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My First Spud-gun. Failsafe question?

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: sjog » Fri Nov 02, 2007 10:35 pm

This started with a fail safe question .
The projectile leaving the barrel is the fail safe.!
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Unread postAuthor: spudgunnerwryyyyy » Fri Nov 02, 2007 11:38 pm

Listen to turbo spudder. If you are using downpipe i am guessing it is abs. when you apply the cement to pipe pipe apply very liberally. It is easy to under applicate the glue.
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Unread postAuthor: BigGrib » Sat Nov 03, 2007 1:05 am

Dude from what I hear you need to scrap the whole deal right now with the exception of the ignitor. Go to your local plumbing supply company and spend like $20 to get yourself some rated PVC or ABS and fittings and the proper glue/solvent combo and do this thing right. That's all the advice I have for you
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: spudgunnerwryyyyy » Sat Nov 03, 2007 1:09 am

ABS sold in stores isnt usually pressure rated but because it is softer than pvc it can withstand a combustion. But on pneumatics only use nsf-pw pvc. ABS is actually preffered for combustions because it can withstand the pressure spike better than pvc. So either use abs or nsf-pw sch40 pressure rated pvc.
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Unread postAuthor: BigGrib » Sat Nov 03, 2007 1:47 am

Definatley agreed. I just think the stuff he's got now needs to be junked
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: jimmy101 » Sat Nov 03, 2007 9:04 am

Like TurboSuper said, the only accetpable glue for PVC is PVC glue. The only acceptable glue for ABS is ABS glue.

And, the glue should be used with the appropriate cleaner and/or primer.
And, you need to get the pipe the full depth into the socket.

There have been a couple posts recently on other glues to use with plastic pipe. Don't believe them. Though it is true that other glues (like epoxy) can be useful for sealing around threadedholes, or attaching handles to a gun etc., they should never be used as the primary glue in the construction of the gun.

Read the directions on the glue can and read the link posted earlier on the proper way to glue plastic pipe.

You should be able to glue up a closed chamber from PVC that will handle the ~120 PSIG peak pressure of propane+air. With a barrel and a high friction ammo the maximum pressure in the chamber will only be about 60 PSIG.

Lubricating the ammo really shouldn't be needed and will decrease the performance of the gun.
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Unread postAuthor: Ragnarok » Sat Nov 03, 2007 9:14 am

I find I get measurable performance increases with a clean lubed barrel, over a barrel that's just been left without.

While static friction isn't a bad thing, as it allows a little pressure to build up, most friction in the barrel will be dynamic, which is undesirable.
I'd happy lose the performance advantages of static friction to lose the larger disadvantages of dynamic friction.

Suggesting that less friction will harm performance must imply that more will help it, which is quite clearly wrong.
If the friction were high enough to hold the projectile still against the pressure, performance would be zero.

EDIT: Horrific grammar - sounded like I was half drunk.
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Last edited by Ragnarok on Sat Nov 03, 2007 10:53 am, edited 2 times in total.
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Unread postAuthor: TurboSuper » Sat Nov 03, 2007 10:47 am

jimmy101 wrote:Like TurboSuper said...


Thanks ^.^
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Unread postAuthor: jimmy101 » Sat Nov 03, 2007 3:00 pm

Ragnarok wrote:I find I get measurable performance increases with a clean lubed barrel, over a barrel that's just been left without.

While static friction isn't a bad thing, as it allows a little pressure to build up, most friction in the barrel will be dynamic, which is undesirable.
I'd happy lose the performance advantages of static friction to lose the larger disadvantages of dynamic friction.

For a combustion gun, I would say that it is pretty well established that more static friction, up to a point, improves performance.

Care to back up the comment on dynamic friction being a significant detremental force in a combustion gun? Typically, the WAG is that dynamic friction is half the static friction. A very high friction spud in a 2" barrel might have 30 pounds static force (~10PSI as a pressure). Dynamic friction would be ~5 PSI, not really all that significant compared to the ~50 PSI peak pressure. Besides, without the high static friction the spud is long gone before you get anywhere near the peak pressure.

Ragnarok wrote:Suggesting that less friction will harm performance must imply that more will help it, which is quite clearly wrong.
If the friction were high enough to hold the projectile still against the pressure, performance would be zero.

Yep, more friction, up to a point, will improve performance of a combustion gun. A zero friction round will perform significantly less well than one with more friction.

