I figured it'd be best that it get it's own thread so that:
1) We don't get splinters in jimmy's topic on combustion spudgun models
2) We can discuss it in further detail.
Anyway, the posts that exist in Jimmy's thread and relate to this topic are reprinted below:
<i>I'm wondering looking at all of this (and I should admit I only understood about a third of it) is there wouldn't be some merit in what I'll call a dual-friction barrel.
Imagine that the first couple of inches of the breach end of the barrel is high-friction, then leading in to low friction. Surely this would allow a higher pressure to build in the chamber during combustion, and thus more gas to be combusted, before the spud makes any initial movement, but also allow the spud to move freely once this high rate of combustion has been acheived.
I'd be interested to see for instance if there was any difference in performance between barrels with internal and external chamferings of the spud knife.
Effectively what you're trying to reproduce is the "burst-disc" effect, but without a burst disc.
Secondary to this, what conclusions can we draw from this on hybrid performance in relation to disc burst pressures, can we model an optimal pressure (as a ratio of peak chamber pressure) for the disc to burst at.</i>
A "dual friction" barrel is a great idea. Any ideas on how? Glue a sheet of sand paper on the inside of the barrel at the breach end? Ream out the breach end of the barrel a bit, for a breech loader, so that the spud in the breech is a bit wider than the barrel?
"I'd be interested to see for instance if there was any difference in performance between barrels with internal and external chamferings of the spud knife." I believe there is evidence that an internal bevel increases the muzzle velocity, presumably by increasing the static friction. Of course, playing with the geometry of the muzzle knife changes both the static and dynamic friction. As you suggest, you probably want high static and low dynamic friction.
The current adiabatic model makes a very simple prediction for the optimal burst pressure of a hybrid. The burst pressure should be as close to the peak pressure as possible. I suspect that that is a bit simplistic and probably is not the best answer to the question. To correctly answer that question I think the model will have to be non-adiabtic so that heat loss is taken into account. The higher the burst pressure the better, but at high pressure the temperature is high and heat is being lost quickly. Hard to say if there is an optimal burst pressure that maximizes pressure but minimizes heat loss.</i>
<i> In terms of the dual diameter barrel, use a 2" sched 80 barrel with SDR 21 or 26 1.5" inside for the first couple inches?</i>
...to which I respond to this:
Perhaps a sub-caliper washer - like, say, .01" undersize - with a chisle grind (a taper on only one side) oreinted so that the ground side faced towards the muzzle would be good.
I'm thinking that you'd hold it in place with a coupler or union or something.
Seems it'd grind off a happy little bit, significantly reducing friction.
The only problem is that you'd have to control both the outside and inside diameter of the washer to several thousandths.