Hybrid geometry

[Pennywise]

I have attempted to find some testing in regard to the geometry of a chamber-barrel transition, but it has been mostly fruitless so far.

I am building a hybrid from a section of 1.5"ID stainless. The barrel will be .5"ID. The actual combustion portion of the chamber will be ~6" long.

From what I have read, nobody seems to have studied the impact of a blunt transition (imagine a 1.5"x.5" reducer bushing, with blunt walls exactly 90* perpendicular to the flow of the gasses), vs a tapered transition (elongation of this transition. Essentially like 'porting' on an engine).

--Blunt transition-I have read that, if anything, this blunt transition causes change from a largely laminar flow to a turbulent flow that increases the flame propagation speed. This sounds to be the very basis of what a high-mix hybrid would strive for.

--Tapered transition-everything I have learned about fluid dynamics suggests that a gradual transition allows for the highest velocity of flow at any given pressure.

So this brings me to my main question. I have machined the plugs for either end of my chamber. Currently, they are both of the 'blunt' design. I am considering 3d printing a 'transition damper' (essentially a piece that could be added in to smooth out the transition between the chamber ID and the barrel port). Would the loss of a couple CC's of chamber volume be more detrimental to performance than any benefit gained by the loss of resistance at the point of the transition?

[jackssmirkingrevenge]

An interesting question and I thought worth investigating, so I made the following modifications to the Mini Hybrid II:

Blunt endcap insert:

Insert bore tapered:

I fired two shots each with both endcap variations, for each shot I used a 63cm long 0.177" barrel, 0.35 gram BBs, one layer of photo paper burst disk and a 19x mix with butane.

Velocity results:

Blunt - 1128 fps, 1143 fps

Tapered - 1146 fps, 1148 fps

There is no real difference between the results, the tapered average is slightly higher but not remarkably so, and technically speaking the chamber volume is slightly larger in the second case.

I'm also amazed by how similar the last two results are.

In order to corroborate the chrono results I also fired through a steel security box. In each case the shot cleanly penetrated one side and dented the other, in a very consistent manner.

Entry holes:

Exit holes:

In conclusion, the shape of the transition does not seem to be a significant variable in practice.

[Moonbogg]

Nice experiment! Now the OP can just focus on simplicity.

[jackssmirkingrevenge]

Now the OP can just focus on simplicity.

With say a 30 inch long barrel and 15x mix one can envisage sending a half inch round ball downrange at just over the speed of sound

[Pennywise]

My chamber walls are around .23" thick, so I should be able to run mixes considerably higher than 15 and remain in a safety margin. (Mathwise anyway)

Another thought: have you experimented with an *inner* chamber liner with a lower thermal conductivity than the chamber material? If the largest portion of forces come from the expansion due to temperature differential, it would stand to reason that a material less effective at sapping heat would aid in keeping temperature longer and thus keep pressure up

[jackssmirkingrevenge]

My chamber walls are around .23" thick, so I should be able to run mixes considerably higher than 15 and remain in a safety margin. (Mathwise anyway)

Given the materials and toolings you've shared previously I was hoping this would be the case

Another thought: have you experimented with an *inner* chamber liner with a lower thermal conductivity than the chamber material? If the largest portion of forces come from the expansion due to temperature differential, it would stand to reason that a material less effective at sapping heat would aid in keeping temperature longer and thus keep pressure up

It would indeed make a difference and for example HGDT is supposed to compensate for it, and I can confirm that the chamber and barrel do tend to heat up after firing so there is definitely thermal transfer.

I have not experimented with insulation but I would think that this would not be too significant a difference.

As you're concerned with these minutia that usually seem to make only incremental differences, is it safe to assume that you're looking to push the limits with this project?

[Pennywise]

Pushing my own limits, at least. Really, I'm just trying to get the most possible out of what I make. If I have to stay within a safe pressure limit, I want to do as much as possible while remaining in that limit. My primary interest lies in handheld launchers. I dabbled with large cannons in my younger days, but now getting as much power as possible without losing all practicality is a lot of fun.

[hectmarr]

An interesting question and I thought worth investigating, so I made the following modifications to the Mini Hybrid II:

Blunt endcap insert:

20231123_211452.jpg

Insert bore tapered:

20231123_214925.jpg

I fired two shots each with both endcap variations, for each shot I used a 63cm long 0.177" barrel, 0.35 gram BBs, one layer of photo paper burst disk and a 19x mix with butane.

Velocity results:

Blunt - 1128 fps, 1143 fps

Tapered - 1146 fps, 1148 fps

There is no real difference between the results, the tapered average is slightly higher but not remarkably so, and technically speaking the chamber volume is slightly larger in the second case.

I'm also amazed by how similar the last two results are.

In order to corroborate the chrono results I also fired through a steel security box. In each case the shot cleanly penetrated one side and dented the other, in a very consistent manner.

Entry holes:

20231123_220645.jpg

Exit holes:

20231123_220659.jpg

In conclusion, the shape of the transition does not seem to be a significant variable in practice.

Interesting tests. I always wondered what would be better... Now I know. The small differences in favor of the conical cap are only due to the small increase in volume.