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Alright, I'll tip my hand a bit...

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 7:25 pm

DYI wrote:It would be great if there was some way to take pictures of GGDT outputs that was easier than actually photographing it with a digicam and then uploading it to Photobucket.

Ummm... You are aware that Windows allows you to put a screen capture of the active application using the Print Screen button, right? Seriously, it's as easy as copy/paste.

(I'll look at that scenario)
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 7:27 pm

pat123 wrote:I found one other thing I kept everything exactly the same except i changed the mix. At 1x it says 346 ft/s and at 3X it says 277ft/s. wouldn't 3x be alot faster?

Not necessarily....especially when barrel combustion isn't incorporated yet.
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Unread postAuthor: DYI » Fri Mar 28, 2008 7:33 pm

Ummm... You are aware that Windows allows you to put a screen capture of the active application using the Print Screen button, right? Seriously, it's as easy as copy/paste.


I've seen the button, but never really understood it's function. Amazing how it could have escaped me for that long...

So any ideas about the negative range?
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 7:41 pm

DYI wrote:Example:

Gun using:
-1x12" chamber
-.3" porting barrel sealing piston valve with 2 ounce piston, 0.25" porting,
1ci pilot volume
-848.3 psi hydrogen at 80 degrees fahrenheit
-0.2401" x 72" barrel
-0.24", 0.2 gram projectile

Predicted muzzle velocity: 3 250 fps.
Predicted maximum range using drag coefficient of 0.4 (perfect sphere I believe): -1 feet

There must be some problem there.


OK, bug identified and fixed. A new version is up on the GGDT webpage for download.

Strictly speaking there wasn't really a bug... What was happening was you were shooting a fast projectile that was very light. Insane drag forces change things very quickly. I was using 0.01 s as the calculation interval for the range calculator. With your scenario.... Well, if the initial drag numbers were maintained for a full 0.01 s, the velocity is negative! That's how fast things are changing!

So I lowered the calculation interval to 0.001 s and everything's good.



edit: I should also point out that the range calculator isn't really intended for supersonic. I play some games tweaking the user-inputted Cd as the projectile goes through transonic and low supersonic regimes but they ARE kind of BSy. Truely supersonic projectiles DO require a more elaborate drag model than the one I've incorporated. So don't just take those results with a grain of salt, take them with an entire salt lick!
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Unread postAuthor: jimmy101 » Fri Mar 28, 2008 8:05 pm

I look at it as if there are three volumes in the chamber; (1) the unburned volume near the breech, (2) the burned volume and (3) the unburned volume on the muzzle end. For this example, all three volumes need to increase by 1% so that the fraction burned stays unchanged. (Fuel can't be "unburned" just because the total chamber volume has increased.) That means the breech end flame front moves towards the muzzle. [...] Of course, the math gets to be a PITA, especially when the flame front actually moves into the barrel.

I considered this approach for a long time and eventually went in a different direction as it seemed like the math would get nasty when multiple ignition points where thrown into the mix. In the end I went with a different approach that is admittedly less accurate for the single point ignition scenario but should be more flexible.

Ya, I've realized the same thing. With multiple sparks it gets real ugly real quick. I figured the only way to do it without driving myself nuts is to model the chamber as discrete volumes then scan over each volume for each time step and see if it is within the current flame front radius for any of the spark gaps. This would be an easy way to handle the flame fronts when they overlap since a particular volume element can only be burned once. That apporach would be easy to code but will probably be real slow. Currently, I'm kind of limited because I'm too lazy to take my model out of Excel and put it in a real programing language.

The other thing I do is that once the spud starts to move the Tmax and Pmax have to be rescaled to take into account the expansion. In a gun in which the fuel is still burning when the projectile is moving the temperature and pressure will never reach the closed chamber Tmax and Pmax values.

Obviously. This phenom is at the heart and soul of GGDT.
<strike>Actually, it isn't quite the same. For GGDT Pmax and Tmax occur at time zero. For a combustion gun it occurs at some later time. P<sub>i</sub> and T<sub>i</sub> rise then fall. For a pneumatic P<sub>i</sub> and T<sub>i</sub> are always falling. Pmax and Tmax start out as maxima and fall as the chamber expands. Of course, Pmax and Tmax are really just bookkeeping devices.</strike>

EDIT: Ignore the section striked out. I was confused but I get it now. GGDT uses Pmax as the chamber pressure and P<sub>i</sub> as the pressure pushing the projectile. The difference between the two is a function of the gas flow through the valve, the pipe etc. well duh.
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 8:13 pm

jimmy101 wrote:Actually, it isn't quite the same. For GGDT Pmax and Tmax occur at time zero. For a combustion gun it occurs at some later time. P<sub>i</sub> and T<sub>i</sub> rise then fall. For a pneumatic P<sub>i</sub> and T<sub>i</sub> are always falling. Pmax and Tmax start out as maxima and fall as the chamber expands. Of course, Pmax and Tmax are really just bookkeeping devices.

It really is the same. The only difference is whether you're adding energy to the system or subtracting it or doing both at the same time. The thermodynamics are the same; it's just a superposition. Seriously, neither HGDT nor GGDT use Tmax or Pmax values even as a book keeping device. I track them strictly because people like to see the results, but nowhere do those terms show up in the programming logic that matters.
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Unread postAuthor: D_Hall » Fri Mar 28, 2008 10:00 pm

Going through minor reported bugs.

