Thinking aloud: the "Mentos Project"

Build a homemade water gun or water balloon launcher and tell us about it.
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insanitys_engineer
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Thinking aloud: the "Mentos Project"

Post by insanitys_engineer » Wed Jun 21, 2006 9:06 am

You all must have heard of the Mentos experiment: empty a packet into diet coke and all the gas goes wham out of solution. It produces a 5m vertical jet.

Could this be harnessed in something like a water bomb launcher, or as an auxiliary gas supply for a weapon? By opening a valve to allow the drink to pour over a pack of mentos, do you think enough pressure could be created to launch a bomb? If anyone wants to try this, please post some pictures :D

I realise this isnt the most practical thing ever, but it would rock if it was possible.

Also, coke have released small 250 ml spherical bottles for the world cup. These are like twice as thick all over than standard drinks bottles (you can't easily squish them), and although being smaller in volume could increase weapon pressure considerably. I'm thinking more for the UK builders as they fit onto 22mm pipe with a bit of hammering.
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joannaardway
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Post by joannaardway » Wed Jun 21, 2006 9:45 am

Yes, I have heard this.

I also understand that it's dangerous if contained.

Well controlled, it might work.

Any powder that won't stick together in water will suffice. Sherbet is good.

However, if you want to do this kind of thing, there are dozens of chemical reactions that produce gasses. Hydrochloric acid and Sodium Bicarbonate produces CO2 gas, and quite a lot of it.

In theory, in the right quantities, you could have a rapid recharge for a wbl.

Have some sort of charger that stores the acid in one chamber, a ball valve to pour it over the sodium bicarbonate, and a hose with a connector that will attach to a bike valve.

If it were air tight, then more or less instant power.
"Over the hills and far away, she prays he will return one day. As sure as the rivers reach the seas, back in his arms again she'll be." - Over the Hills and far away, Gary Moore

"So many people have come and gone, their faces fade as the years go by. Yet I still recall as I wander on, as clear as the sun in the summer sky" - More than a feeling, Boston

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insanitys_engineer
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Post by insanitys_engineer » Wed Jun 21, 2006 10:27 am

Thats true, I'd forgotten about acids and baking soda. It could be an interesting concept, and be quieter than a cordless electric compressor. These you can get for £20ish of the net and will do upto 100psi at least. I'm tempted to get one and try to make a bomb launcher. In the meantime, vinegar it is!
I'm off to do some chemistry thinking...
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joannaardway
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Post by joannaardway » Wed Jun 21, 2006 4:49 pm

You'd need a fair amount of vingear to create much gas. Ideally, you need a fairly strong acid.

I would go for: A copper (?) tube containing 100 ml of 10 mol/dm^3 HCl (really strong), a valve of sorts and 84 grams of pure sodium bicarbonate. This would create 24 litres of gas at room temperature.

I'd vary the quantities used depending on the pressure I needed or my chamber size. Too much would be the fastest way to kill your wbl.

I'd go with Hydrochloric Acid because of the safety of the salt created, and the controlled strength (It also has 100% disassocated Hydroxonium [possible misspell and the spell check has no idea] ions) .

It's standard table salt, so no worries about poisons. Not that vinegar produces anything dangerous.

I'm studying A-level chemistry, so I've got a good idea here.

I could also tell you how to make gunpowder, easy molten iron, a few other explosives, and nitrogylcerine, but I don't think it advisable.

If you want any help calculating the quantities needed, I can assist.
"Over the hills and far away, she prays he will return one day. As sure as the rivers reach the seas, back in his arms again she'll be." - Over the Hills and far away, Gary Moore

"So many people have come and gone, their faces fade as the years go by. Yet I still recall as I wander on, as clear as the sun in the summer sky" - More than a feeling, Boston

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wetmonkey442
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Post by wetmonkey442 » Wed Jun 21, 2006 6:28 pm

Although I'm a bit out of my element in chemical reactions (structural mechanics is more my thing), I would assume that the last thing such a device would produce is consistent results. In my experience, I've noticed that Coke (or any type of carbonated beverage) tends to not be very uniform. Variances in the density of the liquid, along with the unavoidable situation of having your lighter materials rise to the top won't produce a noticeable difference when you're drinking it but it might be the difference between success and failure if tested in a launcher. Another issue I'm worrying about here is the speed of compression. Such a reaction will obviously push gas into the chamber faster than a bike pump (and probably a compressor too). I'd go with Joannaardway and say make the thing out of some type of metal first to test. I wouldn't want to put undue strain on PVC joints with that sort of unpredictability.

