Pete's Easy Peasy Hybrid

[jackssmirkingrevenge]

A 20 gram projectile at 2500 fps has about twice the muzzle energy of a shotgun, we really need to see some videos!

Pretty please?

[PeteS]

A 20 gram projectile at 2500 fps has about twice the muzzle energy of a shotgun, we really need to see some videos!

Pretty please?

I still am wondering if that velocity can be for real. The chrono said so, but it seems kind of hard to believe. It was on two separate sessions though. I was hesitant to post the numbers because I am still finding them hard to believe.

The first with a piece of bubble wrap tightly wrapped in electrical tape. That was the 2609 fps one. It punched a nice hole in a 40 gallon plastic trash can and I never found any of the projectile. Not sure what it weighed, but it was pretty light. It also had a different chamber (a squatty propane tank). I was kind of nervous about the silver soldered connection to the tank so I rebuilt it with a pipe chamber before further testing.

The second reading was with a two inch long slug of potato.

Another slug of potato was the one that hit the tree shown below, but I didn't chrono that one.



The shot pictured was a straight on, direct hit at fairly close range and I am puzzled by the square shape of the damage.

I will try to get some video next time out. I am not sure if the snow and ice have cleared enough to get my car to where I have been testing. If the road is clear enough and there aren't too many hikers there, I'll see if I can get some video today. Otherwise I'll see what I can do later in the week.

[jackssmirkingrevenge]

The first with a piece of bubble wrap tightly wrapped in electrical tape. That was the 2609 fps one. It punched a nice hole in a 40 gallon plastic trash can and I never found any of the projectile. Not sure what it weighed, but it was pretty light. It also had a different chamber (a squatty propane tank). I was kind of nervous about the silver soldered connection to the tank so I rebuilt it with a pipe chamber before further testing.

The second reading was with a two inch long slug of potato.

I would surmise that your readings were inaccurate, the potato for example likely emerged from the muzzle as a cloud of fragments - not the sort of thing chrony sensors can measure reliably.

[PeteS]

I would surmise that your readings were inaccurate, the potato for example likely emerged from the muzzle as a cloud of fragments - not the sort of thing chrony sensors can measure reliably.

You may well be correct. I'll see if I can confirm or disprove the readings. I fired the 1" version a few times today, but had trouble with batteries in the cold, trouble getting the right mix, and the chrony was acting up as well. The mix issues may be the new setup with quick disconnect, gauge, and bike pump. I may go back to using the shock pump with just a presta valve. It took a bajillion strokes, but I seemed to be able to get more consistent mixes.

[jackssmirkingrevenge]

Some careful calculation of dead volume and how air and propane density is affected by temperature should yield more consistent results.

[PeteS]

Some careful calculation of dead volume and how air and propane density is affected by temperature should yield more consistent results.

I did some searching on the forum and was surprised to not find much mention of ambient temperature and syringe fueling. It seems like if it had a major effect in some reasonable range of temperatures, say between 20F and 100F, some discussion would have occurred.

Since density of air and propane at various pressures probably have different curves it seems like there would be some effect. Calculating the mass of the air and propane would allow us to eliminate that effect, right?

All that said I a betting that the bigger issue is slowness of the propane diffusing in the air. I did find a thread on that:
http://www.spudfiles.com/combustion-can ... 13337.html

His results seem to be in line with what I am seeing. That includes the liklihood that ignition will more likely occur if a good bit of time is allowed before trying. It also includes his finding that if the stun gun is activated too much before mixing is complete that it never ignites. He guessed that either air oxygen or fuel were in some way depleted by the arc.

His tests did try to be a worst case scenario, but I was still surprised just how long diffusion took.

[jackssmirkingrevenge]

Since density of air and propane at various pressures probably have different curves it seems like there would be some effect. Calculating the mass of the air and propane would allow us to eliminate that effect, right?

The explosive limits in this table given by volume assume standard temperature and pressure as well as a certain concentration of oxygen in the air: http://www.spudfiles.com/spud_wiki/inde ... n_Spudguns

For extreme temperatures, you would need to see how density is affected.

