A heavy 75x hybrid

[Frozen]

Hi all, I have been following these forums for several years but this is actually my first post. I am from Finland and I have done some simple coke bottle pneumatic launchers years ago and I have also built a small ETG two years ago that can shoot a 4mm diameter piece of steel through a 10mm thick aluminum plate.

Today I started machining a chamber for a hybrid that I have wanted to build for a long time. I recently started studying to be a machinist so I now have access to large lathes and mills to do my projects on. Here is a picture of the partially machined chamber:

Chamber

It is 143mm OD, 75mm ID and about 270mm long at the moment. The OD is in its final dimension but the inside surface has not yet been machined, so it will probably be 77 or 78mm, depending on how much the hole is off center, I havent measured that yet but it is in the 0.5-1mm range. It will have a chamber volume of about 1250-1300cc depending on the final chamber ID.

I have done some pressure simulations on it in AutoCad Inventor, it suggests that I could do 75x shots with butane with over 2.5 safety factor. This simulation is with 700 bar continuous pressure:

The steel the chamber is made of has 355MPa minimum yield pressure, so the max of 170MPa stress suggested by the simulation is nowhere near its breaking point. According to HGDT simulation at 75x my max chamber pressure will be 530 bar so the stresses will be even lower than in the simulation picture above.

Just wanted to share my project here and I will post my progress as I get more machining done. I currently have machining classes every thursday in this period so you can expect more progress next week. I am sure I missed something so be sure to ask.

[jakethebeast]

Awesome little project you got here, and nice to see more Finns catching up he hybrid bug! What kind of projectiles are you planning on launching out of it?

[jackssmirkingrevenge]

There's something about you Nordic types and insane hybrids looking forward to see this fire! What will the barrel diameter be and what are you doing for ignition?

[Frozen]

I dont know about the projectiles yet, I could machine some darts or maybe make a lead casting mold for some balls or supersized airgun type pellets. The barrel diameter will be around 30mm, I have not yet searched for a good barrel pipe from my local hardware store. The gun will be pretty heavy when finished, the chamber piece alone weights over 25kg so the finished cannon will be probably close to 50kg if not more. The weight will help with the recoil so I dont think its a bad thing, I wasnt going to make a handheld cannon either way.

The ignition source will be a thin piece of wire with a small capacitor bank to turn it into plasma to ignite the mix, similar to larda's design. I dont have complete plans for the cannon yet as the design depends on what size and material pieces I can find to be machined.

I was thinking of making the end pieces of the chamber to be two large plates around 200mm diameter and then having multiple large threaded rods keeping the chamber together from outside of the chamber tube. Making threads to the inside of the chamber tube would reduce the volume significantly and I think it will be easier to do it my way and have less chance to for example ruin the threads when cutting them.

[matti]

I've build similar type chamber and connected the endcaps by many M12 screws. You can make threaded bolt hole circle in the chamber pipe ends if the wall thickness is sufficient. It's more compact and cleaner look then tie rod type.

[jackssmirkingrevenge]

You can make threaded bolt hole circle in the chamber pipe ends if the wall thickness is sufficient.

Seconded, your wall thickness is insane.

I could machine some darts

You could!

[Frozen]

You can make threaded bolt hole circle in the chamber pipe ends if the wall thickness is sufficient. It's more compact and cleaner look then tie rod type.

I know I could and the wall thickness would allow it, but it would require me to make the chamber volume smaller to keep the same theoretical burst pressure for the chamber. HGDT simulations suggest that at 75x mix the reduced chamber volume would decrease the kinetic energy by about 10% and at low mixes like 10x the kinetic energy decrease would be about 20%. The bolt holes in the chamber walls weaken that part of the chamber, so to keep the wall strength I would have to do it like this:

If the pressure area would go all the way to the end of the chamber, there would be lots of local stresses in the walls of the bolt holes even larger than those in the inside walls of the chamber. That would make the bolt holes the weakest part of the chamber if not made like the diagram above. For M16 bolts class 8.8 the minimum thread engagement length would be around 25mm so I would have to drill atleast 30mm deep holes in the chamber. That would make the decreased chamber length about 40mm on each end and thus making my effective chamber length about 190mm long compared to the original 270mm. To me the cleaner look is not worth the decreased power. If however I am unable to find a large enough piece of round steel then I would be forced to do it this way.

