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Hi guys, just drew-up a design and would like to hear what you guys think. Constructive criticism, suggestions, and flaming are heavily encouraged!
This launcher is a hybrid type using a classic piston valve design and propane. I designed it to be constructed with relatively few parts with minimal expensive machining. This is a very large and heavy cannon that should be extremely powerful and loud. It will need to be mounted. A good feature for this cannon is that you should be able to use it with only compressed air, combustion only, or hybrid. The blow-off valve will limit the maximum pressure of air or gas/air.
Assume 100psi operating pressure for now. No burst disks anywhere.
Barrel : 12ft sch 40 1.5" nom 304 steel
Chamber : 2ft sch 80 6" nom 304 steel
Design is: http://postimage.org/image/jcliowbu9/
Piston is an Al 6061 cylinder 4" in diameter, 1.5" long, and has o-rings and rubber gaskets. There is a weak spring to help seal the barrel. The pilot valve should be a ~110psi (?) quick release valve with a manual release for air-only firing
Loading mechanism consists of a heavy Al cylinder bolt that is locked in place by 4 x 5/8" removable steel rods. 2 o-rings to seal the bolt assembly. This way the barrel can be removed and reconnected easily.
Mount : 8.5ft Al I-bar with wood mounts and supports screwed-in through drilled holes. suports barrel and chamber. Maybe wheels?
Metal parts will be connected with JB weld, 5/8" screws, and threadlock. I was hoping to use only 5/8" holes and taps (or their metric equivalent) throughout to minimize costs. To hold the inserts to the pipe I was thinking at least 6 5/8" screws around the 6" pipe.... This is the part that I am most unsure about. Thoughts?
The back plate and spacer is removable to take-out the piston and change o-rings and gaskets. All gas inlets are heavy ball valves rated to at least 2000psi on steel.
Chamber volume (loaded) is 6.28L
Barrel volume is 4.8L
~6.7x @ 100psi
~1.3 chamber:barrel volume
This should be LOUD when fired on hybrid. I realize that the ch:b ratio is excessive, but I would like good pressure for air-only. Adding gas would require hearing protection I'm sure. I would consider a longer barrel.
What do you guys think? I'm thinking super-sonic potatoes that can smash cinder blocks. Do any of you veterans see anything wrong here? This would be my 5th cannon, but I'm a newb when it comes to hybrids and metal. Suggestions? Keep in mind I would like to keep this project less than $1500 if possible. Also, does anybody know a good small order Chinese supplier for the metal?
Get rid of the spring, It wont survive. I had a ridiculously strong spring that wouldn't budge with my whole 190 pounds on it and it got absolutely munted on the first shot.
You don't mention how the back plate is secured in place. I would definitely go for something that goes around the outside of the chamber so the bolts can be secured perpendicular to the piston rather than a back plate flush with the chamber with bolts secured parallel with the piston. The reasoning for this is obvious, bolts secured along the same 'axial' as the force exerted from the piston are more likely to come flying out than bolts secured perpendicular to the force; if you get what I mean.
I haven't done the calculations but you're probably going to need a lot more than 100PSI in the pilot if your pre-ignition pressure is also 100PSI (an 8x mix pretty much). An 8x mix will generate ~800PSI if the piston is held back long enough, with only 100PSI and twice the surface area of the piston (compared to the front) keeping the piston back, it will probably actuate prematurely and you wont be getting the most out of the design.
What kind of high flow blow-off valve do you have in mind? To successfully port a 2" diameter piston valve, SpudBlaster15 and I both needed a port diameter of about ~20mm, I assume you'll need something more than that for a ~4" diameter piston; unless your pilot volume is extremely small. If the porting isn't large enough, you'll get piston bounce which will eventually damage the sealing face and other components of the piston.
Oh and potatoes don't survive inside the barrel and freezing them usually doesn't help much.
Thanks for your input Just a few questions if you don't mind...
About the pilot pressure: I see what you mean about having a higher pilot pressure. What would you suggest for for pilot pressure if the chamber pressure was 100psi? I suppose I wouldn't need the spring if I did it that way. Hmmmmm...
About the bolts: The end cap will be bolted (parallel to the piston) to the heavy Al cylinder inside, which is in turn bolted (perpendicular to the piston) to the 6" pipe. I hope that makes sense. It would require a lot of force to shear 6-8 heavy steel bolts at once, right? I could always add more bolts too...
About the pilot valve: I was going to worry about that a bit later, but you're saying a 1/4" flow wouldn't be enough? Keep in mind there won't be much dead space behind the piston. maybe I could do a burst disk with thick foil or scale-up to 3/4" maybe? The displacement of the piston is about 145mL with maybe 50mL dead space after that.
