Pete's Easy Peasy Hybrid

[PeteS]

After having great success with my first hybrid, I decided to build another bigger bore one and add some refinements. I started with the largest galvanized pipe nipple home depot carried, a 2"x8" one I used as a chamber. I picked up some 1.5" aluminum Camlocs from Granger and a piece of EMT and steel compression fitting from Home Depot.

After some tinkering in HGDT I decided to go with a 48" barrel. I like to be able to put the chamber in my backpack and carry a more unobtrusive barrel so as to not attract attention. I can always use the left over 6' piece of EMT if I want to go for more performance, but the numbers looked pretty good for the shorter barrel and supersonic spuds should be well within reach based on HGDT and my previous experience with the 1" version.

The 2" cap and 2" to 1.5" reducing coupling are also from home depot. The main assembly was then just screwed together using teflon tape,

I used a 1/4" pipe fitting with a Presta valve soldered in and a 10mm threaded sparkplug. Holes needed to be drilled and tapped for these.

Ignition was a $9 Vipertek stungun with copper sparkplug wire added in. The connections are soldered and the wire supported with epoxy putty. I added a guard for the sparkplug by forcing a piece of heated 1/2" pvc over it. The guard is notched for the plug wire and split on the other end. A hose clamp clamps the guard on. It also squeezes the ground against the base of the sparkplug.

I am guessing that a single layer of coke can for a burst disk is probably a good starting point for the planned 4X shooting.

Fueling is via a 100 ml syringe that has a schrader to presta adapter epoxied into the end. I ran a drill bit into the end to clear out a bit of platic to allow the presta valve to work properly. That has worked remarkably well for my previous hybrid as well as my combustion guns, so I assume it will work well here too.

A 1" version of this gun has exceeded my expectations, but thus far this larger bore version of the gun has only been fired with no projectile. It was satisfyingly loud in that mode

Performance details and damage pictures to follow when it has been tested further.

[CannonCreations]

Nice looking cannon. I am looking to build a hybrid very soon, although it will be a piston valve . I was just wondering where you found the barrel? I have been through Home Depot and haven't seen anything like that. I also have a quick question about the spark plug, did you have to tap a hole of it? or where you able to find a cap with it already there?

[PeteS]

Nice looking cannon. I am looking to build a hybrid very soon, although it will be a piston valve . I was just wondering where you found the barrel? I have been through Home Depot and haven't seen anything like that. I also have a quick question about the spark plug, did you have to tap a hole of it? or where you able to find a cap with it already there?

It is in the electrical section. It is EMT or thinwall conduit. Not sure how high of a pressure you can go with it but someone said they went 8X with 2" EMT. I will probably stay lower than that.

Yes I drilled and tapped the two holes (sparkplug and air valve). I used a 10mm threaded sparkplug. That size is not all that common, but you can find them if you look. I found the tap in that somewhat odd size (10mmx1mm) at SJDiscountTools.com or autotoolworld.com both at a much cheaper price than most. You could use different size plug though.

FWIW, the plug I am using is an NGK 4629 or C7HSA (both numbers are on the box, but I think my supplier used the first number as their part number.

[PeteS]

It broke the sound barrier today in it's first tests. I had to bump it up to 7X to do so though. There are no damage pics yet. What came out of the barrel at 7x wasn't much more than a cloud of mush. I'll have to find or make some suitable projectiles.

One cool thing I noticed was how completely and cleanly the burst disks blew out at 7X. They looked like they were laser cut. That said, I am pretty sure that I need to use a higher pressure burst disk for best performance.

[jackssmirkingrevenge]

*very* nice!

How are you shooting it, resting on the ground?

Looking forward to damage pics/videos, some "proper" projectiles are definitely in order!

[Labtecpower]

Nice job on your hybrid!

Reminds me of the ones I made hehe
I recommend shooting some ball bearings or saboted darts, they really ramp up the damage this gun can put out.

[jackssmirkingrevenge]

I recommend shooting some ball bearings or saboted darts, they really ramp up the damage this gun can put out.

Yup, at the very least find a friendly local machinist who will make you some decent sabots, a fistful of nuts and bolts travelling at mach 1 is a sight to behold

[PeteS]

How are you shooting it, resting on the ground?

Hand held after a few test shots resting on the ground.

I recommend shooting some ball bearings or saboted darts, they really ramp up the damage this gun can put out.

Yup, at the very least find a friendly local machinist who will make you some decent sabots, a fistful of nuts and bolts travelling at mach 1 is a sight to behold

I have a lathe so I should be able to make some sabots. What material would you recommend?

