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"In the world of spuds today"

A place to ask general spud cannon related questions.
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Unread postAuthor: Ocheeboy11 » Tue Dec 13, 2011 9:53 pm

I have heard that there are major issues with the algorithms that the Gun Design Tool's use. Additionally, I have heard that a user who I know as "Ben" is currently developing a new simulator.
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Unread postAuthor: DYI » Wed Dec 14, 2011 9:52 am

As far as I know, Ocheeboy, btrettel (Ben) is developing a new pneumatic gun model at the moment based on the conservation equations in 1D. Doing a proper first-principles approach to what HGDT does is considerably more involved, due to the chemical reactivity to be considered, and the fact that the gas can no longer be considered a perfect thermal insulator. This makes both the equation of state and the conservation equations describing the flow field much more complicated if tackled by a continuum approach, and particle approaches are still too slow to be of much use for "spudgun" purposes.
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Unread postAuthor: MrCrowley » Thu Dec 15, 2011 10:23 pm

ITWOST,

Tried to do some more chrony work with my small hybrid but this time with a 3.5g s/s ball bearing. Had to go to a location a few km away to do the test.

First test: fail.
I had a plastic sheet thingy in front of and behind the chrony to catch the sabot material but it seems to have changed the path of the projectile as there is a clear entry hole but no exit hole and the chrony displayed 0000, (which is normal, but I had waved my hand over it previously so it displayed a number. It reverts to 0000 if only one sensor is interrupted) so that was a fail.

Went home to get more material, came back. Burst disk failed to burst and this some how jammed tight the union, I couldn't unscrew it with my hand. So back home I went.

Back for the third time, I'm all out of aluminium disks so I have to use tin foil. Can't get it to seal at over 80PSI, one of the o-rings looked damage so I swapped it out and tried again but the tin foil was too crinkly to seal again.

Went home, didn't return. Perhaps I'll try again tomorrow, HGDT says I should be looking at over 2000fps so I'm keen to chrony it ASAP.
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Unread postAuthor: jackssmirkingrevenge » Thu Dec 15, 2011 10:34 pm

MrCrowley wrote:I had a plastic sheet thingy in front of and behind the chrony to catch the sabot material but it seems to have changed the path of the projectile as there is a clear entry hole but no exit hole


Sounds like you''ll be shooting your chrony sooner than we thought :)

Sabot material is likely to follow the projectile through the hole in the plastic anyway. What sort of material are we talking about, and what's the discrepancy between barrel and projectile size?

My tests have shown that anything short of a strong lightweight sabot yields miserable results from a high pressure device.

Compare fail to win, fail being a composite foam/balsawood/duct tape sabot, win being a two piece machined delrin sabot:

[youtube]http://www.youtube.com/watch?v=o5WDmgFREWY[/youtube]

[youtube]http://www.youtube.com/watch?v=b7geiqhE1GA[/youtube]
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Unread postAuthor: MrCrowley » Thu Dec 15, 2011 10:45 pm

jackssmirkingrevenge wrote:Sounds like you''ll be shooting your chrony sooner than we thought :)
I have come within like 2cm of hitting it :D

jackssmirkingrevenge wrote:Sabot material is likely to follow the projectile through the hole in the plastic anyway. What sort of material are we talking about, and what's the discrepancy between barrel and projectile size?
Yeah I expected some of it would but was hoping most of it wouldn't. I found some small pieces on the other side of the plastic and a big piece caught in the hole with most of the sabot having been blown to the side of the plastic screen.

Difference between barrel and ball bearing is about 10mm I think (10mm ball bearing, 20mm barrel).

jackssmirkingrevenge wrote:My tests have shown that anything short of a strong lightweight sabot yields miserable results from a high pressure device.

Compare fail to win, fail being a composite foam/balsawood/duct tape sabot, win being a two piece machined delrin sabot:
This is the main problem I face. As you probably remember, I was alerted to the importance of sabots back when I shot a .303 round through my garage door :D

I'm using several cotton balls with one cotton ball having a small hole cut in to the middle for the ball bearing. Definitely not an ideal sabot, just the only thing I had on hand that would discard quickly.

