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High Velocity Launch Systems

A place to ask general spud cannon related questions.
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Unread postAuthor: DYI » Fri May 28, 2010 2:17 pm

Your design looks quite robust, but I'm curious regarding what seems like a whole mess of slip-fit parts around the capillary tube; how does it all go together when loading?

Also, I note that you've decided to use two insulated conductors instead of using the steel casing as the return path. Considering the dimensions, I can't imagine that it would impact the overall system resistance very much. As it stands, the design will require a true feat of machining to be realized.

I'll be suitably impressed if you manage to build it.

Also: good luck with the tungsten foil. I'd say it's a waste of money, but I've been known to experiment with similar things from time to time :wink:
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Unread postAuthor: axi0m » Fri May 28, 2010 4:13 pm

I was undecided as to whether to make the chamber part of the circuit or insulated. Being that this system would only be a prototype and not an actual field-use unit, I believe I'm going to use the chamber as part of the circuit in my actual build in an attempt to concentrate resources on performance.

For various reasons, I've also decided to make the bore 0.1875" instead of 0.125". A 0.1875" sphere of polypropylene has a mass of 0.4 grams and, as such, could theoretically be accelerated to a speed of 3 km/s (Mach 9) with 20% efficiency given my stored energy of 6,885 joules.

I've decided on what I believe to be the best method for creating pressure. It is very much inline with what I've seen in many other internet ETGs: strips of thin metal directly coated with water. Rather than relying on a single arc radiating and convecting heat into a porous surrounding ablation material, having a proper amount of the ablation material in direct contact with the arcing material theoretically converts water to steam most effectively. Something that I have not seen addressed, however, is how the water beads up on the surface of aluminum and, as such, does not provide a large amount of surface area contact between the aluminum and water. I intend to test different surfactants or detergents to reduce the surface tension of the water and allow it to spread uniformly across the surface of the foil.

Also, I'm no longer interested in the tungsten foil of 0.001" thickness. I've realized that aluminum "leaf" can be readily purchased cheaply. The more surface area contact I can have between the arcing material and the ablation material, the better. The thickness of the leaf I believe to be somewhere between 0.1 and 0.3 microns, as such, this is by far the best choice. Being that the resistance of such a thin arcing material will be high, I will connect multiple pieces in parallel to achieve the same resistance as a thicker piece of aluminum foil, such as that which you'd find in most ETGs, giving me a much higher surface area contact.

Being that I'm planning to use the chamber as a conductor, I've looked into aluminum. Alloy 7068 has a yield strength of around 95,000 PSI, which would suffice well, while providing great conductivity. Also, the machinability of aluminum would reduce costs.
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Unread postAuthor: ramses » Sun May 30, 2010 11:00 pm

This site merits a bookmark.

some can be found on google books. Others are IEEE.

in particular, linky
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Unread postAuthor: axi0m » Thu Jun 03, 2010 2:03 am

I was VERY wrong on my theoretical speed calculations. Somehow the weight of the polypropylene ball was very much miscalculated -- I knew that 0.4 grams seemed heavy for one of the lightest plastics available. It was actually off by an order of magnitude!

Since these numbers are so staggering, I'm going to walk through the calculation very briefly.

Stored energy:
0.5 * (6,800uF/1,000,000) * (450V^2) = 688.5 Joules per capacitor
x 9 capacitors (3 in series by 3 in parallel) = 6,196.5 Joules total

Projectile:
0.1875-inch diameter polypropylene ball

Volume of projectile:
(4/3) * 3.14159265 * (0.1875/2)^3 = 0.003451 cubic inches (0.056559 cubic centimeters)

Density of projectile:
0.85 grams per cubic centimeter

Weight of projectile:
0.056559 * 0.85 = 0.04807515 grams

Projected efficiency:
6,196.5 * 0.25 = 1,549.1 Joules

Kinetic energy of projectile at 8 km/s (26,247 fps; Mach 24):
0.5 * (0.048075/1,000) * (8,000^2) = 1,538.4 Joules

***

Obviously, it is very easy to do this math and obtain these materials. In practice, however, we never see anyone obtain Mach 24 with an electrothermal gun. I believe this has to do with the supersonic flow, or lack thereof, of the expanding gas.

