a supressor.

[apathyboy]

This is an interesting site, it has some info on actual experimental gas guns, maybe you can do something with it

http://yarchive.net/space/exotic/light_gas_gun.html

actually, theres some good info (I think) on the whole thing http://yarchive.net/

[Jacks Complete]

Bizzarely, I met a man who might be able to help with this project. Turns out he is a hydrogen specialist!

If I can work anything out, I will post about it.

Please note that metal and hot hydrogen don't go together well. Look up "Hydrogen embrittlement" on goggle. I am also rapidly coming to the conclusion that the LLG only works by being enormously strong and heavy, so a small version could never be made. :(

Problems to be solved:

Hydrogen is a bugger to keep trapped, as the molecules are really small, and so they even get into teh metal lattice and cause embrittlement after a while - not good for storage of cartridges, as they would become brittle, and you might find they had leaked away, meaning you had a dud.

The ratio of propellant and first chamber to the second chamber needs to be high. Someone quoted 200x. This might be excessive, but it might not be. So for a .17 Bee round (4.5mm), you would need a 35mm piston of the same length, by my calc. I don't know how to work out the lengths, though.

It would be a bugger to move this thing, as the barrel is going to be pretty long, and recoil will likely NOT be man-sized. As a quick calc for helium as your gas (4000 m/s top speed) running at "just" 75% (3000m/s) I reckon a 60 grain bullet should do (some kind of Tungsten rod) the KE of the bullet will be
3.88 grams @ 3000 m/s -> 38571 Joules or 28448 ft-lbs
or twice that of a .50 (18218J for a hot round)

In a 5 Kg rifle, that is just 2.32 m/s of recoil!
For comparison, a No.4 7.62 weights 4.11 Kg and recoils at 2m/s, for a little over 3352 J a shot.

Obviously, this only works for light, tiny calibre rounds, anything heavier than 60 grains is going to need a much heavier rifle. Mind you, making an LLG as light as 5Kg is likely impossible!

Do we have any engineers on here who could do the calculations, and work out what sort of pressure, etc. would be the upper limit? I have seen proof marks that go as high as 20 tonnes per square inch, which is the standard pressure for a 7.62 rifle, but does anyone know what, for example, .338 Lapua Magnum is proofed to?

What is the maximum pressure that modern high-strength steel can take before yeilding?

Also, does anyone know what an "AGARDograph" is? One of the most cited references in LLG things is "A. Seigel, "The Theory of High Speed Guns," AGARDograph 91, 1965", but I have no idea what this even is!

[keith]

I can't even imagin the wear and tear on this type of system. How doesnt the piston slamming down at well over 2000fps destroy the gun?

[Jacks Complete]

That is a very good point.

Logically, the answer is that the piston is already decelerating, as it is using its energy to compress the light gas. Just before the piston stops, and shoots backwards, the seal ruptures, and the bullet gets blown forward. The piston then continues to slow, but not as rapidly, and then probably starts to get pushed backwards a bit as the hot gunpowder charge starts to cool.

Getting this to happen might be somewhat tricky.

[keith]

Ok, now where does the 80,000PSI of gun powder propellant go? 80,000PSI is the standard pressure rating for medium bore rifles ie. 30-06, 7mmRem.Mag., 30-30, and so on. After the gas pushes the projectile out the gun would become a very big pipe bomb. Even if the gass rapidly cooled(which it wouldnt!) there would still be around 60,000PSI in the chamber.
This design is new to me and seems unefficient but with more research could become quite useful.
Does the military use this for anything?

[Jacks Complete]

The military use them for studying hypervelocity projectiles, impacts at very high speeds, and various other things, like what happens if a chip of paint hits a space shuttle window at 15km/s. They can't get that fast, but it is the closest anything can get. The only thing faster would be something non-combustion related, or something weirdly exotic, like a cloud of plasma or neutrons or something lighter than hydrogen.

No-one has ever tried to field one, since they are a bit big.

The gas pressure left in the cylinder will be high, but you could just vent it. Failing that, it is BP they use to push the piston, and the volume changes upwards so far, that they may well just contain it fully.

80,000 PSI seems a bit too high- that is 40 tonnes per square inch, which is more than double the proof on my target rifle (It should be proofed to 20 tonnes, but it is old, and proofed to only 19) in 7.62. Remember, there is no reason they have to go to such high pressures in the first stage. In fact, you want lower presures, as the gun will last longer. You want the scary-ily high pressures in the second stage, where the light gas is at, and that just vents to atmosphere when the projectile leaves.

Must be one hell of a shockwave/muzzle blast! Over 100ft in some reports, as the Hydrogen burns...

[Tended Tripod]

I was watching Nova, a show on PBS (public broadcasting) that is almost every time exploring a nifty branch of science like demolitions, warfare, or any new technology, and this time they were doing tests to see what the temperature of the sun would be like. The only way they could get temperatures even close to the sun was by using a Light Gas Gun to fire a bullet into a block of metal. The impact would heat up the projectile to almost the temp of the sun. I think the gun was about 30 feet long, and was fired into a container with a camera in it. Quite amazing. I don't really remember any more details on it, if you could call any of this details.

[xyz]

Actually electric arcs can reach sun-temperature (6000°C) fairly easily if they have enough current flowing through them.

[Jacks Complete]

I imagine Tended Tripod means pressures and temperatures of the CORE of the sun.

[xyz]

Yep, that temperature is the core, the outside is only 2800°C.

It doesn't create the pressures though.

[Jacks Complete]

xyz,

the whole point about using the gun is that you get the pressure for a fraction of a second when you shoot something with it, as well as the instant heating effect.