Of course someone can always throw this into a physics engine and get the (somewhat) accurate stats.

Don't get me wrong, it seems like a real interesting idea. I look forward to it's implementation, and if anyone tries it remember to start small.

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joannaardway
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Post by joannaardway » Wed Jun 21, 2006 8:22 pm

I'd go with copper for three reasons:

Strength - it's going to dump a huge amount of CO2 out very fast.

Unreactivity - Copper is unreactive - I wouldn't like to bet on what strong acid could do to PVC - which is then flash pressurized.

Availability - It's easily found in the UK in small quantities. I wouldn't want to buy 3 or more metres of PVC of a net store only to use 50 cm.

My design would factor in a safety valve. If you make a mistake with the mix, the last thing you want is an exploding pipe.

Which is what you would get, if the excess pressure weren't released.
One of these would produce massive pressures if misused.

If vented into a chamber of the right size then this would be safe, as long as the hose diameter were sufficent, and flow weren't restricted.

I'm going to go and do some calculations based on rates of reaction, flow, etc.

If I find the results too alarming, then I'll either:
a) Say not a chance, and delete this topic due to the dangers.
b) Give advice and recommend that designs are well built and built to exact specifications.

However, this will be of little use in a wbl. The pressure would rise fast, but probably not fast enough.
"Over the hills and far away, she prays he will return one day. As sure as the rivers reach the seas, back in his arms again she'll be." - Over the Hills and far away, Gary Moore

"So many people have come and gone, their faces fade as the years go by. Yet I still recall as I wander on, as clear as the sun in the summer sky" - More than a feeling, Boston

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insanitys_engineer
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Post by insanitys_engineer » Wed Jun 21, 2006 9:45 pm

joannaardway wrote: ...
Unreactivity - Copper is unreactive - I wouldn't like to bet on what strong acid could do to PVC - which is then flash pressurized.
hehe I made the same calculations at work, but using standard table ingredients. The problem is cos im out of school , its harder to get the whack acids, so I may be boiling vinegar. Yum :eek: . Also, PVC is safer than metals with strong acids. As copper tubes would end up mixing metals at the compression fittings, you may get a battery effect and so all the joints will corrode faster than...well ...fast things. However, copper is unreactive with HCl and so this could work, but watch out for valves that may contain other metals.

Glass and plastics are generally very safe to use with acids, and some of the PVC pipes you can buy is specifically rated for use with strong acids ( i think it was expensive though) because they generally have unpolar molecules: in the case of plastics its basically a barrier of hydrogen making sure there is no site for any acid to attack as there are no differing areas of charge to allow the acid to H+ all over it.

I do agree that PVC could break under the pressure, but if you do good calculations then it shouldn't be a problem and you could create relatively accurate pressures.

If not, have a 6 bar pressure release valve bleeding into an auxiliary tank, which has its own valve on. Also, put tape around bottles and pipes! gaffa or duck tape over pipes will help stop shatter effects.

Edit: Also, if you delete this thread, replace it with one that details the dangers of this sort of gas source
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joannaardway
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Post by joannaardway » Thu Jun 22, 2006 9:36 am

About safety:

I didn't manage to finish the maths, but nothing seems impossible to handle - yet.

In fact my flow calculations seem to suggest that with a 1/2" pipe with minimal choke points, that there is almost no way that anything can go wrong, save for a chamber that is too small.

1/4" pipe could handle it, but here, I'd choose a check valve and hose fittings to allow the uberly high flow. A 1/2 pipe could dissipate a 10 bar pressure spike in a little under 1/6th of a second.

