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What's your Holy Grail of Spudguns?

A place to ask general spud cannon related questions.
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Unread postAuthor: Fnord » Fri Jan 04, 2008 12:17 pm

About piston opening times...

Believe it or not, the opening time/pressure CAN be controlled very easily; you just need to look at the forces involved at ignition.

For example, my piston has a diameter of 2" and a seat area of 1.23 inches^2, and needs 93 lbs of force to open it.
The non-seated piston area is 1.91. This means I will need 49 psi acting on this area to open the valve.
You can cause a piston to open at higher pressures by reducing the diameter; a 1.5" diameter piston would open at 173 psi, and a 1.4" piston would open at 300 psi.

Obviously, a heavier piston will open more slowly, but I'd rather have a light piston that opens at the exact pressure I want it to. I made a new piston that was about 30% lighter after the 6x incident, but I found this piston actually decreased performance. which didn't make much sense. I guess the heavier piston held back the gases longer, helping negate my low opening pressure.
But anyway, After making a bumper out of solid rubber I haven't had any problems at all after dozens of 6x firings.

(Oh and for a 20x-30x hybrid, I wouldn't try using a piston valve. Well, I mean, I would, but I'm just crazy)
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Unread postAuthor: Ragnarok » Fri Jan 04, 2008 1:42 pm

DYI wrote:@Rag: It seems like you have a lot of interesting projects on the go. I look forward to seeing the combustion BBMG, and the gauss gun, as well as your modified compressor. After a little research, I think I've figured out exactly how it will work.

No, I have a lot of interesting ideas for projects, I usually only have one, maybe two on the go at once.

The method of operation depends on which version of the compressor I build. I'll probably cook up a super-charged fridge compressor that should be able to put out a respectable 1 scfm each, and I could easily bolt 3 or 4 together to get real flow rates - Which could fill HEAL to 30 bar in under 10 seconds, not bad going.
I could then hitch that up to a couple of empty CO2 fire extinguishers I have around to give it some volume so I could run higher flow things like BBMGs off it for a short period of time.
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Unread postAuthor: SpudBlaster15 » Fri Jan 04, 2008 2:30 pm

_Fnord wrote:Believe it or not, the opening time/pressure CAN be controlled very easily; you just need to look at the forces involved at ignition....

....You can cause a piston to open at higher pressures by reducing the diameter; a 1.5" diameter piston would open at 173 psi, and a 1.4" piston would open at 300 psi.


So altering the piston diameter is a very easy way of changing the opening pressure of the valve? Just how would you reduce the piston diameter in the manner you suggest without rebuilding the entire valve?
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Unread postAuthor: Fnord » Fri Jan 04, 2008 3:37 pm

You could easily modify an existing piston valve by simply sleeving a smaller pipe inside the back of the valve and making a piston to fit that.

If there is something I'm not seeing here, correct me, but I think that will work.
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Unread postAuthor: SpudMonster » Fri Jan 04, 2008 4:01 pm

DYI wrote:@Spudmonster: Unfortunately, it is very unlikely that any conventional (i.e. non detonation) hybrid could achieve hypersonic (Mach 5+) muzzle velocities. The speed of sound in a post combustion hybrid chamber is about Mach 3. However, I don't know how the massively higher combustion pressures in a 30x hybrid would affect things. And you wouldn't need to machine a custom union, McMaster sells unions rated for 3000 psi in up to 4" sizes. And you wouldn't need a union any bigger than 2" for a golfball barrel.

I still don't really know if I will try a detonation gun. While the novelty would be nice, I suspect that the hypersonic capabilities would only be possible with very light projectiles, which would get carried along at the same speed as the shockwave. And for the amount the machining would cost to build it at a useable size, I could build an uber-powerful coilgun with Mach2+ injection speeds to achieve higher efficiency. (seriously, the cost of machining and parts for a decently sized detonation gun would be in the thousands :shock: )

@Rag: It seems like you have a lot of interesting projects on the go. I look forward to seeing the combustion BBMG, and the gauss gun, as well as your modified compressor. After a little research, I think I've figured out exactly how it will work.


