SpudFiles

Lockednloaded wrote:I doubt that extra ~75psi getting trapped will do anything.

That's what I was thinking, if we assume that 10x generates 1,000 psi and the pre-ignition pressure is a mere 130 psi, I really doubt there would be a visible difference in performance.

Still haven't started prototyping as I'm mulling over materials, currently thinking 5mL syringe piston with a pinhole in the tube for equalisation that would be covered once the piston starts moving to reduce the likelyhood of the volume behind it igniting, and taking the "risk" of making the chamber out of thick walled (0.75" I/D, 0.14" wall thickness) PVC to see how it stands up to the pressure spike, which would have implications for cartridge construction. It would also help with insulation, both of the heat of combustion as well as the ignitor mechanism.

interesting... but have you noticed that the forces acting on the piston even out?

Oh yep, sorry I said the opposite of what I meant to post. As long as the pilot side seat has a smaller area than the barrel seat it should be unbalanced.

The point of this set up would be to have piston opening pressure as close to peak combustion pressure as possible without choking the flow once the peak has passed. Which seems to be what people try to do when experimenting with burst disk thickness ?

But yeah I see the simplicity point.

Mmm, now that I'm back at home and have a few tools and materials I might just have to try a hybrid cartridge, I've been starring at my laptop with envy, now is my chance

coincidentally I made a great deal last week and got 5 3/8 QEV's for 10€ (+6 € shipping) from eBay last week 8), which arrived today. So I think I will give the QEV - one-fillvalve - piston - hybrid a go over the Christmas holidays.
If that my Idea, of a one-fillvalve piston-hybrid with the same pressure in both chambers wont work i will perhaps build a new, airtight piston for the QEV... let's see how it works....

An advantage of using the original QEV-piston, is the fact that it's cup-shape works like a checkvalve, which decreases the possibility of the rear igniting.
An disadvantage of a QEV would probably be the fixed pistonseat/piston ration, so you cant tweak the opening pressure...

@poland: I don't understand what you mean, what is the problem in al-xg's design? if the pressure peak in the main chamber will be high enough to overcome the force from behind the piston, it should work?

Edit: Ok, I got it, it wont close if the pilot is the same diameter as the barrel.

jackssmirkingrevenge wrote:

Lockednloaded wrote:I doubt that extra ~75psi getting trapped will do anything.

That's what I was thinking, if we assume that 10x generates 1,000 psi and the pre-ignition pressure is a mere 130 psi, I really doubt there would be a visible difference in performance.

Still haven't started prototyping as I'm mulling over materials, currently thinking 5mL syringe piston with a pinhole in the tube for equalisation that would be covered once the piston starts moving to reduce the likelyhood of the volume behind it igniting, and taking the "risk" of making the chamber out of thick walled (0.75" I/D, 0.14" wall thickness) PVC to see how it stands up to the pressure spike, which would have implications for cartridge construction. It would also help with insulation, both of the heat of combustion as well as the ignitor mechanism.

I got my chamber materials and some 30mL syringes today, so I can try the epoxy version and my commercial relief valve . prototyping is so exciting when the tests may blow up

I've been doing a lot of work on my commercial made pop-off version, I get my precision gauge for manometric fueling tomorrow, so I can test it pretty soon! I just wanted to check in and see if any prototypes have been made based on this concept

I actually built the design I posted last, unfortunately I neglected to put a spring behind the piston, so some air and fuel is lost while pressurising before the piston seals and I didn't manage to get a ignitable mix. I have some modified designs planned, especially now that I'm playing with 400 psi pre-ignition pressures and reliable burst disks have become a pain in the posterior.

prototyping is so exciting when the tests may blow up

At least you've entered the epoxy user's mindset

I've been having trouble making 100% sealing o-rings for a similar design, but would a normal piston valve with a good fit, light spring, and o-ring that let's air equalize into the chamber work if filled from behind with fuel and air?

Lockednloaded wrote:a normal piston valve with a good fit, light spring, and o-ring that let's air equalize into the chamber filled from behind with fuel and air

That's pretty much what I had in mind, as long as the mix behind the piston doesn't ignite it should be fine.

Exactly what I was hoping, I'm going to use a few more standard fittings then what you tend to use, but I'm mounting a 300psi pop off in the pilot area for safety's sake

one could place a pop off behind the piston, so that in the event of a pilot mix ignition your prototype would still be in once chunk.

This would have the added benefit of venting the pilot volume once the air is compressed by the piston moving back.

Or, even better, place a burst disk behind the piston.

jhalek90 wrote:Or, even better, place a burst disk behind the piston.

The main reason for the piston-hybrid is to eliminate burst disks, this would defeat the purpose.

If all went well, the burst disk would remain intact. If things went awry then replacing a bust disk isn't too bad. Building a device to still be safe even if it fails is good practice, especially if you like all your fingers!

if the burst disk was week enough to break if the mix behind the piston ignited it would also be week enough for it to break even if it didnt

really?
Ill take your word for it... but I would have thought a reasonably strong disk would have held up. For the above example (10x mix that makes 1000psi) a disk that burst at 500psi would hold the pre ignition mix and whatever back pressure resulting from the piston moving back, the pressure should go out the barrel - unless the spring holding the piston shut was exerting such force that it took more than 500psi to start opening the piston. I guess your going for max efficiency and aiming to have the piston open just below the max PSI generated by the hybrid combustion.... in that case a burst disk wouldn't be the best way to go, and a pop off valve would be just as bad. Id sacrifice some performance for added safety, but that's just me.

My point still stands tho - design your launchers to fail safely.

if i was you i wouldnt take my word for it i was just thinking that with a projectile in the barrel and the speed of ignition for a split second the pressure would be the same if not just a little less then if it ignited even if for not as long, but then that would take some sirius testing to get a disk that held to exactly ignition pressure