SpudFiles

Firstly, I'd like to point out a slight inconsistency in the posting rules regarding "explosive propellants":

Solid propellant: Any substance containing all the chemical elements required to enable sustained combustion without the presence of additional oxidizers. This includes but is not limited to gunpowder, thermite, and many commercial and/or homemade pyrotechnic compounds.

1. For this purpose, nitroglycerine, though liquid, would qualify.
2. For this purpose, ordinary flour, though solid, would not qualify.

Black powder is a mechanical mixture of oxidiser and fuel, not a compound which is independently capable of energetic decomposition. It falls under a completely different category than nitroglycerin, and if it is banned for discussion, the wording here would ban any air/fuel mixtures for discussion as well.

Also, the posted definition of "explosive devices" specifically prohibits the discussion of spudguns anywhere on the site. That should probably be fixed...

As such, I'll assume that the actual intent was to ban the discussion of solid explosives and HE compounds.

I'm certain some of you have heard of Al/H₂O reactions before, but here are the basic parameters:
4Al + 6H₂O --> 2Al₂O₃ + 6H₂
By mass, this works out to a roughly 50/50 mix, with a resulting density of 1.85g/cm³. Some simple calculations show a reaction energy of 7.9kJ/g for this mix, and the literature indicates that energy input required to initiate this reaction is at least 5kJ/g in a good geometric configuration, for proper vaporization of the aluminum (this figure is confirmed by calculating the deposited energy required to boil the aluminum). Peak pressure for this energy level (with an absurdly fast current pulse, as the authors were investigating its use as a high explosive) was listed as 113 kbar for the minimum electric pulse energy. With systems more accessible to the average hobbyist, pressures generated would be no more than a few kilobar for a reaction lasting tens of microseconds, and thus containable with easily built launchers (and with internal ballistics characteristics more similar to those of traditional propellants).

The propellant gas in this case, as a few of you have undoubtedly noticed, is hydrogen. Very hot hydrogen, at that. The aluminum oxide is an unfortunate byproduct, but very high speeds may yet be attainable through this method, and the firing energy when compared to an ETG using the same capacitor(s) would be at least doubled, making this reaction a more economical alternative, if it turns out to be effective. The most obvious configuration for the aluminum is in the form of a single strip of foil, fit into the correct volume between two electrodes. The resulting geometry will have lots of sharp edges, twists, and general asymmetry, leading to an excellent mixing process.

As such, there will be some testing done over the break to ascertain the efficacy of this reaction for propulsion purposes. The system used is capable of discharge times not exceeding 40μs for pulse energies up to 10kJ, and the chamber volume of the test device will be set at 0.5cm³. Watch this space, as results will start appearing in a few days.

One excellent reference to check out on the topic is this paper. Any (relevant) discussion on the topic is appreciated, as well as links to instances of this being tried by others. Also, if you're not a moderator, DO NOT comment on the first section of the post, regarding the rules. On that subject your opinion is neither relevant nor appreciated, and I ask you to kindly keep it to yourself.

DYI wrote:Peak pressure for this energy level (with an absurdly fast current pulse, as the authors were investigating its use as a high explosive) was listed as 113 kbar for the minimum electric pulse energy. With systems more accessible to the average hobbyist, pressures generated would be no more than a few kilobar for a reaction lasting tens of microseconds, and thus containable with easily built launchers (and with internal ballistics characteristics more similar to those of traditional propellants).

I think I would put that burn rate, "tens of microseconds", in the "detonation" range. For combustion guns the burn times run in the tens of milliseconds range.

That's a pretty neat idea, though I wonder if the aluminum would tend to corrode the containment walls and electrodes at a high rate? Not really a problem for rough experimental models but if you want to do a lot of testing it could get tedious. I can't really see any other obvious problems(other than the limited tensile strength and SoS in modern materials).

I always enjoy your little ludicrously-high-energy-density ventures, though I think most of us can't fully appreciate the thought you put into the designs.
You know, like the media/police


On an unrelated note, just what the hell are you going to be doing when room temperature SC's start appearing? lol.

I think I would put that burn rate, "tens of microseconds", in the "detonation" range. For combustion guns the burn times run in the tens of milliseconds range.

