Stone Age Tech

[Aethir]

after spending the last couple days getting thoroughly lost in mountains of threads, I thought I'd throw my 2c worth down memory lane, crossed with a question...

way back when, I used to make my spudguns a bit different to the endcap/piezo norm...

Prehistoric arquebusses were first crafted from steel (yes, prehistoric steel) soda cans! We'd cut the tops and bottoms out of 4 cans, stacked and taped onto a 5th can, left whole but with a small port in the side for fuel and ignition - we just used a cigarette lighter as an ersatz flintlock for a methylated spirit propellant; average range was ~100' with a bit of windage.

The Mk.II started with a sturdy steel aerosol can - deoderant cans worked well, as they were slim enough to be taped into the end of a PVC 50mm expansion chamber that was approx. 8-9" long, with a further 2" of 40mm PVC as actual barrel...

It too was fuelled by ~2-4ml of (somewhat more accurately measured) methylated spirit, retained the open-port "flintlock" ignition system, and was good for launching spuds 2-300m.

ahh, memories... now to the question, which regards propellants.

(and yes, I read the wiki...) has there been much success/experimentation with "wet" fuel injection? By this I mean, liquid as opposed to gaseous-at-room-temperature propellants...

What I'm working on are plans for a Bofors-style truck-mounted launcher that utilizes pre-loaded 'cartridges' (essentially a sleeve holding the projectile, which varies from tennis balls to sabot-spuds), and I'm looking into various methods of fuel injection to suit.

I know that propane/butane injection will be both cleaner burning and more powerful for a given pressure and chamber; I'm just curious as to alternatives to gaseous propellants.

[jackssmirkingrevenge]

(and yes, I read the wiki...) has there been much success/experimentation with "wet" fuel injection?

In spite of the fact that liquid fuelled internal combustion engines are omnipresent, surprisingly little work has been done. The fact that the liquid fuel needs to be physically atomised in order to effectively vapourise and can condense again over time on the chamber walls means that it is not the most reliable fuel for a combustion spudgun.

This is an interesting one though: http://www.spudfiles.com/forums/auto-t1278.html

[Technician1002]

I toyed with a design for using an auto fuel injector with a custom timing circuit to auto vent the chamber, inject fuel, fan vaporize, and ignition for a full auto hopper fed tennis ball gun. Due to the fuel, it would have to be metal. Due to time and budget, it is still just a concept drawing.

[Aethir]

In spite of the fact that liquid fuelled internal combustion engines are omnipresent, surprisingly little work has been done. The fact that the liquid fuel needs to be physically atomised in order to effectively vapourise and can condense again over time on the chamber walls means that it is not the most reliable fuel for a combustion spudgun.

This is an interesting one though: http://www.spudfiles.com/forums/auto-t1278.html

Aye, interesting take on the autoload... I used to be able to do similar things with my methocannon; in that either overfuel, or incomplete combustion, would sometimes negate the need for refueling between shots.

I'd had in mind the use of an actual fuel injector/pump to handle the atomising... Your point about the condensation is quite valid; and is one factor of the process I'd have to take into consideration. I assume then that chamber fans wouldn't be sufficient to maintain adequate fuel dispersion after injection? I don't need something that can stay loaded for hours; I'd be expecting delta T between loading and firing to be <60s on average.

I must confess, a lot of my reasoning behind liquid fuel consists of equal parts curiosity and nostalgia (given my previous experience with alcohol guns), combined with a hint of wanting to push the envelope a little regarding design and execution.

Thanks heaps for the input, peoples

[jackssmirkingrevenge]

If you have the time and the inclination, why not give it a go a chamber fan would certainly be helpful.

Here's a guy using a spray bottle to fuel his spudgun:

[youtube][/youtube]

Very "spray and pray" but hey, it works. Using a small bottle, you could calculate how much volume per squirt is injected and be a bit more scientific.

I would go for a relatively large scale, in order to facilitate correct fuel injection.

[Aethir]

I would go for a relatively large scale, in order to facilitate correct fuel injection.

In a self-admittedly brilliant flash of insight, I'd figured on using a chamber that equates (to reasonable accuracy) to the volume of the cylinder the injector was originally fitted to... eq were I to filch the injector pump from the 2.4 litre Hilux, I'd end up with a chamber ~740cc (that's 45.25ci to the non-metrically-inclined) - being 2399cc/4cyl=741.5cc

The reasoning behind this is minimising variables from the injection setup; if I know the injector pump outputs X amount of fuel per cycle, then I can use that as a baseline and fine-tune chamber size for optimal combustion, as opposed to monkeying with flow rates/pressures to suit a particular chamber.