Thanks for your help, I appreciate a second opinion!
Exactly, I only ever used a spring as a bumper, not a means to help push the piston forward. To calculate how much pressure you will need in the pilot, you just need to do some basic math.
Calculate the surface area of the piston and then calculate the surface area of the barrel (using outer diameter of the barrel to account for wall thickness). Remove the barrel surface area from the piston surface area and this is the area on the front of the piston that the combustion gases will act upon (lets call it FSA (frontal surface area). 100PSI pre-ignition pressure is pretty much an 8x mix which will generate about 850PSI (roughly). If you calculated the FSA using inches, multiply the FSA by 850 pounds per square inch. You now have the force acting on the front of the piston. Now, go back and get the number for the surface area of the piston; this is the surface area that the pressure in the pilot will push against and because there is no barrel in the way it will have more surface area to act upon than the combustion gases. Multiply this surface area number by a pressure (e.g. 300PSI) until it matches the force output of the combustion gases on the FSA; it should be less pressure than the 850PSI required by the combustion gases. This gives you a rough idea of how much pressure you need to put in the pilot.
Sounds alright but I'm not good at this engineering stuff, another member should be able to give you a better idea of how safe this is.
145ml could be about the same amount of pilot volume in my piston valve as well. You can always stick with the original pilot valve and change it if you start getting problems.
You might have trouble stopping such a large piston in such a small area. A 4" diameter piston will have roughly 10,000 pounds of force acting on it at a 8x mix and if it is made from aluminium it will be quite heavy. You need a bumper that can absorb the impact so the piston isn't damaged but it also needs to stop the piston from bouncing; which can happen with a rigid bumper or a bumper that is too weak (where the piston bounces of the backplate).
Thanks again for your detailed reply! "850PSI" Uh yeah... that sounds like it would be very dangerous to anything behind it! I would have to do some better designing for those kinds of forces. I was generous with the piston travel distance and diameter, but perhaps I will do a redesign.
I had it in my head earlier that having the piston wider would open it faster, and that most of the fuel would burn as the projectile accelerated, with the maximum force being delivered at the end of the barrel, rather then at the breach.
I can see that what you're suggesting would deliver a more powerful shot. However, do you think a 120psi pilot pressure would still not be satisfactory for hobby purposes? I'm looking for something that's fun, loud, and powerful... but I'm not trying to break any records or anything. I really thought I was going overkill without extra pilot pressure, 850psi sounds like it would be bananas.
I'm gonna do some research and brainstorming and I'll post in a day or two
Well if your barrel had an outside diameter of 2" and your piston was 4" in diameter...
Area of piston face = (π2^2) - (π1^2) = 9.42in^3
Area of piston rear (the area which the pilot pressure acts on) = (π2^2) = 12.56in^3
Assuming an 8x mix generates 850PSI:
Force on piston face = 850 x 9.42 = 8007lbs
So you would want the pilot pressure to have at least 6000lbs of force on the rear of the piston for optimal performance.
6000/12.56 = 477PSI.
If you only used 120PSI in the pilot, you'd only have 1500lbs pushing the piston against the barrel while the pre-ignition pressure would be pushing against the piston with 1250lbs of force. Basically, as soon as the mix is ignited the piston will fly back and the combustion of the fuel will be incomplete in the sense that either all the gases wont ignite or that they will ignite in an 'open system' where the gases can expand out the barrel without increasing pressure significantly. In other words, you'll have quite bad performance at an 8x mix with only 120PSI in the pilot.
If you want it so that the pressure in the pilot is less, you need to make the barrel outside diameter much larger. This could be as simple as welding/epoxying a wider diameter adapter to the end of the barrel which will act as the seat. So you could use the 1.5"nom diameter barrel with a 1.5"nom - 2"nom diameter adapter so the sealing face area is much less.
At the moment, your barrel would take up 1/4 of the surface area of the face of the piston. In my piston hybrid, the barrel takes up more than 2/3 of the surface area of the face of the piston; meaning I only need about 300PSI maximum in my pilot for a 10x mix.
Well it all depends on how much pressure you have in the pilot. You must remember that in a hybrid piston we are trying to achieve different goals to that of a pneumatic piston. In pneumatic piston valves, you want the valve to open as fast as possible because the pressure is already stored and ready to go. In a hybrid, we need to wait until the combustion reaches peak pressure before allowing the valve to open. Best way to do this is by having enough force keeping the piston closed until peak combustion is reached and its force is greater than the force keeping the piston closed (and any friction), then and only then is the goal to have the valve open ASAP; this is achieved by a high flow pilot valve.