Any ideas for a better sabot?
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Unread postAuthor: jackssmirkingrevenge » Thu Dec 15, 2011 11:24 pm

MrCrowley wrote:I'm using several cotton balls with one cotton ball having a small hole cut in to the middle for the ball bearing. Definitely not an ideal sabot, just the only thing I had on hand that would discard quickly.


booooooooo...

Any ideas for a better sabot?


Since I had a few minutes to kill before work, here you go :)
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Unread postAuthor: MrCrowley » Fri Dec 16, 2011 12:04 am

Ah yes I have seen that before, pity I didn't think of it earlier today. Do you know what the discarding is like with a sabot such as that? I wouldn't want to fire more than 2m away from the chronograph but that might be too close for a sabot such as that.

My dad needed something from a store nearby so I went back to the place to try again but since I couldn't find anything sabot worthy, I found some rubber thing with a hole in it that fit in the barrel. I put the ball bearing in the hole and stuffed the back of the hole with cotton ball. I wasn't planning on the 'sabot' discarding, so the projectile weight is probably closer to 10g.

I got a reading of 1200fps at a 9x mix, which seems pretty poor; even for a 10g projectile with cotton wool hanging out the back. There were little pieces of cotton wool covering the chrony as well, so there could be interference. Looking at the replay, the muzzle blast engulfs the chrony and pushed back grass up to 2m behind it. The barrel was also on an angle pointing down, increasing the distance between the sensors. Not exactly ideal chrony conditions.

I think the best sabot that can be trusted to discard is either a 3 or 4 piece light-weight sabot with a plastic screen in front of the chrony and the muzzle 2m away from the chrony.
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Unread postAuthor: jackssmirkingrevenge » Fri Dec 16, 2011 12:52 am

MrCrowley wrote:Ah yes I have seen that before, pity I didn't think of it earlier today. Do you know what the discarding is like with a sabot such as that? I wouldn't want to fire more than 2m away from the chronograph but that might be too close for a sabot such as that.


If you cut slits in the front part with any luck it will do this:

[youtube]http://www.youtube.com/watch?v=kMac29qi49w&feature=related[/youtube]

Some more high speed footage of shotgun wads slowing down, you can clearly see the shot go through the target before the wad reaches it.

[youtube]http://www.youtube.com/watch?v=nK9ev2ZZXLI&feature=relmfu[/youtube]

The combination of larger drag and poor sectional density should allow the projectile to go through both sensors before the sabot reaches it.

Good idea to have a high speed on the chrony display, in case the chrony interprets the sabot as a second shot and the result is lost.

I got a reading of 1200fps at a 9x mix, which seems pretty poor


Don't expect any better without a "proper" sabot.

I think the best sabot that can be trusted to discard is either a 3 or 4 piece light-weight sabot with a plastic screen in front of the chrony and the muzzle 2m away from the chrony.


Again, make friends with someone local that has a small lathe and some acetal/UHMWPE to spare ;) isn't there a craigslist equivalent in Kiwiland?
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Unread postAuthor: MrCrowley » Fri Dec 16, 2011 1:42 am

If you cut slits in the front part with any luck it will do this

:idea:

The combination of larger drag and poor sectional density should allow the projectile to go through both sensors before the sabot reaches it.

Good idea to have a high speed on the chrony display, in case the chrony interprets the sabot as a second shot and the result is lost.

How fast can the chrony compute results? The first time seems to take a few seconds to count up to, for example, 1200fps. But I've also heard of people using it to figure out RPM leading me to believe it can compute quick semi-auto firearms or such.

I noticed that waving my two hands over the sensors, one hand after the next with a gap between the two hands, it would compute the first hand only and ignore the second. It must be wired in a way that it only calculates from the first interruption on the first sensor to the first interruption on the second sensor with a default time set before it is allowed to compute a new result. Can't find any data from the chrony website about it though.