Although it's somewhat faster than the reaction occurring in traditional spudguns or firearms, it falls firmly outside the definition of detonation - regardless of energy input rate, this reaction does not form a detonation wave. This reaction occurs on the same timescales as the firing process of my ETG, which has shown no evidence of the "strange" mechanical failure properties or projectile damage that are traditionally associated with detonations. It's still a "gradual" pressure buildup, and there aren't any of the localized extreme pressure regions that occur in detonations. Part of the difficulty of applying these reactions commercially IS the lack of a detonation wave, which forces users to resort to extremely fast pulse discharge systems to achieve similar effects.

That's a pretty neat idea, though I wonder if the aluminum would tend to corrode the containment walls and electrodes at a high rate? Not really a problem for rough experimental models but if you want to do a lot of testing it could get tedious. I can't really see any other obvious problems(other than the limited tensile strength and SoS in modern materials).

Perhaps you mean the aluminum oxide deposits would cause problems? If so, you're correct, but the electrodes in these things typically need cleaning every shot anyway. The use of steel all-thread for the cathode simplifies electrode replacement/repair (resurfacing is required after every shot in the ETG). What's being done now is a very rough design, which will in all likelihood be fired no more than three times simply to assess whether the concept is valid. The anode is the chamber itself, in this case a 2.5" diameter steel block with a yield strength of around 80KSI. A similar chunk of this same block was used to fully contain up to 1g of [redacted] twenty times (at well over this yield strength), so mechanical failure of the chamber is not something I'm worried about.

The insulator for the test device is complete, and I played around with some packing configurations tonight. Seems that the best bet in terms of eliminating air space is to fill the chamber half way with water, then slowly pack in very small chunks of cut aluminum foil. I'll be testing such a configuration tomorrow morning at 8kJ energy input, and hopefully finishing the chamber for the test device either tomorrow or Tuesday.

Also, "ludicrously high energy density"? A chamber made of 350 series maraging steel could contain over 20 kilobar without any deformation. I'm just getting started

Rule 8-E states:"These rules may not be complete. Intentional exploitation of loopholes may result in new rules."

I'm still waiting for PCGUY to take a look at this. For now keep on topic.

Thread cleaned.

Testing today indicates the effectiveness of the reaction for generating pressure with much slower burn rates than those used in the paper. The test consisted of one gram of the 50/50 Al/H₂O driven by an 8kJ pulse (at 18kV). The aluminum foil had been cut into fine chunks and sunk in a tube full of water. The electrodes were 3/8" bolts clamped into either end of the tube, for what appeared to be a watertight seal.

The containment vessel was 15L in volume, and capped by a 50kg concrete block with 780 cm² exposed to pressure, which lifted ~15cm off the top, a height that was attained after 105ms. The force pushing the block up, by some rough calculations, would have momentarily exceeded ten times its weight. Some areas of the reaction products were visibly incandescent up to 150ms after the discharge. A sheet of 1/2" plywood with its face roughly 2cm from the outer surface of the tube was punched through and cut in half, and a large plasticine block placed under the tube was left with a shallow (~15mm) crater 50mm across. Several intact fragments of aluminum were found, confirming that this configuration was non-ideal (the relatively slow rate of the reaction also contributes to this). After the containment vessel is rebuilt, this will be tried again with the foil cut into fine strips running from one end of the tube to the other.

The long-lasting incandescence is attributable to the same effect that makes [magic silver dust] so bright - unusually high temperature causing the cloud of fine aluminum oxide to glow. However, the real factor in whether or not this is a good propellant is the hydrogen, and until the test launcher gets put together (tomorrow), we can't draw any solid conclusions about that. Expect a test of that by Thursday. Regrettably, yours truly is back to university on Sunday, and it's starting to look like this testing might get cut short.

OK...

To be honest, this is a bit over my head. I understand parts of what you are saying, but during your testing what exactly are you mixing to what exact to create this hydrogen? I see the electricity is applied (I believe), what exactly is it being applied to and how?

As far as discussion of this, for sake of development and interest we can continue this conversation. You should all understand what I am trying to "ban" with the rules, and that is basically someone making what the media/police would consider a homemade firearm, such as the "typical" black powder (or flash powder, whatever type of high explosive) in a tube type of idea.