For when you run this cannon as a pneumatic, just put the same pressure in the pilot as the pressure in the chamber. To actuate the piston, you can vent the pilot volume by opening the 'fill' valve that is used to fill the pilot volume with air pressure. On my cannon, I have a 1/4" ball valve that closes off the fill valve (a quick connect), I can use this ball valve as the pilot valve if I run the cannon as a pneumatic.
Again, killer advice man!
I will redesign the piston and go with the 2/3 barrel to piston I think. That makes much more sense. I'll post a scan when I'm done with some numbers too, may take a bit though.
I looked at some of your cannons, very nice!
Ok, here is a better piston, i hope?
This design is roughly to-scale, each blue line is 1/4"
I added a flange to the breach to reduce piston area, and reduced the piston to 1" long. I'm also thinking of using high-impact plastic instead of Al.
The bolts are shown, but will be offset in reality (so that they don't run-into eachother). That's 12 perma-bolts to hold the insert, and 6 removable to hold the backplate.
I know how to calculate the pilot pressure now, but I'm curious as to how you calculated an 8X mix producing 800-850psi? At 100psi, isn't it only a 6.9X mix? I looked through some calculators but maybe I'm not searching for the right one? Thanks again for your help.
100psi/ 14.5psi = 6.90 ?
You're limiting the flow quite a bit there, how about having the piston protrude forward a bit more ?
I agree with al-xg; try have the piston come further out by cutting an inch or so off the barrel. Apart from that it looks good.
Well it's a general calculation that you multiply the mix number by 100 to get the resultant combustion pressure but I find that as you get to higher mixes you can push the resultant combustion pressure slightly higher. For example, HGDT will say there's 1100PSI in the chamber at a 10x mix if the valve is set to hold back to late as possible.
100PSI is just short of an 8x mix. You should know that for a given mix (let's called it n), you inject n-1 atmospheres of air. So for an 8x mix, you only add 14.5PSI x 7 in air as there is already an atmosphere of air inside the chamber. So 14.5 x 7 gives us a little over 100PSI and then on top of that you would have added ~5.7PSI of fuel giving a resultant pre-ignition chamber pressure of about 108PSI.
One mistake a lot of people make with hybrids is not accounting for the chamber pressure when just fuel has been added. People read that they need about 101PSI of air for an 8x mix so they pump up the cannon until it reaches 101PSI; not realising that they've only added 95PSI of air as the other ~5.7PSI (dependent on fuel used) is the fuel.
Ok thanks guys
About the limited flow... I know that it looks restricted from the drawing, I thought so too, but the flow area is still over twice as much as the inner barrel area (4.3 in^2 and 2.0 in^2 respectively). If I beveled the Al block a 1/4" around it would be more then enough, I thought. Do you still think it's too small?
Area is one thing, but changes in direction, sudden changes in diameter, sharp edges will all have an effect on the flow.
A bevelled edge would be good, but is there an issue with moving the barrel seat slightly forward ? I'd do it any way.
Oh also, I'd go for O-ring seals on the piston housing and plug, it'll be more reliable than a gasket.
Thanks for helping out a noob, you guys.
I will bevel the edge and extend the piston.
I just realized that going all-aluminum may be cheaper and easier than steel. 6" sch 40 Al is about 1/4 the price of sch 80 steel and could do 1000PSI easily, much lighter too. I imagine drilling 30-40 holes through sch 80 steel would be quite a chore. Any thoughts?
I'd go with the aluminum since it's lightweight and strong. Much easier to machine too. Save the money. That's just what I'd do
Hell ya, 6061 Al is cheap! I didn't realize that it would be as strong as steel so this will probably save me $500.
I promise to post pics when things start to come together more! Maybe if you're lucky there will be some test shot vids in maybe a month or 2.
Any last suggestions before I start ordering parts? I thought you folks would be asking for a longer barrel or different locking mech? No?
I'm going to do the machining and build the cannon and mount first, the pilot valve, electronics, and gas fittings will come later. I'm reconsidering burst disks for the pilot valve so that different mixes may be tried, I'm guessing multiple exhaust valves are expensive, where different burst disks could be used for different mixes.
I've modified the specs on my original design to include:
-pilot valve for 8x mix that triggers when chamber pressure reaches 750-800psi
-smaller piston (1" x 4")
-barrel seal/rest (for ~2/3 barrel/piston area and lower pilot pressure)
-all aluminum for cost, weight, and machinability, chamber is still good to 2500psi working pressure if I'm not mistaken.
-extruded piston and beveled edges to maximize piston valve flow
-o-ring on removable end cap
I'm getting pretty excited about this folks, one rough calculation I did earlier today predicted a muzzle energy of 8.2 X a 12gauge shotgun slug. Sounds like fun I wonder what would happen if you fired a pound of metal bb's at a delapitated shed or a liter of gasoline? Stay tuned
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