Again, make friends with someone local that has a small lathe and some acetal/UHMWPE to spare isn't there a craigslist equivalent in Kiwiland?
Na, not really. I could try some forums.
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Unread postAuthor: jackssmirkingrevenge » Fri Dec 16, 2011 2:29 am

MrCrowley wrote:How fast can the chrony compute results? The first time seems to take a few seconds to count up to, for example, 1200fps. But I've also heard of people using it to figure out RPM leading me to believe it can compute quick semi-auto firearms or such.


The advanced models can certainly do it:

[quote="USER’S MANUAL
FOR SHOOTING CHRONY® CHRONOGRAPHS,
MODELS: BETA and GAMMA SHOOTING CHRONY®S and
BETA and GAMMA MASTER CHRONY®S
"]When in Timer Mode, the Chrony is automatically also in Rapid-Fire Mode, and able to record Time and Velocity at a rate of 1800 rounds/minute. [/quote]

Some good advice from the f1 manual:

because flying wads may go in any direction, protect the face of the Chrony by taping a thick piece of clear acrylic over the front of the unit. The acrylic must not extend above the face. Shooting Chrony Inc. offers such protection (see Accessories List). NOTE: When shooting without Diffusers one may substitute soda straws or wooden dowels for the Wire Rods. Hitting the Wire Rods with a bullet could break the Sensor Housings, whereas straws and dowels provide a safer sighting guide
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Unread postAuthor: MrCrowley » Fri Dec 16, 2011 9:47 pm

ITWOST, back to the same place for more testing.

This time I tried your sabot idea, Jack. It looked like it would work well and only doubled the weight but I don't think they discarded from the ball bearings. Even though I cut the end four times to try and make it flare out upon leaving the barrel but it doesn't seem to have worked; I got two shots in both at around the 1200fps mark. HGDT says 1700fps for an 8g projectile so this sounds about right considering the severe drag on the projectile and 2m distance to chrony.

Unless someone else has got sabots working, perhaps they can only be used when the chrony is at least 5m from the muzzle. Not only does that make aiming difficult but it would give a reading a fair amount less than the actual muzzle velocity.

Perhaps if I get someone to help me I can film some high speed videos to see what's happening with the sabot, although I'm doubtful it will be clear enough to interpret much.

What I will do now is find a light projectile that fits the barrel and weighs similar to the ball bearing, it should give fairly similar results.
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Unread postAuthor: jackssmirkingrevenge » Sat Dec 17, 2011 3:46 am

Have you thought of adding something to the muzzle to slow down the sabot?

Drill say three holes about 5mm from the end of the muzzle and put in three short bolts, that project say 10mm or less into the barrel.

This won't stop the sabot, but it should slow it down enough to allow projectile separation.

The disadvantage I see is that the sabot will disintegrate into a cloud of chaff :D maybe a that screen is a good idea.
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Unread postAuthor: SpudBlaster15 » Sun Dec 18, 2011 6:10 am

ITWOST, I picked up 10ft of 1.5" galvanized steel EMT conduit for ~$20.

The interior is fairly smooth, the weld bead on the seam is almost nonexistent, and the (very conservative) calculated yield pressure is about 3KPSI, so a 4' section should make a nice hybrid barrel.
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Unread postAuthor: ramses » Sun Dec 18, 2011 4:40 pm

SpudBlaster15 wrote:ITWOST, I picked up 10ft of 1.5" galvanized steel EMT conduit for ~$20.

The interior is fairly smooth, the weld bead on the seam is almost nonexistent, and the (very conservative) calculated yield pressure is about 3KPSI, so a 4' section should make a nice hybrid barrel.



I'm very sure you meant a 10' section. :D
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Unread postAuthor: SpudBlaster15 » Sun Dec 18, 2011 4:57 pm

Maybe if I had one of these to transport the cannon. :lol:
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