The laws around spud cannons in the USA are very gray, but we try to create these walls to try to keep our cannons separated from "homemade firearms" or just plain "deadly weapons". There is no exact answer for every situation, and if you pick apart the rules as you did with your Blackpowder example it then just becomes up to the moderators/myself to deem what is "safe" and "sporting/spudding like" vs flat out dangerous and possibly illegal. I enforce the rules around what I would figure local police, media, and laws would consider "crossing the line" into dangerous/illegal activities. Of course as well, I want to keep users safe and your typical combustion/pneumatic cannons made out of PVC have proven fairly safe for years.

What DYI is doing is using electricity to super heat a mixture of aluminum and water. When a critical temperature is reached, the water basically "burns", by reacting with the aluminum to produce hydrogen and aluminum oxide. Extra energy is released by this reaction, and this heats the hydrogen to increase pressure.

I suppose the closest thing most would relate this to is thermite, which is legal in the u.s.
Nothing (that I know of) in U.S law would define aluminum and water as an explosive, and the energy needed to actually get it to react is very high; so high that you can't compare it to something like BP.

The 'gun' is immobile, so I don't think it's likely to be viewed as a weapon. The extreme voltages/currents are of far more concern than anything else .

I'd just like to vouch for DYI by saying I believe he knows what he's doing and will be able to carry out this experiment in an entirely safe manner.

In an electric discharge of that energy level, aluminum is vaporized and water forms a steam plasma.

but during your testing what exactly are you mixing to what exact to create this hydrogen?

Much like thermite where at high temperature aluminum combines with oxygen from iron oxide and leaving molten iron, Aluminum reacts with Oxygen from water with the same release of heat. This takes the O2 from water and leaves Hydrogen as a byproduct.

The energy required to vaporize the aluminum and create plasma from water is not the simple reaction of flash powder or thermite. You can't start the reaction with a sparkler or magnesium strip.

Specifically it electro thermal with a boost in energy from a chemical reaction that normally does not accur. Wet aluminum normally becomes dry aluminum long before it melts or turns to a vapor.

I understand the reasoning behind these rules, and it certainly is difficult to make them perfectly airtight. Perhaps wording along the lines of "solid oxidizers, solid explosives, and any compound capable of rapid, exothermic decomposition which is not gaseous under standard conditions". That'd exclude pretty much any "unwanted" energetic that I can think of - all firearm propellants, pyrotechnics and high explosives, even the strange ones like tannerite and peroxide watergels, while not barring any discussion of "legitimate" spudgun fuels, and non-dangerous propellants like the subject of this topic. I support the prevention of discussion related to "explosives" and "weapons" on the site, but I felt that a little hair-splitting was potentially useful in this case. I won't go discussing black powder any time soon - not enough energy density for my liking

Now, on to the explanation: the reaction between aluminum and water is very energetically favorable, but very difficult to initiate (the ignition energy being similar to the reaction's energy output). The ability to use water as an oxidiser is due to aluminum oxide's very low enthalpy of formation - oxidising it is almost always exothermic, as you can see by the thermite reaction, which strips oxygen off of rust.

Part of the reason that it's so difficult to initiate is that aluminum is always covered in a thin layer of its oxide which, as you probably gathered from the above paragraph, is VERY stable and unreactive (hence the vast power consumption of arc furnaces to get pure aluminum out of bauxite). If this layer is ground off or removed somehow, a new one forms instantly (picosecond timescale). We've got a very reactive metal here which we hardly ever think of as such due to the protective oxide layer. To get it to react in the manner I'm proposing, it's necessary to boil the aluminum first.

This is where the electric discharge comes in. The aluminum is cut into small flakes, which are packed into the water-filled tube. They're all in contact due to the packing, with water filling the gaps. When current is passed through the foil (about 70,000 A in this case) it begins to heat up. The hotter areas increase in resistance, shunting current through the cooler parts and tending to cause a fairly even heating. Enough energy is deposited to heat the wire past its boiling point and partially ionize it, and this process is accomplished over the course of about 30 microseconds. This boiled aluminum now has enough energy to react spontaneously with the water (analogous to the auto-ignition of "traditional" fuels) and when it does so, the main products are aluminum oxide, and hydrogen gas.