[Frozen]

Here is the barrel I got today, it is larger than I had planned but when they didnt have any smaller pipe with thick walls and smooth inside surface I got this for my cannon. It is 55mm ID and 70mm OD, meaning 7.5mm wall thickness. It is 355MPa yield strength steel like the chamber. The inside surface is a mirror finish after I ran a towel through it a couple times:



I also got some pieces of round steel to do the end pieces, they will be done like in the diagram in my last post because I didnt find a small enough piece of 220mm round stock. I have much more machining time at thursday than I did last week so I am pretty sure I will get the chamber piece done except the bolt holes and then I will start turning the end pieces.

[matti]

Have you ever fired a hybrid cannon of any size or any mix ? . I mean that I hope you do understand what kind of power and bang you would really get with a 75x hybrid that size.. I have fired a large hybrid with only at low mix (4-10x) and the booom is so loud that I have to drive about 20km to fire it.

I really don't know where you would shoot it or to what.. It's just little bit too powerful for just "fun". And hybrids are not allowed in Finland, so you might get into some troubles if caught shooting it.

[Frozen]

I have never fired a hybrid before but I have experience of real large cannons, I have myself fired 130mm and 122mm cannons and seen probably several hundred shots, they make some noise. I have lots of space to shoot at where my parents live, there is lots of forest and fields and the few neighbors dont mind the noise of guns there. The first shot will be with a pretty low mix, probably 5x or at max 10x, I will then see what kind of noise it makes and I will then up the mix until the noise is too big.

I know they are not legal here but I have never seen a police near the house in the 18 years that I lived there and the neighbors are nice, they will come asking me what the sound is before calling anyone.

I got some machining done today, the chamber piece has now fully machined surfaces and tomorrow I will drill and tap some holes to the ends for attaching the end pieces. The chamber ID ended up at 77.6mm. There is not much more to see at the parts at the moment, I will probably take a picture of the chamber tomorrow if I get the bolt circles all drilled and tapped.

EDIT: Bolt circle is now done on one end, it has 12 M12 bolt holes.

Bolt circle done

[Frozen]

I have done some progress during the few weeks since my last update. The chamber cap is now in good shape, I still have to bore the hole to 55mm ID and then do some drilling and tapping and an o-ring groove.

A few pictures of it:

The smaller OD goes inside the chamber

The barrel will be attached here

It will be placed like this

[jimmy101]

Will propane survive 75x without auto-igniting or liquefying the propane?

[Ragnarok]

Will propane survive 75x without auto-igniting or liquefying the propane?

Larda ran Hygac20 at 200x, so I'm going to say yes.

The partial pressure at 75x isn't enough to liquefy propane (depending on temperature, that happens at around 220x), and the concern with auto-ignition is gas temperature (as that makes individual molecules are energetic enough to start a reaction) rather than density (which has no bearing on the energy of any given particle, just the total number of them).

[mark.f]

Will propane survive 75x without auto-igniting or liquefying the propane?

I don't think dieseling is a factor at the slow rates of compression associated with fueling a hybrid, and liquefication depends on the partial pressure of propane, so 75x is still good (as Ragnarok said, Larda used 200x without problems).

[Frozen]

Some time has passed and I have done lots of progress on the cannon:

There are some holes to be drilled and tapped so that I can get all the bolts screwed in but this is basically what it will look like. I am still not sure if I will weld the barrel in place or if I will make some kind of bolt clamp or something to keep it in place.