Amateur question...

[EternityTransfer]

Hello, I am back, and I am finally going to complete my advanced combustion that I have been working on for the last... well... forever, really...

Shortly my potatoes shall fly. (hopefully )

I just have a quick question:

I am wanting to power my advanced combustion with Butane gas. At the optimum concentration (3.23% Butane), what is the maximum pressure I should be expecting in the combustion chamber?

I ask this because i don't want my cannon to be dependant on the projectile completely sealing the barrel, so I am using the brilliant idea (not mine) of a camlock-burst disc (see starman's noise cannons for an example), and I don't want to put too much material (Aluminium foil: bursting pressure ~6PSIG/layer [???]) in at the first shot, and blow my precious creation sky-high. Also, I know that if I try layer by layer, working my way up, I will probably keep pushing it until something gives way. (It's how I am )

Any ideas?

Many thanks in advance.

[john bunsenburner]

Seeing as how a

advanced combustion

is operating at atmospheric and it takes another 6psi to burst the alli, one layer will suffice. In simple terms:

A combustion gun is not pressurized before firing, it takes 6psi to burst the alli foil, no pressure is less than 6psi, so one layer will be enough.

[niglch]

Well the maximum pressures you're going to see with a perfectly stoicheometric (1x) mix of butane/air is probably about 90-100psi in a sealed chamber. These are pressures that PVC can handle provided you are using pressure rated fittings and pipe. Therefore, you probably won't blow up your cannon even if you had a "perfect" burst disk that broke just as the chamber was approaching it's max pressure.

I'm not sure what would happen if the burst disk failed to break. The heat from combustion might start melting things in the chamber after a while so you might want to include some kind of release valve to vent the chamber in such an event. Not sure if it would actually cause the chamber to fail though . . .

Actually, does anyone know what does happen in PVC if the heat and pressure of combustion were to be held inside the chamber? Would the gasses cool quickly or just start melting plastic?

[jimmy101]

Actually, does anyone know what does happen in PVC if the heat and pressure of combustion were to be held inside the chamber? Would the gasses cool quickly or just start melting plastic?

Why yes I do know what happens in a closed PVC combustion chamber buring propane+air.

In a nutshell, nothing interesting or all that dangerous.

The chamber pressure is back to near atmospheric pressure within a second or so.

Peak pressure is about 100 PSIG.

The chamber warms up slightly, just barely detectable with your hand.

Nothing gets burned or melted.

The explosion doesn't make any noticable sound.

http://www.inpharmix.com/jps/Chamber_Temperature.html
http://www.inpharmix.com/jps/Closed_Cha ... udies.html

[ramses]

If the burst disk were to fail, the chamber would rapidly absorb the heat of the CO2 and H20 that caused the pressure to spike. The pressure would decrease within a few seconds to slightly above atmospheric pressure since you created more moles of gas by splitting the butane. the plastic would most likely not melt.

[niglch]

I stand corrected. No melting. Anyway just use pressure rated PVC and you should be OK even with a burst disk.

[EternityTransfer]

Well, thanks to all for the speedy replies.

To summarise: even if the burst disc does not burst, there will be no dangerous rise in pressure once the gases cool.

So, will any noticeable performace gains be experienced if the disc bursts at 90PSIG as opposed to 100PSIG?

And if there was no burst disc? Might the burst disc actually decrease performance?

My reason for using the burst disc is that the entire volume of hot expanding gases hits the projectile at the same instant. (As well as the barrel-sealing issue)

But if the burst disc burst at say 20 PSIG, the only purpose it serves is to hold back the fuel from the barrel before firing, and the cannon performs as if it had no disc at all.

MY word, I have just logicked myself into a circle...

Help...

Please?

[jackssmirkingrevenge]

Help...

Please?

I think as much data as can be given has already been shared, you know there's no danger if you're using pressure rate parts of a catastrophic failure if the disk fails to burst, so perhaps a bit of R&D is in order.

I would find a burst disk material of which a single layer will hold 10 psi or so, then go incrementally up or down to determine what is the optimum burst pressure, which is pretty much what you suggested to begin with

Some sort of chrony to determine velocity is in order if you really want to optimise performance, as well as projectiles of reasonably consistent shape, weight and friction in order to reduce the variables that could interfere with your data.

You might also want to include a schrader valve in the chamber so you can pump in air to determine at exactly what pressure various combinations of disc material will burst (plus you can also use your launcher as a pneumatic )

[niglch]

Yes, but don't go trying to use it as a pneumatic AND a combustion at the same time
And I doubt the burst disk would decrease performance. They are about the best way to release pressure quickly. You don't see them in a lot of (atmospheric pressure) combustions because the performance gain is small (probably around 5-7%). They are very widely used in hi-performance pneumatics and hybrids where every ounce of power is desired. So, if you want to get the most power you can out of your cannon, a burst disk is a great way to do it.

[EternityTransfer]

Many thanks, R&D it is for me ahead. I feel that this is the most enjoyable part of the whole cannon-building exercise, tweaking the countless variables to ascertain precisely which combination yields the furthest shot, the fastest shot, the most accurate shot. And, since I now have a rough estimate of where to start from thanks to you all, I can begin!

I'm excited.

[psycix]

Most people here enjoy designing cannons more then shooting them.
Designing/theories > building > shooting.

Have fun!
If you stumble upon any questions, feel free to ask (I guess you could use this thread for small questions.)