I understand that once a flow of gas through an orifice has reached the local speed of sound, constricting the orifice will not increase the flow rate of the gas. This does not mean, so I believe, that somehow increasing the pressure behind the orifice, for a fixed orifice size, will not increase the flow rate of the gas. If this is the case, why can't conventional powder-based cartridges simply be expanded further (thus holding more powder) with a fixed neck size, thus increasing the pressure behind the neck?

I believe the answer to this is that conventional propellants can burn (ablate) at limited speeds. Thus, the only improvement electrothermal guns offer is that they can ablate any given amount of material infinitely fast, creating infinitely high pressure.

Why, on the other hand, can't a bullet be packed more tightly into conventional cartridges such that they won't budge until some higher pressure is reached? Does this simply come down to the strength of available metals with regard to stresses experienced thick-walled pipes?
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Unread postAuthor: inonickname » Thu Jun 03, 2010 9:38 am

Perhaps this doesn't belong here, but a thought came to my mind. How about rapidly decomposing a chemical? Not a conventional high explosive or anything- but perhaps something such as nitrous oxide. Decomposition could be begun by electronic heating (resistance) or a setup similar to an ETG. Once the reaction begins it will self sustain (as it is highly exothermic). Release could be via a burst disk or similar.

The space between the projectile and disk could perhaps be filled with a propellant such as methane or hydrogen to take advantage of the oxygen released (a liquid such as ethanol would work best).

With liquid nitrous oxide, the available power could be substantial.

Thoughts?
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Unread postAuthor: ONEWING » Thu Jun 03, 2010 12:39 pm

N2O isn't flammable and wont decompose to any useful extent. H2O2 would in higher concentrations. But these are still firing under the same principles as any chemical propellant gun, self sustained "decomposition" could define any explosive.

H2O2 with Al powder hit with a huge jolt would be interesting none the less, though quite likely a detonation would occur. But I assume thats the problem with ETG's in general, slowing down the energy release to propel rather then explode.
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Unread postAuthor: axi0m » Thu Jun 03, 2010 2:57 pm

ONEWING wrote:But I assume thats the problem with ETG's in general, slowing down the energy release to propel rather then explode.


If the objective of any propulsive system is to attain maximum speed, an explosion, assuming it is confined within the chamber and is only allowed to escape via displacing the projectile, is exactly what one seeks. Matter-of-factly, the more explosive it is, the faster the projectile will go.

Perhaps this answers my previous question. Do gun makers not use more "explosive" propellants simply due to the strength limits of conventional materials? If the current limit on speed (around 6,000 fps) is due to the strength of the materials, then ETGs offer nothing advantageous to attaining higher speeds.
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Unread postAuthor: ONEWING » Thu Jun 03, 2010 3:31 pm

Sorry when I said "explode" I was refering to the whole thing going kablooey. Even with extremely high pressures it does take some time to push the projectile up to speed such that a lower pressure over a longer time will be better, this was the problem when nitrocellulose was first used until ways were found to slow it down. Shrapnel travels no faster then a bullet though it experiances much higher pressures, just for not as long (munroe effect is used to create hypervelocity "shrapnel").

So its not just more pressure and stronger materials, the main problem is maintaining the pressure which solid deflagrating propellants do so well by continuing to burn creating gasses as the bullet travels up the barrel. Thus the problem with single stage ETG's, all the energy is released instantaneously thus the initial high pressure can't be maintained.
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Unread postAuthor: axi0m » Thu Jun 03, 2010 4:44 pm

ONEWING wrote:Even with extremely high pressures it does take some time to push the projectile up to speed such that a lower pressure over a longer time will be better, this was the problem when nitrocellulose was first used until ways were found to slow it down. [...]

So its not just more pressure and stronger materials, the main problem is maintaining the pressure which solid deflagrating propellants do so well by continuing to burn creating gasses as the bullet travels up the barrel. Thus the problem with single stage ETG's, all the energy is released instantaneously thus the initial high pressure can't be maintained.


So, let us examine two different modes of conventional propulsion.

Firstly, let's assume we have a solid propellant that releases its heat and gas nearly instantaneously -- very explosive in nature. This will create a very high pressure behind the projectile and accelerate it at an extremely fast rate for a relatively short period of time, because by the time the bullet has traversed the length of the chamber, the pressure is already half of what it was.

