03cumminsguy wrote:
Pros and cons of Burst Disk vs. Piston
Barrel connection? Camlock? any other ideas or options
Metering techniques? Pros and cons of those?
Burst Disk:
Pros:
Has the potential for more flow so you get better performance out of the valve.
Easy, simple, cheap.
Cons:
Requires a new disk to be loaded each shot.
Can sometimes be quite tricky to get the disk to seal, the use of o-rings or gaskets in conjunction with modifying the union may be necessary.
The disks are a pain in the ass to make. Making disks for 2" unions or larger would be quite annoying. If you're happy to dedicate a Sunday afternoon to cutting circular disks from soda cans, milk bottles, several dozen layers of tin foil or other suitable material, then go with a burst disk.
Burst disks can also be more difficult to 'fine tune' for each mix number. With a piston valve you can calculate roughly how much pressure you need in the pilot for a particular fuel mix so that the piston will be forced open upon combustion.
Piston Valves:
Pros:
Potential for a higher rate of fire
Performance is still pretty good but not as good as burst disks
Easy to fine tune for each fuel mix
No disk cutting
Cons:
Probably more expensive to make
Much harder to make as making the piston requires a decent amount of experience so you can easily troubleshoot the problems you will undoubtedly run in to
Can be destroyed by the combustion pressures fairly easily. It's taken me about 9 months to finally make a piston that doesn't wear out after a dozen shots.
More things to go wrong, if an o-ring is buggered or a leak appears you can't fix the problem in the field.
I'm sure there's more that I missed.
In short, if you have no problem making disks for the union, go with a union. For a 3/4" union and a 10x mix, I use 64 layers of aluminium foil. DYI would be able to tell you how many you need for a 2" union but all I can say is that it would be more than 64 layers and you'd cut a much larger disk.
If you can afford it, go with a 2.5" or 3" valve.
You could use a camlock for the barrel, it is a bit dodgy as the aluminium ones aren't rated very high but I believe SpudBlaster15 has used them on his hybrids. If you're planning on changing the barrels in the field, it might be easier going with a camlock connection although if you don't, you only have to bring a plumbing wrench with you to change the barrel.
For fuelling, I recommend manometric metering. Some will agree with me, others might disagree. I've used volumetric and manometric in the past and I much prefer the latter. I've got consistent results every time and I find it much easier and quicker to fuel. Manometric metering will require a fairly accurate (1% F.S. would be ideal) pressure gauge that reads no more than 15PSI.