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Mjöllnir - 1.5" porting Handheld Piston Hybrid

Built a hybrid cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: MrCrowley » Thu Dec 08, 2011 4:17 pm

Sure, no worries. I had filmed it about a year ago at this same place but the cannon has changed so much since then that I've been meaning to re-film the process.
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Unread postAuthor: SpudBlaster15 » Thu Dec 08, 2011 4:26 pm

MrCrowley wrote:Bad idea putting a pressure gauge on the pilot, obviously the piston compresses the pilot air to well over 300PSI which caused my gauge face to be blown off and the gauge to read 30PSI by default.


I had this problem until I increased the pilot vent area to ~0.4in<sup>2</sup>, then all issues with piston bounce and blown gauges disappeared. Still might be a good idea to use some type of isolation valve though. A small high pressure check valve or shut off valve should do the trick.

After the first shot the pilot air started leaking in to the chamber, I assume from a failed o-ring.


This happened to me quite often when I was using the ABS piston in the first version of my piston hybrid, and I ended up determining that it was a result of insufficient o-ring compression. The newer design achieves ~20% compression around the circumference of the o-ring, and it has never leaked, despite several failures of the center rod.

Not sure if your issue is related to this, but it might be worth looking into.
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Unread postAuthor: MrCrowley » Thu Dec 08, 2011 4:33 pm

SpudBlaster15 wrote:
After the first shot the pilot air started leaking in to the chamber, I assume from a failed o-ring.


This happened to me quite often when I was using the ABS piston in the first version of my piston hybrid, and I ended up determining that it was a result of insufficient o-ring compression. The newer design achieves ~20% compression around the circumference of the o-ring, and it has never leaked, despite several failures of the center rod.

Not sure if your issue is related to this, but it might be worth looking into.
Hmm interesting. This is the first time it has ever happened to me, it may have something to do with when I cleaned up the piston back when I changed the sealing face (though, the cannon has been through a dozen shots since then). I can't remember whether I removed or added teflon tape around the washers to push the o-ring out more for better compression. The piston also slides back far too easily so it's possible I've actually lost an o-ring. I'll see if I can open it up today and narrow down the problem. I just hope the epoxy isn't leaking as there's no way to leak test a piston.
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Unread postAuthor: mobile chernobyl » Thu Dec 08, 2011 6:07 pm

MrCrowley wrote:I'm quite excited about trying this, though it's unfortunate that most of my projectile have sabots that may screw up the reading. Lemons could disintegrate sending some pieces faster or slower than the main piece, golf balls need a sabot, D-Cells have sabots that, although not discarding, may break apart upon launch.


I had considered this and I'm planning on setting up a few different test barriers to shoot through and try and catch the sabot's for items other than golf balls for mine, or just when I would like to get a better barrel seal.

I'm considering a sheet of 1/2" low density insulation foam with the backside covered by vinyl adhesive as the catch barrier.

The idea is that this shouldn't hinder the speed of the projectile all that much but it should be able to divert the gasses and catch a very lightweight sabot or wadding.

We'll have to see lol. :)

::EDIT::

Well a catch screen is good for accurate shot readings but having consumable sky screen supports also seems like a good idea should anything fragment before entering the pathway, as a form of insurance for the near hundred dollar speed reader lol. Check out this thread
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Unread postAuthor: artillerycreations » Thu Dec 08, 2011 6:58 pm

Just curious as to if you use a bumper of any sort for the rear of your piston to protect the pilot area from shock.
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Unread postAuthor: DYI » Thu Dec 08, 2011 8:20 pm

This has probably been answered somewhere already, but the video brought the question back to mind: My ME20H-41 operated in almost exactly the same performance envelope as Mjöllnir, with similar chamber volume, pressure, and barrel diameter. It was very loud, and produced quite a pall of smoke after a shot. I recognize that a bad enough camera can make Krakatoa erupting sound like a potato falling off the counter, but there seems to be a nearly complete absence of smoke in all your shots. SB15 may have some insight here; does the oxygen enrichment I used make that much difference in terms of the visible and audible qualities of the muzzle blast? I wasn't using mixes any richer than what you would be using (roughly 5% chamber volume in propylene per X).

Mjöllnir appears quite "tame" in comparison... :roll:
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Unread postAuthor: MrCrowley » Thu Dec 08, 2011 9:53 pm

DYI wrote:This has probably been answered somewhere already, but the video brought the question back to mind: My ME20H-41 operated in almost exactly the same performance envelope as Mjöllnir, with similar chamber volume, pressure, and barrel diameter. It was very loud, and produced quite a pall of smoke after a shot. I recognize that a bad enough camera can make Krakatoa erupting sound like a potato falling off the counter, but there seems to be a nearly complete absence of smoke in all your shots. SB15 may have some insight here; does the oxygen enrichment I used make that much difference in terms of the visible and audible qualities of the muzzle blast? I wasn't using mixes any richer than what you would be using (roughly 5% chamber volume in propylene per X).

