Single-pump hybrid concept

[Skywalker]

I like @hectmarr 's work on these autonomous hybrids. I'm thinking about a design for a single-pump, high-mix hybrid. It seems to me that going for multiple pumps is a waste, since the resistance of the first few strokes is low, so one's arm is getting a lot of exercise just going back and forth. [ahem] I'm thinking of using a pump like the Crossman multipump BB gun, but scaled up. I wrote a little calculator in an iPython notebook to calculate the required lever force for a particular piston diameter, stroke length, lever arm length, and chamber volume. It also calculates the resulting chamber pressure.

I found a nice solution that I could use: with a chamber volume of 0.5 cubic inches, a piston diameter of 1.25 inches, a lever arm 16 inches long, a connecting rod 6 inches long, attached 4 inches up the lever arm from the pivot point, I get a 20x mix in a single stroke, with about 20lb peak force on the lever. That's less than many upper-end spring air rifles, b/c I'm a weakling, lol. This is assuming isothermal compression -- if the air heats up instead, the peak force would be more like 38 lb, and the chamber pressure would start out at 67 atmospheres and drop to 20 after cooling down. The truth is somewhere in the middle probably. I could probably increase the lever arm a bit, reduce the travel, or increase the chamber volume if that turns out to be too much for enjoyable shooting.

With such a small chamber volume, I need to minimize deadspace as much as possible. At 0.5 cubic inches, that's only 0.4 linear inches of space inside a 1.25 inch diameter cylinder. So, I think I would need to make the piston with the o-ring groove very close to the end, and make the end of the piston fill the bottom end of the cylinder very well, and have a check valve built into the wall or end of the cylinder. I considered simply letting the chamber be the dead space at the end of the piston, but I'm worried about the force of the combustion blowing the piston back.

On the other hand, for a 0.22 inch diameter barrel, that equates to 13 linear inches. I figure that's pretty reasonable for a 26 inch long barrel: the gas volume will expand by 3x.

For fueling, I like the idea of metering the fuel into the pump before compressing, provided it doesn't diesel on me. At a 20x mix, if I assume adiabatic compression, the peak temperature is 1000 K, which is above the ignition temperature for butane (678 K) and propane (743 K) and even hydrogen (809 K) and methane (810 K). I can get the peak temperature down to 748 K if I increase the chamber volume to 1.1 cubic inch for the same pump parameters. That limits me to a 9x mix. I figure the peak temperature will be somewhat less than isothermal, especially if the air is flowing through a checkvalve into the chamber, which should provide some mixing & cooling.

[Skywalker]

A few more thoughts: I looks like Hector's little pistol has 16mL chamber, or ~1 cubic inch. I feel a bit silly for planning a rifle around a chamber with half that volume. So, I eked out a bit more performance by extending the lever arm to 24," the attachment point to 6.5," and increasing the cylinder to 1.5" diameter. This will now fill a 1 cubic inch chamber with 23.7x in one stroke, with 22lb peak force.

Propane and butane combust around 1980 Celcius, so the peak pressure is ~2,600 psi. For reference, the chamber pressure of a 22 rifle is in the range of 10-20 kpsi. So this won't have the performance of an actual rifle, but it'll definitely scoot a pellet right along.

At 23.8x mix, I can use the chamber itself as the metering volume. That kills two birds with one stone: I also blow out the exhaust that way. I'm not sure yet how to do all the valving, but I want it to be simple. I'm toying with the idea of using a lighweight piston inside the chamber that is used to push the exhaust out the barrel after the shot, meanwhile drawing fuel into the rear of the chamber. Then this piston retracts, allowing the fuel to pass through using a check valve. This scheme requires a firing valve that stays open after the shot, and is then closed after this operation but prior to the pump stroke. Perhaps a pop-off-style valve that sticks to a magnet & then gets bumped closed by the exhaust piston? I think this fuel/exhaust piston could be driven forward and back at the same time the bolt is cycled to load the next pellet.