According to the websites I found on google, propane has a maximum combustion temperature of over 2800 degrees centigrade, or over 4400 degrees Fahrenheit. According to http://hyperphysics.phy-astr.gsu.edu/hb ... ouspe.html that calculator, the speed of sound at 2800 degrees F is over 5 times that of the speed of sound at 75 degrees F.

As for the unions, the reason I would consider remachining a 3,000 PSI union is to increase the area that grips the disc to keep if from slipping out due to the 300+ PSI pre-combustion pressure, and also because the thickness of the disc would probably prevent full thread engagement of the union nut if it wasn't modified.
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Unread postAuthor: Ragnarok » Fri Jan 04, 2008 4:22 pm

SpudMonster wrote:According to the websites I found on google, propane has a maximum combustion temperature of over 2800 degrees centigrade, or over 4400 degrees Fahrenheit. According to http://hyperphysics.phy-astr.gsu.edu/hb ... ouspe.html that calculator, the speed of sound at 2800 degrees F is over 5 times that of the speed of sound at 75 degrees F.

Aside from the fact that GasEq will only quote me a speed of sound of about 1000 m/s (three times the speed of sound at 20 deg C) for a 20x propane air combustion, you are forgetting that you'll lose a lot of heat from the gasses, so I wouldn't hope for more than Mach 3 from any propane/air hybrid. Propane/oxy, perhaps, that has a speed of sound of about 4x the SOS at RTP.

A high mix hydrogen powered hybrid might just pull off hypersonic because the gasses are less dense, but that's only if it didn't do an impression of a dustbin full of nitroglycerine being dropped from a plane.

I believe hypersonic is the realm of light gas guns, electromagnetic power, and lastly plasma launchers, an exclusive club to which propane hybrids aren't welcome.
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Unread postAuthor: Novacastrian » Fri Jan 04, 2008 4:55 pm

Can i join, my Hybrid uses Butane :D
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Unread postAuthor: Ragnarok » Fri Jan 04, 2008 5:04 pm

Novacastrian wrote:Can i join, my Hybrid uses Butane :D

Hardly, butane hybrids will have a lower speed of sound again.

Maybe if you can make a 100x hybrid, you would go hypersonic... when the police sent you to a cell so fast your ears would be left behind.
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Unread postAuthor: jrrdw » Fri Jan 04, 2008 5:39 pm

I want something in a 1" bore, runs off of compressed air and cracks like a rifle without chipping the end of the muzzle a way little by little as I shoot.
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Unread postAuthor: Spitfire » Mon Jan 07, 2008 3:57 pm

Supersonic Golfballs! nuf said
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Unread postAuthor: Zen/// » Sat Jan 12, 2008 11:51 pm

A portable marble/bb/pellet gun that shoots 500m+ accuratly, and still have enough power to turn soda filled cans inside out.
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Unread postAuthor: Ragnarok » Sun Jan 13, 2008 2:15 pm

If it can shoot 500 metres, you won't be wanting for power to destroy drinks cans.
With my current cannon I could probably shoot over 500m using a pretty shallow trajectory if I had the right ammo, and I could still split drinks cans in two with my old cannon - which was only about half as powerful.
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 13, 2008 5:52 pm

pretty shallow trajectory


I think it would be more howitzer-like if you want to reach 500 metres.
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Unread postAuthor: benstern » Sun Jan 13, 2008 9:37 pm

THIS
'nouf said.
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Unread postAuthor: Ragnarok » Sun Jan 13, 2008 9:44 pm

@JSR: Not really, if I get my 30 bar mods done, with HEAL and a dart, a trajectory of 3 degrees would get it out to 500 metres, or so GGDT and my range calculator tell me.

The range calculator is pretty accurate, I can't fault it's predictions, but it still has a few last bugs to handle before I'm happy about releasing it. Sorry to keep it from you for so long, for that I apologise, but I want to get it all as streamlined as I can first, I'm a bit more of a perfectionist than my relatives.
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