Mjöllnir appears quite "tame" in comparison... :roll:

SB15 did get some massive muzzle blasts (flames) when using oxygen, they were quite impressive. Since it doesn't seem oxygen makes all that much difference in cannons like SB15's or Mjöllnir, I'm not worried about muzzle blasts or sound output if it doesn't change the performance.

This cannon is loud, seriously loud. You can barely hear the echo in this video but in my previous video up at the same location you could hear the tearing sound of the golf ball through the air or the echo of the cannon as loud as a distant thunder or a low-flying Cessna aircraft. Would there be as many as three distinguishable sounds heard when firing a cannon like this (sonic boom, cannon 'boom' and the echo of the cannon 'boom')? Or is the sonic boom mixed with the cannon report? In that case, the tearing sound of the projectile would be another one to add to the list.

In this video you can see I walk hurriedly away, grabbing the camera with one hand and the gun in the other to go chuck it in the garage and close the door. I do this because it could easily be mistaken for a bomb going off (most farmers would know it doesn't quite sound like a rifle but holidaymakers wouldn't). Last year when I was at this house firing the cannon, the 'neighbours' 750m away could be seen standing on their porch looking at us.

I think it's typical of most cameras to not be able to record such reports accurately, although I have seen a few videos that have done well. Wind and acoustics may be bigger factors than you account for as I can easily notice the difference when firing in to the wind, in to a valley, at a fence or in to the air. As you can hear in the video, it was pretty gusty during filming.

As for the smoke, not sure. I've had lots of smoke in the muzzle blasts on some shots (lemon shooting at the mangroves for example) but not much in these shots, one could put that down to the aforementioned fuelling issue resulting from the piston leak.

Now that I have a chronograph I can be sure of the performance of this cannon regardless of whether it has a large muzzle blast or not.


Cheers for the link and ideas, MC. My other option was trying to find a ball that would fit the barrel and weigh similar to a golf ball.

artillerycreations wrote:Just curious as to if you use a bumper of any sort for the rear of your piston to protect the pilot area from shock.
Yeah, I do. It is a weird array of rubber stuff. I'll try take a photo today when I inspect the piston if you want.


Edit:

As suspected, the rear piston o-ring had failed. The back large washer for the rear o-ring and the back large washer for the front o-ring have also come loose and are being epoxied back together although they are not required for a seal (as long as the forward washers are sealed and the o-ring seals against the pipe wall, no air should pass through).

It appears I may need to re-think my piston has the fault for the o-ring coming loose is either due to the large washers not being a large enough diameter (about 1.5mm diameter smaller than the housing) meaning the o-rings can come off and jam, causing them to fail, or the piston is compressing the bumper enough to get the rear o-ring out of the pipe housing allowing the o-ring to come off when re-seating the piston. I believe the problem is the former and not the latter but there is no way to fix it except for maybe some epoxy (which might do more harm than good).

So, add a little epoxy to the outside of the o-rings or just let them be?

Image

Picture of the bumper for anyone who was curious.
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Last edited by MrCrowley on Fri Dec 09, 2011 12:43 am, edited 1 time in total.
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Unread postAuthor: artillerycreations » Fri Dec 09, 2011 12:29 am

Interesting bumper. Have you checked the diameter of your piston cylinder. If it isnt very uniform you will have trouble getting an oring to seal whether the washers are close to the diameter or not.
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Unread postAuthor: MrCrowley » Fri Dec 09, 2011 12:34 am

I believe the bumper is also similar to the one SB15 uses in his cannon (one of the many things I have borrowed from him in this design).

As for the cylinder, it is uniform as it is a galv. iron barrel nipple although the seam has been removed. I know the seam isn't the problem as previous to the shots in the recent video, the piston was, more-or-less, 100% air tight.

It doesn't help that the washers aren't centred perfectly as well. They came in different outside diameters but the same inside diameters. The I.D. of the washers are about 15mm or so while the bolt they go through is 10mm so it was difficult keeping them all centred while I epoxied it all together.

On another note, anyone familiar with the practice of keeping a part that has epoxy drying on it in your room so you can rotate it throughout the night/day to stop epoxy dripping? :D
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Unread postAuthor: artillerycreations » Fri Dec 09, 2011 12:40 am

I dont know if you guys in New Zealand get different grade stuff from what we do in the states but pipe here isnt as uniform. I checked the id of a sch 80 seamless pipe nipple and it varried from one end to the other a bit.

It could be there is a low spot where the force due to the pressure acts on the oring unevenly enough to eventually weaken it and blow it out.