I'll keep editing this post as I find/fix them.

When done I'll make notice of updated file uploaded.

Carlman wrote:when you change the configuration to metric the inner diameter of the chamber stays measured in inches

and when you configure settings for projectile mass it doesn't show anything other than ounces although the number changes

Fixed (both of 'em).

_Fnord wrote:I don't get an error/warning when I set the burst pressure below the mix pressure.

Fixed.

jimmy101 wrote:How about putting the version number in the GUI somewhere?

Done.


Uploaded. The "guts" are unchanged at this moment. Changes are simple corrections to the issues quoted in this post.
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Unread postAuthor: D_Hall » Sat Mar 29, 2008 1:16 am

OK, I'm still evaluating ways to deal with combustion in the barrel. The new program architecture has allowed me to see some things in ways I wasn't seeing them before. That's nice as it opens my eyes to some different methodologies. For the moment I'm just digesting it (or at least, attempting to).

In the meantime, I amused myself by adding a new feature that I believe will be very useful for those who care about the plotted data.... You now have the ability to manually zoom in on an area of the plot. So if there's something interesting happening but you're having troubles seeing it because it's only happening in a small section of the plot? Hey, no worries! It's a bit clunky, but it works.

File has been uploaded.
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Unread postAuthor: starman » Sat Mar 29, 2008 2:37 am

Just downloaded the program update. This one seems to be usable for the most part...the first one had too many conversion bugs to do anything useful.

Awesome tool!! It's great to be able to change parameters and watch the effect on performance. What was eye opening to me was the effect a stronger burst disk had on even small mixes. Seems logical to me now...I'm sure obvious to hybrid vets.

I had a relatively small 3x mix and a 2 ounce projectile going super sonic with a long enough barrel and a 150 psi disk...I suppose that's the trick though, finding 150 psi burst disks and a way to mount them reliably!

I'm seeing the advantage of burst disking even 1x advanced combustions, something I have already been experimenting with. A rather modest 30 psi disk will gain you a very healthy muzzle velocity, varying of course on other factors. A fairly simple addition for a big gain.
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Unread postAuthor: SpudFarm » Sat Mar 29, 2008 8:48 am

3x SOS? :shock:
that has to be a BIG cannon..
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Unread postAuthor: starman » Sat Mar 29, 2008 9:08 am

spudfarm wrote:3x SOS? :shock:
that has to be a BIG cannon..


Yeah, I just plugged in the dimension numbers (~225 ci) for my advanced combustion to see what popped out. Projectile was a heavyish golf ball, barrel 160" long, and started running up the burst disk number. There's a massive amount of energy in there, just being able to tap it before the ball leaves the barrel is the key...high pressure disk allows that.

Even a 2x mix with the right disk was a very impressive 3/4 SOS or so. Makes me think all these efforts to accomodate 5x and beyond may not be necessary...higher x mixes typically lost performance when nothing else changed. Just need a biggish chamber to do it....
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Unread postAuthor: Jared Haehnel » Sat Mar 29, 2008 9:18 am

It makes a lot of sense of you build a large pneumatic the barrel is usually very long if you want to get the best performances. Often times in excess of 240 inches...

If you want a smaller gun to achieve better results your going to have to up the mixes. 160 would be clumber some but if you upped the mix to 6x you could shorten the barrel and still get the same performance from a longer barrel at a lesser mix
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Unread postAuthor: DYI » Sat Mar 29, 2008 9:43 am

I used a 1 000 psi burst disk for most of this - that's quite close to what 96 layers of alu. foil would burst at in a 2" union, give or take 100 psi or so. Higher mixes are always better, unless you're using really pathetic burst disks.
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Unread postAuthor: D_Hall » Sat Mar 29, 2008 10:26 am

starman wrote:I'm seeing the advantage of burst disking even 1x advanced combustions, something I have already been experimenting with. A rather modest 30 psi disk will gain you a very healthy muzzle velocity, varying of course on other factors. A fairly simple addition for a big gain.

Do keep in mind that I'm not currently barrel combustion. On a hybrid that bursts right at max pressure, this will have zero effect on muzzle velocity. On simple combustion with a light weight projectile it will have a huge effect. Between those two extremes are shades of gray, of course.
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Unread postAuthor: D_Hall » Sat Mar 29, 2008 2:35 pm

OK, folks...

Barrel combustion is being modeled now. New version is 0.1.0.

I'm sure there are still a lot of problems (haven't even started to address the scenarios already presented in this thread that gave severe problems), but I think I'm a lot closer to the finished product than I was last night.

Interesting result I'm seeing so far: As long as all fuel burns before the projectile clears the muzzle, velocity is pretty constant regardless of burst diaphrams. Kinda makes sense as we're talking about similar energy releases.... But does this match observations?

Also, I'm seeing optimal C:B ratios that depend upon projectile mass. This makes sense in that heavier projectiles spend more time in the barrel and as such lose more energy to their surroundings. HOWEVER, I suspect my dependence is stronger than it should be.

Again, still a lot of work ahead....
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