The goal is to reload the gun simply by swinging the pump lever once.

[hectmarr]

Glad my antics serve to inspire and can be improved upon.
What I suggest is trying to keep everything as simple as possible.
For the rest, surely a lever will increase the compression and the complexity and size of the weapon, which does not have to be a problem, unless like me, you are looking for something extremely simple.
My pistols have between 40 - 50 cm3 of mixture, they are short weapons. Jack posted an HGDT simulation of an equally small gun, you can see it on my "Strong Boy" topic.
The issue of the automatic dispenser is a separate issue. It is what will give you the most work.
I think that a long gun has the possibility of incorporating a lever, like the ones I use in my autonomous, multi-pump hybrids.
It is interesting that you add drawings, because it is easier and faster to follow your thoughts. A simple drawing, with measurements, is what I like.

[Skywalker]

Thanks for chiming in!

> What I suggest is trying to keep everything as simple as possible.

Have you seen your own diagrams?!
Seriously though, I want the operation to be simple for the user, even if it complicates the construction a bit. I got spoiled plinking with my brother's 22 rifle & 9mm pistol last year. It's nice to be able to focus on target practice w/o going through a whole checklist of steps between shots.

Here's what I was thinking for the fuel/exhaust cycle:

When the gun fires, the firing valve piston gets trapped by the detent, so it stays open post-shot. Then, the push rod is pushed forward, drawing in fuel through the check valve from a near-atmospheric-pressure fuel reservoir (eg, syringe or balloon). At the end of the stroke, the push rod releases the firing valve from the detent, closing it. Then the push rod is withdrawn, and the fuel flows through the check valve in the fuel/exhaust piston and occupies the chamber. Finally, the pump pressurizes the chamber with air.

I'm thinking that the push-rod could be coupled to the loading mechanism, so that a new projectile is loaded at the same time. The push-rod could also be coupled to the pump, so that as the pump moves forward, it pulls the push rod forward until the push rod hits its limit of travel. At that point, the push rod disengages from the pump and returns using a spring. I'm still kinda fuzzy on the details of making that catch/release mechanism.

Hybrid_pump_coupling_action.png (24.31 KiB) Viewed 9858 times

[hectmarr]

Have you already got the materials to build the prototype? It may work, but to be safe and learn from the facts, you need to build and test this idea. Surely, while testing or building, new doubts and problems will arise. I will be attentive to your updates on the prototype.

[Skywalker]

No, I don't have plans to build anything like this soon -- I'm just thinking about how I would do it. I live in a small apartment in the suburbs. I have neither a place to shoot nor to do metal work. I made a small bolt-action cartridge hybrid once upon a time, though. My hope at the time was to be semi-autonomous, but making enough cartridges would have been a pain. That's why I've been inspired by your recent designs.

I guess if I wanted to design something I could both build and use in my current environment, it would probably be a simple low-mix prototype, or maybe even a pneumatic. Maybe once the pandemic is over I could get a membership at a 'maker-space' and actually do some serious work on this. There are probably ranges where I could go shoot, also.

Edit: The more I think about it, the less I like the idea of having a bunch of moving parts and seals in the chamber. Maybe the way to go is a manometric metering with a 3/2-valve that empties into the pump air intake. At 24x or so, the presence of exhaust gas in the chamber is pretty much a rounding error. I also don't like the idea of being stuck with a single mix ratio that I can't adjust.

I also had a look with HGDT (thanks for reminding me about that!). It should be plenty powerful with 1 cu in chamber and 24" long barrel.

Baseline_hybrid_calculation.png (22.99 KiB) Viewed 9851 times

[hectmarr]

Too bad I can't try. I have very limited resources, and I only think about what I can build. The basic model I have built is simple to build, with few inexpensive things. It really is one of my best weapons, to play around a bit. It's not very powerful, but with everything optimized it shoots at around 450fps with just 14kg of force of mine to compress.
Its design seems to work, but if you can't try to do it, it's all very theoretical, but just as interesting.