What I did when i had to cast some epoxy was use duct tape to help it keep shape. The adhesive will let go after it has set up.
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Unread postAuthor: MrCrowley » Fri Dec 09, 2011 12:48 am

Hmm, I don't have anyway to check that as the pipe nipple has a fitting on the other end and is too much of a P.I.A. to take off. How much of a difference did you observe? The o-ring compresses fairly significantly so it might be able to counter any diameter changes.
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Unread postAuthor: artillerycreations » Fri Dec 09, 2011 12:55 am

Sch 80 1 1/4 id 1.267 and 1.255 for each end. Sch 80 1 1/2 id 1.479 and 1.490 for each end. I think that would be enough to blow my o-rings out. Im getting the 1 1/2 machined to 1.5 all the way through. If my washer and bolt setup similar to yours doesnt work then i will have an aluminum one machined.
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Unread postAuthor: MrCrowley » Fri Dec 09, 2011 1:05 am

You're talking about one end of the fitting being a different diameter than the other end of the fitting, right? If so, I'm still not sure if it would be a problem as that's less than half a millimetre difference which my o-rings can account for in compression/decompression.

I think the large washers being slightly too small are a significant factor for this problem, whether they cause it or not they could prevent it if they were a better diameter. I could probably wrap them in a layer of tape and see if it makes much difference.

It could be there is a low spot where the force due to the pressure acts on the oring unevenly enough to eventually weaken it and blow it out.
This has me thinking you might be saying the actual pipe diameter is not uniformly round and not that the diameters are different at each end. If that's the case, I believe this could be a problem.

It's interesting I haven't had this problem 'till now, perhaps the o-ring was just worn.

If you can get a piston machined for less than $50USD, I'd recommend taking that route. Much easier in my opinion.
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Unread postAuthor: artillerycreations » Fri Dec 09, 2011 1:15 am

Not only diameter different end to end but also around the inner curcumference of the pipe. My sch 80 "seamless" still has high and low spots. I will probably end up going with a machined piston at some point for overall ease of use. I wanna try to keep as much of the work in house as possible at first though.

One main thing you should remember is were dealing with pipe and not tube. Not only that but we use washers to create a piston with a threaded rod. The outside diameter of the washers isnt necessarily round or exact. It sure makes for a challenging environment. The fact that you and a few others have overcome that and created a working gun is pretty good.
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Unread postAuthor: SpudBlaster15 » Fri Dec 09, 2011 3:06 am

DYI wrote:SB15 may have some insight here; does the oxygen enrichment I used make that much difference in terms of the visible and audible qualities of the muzzle blast?


From testing my own hybrid, I've noticed that oxygen enhanced shots tend to produce much louder and sharper reports than ordinary air/propylene mixes, even if the predicted adiabatic chamber pressures are roughly equivalent. It's possible that the higher temperature and subsequently the higher SOS of the exhaust gas is resulting in the formation of a pressure wave with greater intensity, thereby increasing the quality of the report.

As for the visual aspect of the muzzle blast, I haven't noticed any significant differences in smoke output across the board, but mixes containing >40% oxygen often produce very large quantities of visible flame. Here's a short clip of one such shot:

Air/O<sub>2</sub>/Propane Blank Fire

MrCrowley wrote:SB15 did get some massive muzzle blasts (flames) when using oxygen, they were quite impressive. Since it doesn't seem oxygen makes all that much difference in cannons like SB15's or Mjöllnir, I'm not worried about muzzle blasts or sound output if it doesn't change the performance.


The appeal of oxygen mixes is more so the lower preiginition pressures required to achieve similar performance, as well as the substantial reduction in pumping effort.

MrCrowley wrote:It appears I may need to re-think my piston has the fault for the o-ring coming loose is either due to the large washers not being a large enough diameter (about 1.5mm diameter smaller than the housing) meaning the o-rings can come off and jam, causing them to fail, or the piston is compressing the bumper enough to get the rear o-ring out of the pipe housing allowing the o-ring to come off when re-seating the piston.


I still suspect it to be an issue of insufficient o-ring compression, or perhaps your o-rings are simply too small and are being sucked through the gap between the pipe wall and the circumference of the large washers.

To give you an idea of what I've found to be a reliable setup, my piston uses 1.75" diameter washers to form the inner circumference of the groove, leaving a gap of (2.067-1.75)/2 = 0.16" for the ~0.19" thick o-ring to squeeze into. Even though the 2" diameter washers are somewhat smaller than the 2.067" diameter pipe, the seals have never slipped and never leaked.

MrCrowley wrote:On another note, anyone familiar with the practice of keeping a part that has epoxy drying on it in your room so you can rotate it throughout the night/day to stop epoxy dripping? :D


Yep. :lol:
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