mobile chernobyl wrote:The inertia of the projectile itself does more to aid in building up chamber pressure than the burst disc does.
Another effect that I discovered with VERA that is not accounted for in HGDT... Increased turbulence once the gases start to move. Burst disk in place with no projectile at all. Projectile in... The pressure ramp looked the same. The decay looked very different, of course... But the ramp? Virtually identical. Once that disk goes and the gases start to move the turbulence goes nuts and the burn rate follows. Seriously, on the shots where I had no projectile at all, once the disk blew (visible due to accelerometers) the pressure rise took a step function INCREASE. Seriously. I had an 18" diameter hole in my chamber but the pressure was increasing faster than before the hole appeared!
In summary: Based on my experience, whether you have an optimized disk or not makes even less difference than HGDT indicates.
Also - the "contoured" selection is certainly not correct I would think lol. with a blunt profile, 750psi bursting disc is needed to maintain >100K ft-lb performance.
You can get your system to perform as if it's contoured using a trick... I don't know if it's the correct name (probably not), but at the office we refer to it as using a "dork."
So what is a dork?
OK, see that small length of pipe you have welded to your chamber that provides the breech flange? Yeah, that pipe. When you're building your chamber, extend that pipe into the main portion of the chamber maybe 2 or 3 diameters. Now, for the length of pipe that is inside the chamber, drill a bunch of holes in the pipe.
That's it. That's a dork.
Thought experiment to explain function: If you don't have the holes drilled, the pipe will act very much like it did before. You'll have a choke point at the entry of the pipe that will neck the flow down. OK, fine. But past that choke point the pressure will be lower than it was at the mouth of the pipe. If this were not so, there would be no flow. OK, now imagine the holes... You have a pressure differential between the main chamber and the main flow area of the pipe. And a hole between the two. More air is going to flow in via the drilled hole. Voila, an increased flow coefficient.
Done perfectly it will give you flow that matches a perfectly contoured entrance. Of course, you won't be doing it perfectly, but the point is that significant improvements in flow are available with a bit more work with no increase in build complexity.
