SpudFiles

go for it, you'll be finished far before I will. My first one failed horribly; the syringe tube was launched out of the back, so I put in an order for some more syringes. No pictures, had to clean it up before my parents could see that one of my creations failed

Lockednloaded wrote:the syringe tube was launched out of the back

during pressurisation or combustion?

This is what I had in mind, something the size of a pocket pistol which could have a barrel extension/suppressor added on.

combustion. in that design, is your piston supposed to be airtight?

Lockednloaded wrote:combustion.

Ouch. Any idea what went wrong, epoxy not sufficiently cured/thick, surfaces not cleaned/abraded properly?

in that design, is your piston supposed to be airtight?

No, I'm not sure it would work but it will certainly simplify construction. In practical terms it will probably look something like this, still looking at chamber materials.

sorry, but I found an easier way to go about this without epoxy. I'm using a pre-made 1/2" relief valve. Its basically a pop-off valve set at 75psi with an outlet port, so I'll probably go with 5X max unless I can find a stronger spring

That valve may close with a projectile in the barrel, as that would reduce the effective pressure differential. Look for a spec of closing pressure, that is the minimum pressure drop across the valve for it to stay open.

Unless the back of the popoff is ported to atmosphere, which it is likely not.

Would an unbalanced piston be a good way to vent the pilot volume once the shot goes off ? Or is that making it too complex ?

he's using a pre-made relief valve. There is no pilot. I'm not sure what you mean by an unbalanced piston. Both of the recent 1.5" porting hybrids vent the pilot with the main piston as it begins to move.

And I retract my previous comment; the back of the relief valve may well be ported to atmosphere. In that case, the valve will close at a whatever pressure above atmospheric.

I think that is what I'm referring to. The piston has a rod through it and a second sealing face larger than the barrel diameter but smaller than the chamber diameter. The second sealing face blocks off the pilot port. Combustion pressure moves the main piston back and the smaller sealing face comes off the pilot seat releasing the pilot volume. A sort of hybrid QEV. ?

You could add a spring to a QEV pilot:

I think he's using the popoff as the main valve.

ramses wrote:I think he's using the popoff as the main valve.

I had modded a pop-off as a main valve here, the problem is that the piston isn't airtight so it would tend to vent out past the piston as well as the port.

Here is what I meant, I'm pretty sure I've seen this set up before.

Oh and not too sure why I edited the QEV schematic, I should have just used the proposed design.

I guess I would be less silent... But at least it is closer to a burst disk in that once it has opened it doesn't require any energy to stay open.

i get that those ways would be more efficient, and offer full flow the full time, but it doesn't bother me to sacrifice performance for simplicity in this project. I know the valve will close back before all the gasses are exhausted, but I doubt that extra ~75psi getting trapped will do anything.

@al-xg
interesting... but have you noticed that the forces acting on the piston even out?