SpudFiles

starman wrote:

fogus wrote: I'm clueless, sorry.

Then a hybrid, especially one like this is no where for you to begin. Have you built a standard combustion or pneumatic spudder before?

If you are indeed new to the hobby, take a good long look at the burntlatke site. That will get you well on your way.

Thanks. Yes, I have built two regular combustion spuds before, and a pneumatic. I do understand grade 12 chemistry and I have worked lathes, mills, drill presses, etc. I am clueless as to the specifics of hybrid spud guns.

Ragnarok wrote:

fogus wrote:How do you stop the propane from liquidizing at 177psi?

The important factor is the partial pressure, not the total pressure. Partial pressure is total pressure times percentage of gas mix.
This means the partial pressure on the propane is only about a 25th of the total pressure in the chamber.

Hope that helps.

I see, so the maximum pressure would be 25*177 or 4425psi? Would I be right in saying that the maximum mix would be a 295 mix at STP?

How do you put the propane into the gun without the high pressure air going back into the propane tank?
Do you have to do them one at a time (propane first)?

If you use a stoichiometric mix (which probably isn't quite ideal, slight overfueling, by 0.5% or so, can improve power), you would need 1 mol propane for every 5 mol O₂. Assuming that air is 21% O₂, you would need:

(21*0.167 + 21 + 79 (for the nitrogen and other inert substances))/100.
Multiply the quotient by 14.7 (for atmospheric pressure), and then subtract 14.7 from the product. This gives you the pressure that each "x" of fuel will add (0.5145 psi).

Multiply that pressure (it varies depending on atmospheric pressure, oxygen content, the desired fuel percentage, and all sorts of other fun stuff) by the mix number you want, and then add (nX * 14.7), if you're using air as an oxidiser.

If you plan on using oxygen instead, and want to stick with the traditional mixes numbers (which is unnecessary, and some would argue, useless when you get into varying the inert gas percentages and using a different oxidiser), add (nX * 3.09) to the propane pressure.

Why I bothered to write that all out for you, I have no idea, as you're not exactly taking the initiative and doing your own research. I suppose that what it all boils down to is that a 200x, stoichiometric mix will have a starting pressure of 3043 psi, and that the maximum mix possible with propane varies by temperature. Assuming a liquification pressure of 120 psi (higher than it would usually be where Larda lives), you could achieve a mix number up to around 230x, which would have a pre-ignition pressure very close to 3500 psi.

All numbers in absolute pressure.

fogus wrote:I see, so the maximum pressure would be 25*177 or 4425psi? Would I be right in saying that the maximum mix would be a 295 mix at STP?

Not quite. The usual reckoning is that propane has a max pressure of 130 to 140 psia (that is, against absolute vacuum, rather than normal air pressure) at normal temperatures.

That puts the maximum mix at about
~135 psia * 25 / 14.7 psi per mix = 230x

You need to remember to work from Absolute, not gauge pressures for this.

There are ways to boost this by using a mix of propane and butane to about 330x, but in many ways, it may be simpler to use hydrogen as the propellant.

Do you have to do them one at a time (propane first)?

Yes.

EDIT: Darn, it appears DYI beat me to it.

I bet its been said, but I just have to say it.

Have you checked this thing for DWV yet????

pizlo wrote:I bet its been said, but I just have to say it.

Have you checked this thing for DWV yet????

Yes. Its been tested for DWV and is positive.

Why the thing hasn't blown up is a mystery to me.

In all seriousness, these parts were all machined by yours truly, Larda. (Im pretty sure he did it himself, but i know for sure they are machined.)

Okay, I'm new here, i saw this linked from another forum and my jaw dropped. The thing i am wondering is how does larda keep this thing from a DDT? i read a few thread down that over a 4x or higher mix that DDT is a major concern and i was wondering how he got away with the 200x mix? Or is the cannon just built to withstand a DDT, and if and when it happens it may not even be noticed?

blafen wrote:Okay, I'm new here, i saw this linked from another forum and my jaw dropped. The thing i am wondering is how does larda keep this thing from a DDT? i read a few thread down that over a 4x or higher mix that DDT is a major concern and i was wondering how he got away with the 200x mix? Or is the cannon just built to withstand a DDT, and if and when it happens it may not even be noticed?

Even at 190x with air-propane i haven't got this problem , so i think its the Nitrogen in the air that's prevent DDT from happening

That help?

I knew the answer was somewhere in those 12 pages i just read, too much information at once i suppose.
I see that the chrono'd shot was at 100x does he have data for the higher mixes?
With this kind of velocity the only thing keeping you from .50BMG type power is the projectiles mass, if you were to make prjectils that were hollow with a solid point (like a fostner shotgu slug) and fill he chamber partway with led you would have some real FPE, not to say that a steel projectile at 3,000 FPS is anything to be scoffed at.
Or perhaps some shotgun slugs or a .50 bmg round in a sabot, or hell maybe even try to get some non explosive 20mm vulcan surplus ammo.

or hell maybe even try to get some non explosive 20mm vulcan surplus ammo.

That may be a little hard to find, especially in Europe. Where could someone get such an ammo?

Oh my various gods, I hadn't seen these!



Just when I thought I had run out of expletives, I'm rendered even more speechless!

One question though, why the bulge in the nose? Are you worried about Area Rule?

This is an actual 20mm APFSDS round:



If you look at the 25mm M919 APFSDS round it's of similar proportions. As you can see, there is little in the way of aerodynamic concern - just a straight pointy cylinder with fins. A more slender dart would make a smaller hole, granted, but would penetrate much deeper. Any results for your own darts?

That may be a little hard to find, especially in Europe. Where could someone get such an ammo?

Many military surplus stores carry inert cannon ammunition, and 20mm is not a rare calibre in Europe either, indeed it was the much used and copied Swiss Oerlikon that set the trend.

The concern would be though whether they would actually fit the barrel. A 20mm cannon does not necessarily have an exact 20mm bore, and since projectiles have to bite into the rifling, they're usually a little larger too. Then again, Lardo should have no problem lathing them down.

Where have you been to have missed these JSR, let me guess. Probly in some foreign country paying women of questionable character for favours that you don't deserve.....

IMO the darts were made like that to keep all the weight towards the front.

CasinoVanart wrote:Where have you been to have missed these JSR, let me guess. Probly in some foreign country paying women of questionable character for favours that you don't deserve.....

I'd better withhold comment on that

IMO the darts were made like that to keep all the weight towards the front.

There's no need for that really once you have fins. Take a look at flechettes from beehive rounds, they've simply sharpened bits of wire with the rear end pinched to create fins:

well one thought that just came to mind.
If you fire harden everything but the fins, then you will get better penetration, as they don't need to pass through, and will just bend or break off.

Great job! I envy you very much (though I don't envy your wallet...)

*drools* those are some seriously cool darts you have there larda... i woulnd't want to shoot them because it would destroy the beauty

Lentamentalisk wrote: Great job! I envy you very much (though I don't envy your wallet...)

From my understanding Larda has been very lucky with aquiring the materials needed for next to nothing. If one were to get a qoute for the manufacture of one of these they would be looking at around $5000 US.