*edit* derp... I'd probably still need a method to adjust fuel volume in instances where ambient air pressure has changed. Not that it's much an issue living some 25' above sea level, but still... I guess I can find a best-compromise point whereby fuel/air ratios are largely irrelevant in terms of ambient conditions.
/edit

Besides which, ~750cc should allow sufficient room for chamber fans etc.

Is there a (relatively easy) method for estimating the energy potential such a setup would produce, given the variables: chamber size - fuel octane - fuel-air ratio? My maths is rudimentary at the best of times, and the google-fu required to answer this escapes me...

[jackssmirkingrevenge]

Don't forget that the cylinder compresses the fuel before ignition, so the chamber volume you should aim for should be that at the bottom of the piston stroke if you plan on a non-hybrid atmospheric pressure launcher.

As to predicting performance, a good place to start is downloading HGDT: http://www.thehalls-in-bfe.com/HGDT/

It has no provision for petrol to my knowledge, but will at least give you an idea of how power is altered when you tweak variables like chamber size, barrel length, projectile weight etc.

You can (very roughly) translate it to petrol performance by looking at the relative energy of fuels.

http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns

[Aethir]

the chamber volume you should aim for should be that at the bottom of the piston stroke if you plan on a non-hybrid atmospheric pressure launcher.

so measuring at TDC could prove problematic in this case?

I'd already assumed that a gas/petrol-powered Otto-cycle motor is essentially a hybrid with the burst disk (ie the piston) attached to a crankshaft... my theory is the molecular mass of air inside the chamber stays constant whilst the pressure changes with compression; thus the actual volume of fuel used would stay constant as well - essentially a 1x mix in a 45ci chamber being equivalent to a 10x mix in a 4.5ci chamber. Would this be a fair statement, or have I missed something vital along the way? Also; how would the displasive volume of the fuel factor into all this? That bit just confused me

Cheers for the HGDT link, too

regarding the relative energy thing... oy, chem was never my strongest subject, nor math. could molecular weight/adiabatic temp be used to extrapolate? The wiki alas considers such information irrelevant based on poor shot-to-shot reproductiveness, which is one of (if not THE) key areas I hope to substantially improve/expand upon...

I'm also interested in exploring/researching the relative differences between liquid fuels; although at this stage that essentially requires completely independent injection systems for each type of fuel (primarily, petrol of various octanes, and IPA/metho) it still seems an avenue worth pursuing. Amateur rocketry accelerometers and a chronograph go onto the xmas list, lol.

(so many questions! still, better to ask and appear stupid than act and prove it so)

[jackssmirkingrevenge]

so measuring at TDC could prove problematic in this case?

Make that "T" a "B" and you've got it

I'd already assumed that a gas/petrol-powered Otto-cycle motor is essentially a hybrid with the burst disk (ie the piston) attached to a crankshaft... my theory is the molecular mass of air inside the chamber stays constant whilst the pressure changes with compression; thus the actual volume of fuel used would stay constant as well - essentially a 1x mix in a 45ci chamber being equivalent to a 10x mix in a 4.5ci chamber. Would this be a fair statement, or have I missed something vital along the way?

Spot on, that was my thinking here.

By having a 1x mix in a syringe and using that to pressurise a cartridge, any pressure you reach would technically be at the right ratio. Using it manually would naturally limit you to the pressures you can achieve through the pressure you can exert, so a possible solution to this would be modifying the syringe to enable the plunger to be pushed by air pressure - that or closing it in a vice

Also; how would the displasive volume of the fuel factor into all this? That bit just confused me

Let's say you need 15g of air for 1g of petrol. Air at atmospheric pressure weighs around 1.2 grams per litre.

For a 2 litre chamber say, you have 2.4 grams of air, which means you need to inject 0.16 grams of fuel. This equates to a paltry 0.22mL, which is only 0.01% of the total chamber volume, so nothing to worry about.

regarding the relative energy thing... oy, chem was never my strongest subject, nor math. could molecular weight/adiabatic temp be used to extrapolate? The wiki alas considers such information irrelevant based on poor shot-to-shot reproductiveness, which is one of (if not THE) key areas I hope to substantially improve/expand upon...

A bit of research will give you the heat/mole oxygen for petrol so you can compare to the list of fuels.

This was Jim's (still thankfully an active member of the forum) original article exported to the wiki:

http://www.inpharmix.com/jps/Combustion_fuels.html

I don't think the shot-to-shot unreliability statement is fair, if you're using the consistent amounts of fuel in a well ventilated chamber, with a consistent ignition source and projectiles of similar size and weight, there is every reason to believe muzzle energy will be consistent. If you're using a spray and pray to shoot potatoes, that's different from using syringe metering and shooting BBs.

