Page 1 of 3

Hybrid WIP.

Posted: Sat Mar 29, 2008 8:37 am
by SpeedyFerrite
I'm building a hybrid that's 35 mm in diameter with 5 mm thick walls, The thing is aluminium, what will be the highest mix I could use safely?

Posted: Sat Mar 29, 2008 8:41 am
by SpudFarm
8 or 9x...
5mm thick walls is really thick for steel but i don't know about alu..

Posted: Sat Mar 29, 2008 8:48 am
by bigbob12345
Ill say 12-13x but dont quote me on that
5mm is quite thick, even for aluminum.
I mean look at copper, thats probably not even 1mm thick. Aluminum is a little bit softer than copper but a 5mm wall of aluminum will hold at least three times as much pressure safely.

Posted: Sat Mar 29, 2008 8:51 am
by SpudFarm
AND copper is rated REALLY high..

i think he wants a good safety margine...

Posted: Sat Mar 29, 2008 9:45 am
by DYI
1" copper pipe with walls about 1mm thick is rated for ~1 000 psi. This will be able to take higher mixes than a conventional air compressor will be able to fuel it to. I'm assuming you're machining it out of bar stock?

Posted: Sat Mar 29, 2008 11:09 am
by SpeedyFerrite
DYI wrote:1" copper pipe with walls about 1mm thick is rated for ~1 000 psi. This will be able to take higher mixes than a conventional air compressor will be able to fuel it to. I'm assuming you're machining it out of bar stock?
No, it's piping, but i did machine the fittings. :)

I'll have to install the spark plug tomorow and it'll be good to go.

*edit* what about DDT?

Posted: Sat Mar 29, 2008 11:19 am
by SpudFarm
how long is your chamber?
you need to have a really long chamber for DDT

Posted: Sat Mar 29, 2008 12:02 pm
by Lentamentalisk
your pipe is almost never what you have to worry about pressure wise. The fittings are always much weaker than the pipe. For example, threaded unions at the 2" size can generally only take around 200psi, while their corresponding pipe can take well into the thousands.

Posted: Sat Mar 29, 2008 12:13 pm
by SpeedyFerrite
Lentamentalisk wrote:your pipe is almost never what you have to worry about pressure wise. The fittings are always much weaker than the pipe. For example, threaded unions at the 2" size can generally only take around 200psi, while their corresponding pipe can take well into the thousands.
I machined the fittings from solid aluminium.

Posted: Sat Mar 29, 2008 12:20 pm
by Lentamentalisk
you should be very well covered on that account...
Cant wait to see it.

Posted: Sat Mar 29, 2008 2:03 pm
by SpeedyFerrite
Picture of it right now.

Posted: Sat Mar 29, 2008 2:10 pm
by bigbob12345
oh its low diameter
that makes things even better, i was assuming thjat it was 2in or something.
Im going to stick with my previous opinion of 12-13x maximum although it could probably even take DDT a couple times

Posted: Sat Mar 29, 2008 2:46 pm
by DYI
Low diameter actually shortens the DDT runup distance, making it more dangerous. It should be able to survive a DDT, but I wouldn't be too confident in its safety afterward. And the ball valve would obviously need to be replaced if a DDT did occur.
For example, threaded unions at the 2" size can generally only take around 200psi


Where did you get that figure from? How many tests were done in the study that came to this conclusion? If that number's right, I either have an extraordinarily strong union, or a metric assload of good luck, because I use them at 450psi regularly on the SCTBDC.

@SpeedyFerrite: If you can find the fatigue yield strength of your aluminum alloy, you can use Barlow's formula to get an approximate static burst pressure (shock would lower that number).

Posted: Sat Mar 29, 2008 2:50 pm
by bigbob12345
DYI wrote:Low diameter actually shortens the DDT runup distance, making it more dangerous.
yes I realise that but this isnt nearly long enough for there to be any real danger of 12-13x mixes. If it was twice as long i would say otherwise

Posted: Sun Mar 30, 2008 6:14 am
by sniperjosh
Looks awesome, what FPS does it pump out? I think your barrel should be a lot longer for that chamber setup.