And there is little way that the spike could rise that fast - and there is only one point at which it could occur - the reaction start, when flow is potentially highest.

However nonetheless, I would suggest extreme care when doing this for the first few times. Do something like: Open the valve remotely with a bit of string until you are certain it's safe.

Very little can stop a shatter effect. Duct tape - sorry, but shrapnel will rip straight through that.

Chicken wire is next to useless, so go figure with regards to duct tape.

Practicality/more safety:

I'd use a pop-off valve that was working in the 8-10 bar range, as long as the pipe could handle it.

I might be tempted to install 2 or 3 of them to really insure that it could vent excess pressures quickly.

The hose would benefit from being high ID. Not only would pressures be equalised quicker, but also pressure spikes would be smoothed.

Materials:

Getting high concentration acid isn't hard. Mind you, I can't get stuff that strong from school. However, that strength is more than possible.

The main reason for using high conc. acid is to reduce the required volume of acid, so a smaller tube could be used.

I suppose special PVC pipe might be more sensible than copper - I hadn't considered battery effects.

Thoughts:

The next issue is with the reaction itself. The reaction will mass fizz, so the next trick is to stop the liquids from fizzing through into the feed pipe.

CO2 isn't the best gas for launching. In general you get lower muzzle velocities with CO2 than air.

Mind you, with the ability to fill a chamber in a mere few seconds, who cares?
Last edited by joannaardway on Thu Jun 22, 2006 9:49 am, edited 1 time in total.
"Over the hills and far away, she prays he will return one day. As sure as the rivers reach the seas, back in his arms again she'll be." - Over the Hills and far away, Gary Moore

"So many people have come and gone, their faces fade as the years go by. Yet I still recall as I wander on, as clear as the sun in the summer sky" - More than a feeling, Boston

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insanitys_engineer
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Post by insanitys_engineer » Thu Jun 22, 2006 11:39 am

Having drawn stuff, I have made a plan. The DCP SL (dual charge/purpose support launcher)

The whole thing would be about 4' long (stubby 2' barrel, made of 70mmish drainpipe cos i have some) and be able to fire stomp rocket type things off the tube under the main barrel (as a by-product of the valve setup), balloons from the top tube, and be charged either by reaction or compressor of some type.

I'm thinking of having the barrel, connected to a ball valve, connected to a tee pointing up (upwards branch for for Schrader, pressure release valve, gauge attachments, pump etc), to a tee pointing down, to the 1.5/2L gas reservoir (horizontal in line with barrel). From the tee pointing down, the tube drops and meets another tee(main direction is horizontal). Horizontally undet the main barrel is a tube, valve, the tee, another valve and the reaction tank (500ml?) under the pressure tank. If that makes sense. I'll draw a diagram later.

Basically to load the weapon, you generally have to stand it uprightish. This means filling the reaction chamber, then gap between valves, is done by gravity. Then load the projectile, and allow acid to mix using the valve next to the reacton tank. Gas will fill the weapon, which i suggest you do before you need to fire it. Then when youre done, drop another sachet of powder down into the chamber, add acid, etc. Sachets would be the basic powder in a tissue wrapped stick like parcel, that you simply ram into the reaction tank.

Once fired, liquid is emptied by opening both the bottom valves and letting it drain for a second.

I am thinking of having a different method, but its taking a while to develop.
There are problems with this design, and i will endeavour to come up with a similarly cheap design. Less valves the better, cos they are costly.
Sanity is just an excuse for not having an imagination

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joannaardway
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Post by joannaardway » Thu Jun 22, 2006 12:20 pm

If anyone is thinking of doing this kind of thing, I present simple maths to calculate your mixes.

I'm working by the Bicarbonate rather than the acid as the acid concentration is harder to measure.

Calculate your chamber volume in litres and multiply by the number of atmospheres of pressure you want in it.

Example: 2 litre chamber * 5 atm (about 75 psi) = 10 litres of gas at room temperature.