Secondly, let's assume we have an identical amount of solid propellant that releases its heat and gas slowly and linearly, such that it has completely decomposed exactly when the projectile reaches the end of the barrel, essentially maintaining the same pressure for the entire length of the barrel.

There are still many variables being ignored here, but this is a simple thought experiment. With that being said, I say that the ideal system (to achieve the highest speed) would use the most explosive material possible and implement additional amounts of it in stages along the barrel, maintaining the highest pressure possible given the structure throughout the entire length of the barrel.

Ignoring the stages concept, however, I still do not see how the second scenario would attain a higher muzzle speed than the first scenario, or how it would be "better," to quote you, unless you are considering the erosive effects of higher pressure. Thanks for your time.
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Unread postAuthor: DYI » Thu Jun 03, 2010 6:16 pm

Firstly, to respond to questions on conventional firearms:

Much of the problem is barrel life. The large gun used in the HARP project pushed projectiles up to 3km/s, using what we would consider "conventional" propellants. Increasing the propellant quantity or necking down the bore for a given cartridge increases barrel wear and necessitates a heavier chamber. Looking at firearm design over the past 200 years, this has indeed been the case. We simply have to be careful to not let our cartridges exceed the capabilities of our barrels. ETGs can get around the problem somewhat by producing lighter propellant gases, maintaining a relatively stable pressure for longer, and reducing the pressure gradient between the breech and the projectile base.

The real attraction in ETGs is the ability to generate extremely hot, low density gases at very high pressure, and to maintain this pressure longer and more stably than would be the case with any sort of chemical reaction. Extreme pressure is not the goal so much as is the maintenance of high pressure - with no relevant expansion rate limitation, 60k psi could push a 45g slug from a barrel with the same dimensions as that of a .50 M2 machine gun at 1630m/s. The actual weapon achieves 890. We didn't have to make the gun significantly stronger, we just maintained the existing pressure. The temperature required will destroy the breech, but replaceable inserts should essentially eliminate that problem in a lab gun.

Now, onto this whole "explosiveness" issue; we don't use "more explosive" propellants because there's little to be achieved by doing so. You pack roughly the same amount of energy into the same chamber and create very similar reaction products, while producing a much higher initial pressure spike through a detonation. The result? Peak pressure is vastly higher, average propelling pressure (what matters) is essentially the same, and you destroyed the chamber (whether or not it actually ruptured) to achieve the same results you'd get with a proper gun propellant. HE certainly has its potential applications in the field of high speed launch, but it isn't used for firearms simply because as a gun propellant, it is useless.

@Axiom: The only reason that the second scenario is "better" is that, in the second scenario, the gun ends up in the same shape it was before firing. :lol:

I'd suggest a look at this page for a very in depth treatment of the topic (albeit rather dated).
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Unread postAuthor: Zeus » Thu Jun 03, 2010 6:28 pm

Well, with current propellents you can't make any projectile go faster
than about 6000 metres per second. I think you suggested a
"Flessiges Lieschen" like setup with your last idea.

A multi-chamber gun has been constructed, that was a working scale
model of Busy Lizzie, it had suprising power. The main problem with such an arrangment is flashover. That would simply slow down the
projectile as the charge ignited in front of the shell.

Timing was brought up as a potential issue, but with today
igniters that would be no problem.

On another type of hypervelocity projection, I saw mention of
light gas guns, and that they would be feasible for a home builder.

I am designing a cascading burst-disc setup (50mm all the
way through, 700 psi capable). I can mould a HDPE piston,
and use a 50mm barrel as the compression chamber.
And I can get 3m lengths of 7.35mm aluminium. So why not.

Sorry about the essay-like post.

Lachlan

[Edit: I hate forgetting to say something]
[Edit 2.0: DYI, you are a bloody fast typer]
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Unread postAuthor: axi0m » Thu Jun 03, 2010 10:15 pm

When we look to achieve ultrasonic muzzle speeds, we will need propellants that can burn at a rate such that they can fill the void created by the traveling projectile much faster than it creates it, otherwise they will not continue to apply pressure to the projectile as it travels down the barrel.