The Burnt Latke study he quotes was done using potatoes as ammunition, and the weight of each projectile is not considered in the data: http://www.burntlatke.com/lpmapprussets.html

I'm also interested in exploring/researching the relative differences between liquid fuels; although at this stage that essentially requires completely independent injection systems for each type of fuel (primarily, petrol of various octanes, and IPA/metho) it still seems an avenue worth pursuing.

I think it's safe to say that what works best for internal combustion engines can be extrapolated to spudguns, because in both cases MOAR PRESHAH -> MOAR POWAH.

Amateur rocketry accelerometers and a chronograph go onto the xmas list, lol.

A good chrony is all you need, coupled with a reliable balance to weigh your projectiles.

[Aethir]

I am sponge, watch me absorb!

I really must stop building things using Worldcraft, it makes 2D blueprints impossible... the cartridge system I had in mind is very similar to the one in your pic, with one crucial difference: the rear of the cartridge is completely open. The design I've been working on uses a kind of "breechface"; essentially a flat plate with fan, injector, sparker all mounted in such a way that when the breech is closed, the components project slightly into the cartridge, and gas seal is achieved by a conveyor-belt O-ring mounted around the breechface pressing against the base of the shell when the breech is closed. Premature loading/firing is prevented by a microswitch on the breechface that disconnects the firing/fuelling solenoids when the action is open. (I hope that makes sense!)

And the reasoning behind the accelerometers was to determine how burn rates of various fuels affect acceleration (as opposed to final muzzle velocity) as a possible lead-in towards paint-bomb sabot rounds
(think hollowed spud, with waterballoon filled with marker)

This way a switchable dual injection setup could be used to swap between ammo types and related propellant at will... but these are thoughts for a Mk.II

much appreciated info

[jackssmirkingrevenge]

That sounds far too complex, much simpler to have a preloaded cartridge.

http://www.spudfiles.com/forums/cartrid ... 20150.html

[Aethir]

That sounds far too complex, much simpler to have a preloaded cartridge.

Indeed it would; but if everyone took the simple route where would the innovation be? remembering too that implementing a liquid-fuel combustion system brings with it its own constraints as regarding pre-loading.

I'm not setting out to make it as difficult as possible - I agree, it's a complicated concept - but I am working to certain design parameters, one of which is to utilize a cartridge system that doesn't require pre-loading as a safety factor. Even were I to use propane/butane injection, it'd still be via a fuel nozzle that would seat into a corresponding inlet on the cartridge upon the breech action being closed.

[jackssmirkingrevenge]

Indeed it would; but if everyone took the simple route where would the innovation be?

Simplicity in my book implies elegance, not crudeness - I'm of the Soviet "Tovarish, it works!" school though, so don't take my word for it

I doubt a preloaded cartridge can be considered a safety risk, given the fact that you need a specifically hook up a high voltage ignition source in order to make it go bang.

You could still go for liquid fuel, here's a thought for some experimentation:

Make a "cartridge" - essentially a tube closed at one end with two electrodes poking in it. For initial trial purposes, it can be as simple as a soda bottle with the mouth closed off with duct tape. Even cooler because it will be transparent and you can see the flame.

Add the correct amount of fuel in liquid form, and wait! Does it ignite after 15 minutes, one hour, four hours, 24 hours? Does it ignite at room temperatue, will it still ignite after being left outside in the cold? What about using a little less or a little more fuel? Does it help if you shake it, or will it still ignite if you just add the liquid and let it stand?

There is a LOT of scope for experimentation even with a simple process, the variables of time and temperature are already worth exploring.

[D_Hall]

http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns

At the office there is another large spud gun (not VERA). The designer of that gun actually did a fair amount of work regarding fuel choice. I don't remember all the gases he tried, but I know he studied Hydrogen, Methane, Butane, and Propane. What he found was that the dominant factor in performance was Oxygen. Fuel choice made very little difference as long as the quantity of Oxygen was fixed. When you think about it, it makes a lot of sense.

I would expect similar results for any liquid fuels.

Anywho... After spending a fair chunk of money his conclusion was basically, "Use any fuel that is cheap/easy to get your hands on and easy to work with." He ended up using Propane.

I came to the same conclusion without spending the money.

Liquid fuels make a lot of sense when storage of large quantities is an issue. For spud guns, such storage is not an issue so gaseous fuels (which are much easier to handle/mix/etc.) make the most sense.

[jackssmirkingrevenge]

^ the man speaks the truth ^

Of all the parameters you can alter, fuel type is not the most significant. Though of course, you seem to be more inclined towards the "because I can" mode of experiment, so whatever floats your boat