Fnord's post is a good summary of the process. He makes a strong point on the difficulty of comparing this to the compounds and mixtures most of us would view as explosives - the energy release from, say, a black powder reaction can be millions of times the energy required to ignite it. In this case, the ignition energy is roughly the same as the reaction energy, and for any significant output a huge ignition pulse must be used. The capacitors used here can theoretically ignite up to almost two grams of this mixture, and weigh a total of about 120 kilograms.

@Tech: I did read a report of several tens of grams of the mixture being ignited (and running to completion) by large, fast HE charges under total confinement (the pressure vessels must've weighed tonnes...) - the reaction energy is higher than the ignition energy, but only just. Not enough for a self-sustaining reaction under all but very special circumstances.

In other news, the new containment vessel (large, heavily built box inside which the launcher is placed during firing for near-total silencing) is more than halfway done, and the tests are still planned for tomorrow. The chamber/barrel assembly for the launcher is complete, with just the endcap to be added. The cap is only 13mm steel (covering a 3/8" hole) and may be insufficiently thick, but it can be replaced with 30mm plate if necessary .

Roughly translated, you can't make a pipe bomb out of aluminum and water.

how much pressure do you expect it to produce, because 13mm of steel and it might be to weak thats insane, also do you have a valve or is it just tight fitting ammo

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Due to various issues with tooling and minor failures in the system, only one test was run. The launcher and containment box were both finished yesterday morning, and tested at night. A small (and easily corrected) ground fault damaged the voltage monitoring system (either the DMM itself or the voltage divider) during the charge, so the launcher had to be discharged at approximately 5kJ rather than the planned 8kJ for safety's sake.

The container held up admirably, and the pressure generation capabilities of the mixture were apparent - the 3/8" steel ball was flattened to over 1/2" diameter and the 10mm target plate was left with a significant impact mark, pictured below. Based on previous testing with these projectiles, the impact speed is estimated as 500-700 m/s (with an 85mm barrel length and a 0.90cm³ chamber containing 1g of the mix, and the remainder filled with water), although it won't be possible to tell for sure until the summer. This indicates low speed propulsion capabilities better than [low performance HE] by mass, and vastly superior by volume. At low speeds such as achieved here, it seems to be on par with modern rifle propellants (and with adjustable pressure and burn rate ). This bodes well for the high speed side of things, as the hydrogen is undoubtedly very hot, and seemingly at a few kilobar starting pressure, as predicted.

The launcher was poorly designed and suffered a mechanical failure which did not result in unwanted escape of gases, but did increase the chamber volume. Essentially, a 3/4" diameter polycarbonate rod was forced roughly 1" through a 9/16" diameter hole, with the excess material being squirted out the sides through a very small gap in an impressive manner. It would have needed to travel another inch to escape completely. The roughly 2.5" of engagement on the 3/8" coarse threads held up perfectly, and the slip fit on the outside of the insulator was so close that no cracking or noticeable expansion occurred.

In short, although the launcher sucks, and takes about 3 hours of painstaking work to load, it does demonstrate the success of this concept as a propulsion method - a very energetic one at that. Below, the requisite pretty pictures.


The ill-fated insulator. Sorry about the picture quality.


Back of the launcher, showing 1/2"-13 threaded holes for attaching endplate.


Assembled launcher in box prior to test. Does that steel bar look familiar to anyone?


Post-shot. Note the copper strip, deformed by the insulator trying to escape.


Target and used projectile, along with undamaged round for comparison. (More on the Photobucket page).


And the remains of the insulator. It seems to be stuck...
I may just drill two more small holes and hang this on my wall.

Impressive low power shot you have there

I wonder how much energy was lost due to the polycarbonate rod failing? What are your options(material wise) for creating an insulator that doesn't deform under pressure so badly?
I imagine ceramic wouldn't take the shock load, but I don't know of many plastics that would suffice. Maybe garolite or similar?

Oh, and DYI, I can't view your photo bucket. I get a 403.