Actually, that isn't true. You could use slower-burning propellants and simply create a very large cartridge that is necked down to the bore diameter. More slow-burning propellants gives off the same amount of gas and less faster-burning propellants.

So, we've established that the burn rate of conventional propellants isn't a limiting factor because we can simply increase the amount of propellant that is being burned at any given time. We are temporarily ignoring the effects of the local speed of sound in the chamber and barrel. This, then, leaves us with the strength of the material as the only thing that limits muzzle speed, because it limits the pressure that can be contained.

"Parts which are expendable may be designed to deform but not rupture at transient pressures as high as 1,000,000 PSI," according to the paper to which DYI linked us. (Page 19)

So, we would have to design a system with a satisfactorily high maximum pressure (anywhere from 100,000 PSI to 1,000,000 PSI) and arrange an amount of propellant such that immediately upon ignition, the chamber pressure reaches this maximum pressure but does not exceed it. To maintain this pressure, we would have to ignite additional chambers such that an amount equal to the original amount of propellant is ignited for every volume of the barrel equivalent to the chamber volume that is traversed by the projectile. This will essentially maintain an average of 75% of the maximum allowable pressure for the entire barrel.

Anyone with a machine shop and a near-by RadioShack could create this system, which would be capable of unearthly muzzle speeds with the proper selection of bore area, barrel length and projectile weight. Again, I'm completely ignoring the effects of the local speed of sound, which I think would ultimately be the limiting factor.

So, to narrow it down again, what advantage could an ETG have over the previously mentioned system which could use any conventional propellant?

Here is some truly next generation stuff:
http://arxiv.org/ftp/arxiv/papers/0910/0910.3961.pdf
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Unread postAuthor: inonickname » Fri Jun 04, 2010 9:33 am

ONEWING wrote:N2O isn't flammable and wont decompose to any useful extent..


Wont decompose to a useful extent?

It is used in monopropellant rocket engines;
Nitrous oxide can also be used in a monopropellant rocket. In the presence of a heated catalyst, N2O will decompose exothermically into nitrogen and oxygen, at a temperature of approximately 1300 °C. Because of the large heat release the catalytic action rapidly becomes secondary as thermal autodecomposition becomes dominant. In a vacuum thruster, this can provide a monopropellant specific impulse (Isp) of as much as 180s. While noticeably less than the Isp available from hydrazine thrusters (monopropellant or bipropellant with nitrogen tetroxide), the decreased toxicity makes nitrous oxide an option worth investigating.


Correct me if I'm wrong, but wouldn't going from a room temperature liquid to an equal mass of gasses at 1300 Celsius would result in a very substantial increase in pressure?
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Unread postAuthor: ONEWING » Fri Jun 04, 2010 10:47 am

inonickname wrote: thermal autodecomposition becomes dominant.


Then I must eat my words :) Thats from wikipedia, though I looked further and indeed found reference to it decomposing explosively under welding temperatures. I was under the mistaken impression that it was much more stable then H2O2 (which if taken as pure compound I guess it still is).

Easiest test of your theory would still be to run straight H2O and H2O2 (say 50%, pure detonates) against each other. You would have to hope that the thermal decomposition would be slower then the flash boiling for the reasons stated above, to give a more controlled energy release. The reason I said to mix with aluminium powder while sure to screw up your chamber and ream your barrel while coating it in sapphire was to give H2 as the gasseous product.
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Unread postAuthor: axi0m » Fri Jun 04, 2010 7:07 pm

I'm on the fence about an ETG or a railgun because the latter can maintain a constant acceleration for the entire length of the barrel, regardless of the acceleration or speed of the projectile with much less difficulty than with a conventionally propelled projectile or ETG, which I believe would require a multiple chamber design.

Are there any other ways to maintain constant pressure for these systems without having multiple chambers?

Thinking about it briefly, one could make a cartridge that has a severe conical shape, larger as the base so that more propellant is being burned when the projectile is further down the barrel.

Also, with a cylindrical charge in a cartridge, one could have an igniter that was placed along the length-wise axis of the cartridge and ignited the entire length of the inner diameter simultaneously. As the propellant burned outward it would increase surface area giving off more gas and maintaining the maximum pressure as the projectile is further down the barrel.

Are there any other methods than these two, with the exception of multiple chamber designs, that could provide a constant pressure system?
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