Registered users: Bing [Bot], Exabot [Bot], Google [Bot], Majestic-12 [Bot], MSNbot Media, Yahoo [Bot]
Who is online
In total there are 37 users online :: 6 registered, 0 hidden and 31 guests
Most users ever online was 218 on Wed Dec 07, 2016 6:58 pm
Registered users: Bing [Bot], Exabot [Bot], Google [Bot], Majestic-12 [Bot], MSNbot Media, Yahoo [Bot] based on users active over the past 5 minutes
I downloaded GGDT version 4.6 today and was playing around with it a little. Funny thing I noticed was when you changed your chambers od the characteristics changed for the volume and fps and the like.
Can someone explain this to me? Am I doing something wrong?
I read the help file and found out that it did not mean od of the combustion chamber So now that is clear.
Also I need some program to help me figure out what size combustion chamber I need and when compared with my barrel the chamber size is to large and wasting pressure after the shot is out the barrel. Also it would be nice to be able to figure out what my peak combustion pressure is based on the propane mix I am using. I will probably be doing a 10x or so.
Yes. It's not necessarily entirely intuitive, but chamber OD is the internal diameter of your chamber.
Chamber ID is only used when modelling a co-axial configuration, and is the outer diameter of the barrel running through the chamber. If you are not using such a configuration, it should be zero.
If you read this D_Hall, I would probably advise making a couple of changes for the next version. Simply calling Chamber OD "Chamber Diameter", and Chamber ID "Coaxial diameter" would help make this common confusion less common.
Also, if you're modelling hybrids or combustions, you need HGDT, not GGDT.
Does that thing kinda look like a big cat to you?
I am looking for HGDT right now but I have not seen it yet. Do you have a link?
Yes! I have just found it!
Now, do you know where I can get some information on ignited propane pressures for different x charges? Or is ther a formula that you can work out to determine the peak pressure of the ignited mix?
ps: what is a spud bux?
One of HDGT's graphs shows you that. However, as a rough rule of thumb, you can expect a peak pressure of about 7 bar (100 psi) per X.
<s>They ward away evil sprites.</s>
They're not much use. They can buy you (see here) signature privileges or a custom user title, but after that, there's not much more to them other than demonstrating quite how much of a nerd you are*.
*Given that they're based on word and post count, having a lot of them demonstrates you spend far too much time on the forums.
Does that thing kinda look like a big cat to you?
As said, it's displayed on there (peak chamber pressure). This is also largely a function of your burst disk or piston pop pressure and projectile friction.
You can also use GasEq to calculate the theoretical peak pressure for any propane + air mix at any reasonable starting pressure.
For propane in air at 1X the peak theoretical pressure is 136 PSIA (9 ATM). That peak pressure more or less just scales with the X of a hybrid. So a 3X gun will peak (theoretically) at 3x136=408PSIA (390PSIG). (GasEq say 421PSIA).
In practice you can't get the pressure as high as the theoretical pressure. Even with a closed chamber heat is lost very quickly, which drops the maximum pressure. In a functioning gun movement of the ammo also drops the peak pressure.
GasEq says that at 3X the peak closed chamber temperature is 4364F (2406C). Gases have pretty low heat capacities so it doesn't take much heat transfer to the chamber to significantly drop the chamber pressure.
Thanks alot guys I am finally understanding hoe to use SGTC, very nice tool. I am downloading GasEq right now and will try that out.
I think I have got it all now, Thaks again for ya'lls help.
Another thought, In SGDT, how accurate do you think the barrel length is calculated? I mean I was doing some calculations with the probram and a 3 cubic inch chamber at 1000psi combustion pressure and the fps kept increasing to like 240 inches or so, what do you think?
Spud Gun Technology Centre? (Better known as Spudtech) You're mixing your acronyms a bit.
I would take any result from a simulation with a pinch of salt. I've written calculators of my own, and they almost always give me velocity good to within about 2%.
However, even then, I don't inherently trust them. There is the underlying principle of GIGO - Garbage In, Garbage Out. Basically, a simulation's upper limit on accuracy is as accurate as the information it was given (imperfections in the program will worsen that of course).
Given the likelihood that your numbers are not entirely accurate, the answers it will give you will not be perfectly accurate either.
I'd expect it's in the right ballpark. However, I wouldn't expect any more of it.
You can try, but bear in mind the impracticality of a 20 foot long barrel before you start...
Does that thing kinda look like a big cat to you?
For the sake of safety, note that detonation can cause a shock wave of pressure that exceeds the theoretical combustion pressure and cause barrel or chamber failure at higher mixes.
This is often seen as a pressure spike in a barrel that causes a local blowout or deformation. This is often not uniform in shape like you would expect from just a pressure blowout.
nuked a dup
Last edited by jimmy101 on Mon Jul 12, 2010 7:49 pm, edited 1 time in total.