Each 84 grams of Bicarbonate (assuing 100% purity - I don't know how pure the stuff normally is) that you react will produce 24 litres of gas

Multiply your previous number by 3.5 - this is the number of grams of Bicarbonate you need for one charge.

I'm assuming that you don't know your acid concentration. If you did you probably wouldn't need this maths.

To find how much acid you need, pour acid slowly over the bicarbonate, waiting for it to stop fizzing before you add any more. Once it no longer fizzes, you have added the right amount of acid. Find this quantity, and add a little to have an excess.

Then just find the right way of mixing it.
"Over the hills and far away, she prays he will return one day. As sure as the rivers reach the seas, back in his arms again she'll be." - Over the Hills and far away, Gary Moore

"So many people have come and gone, their faces fade as the years go by. Yet I still recall as I wander on, as clear as the sun in the summer sky" - More than a feeling, Boston

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Silence
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Post by Silence » Thu Jun 22, 2006 7:39 pm

I wasn't here most of yesterday, but I wish I had been. Anyway, I think this sounds quite a bit like a Duxburian-style water cannon--although you would have tough luck trying to make it a MS (multi-shot) one. Also, if you're concerned about keeping the water well-separated, then you could add a plunger--but then you would need another system to get rid of the pressure afterwards. I'm pretty sure that if you try to contain the pressure, no practical material could handle it; on the other hand, if you release it immediately, both PVC and metal could. I'm not the best person to comment on the various reactions, though...

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wetmonkey442
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Post by wetmonkey442 » Thu Jun 22, 2006 8:08 pm

Rough design of compression chamber design. The front bushing connects to the valve, the back would connect to a threaded male coupler and cap. You load/unload soda in the back, and drop mentos in through the "magazine" on top (which would also have a threaded cap and ball valve). A plunger separates the coke from the air. It's 3" PVC, so a dual hockey puck piston would make a good plunger, wrapped with some electric tape.

http://img.photobucket.com/albums/v258/ ... hamber.jpg

Another view:

Image
Last edited by wetmonkey442 on Thu Jun 22, 2006 8:16 pm, edited 1 time in total.

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insanitys_engineer
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Post by insanitys_engineer » Thu Jun 22, 2006 9:01 pm

Powder and liquid are not suitable for use: ruling out acid reactions (I realise there are solutions, but that is just more effort. Also, vinegar and baking soda smell bad, and make mess.

Just knocked up a short launch tube (14", just an offcut) and have successfully used a wooden plate as an adapter for the tube: its the end of a downpipe that has the bit wider. Using a heat gun, i hammered a circle of wood the same outer diameter as normal downpipe into the end while it was soft, and pushed the plastic to a tighter shape before setting it in water. My largest shot so far (i duno the pressure-the gauge broke so i didnt overpump) with a tennis ball is about 120 ft

I am going to abandon the idea of chemical propulsion, and if you dont want to end up smelling like a bad fish n chip shop or a cola factory, I suggest you stick to air
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Post by powersurge919 » Fri Jun 23, 2006 10:04 am

what i would do is have two electrified carbon rods under water creating hydrogen in the water tankywhen enough hydrogen is produced the pressure will be enormous and would shoot far. but you will have to have a number of good 9v batterys to make enough. when the carbonrods are charging it will transform impuritys in the water into hydrogen thus puriffying the water for more acurate shots. you won't have to use any chemicals. just electric. hydrogen is safer then acids or any thing.

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insanitys_engineer
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Post by insanitys_engineer » Fri Jun 23, 2006 11:10 am

The hydrogen is a nice idea, but i don't see how purer water would make much difference: i use skanky water butt water (hose pipe ban) and it works fine in my weapon.

Also, you'd need to put valves on the tubes, or water will flow out. This makes it more awkward for detonating it aswell. Car batteries are also needed for a good high speed reaction, but the explosion could rip the whole weapon apart. If you could get it to work it could be highly dangerous to the user- I imagine the explosion is more powerful than that used in aerosol cannons.

Good for thinking outside the box though. If anyone else has any interesting power sources, post away!
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