[jackssmirkingrevenge]

Excellent!

UHMWPE is excellent, Acetal (Delrin) also works but is less flexible and more prone to shatter, as well as being denser than UHMWPE.

Might I recommend some darts...

[youtube][/youtube]

[PeteS]

Another refinement...
I added a quick disconnect with a tee, a presta valve, and a pressure gauge (with a ball valve). I figured that I would close the ball valve when injecting the propane, start pumping the air then open the valve after all the propane is in the gun. Not sure if the valve was necessary, but was concerned that propane going into the gauge might not be a good thing, possibly throwing off the mix. I expect to pop the whole assembly off before firing.

I figure that way I can use a small bike pump that has no gauge. I had been using a shock pump but it took a bajillion strokes to get up to even 4X.


Oh and I also made some quick and easy darts from 1" pvc pipe caps. They needed to be turned down just a bit. In the picture two have a 1/2" bolts and a nut making them into a dart and the other two I plan to fire just the caps with no bolt. I figure the darts should do a good bit of damage, but an not sure what to expect from the plain caps. I think the darts will fly straight, but the plain caps will probably tumble. I may make some more plain ones to use as sabots for various junk projectiles or lead shot.

[jackssmirkingrevenge]

Don't forget to factor in the pump pipe volume when calculating fuel volume.

Those little projectiles should fly reasonably well, I would add another nut though, sectional density is a little poor so unless you up the weight they will lose velocity quickly.

[PeteS]

Don't forget to factor in the pump pipe volume when calculating fuel volume.

Those little projectiles should fly reasonably well, I would add another nut though, sectional density is a little poor so unless you up the weight they will lose velocity quickly.

Will do.

If I can find or buy a couple more 1/2" nuts before I shoot the darts I will put them on. That makes me wonder if maybe bigger and or longer bolts might have been a good idea. Maybe 5/8" bolts 1" longer next time? That would significantly increase sectional density.

I am a little unclear on the volume issue. My thinking was that the propane would all be in the chamber and not migrate back into the tee, gauge, or male part of the quick disconnect before I disconnected it (the plan was to pump it up and immediately pop off the disconnect). Does the propane really migrate back through that small opening fast enough for that to be an issue? If so the ball valve may be ineffective and unnecessary because the propane would wind up in the gauge any way. Is that right? Do I count everything back to the presta valve including the ball valve and the gauge or where do I draw the line?

[jackssmirkingrevenge]

That makes me wonder if maybe bigger and or longer bolts might have been a good idea. Maybe 5/8" bolts 1" longer next time? That would significantly increase sectional density.

Just remember to keep the centre of gravity ahead of the centre of pressure.

http://www.rockets4schools.org/images/B ... bility.pdf

I am a little unclear on the volume issue. My thinking was that the propane would all be in the chamber and not migrate back into the tee, gauge, or male part of the quick disconnect before I disconnected it (the plan was to pump it up and immediately pop off the disconnect). Does the propane really migrate back through that small opening fast enough for that to be an issue? If so the ball valve may be ineffective and unnecessary because the propane would wind up in the gauge any way. Is that right? Do I count everything back to the presta valve including the ball valve and the gauge or where do I draw the line?

http://www.spudfiles.com/how-to-databas ... 22301.html

The line stops wherever the pump check valve is. The propane will stop there, but will go everywhere else once you start pressurising. Since a presta is also a check valve, you can consider it to the endpoint of your chamber - but include the volume of the tee in your pressure calculation.

A shock pump has a relatively small volume of tubing, I measured 1.75mL on mine. This is a relatively small proportion of your chamber so you wouldn't have noticed, I only saw it when making much smaller hybrids, some with less volume than the pump dead volume itself.

[PeteS]

Thanks again.

[PeteS]

After some testing I decided to swap some parts between my two hybrids to have the best of both and then to rebuild the remaining parts using some different ideas. That rebuild of the remaining parts will come later though.

The 1" incarnation had much higher velocities at a lower mix, so I am swapping that barrel and associated parts onto the chamber with nicer ignition. Also the projectiles I made so far for the 1" one have a better sectional density. They used the same bolts with two nuts and a 1/2" pvc cap turned down to fit the bore.

The 1.5" EMT barrel did 1209 fps at 7 x. The 1" EMT barrel version did 2609 fps and 2953 fps on it's first tests and that was at 4 x and 4.5 x. I figured that 2609 had to be a fluke, but the second reading makes me think it is for real. The comparison wasn't fair though since barrel length was greater on the 1" gun (6' vs 4 ') and the burst disk was probably closer to optimum.