Though with propane in air at reasonable X the chances of getting detonation are zero.
Besides, failure spec's are different for a shock wave than they are for static pressure. Heck even a hunk of PVC pipe rated at say 200 PSI will take an air shock pressure that is at least ten times that pressure. Shock and static pressure are not the same and the failure characteristics are substantially different for the two. In a shock event you have to take into account the duration of the shock wave (typically much less than 1/1000th of a second), the distance the chamber wall must deform before it will fail, the energy required to accelerate the mass of the chamber wall, etc.
Small scale detonation events (like "pinging" in a gasoline engine) generate fairly wimpy shock waves. They'll significantly exceed the static pressures but the duration is extremely short.
Ping events are fairly short. With continued operation, the local area may break down and fail. Here is a piston that failed due to pinging near the spark plug. A single ping does not do this. Over time the material fails with the beating. You can do the same thing with a small ball-peen hammer
It is true pinging rarely occurs at lower mixes.
Yeah sorry I am getting mixed up a bit, I meant SGDT.
I am not thinking of making a 20' barrel, I was just wondering if those calculations were correct.
Tech: I was just thinking about the shock wave effect the other day when I was calculating how much pressure I neede my chamber to be able to withstand. Thanks for the thoughts. I am looking at a max mix of 10x, and normaly using 7x. I am thinking of building the chamber out of 304 stainless steel and it will be 1-1\2" OD x .188 wall thickness. The calculated yield pressure can handle 2500 psi with a safety factor of 4.
NO! or at least, not with PVC
Some materials can take the spike better than static, but some will shatter with spikes well below what they can take statically.
DYI's (I Think) ETG produced peak chamber pressures he estimates to be 2x greater than the theoretical failure pressure ( SF=1). He got a bunch of plastic deformation, but nothing exploded. It was made of metal, either SS or brass, which tolerates shock fairly well. The mass probably allowed for some inertial confinement, which can in some cases contain fusion pressures in nothing more than a very low pressure gas.
PVC on the other hand, doesn't like shock. It is brittle.
Case in point: brittle things don't like shock. Ductile things don't mind shock too much, but fatigue and fail eventually.
We have to be careful because we are drifting into discussion of things which are generally treated differently. Smacking a piece of PVC with a hammer produces a shock load. It is a fairly mild total load that is concentrated on a small area and the local pressure can be extremely high. A hammer blow could easily put as mush as a couple thousand PSI onto a small spot of the PVC. That is in excess of what the PVC will handle but it is also much higher than what the PVC is rated for in static pressure. If you hit the PVC with a hammer but don't exceed it's pressure/in^2 rating it'll probably be fine.
I wasn't referring to that kind of shock though. I was referring to the shock wave produced by a detonation event. That type of shock wave is supersonic (often tens to hundreds of times the speed of sound). The duration of the pressure spike depends on how it is generated. A detonation event produces a single shock event with a very short duration. Over pressures of tens of thousands of PSI can be produced but the damage those waves produce is much less than the damage that a static pressure of that same amplitude would produce. So, in general, when talking about shock wave loads the effective pressure at failure of say a container is typically much high for a shock wave than it is for static pressure.
I suspect that "ping" events in an ICE produce local shock waves of perhaps 10,000 psi, compared to the "static" peak pressure of what many hundred PSI? The shock peak pressure probably pushes, or even greatly exceeds what the engine is designed for but it still takes many (probably thousands) of ping events to do any damage to the engine.
So it all depends on what you mean by "shock". A hammer puts a very high force on a very small area (a high pressure) but the event also lasts a "long time" (relatively speaking), perhaps as much as a tenth of a second. A detonation shock wave often produces much higher peak pressures but the duration of the shock wave is very short, I would expect it is typically less than 1/1000 second. At a sufficient short shock duration the mass of the pipe starts to contribute significantly to the strength of the pipe. Not because mass=strength but because mass="pipe doesn't want to move" and it can't fail if it doesn't move (deform, stretch...). With a static pressure (defined as any pressure that lasts for say a tenth of a second) the mass of the pipe is irrelevant since there is more than enough time for the pipe to deform until it fails.
So, if a spud gunners concern is DDT giving a shock load in excess of the pipes rating then they need to think carefully about exactly what the shock wave failure load of the pipe is. It generally is not the same as the static pressure failure pressure and is usually considerably higher.
Personally though, I would say that no spud gun, regardless of what it is made of, should be operating at DDT. As tech pointed out with the car piston, a single event might not be a problem but if you keep repeating it you will fatigue the pipe and it's rating will drop. (Most cars ping occasionally, very few cars blast a hole through a piston of throw a bearing or do any of the other things that a chronically pining engine will do.)
Who is online