All that said I a betting that the bigger issue is slowness of the propane diffusing in the air. I did find a thread on that:
http://www.spudfiles.com/combustion-can ... 13337.html

I don't think Jimmy's data is relevant to you because the parameters are different:

I was purposely trying to get the fuel to diffuse as slowly as possible. No mechanical mixing (e.g., a fan), the chamber was held still during fueling and equilibration, the fuel was injected slowly at one end of the chamber, the spark gap was at the other end of the chamber, the chamber is long and skinny, it was pretty cool in the garage (40~50F), etc.

Also, he was testing at atmospheric pressure. In a hybrid, not only does every pump stroke introduce turbulence into the chamber, but you also have more densely packed mixture:



My conclusion would gravitate more towards incorrect mix ratio.

You could do a small practical experiment, try injecting 80%, 90%, 110% and 120% of your calculated fuel volume and see which gives the best shots.

[PeteS]

My conclusion would gravitate more towards incorrect mix ratio.

You could do a small practical experiment, try injecting 80%, 90%, 110% and 120% of your calculated fuel volume and see which gives the best shots.

I did try "bracketing" the mix a bit. I varied the air pressure rather than the fuel though. The results were inconclusive. It did seem that more air was needed than my calculations suggested, but it seemed a bit sporadic at all mixes tried. I will have to try a bit more formal testing of that.

I am probably on the wrong track, but I still can't help but think that it behaves like a mixing issue, so I will try putting something in the chamber to allow me to shake it up and agitate the gasses to rule out that possibility.

UPDATE: I did some testing at 31 F ambient temperature today with both the 1.5" EMT gun and the 1" EMT one. It seemed like the mix was off on the 1" EMT one. Upon checking I found the volume off by 5% or so after having reworked the gun. I suspect the fittings got tightened that much more.

I put a plastic bottle cap in the chambers as an agitator and It seemed to help. The 1.5" EMT gun fired every time. Next time I will try it without the agitator and see if the improvement from agitation was in my imagination

I'll have to do some more testing with the 1" EMT one now that I have remeasured the volume.

With a heavy projectile (pvc cap with 1/2" bolt and two nuts) and a 4X mix the 1.5" EMT (4' barrel) version was just under the sound barrier at 1087 fps. This one should be fun for punching holes in stuff and should be supersonic at anything above 4X.

[PeteS]

Damage pics:

Front view of decent 6 ply 1/2" fir plywood the big hole is the entrance. It is bigger because the pvc cap hit it and shattered allowing the bolt to proceed without it. It easily passed through both sheets and continued on.


Back view of plywood damage.


Tree damage, 1/2" bolt with two nuts in pvc cap on left, 3" long slug of spud on right. Both from 1.5" EMT gun. Range was about 25 feet.

[jackssmirkingrevenge]

Ah, penetration... nice ones!

One other thing to consider about inconsistent firing, are you sure it isn't leaking somewhere? A tiny leak can be enough to throw your mix off.

[PeteS]

Ah, penetration... nice ones!

One other thing to consider about inconsistent firing, are you sure it isn't leaking somewhere? A tiny leak can be enough to throw your mix off.

I went over all the threads carefully when I built it and tested it by submerging it is water, so I think all the parts that screw together were OK. I'll test again it to be sure it holds pressure.

That said, the 1" camloc seems kind of iffy at times. It is has more give when closed than the 1.5" one. I have also have seen a little wrinkle in the burst disk where it seals. I think it may just be a poor fit. That could possibly be leaking some of the time and not others. I may try making a shim to tighten it up a bit. That or just go back to using the union that I used in the previous tests.

The union is a PITA because it means taking along big honking wrenches or channel locks, but it seals better. Maybe I could braze a lever on the union to tighten it with no tools.

Another option is to just put the 1" barrel on a 1-1/2" camloc using a reducing bushing and use it on the gun that is working reliably, if I want to use the 1" barrel. I could then just abandon the 1" camloc and use the rest of that gun as the chamber on my next hybrid (probably a piston hybrid).