Besides, friction is usually highly correllated with how well the round seals the barrel. I will gladly take the minor affect of 10 to 20 pounds of dynamic friction in exchange for a tightly sealed barrel.

Similarly, a low mass round will perform less well than a high mass round. Again, because the combustion process is slow and it helps a lot if the movement of the ammo is delayed (by either high friction or high mass).
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Unread postAuthor: blackosprey » Sat Nov 03, 2007 3:15 pm

So much for staying on the topic of fail-safes. And I agree with sjog, the best failsafe is going to be the projectile leaving the barrel, unless you manage to find a pop-off valve that's got a bigger diameter than your barrel :shock:
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Unread postAuthor: BigGrib » Sun Nov 04, 2007 3:28 am

Do you really think that you can be a pop off that big and that effective if it did need to go off and what pressure would you set it for. I mean even with a meter that pressure spike is difficult to keep consistent dont you think??
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Yea, that's definitely going to get you at least a tazer.

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Unread postAuthor: Ragnarok » Sun Nov 04, 2007 10:23 am

@jimmy: My logic is as follows.

Lets say it takes a millisecond for combustion pressure to rise 10 psi - slow, but it's a number. Also, 36" long 1.5" barrel, 100g spud.

Comparing a round with 20 psi static friction against one with 10 psi:

The 10 psi round will take 1 ms to start moving at the beginning. By your rule of thumb, the dynamic friction is 5 psi. In the 1st millisecond (before the 20 psi friction round were to move), the spud will move a sum total of 0.4mm (0.015") from the force. If the pressure rises faster, this distance is even less.

Both systems now have equal pressures, and only difference is that the spud in the 10 psi one has moved a few fractions of an inch. Force x distance shows that the energy losses will be higher in the high friction barrel.
Now, the high friction spud will stay in the barrel longer, which would allow pressures to rise more again, but by the simple linear model I'm going by, the energy is still lower - not massively so though, my numbers give a 2.5% loss for the higher friction barrel.

But speeding up that pressure spike, and putting a crude heat loss representation into the mix, the difference gets quite a bit larger. Depending on what exact numbers I put in, losses can grow to over 10 to 20% for twice the friction.

It's a very simple model, but even for slow rising pressures, I cannot see how extra friction is beneficial.
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Unread postAuthor: jimmy101 » Mon Nov 05, 2007 12:05 pm

Ragnarok, well though out reply.

Lets take an extreme case. A zero friction round versus a 30 pounds static, 15 pounds dynamic friction round.

For zero friction, the round starts to move as soon as there is any pressure rise at all. That means you are "consuming" barrel length with very little pressure actually working on the ammo.

For the round with friction the ammo won't start to move until the pressure can overcome the static force. For 30 pounds static in a 2" barrel that works out to be about 10 PSIG. Once the round starts to move the friction drops to 15 pounds and the "cost" of the dynamic friction is ~5 PSIG. So, with these friction values, when the projectile starts to move there is at least 5 PSIG actually pushing it.

The average pressure pushing the spud in a typical combustion spud gun is about 30 PSIG.

The second factor is the burn rate. If the projectile starts to move at low pressure then the movement of the projectile expands the volume of the chamber which slows down combustion. If the movement of the projectile is delayed then when it does start to move the rate of change of the combustion versus time is at a higher value. In other words, the combustion process is accelerating at a higher rate if movement of the ammo can be delayed a bit so that the chamber temperature can rise.

It is easy enough to estimate the energy budget for a combustion gun with and without friction. I did it a long time ago and 15 pounds dynamic friction in a 2" barrel represents less than 1% of the total energy in the chamber. Basically, work = force x distance where the force is the frictional force and the distance is the barrel length. For 15 pounds of dynamic friction, 2" D barrel that is 3' long, the work is about 15 foot pounds (20 Joules). Latke's L1 cannon (2600cc chamber) generates about 10,000 joules of energy if you assume there is no heat loss. Latke's muzzle velocities give a kinetic energy of the ammo of about 1400 joules (combustion spud guns are only 10~15% efficient). The 20 joules "consumed" by the dynamic friction is really not significant compared to the 10 KJ in the chamber or 1.4 KJ that actually ends up in the projectile.

This is one of the many questions in spud gunning where some actual numbers would be nice. You say your gun fires at a higher muzzle velocity with a lubed barrel. Others have posted in numerous places that a double beveled spud cutter, which significantly increases the frictional forces, improves the performance of combustion guns.

I don't believe anyone has ever posted actual muzzle velocities as a function of frictional forces.
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