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GGDT v4.5

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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GGDT v4.5

Unread postAuthor: D_Hall » Fri Apr 18, 2008 5:54 pm

Just uploaded.

Changes....

1 - "Efficiency" removed from output. It was wonky from the beginning and I don't think anybody cared much anyway.

2 - The ballistic calculator now uses dynamic integration intervals. Should be a bit more reliable than the old while maintaining fast computation times.

3 - A manual rescale ability has been added to the output plots (identical to the HGDT code).

4 - Improved accuracy for long barreled systems. Not perfect, of course, but it should be better.
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Unread postAuthor: starman » Fri Apr 18, 2008 6:16 pm

Thanks for the update. GGDT and HGDT are both awesome tools!
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Unread postAuthor: benstern » Fri Apr 18, 2008 6:45 pm

YES!!!! We've been waiting for an update for so long!! Thanks!!!!
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Unread postAuthor: D_Hall » Fri Apr 18, 2008 6:50 pm

Don't get too excited. The changes are minor. Mostly they're just incorporations of logic used in HGDT into GGDT.
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Unread postAuthor: Ragnarok » Fri Apr 18, 2008 7:27 pm

Looks pretty good to me. The manual re-scale might prove useful.
I do sometimes wonder about the efficiency of my launchers, but I tended to do that calculation myself as I knew the output on GGDT was problematic.

While we're on the subject - you're probably aware of this, and I don't want to cause a problem (I recognise there is a lot of work put into doing something like this, and I'm wary of saying this lest I sound ungrateful) but I often find GGDT can give quite "generous" velocities around or above the sound barrier.

When I compare GGDT results above around 300 m/s to real world data, I often find they're quite a bit (although not hugely) higher than my home-made chronograph reports.
It's possible that the difference is partly related to my chronograph, which isn't completely accurate, and I might not have perfect inputs for GGDT - but the real world and simulated information tend to match quite well at more modest velocities.

More occasionally, outputs are generous enough my understanding tells me it's impossible (results of higher than the particle speed in the accelerant gas), although that only really arises when I'm being a bit slapdash with parameters for "daydream cannons" (Low mass projectiles, high pressures, multiple valves).

Is there a future fix planned for this?
It's not really a huge concern for me, as I just tend to use GGDT for a "ballpark figure", but it would be nice to see an adjustment for that.
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Unread postAuthor: pat123 » Fri Apr 18, 2008 7:29 pm

just wondering how accurate is hgdt in the supersonic range.
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Unread postAuthor: D_Hall » Fri Apr 18, 2008 7:39 pm

pat123 wrote:just wondering how accurate is hgdt in the supersonic range.

That's totally unknown at the moment, but it should be much more accurate than GGDT. Not because it uses any improved theories, but rather because combustion guns utilize hot gases which in turn means that the internal ballistics are safely subsonic (assuming we're talking about reasonable guns and not super light projectiles and such).
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Unread postAuthor: Ragnarok » Fri Apr 18, 2008 7:41 pm

@pat123: Well, I couldn't tell you, but my concern was about supersonic effects in the propellant gas, not relative to the atmosphere.

With the heated gasses of a hybrid, supersonic would be closing on 3000 fps, depending on exact heat losses.
So, it's unlikely there will be a concern like I have here for HGDT.

As for HGDT's natural accuracy? Well, it seems to meet Latke's results reasonably well, but as D_Hall has said, you should bear in mind, it is still a Beta release, so things are still being ironed out.
As with anything else, Time will tell on this matter - as more test data and feed back arrives...

EDIT: Ah, beaten to it. Ignore me.
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Last edited by Ragnarok on Fri Apr 18, 2008 7:43 pm, edited 1 time in total.
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Unread postAuthor: pat123 » Fri Apr 18, 2008 7:42 pm

it says that both of the hybrids that i have built are supersonic. you can see the results in my sig. that is with a .1 oz projectile though.

edit: ok so by the time the gases temperature gets back to normal it wouldn't be going that fast anymore? wouldn't it still be going faster then the speed of sound though?
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Unread postAuthor: Ragnarok » Fri Apr 18, 2008 7:56 pm

That's a very light projectile... It might be ok, but my instinct says that's probably outside what is likely to be an accurate range.

Bear in mind, although simulations are very useful, and many are pretty accurate, you shouldn't rely on the results being right - that goes double when the program is a relatively early beta version.
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Unread postAuthor: D_Hall » Sat Apr 19, 2008 10:46 am

Ragnarok wrote:When I compare GGDT results above around 300 m/s to real world data, I often find they're quite a bit (although not hugely) higher than my home-made chronograph reports.
It's possible that the difference is partly related to my chronograph, which isn't completely accurate, and I might not have perfect inputs for GGDT - but the real world and simulated information tend to match quite well at more modest velocities.

Mind forwarding this high speed data to me? That's the hardest part for me: Getting my hands on real-world data. Without it, I can't make any realistic efforts at improvement for the scenarios in question as I've got no way to gauge which theories are working and which ones are crap.

And when it comes to transonic, the ONLY data I have is the published data for the Condor (which is obviously incomplete).
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Unread postAuthor: Ragnarok » Sat Apr 19, 2008 10:58 am

D_Hall wrote:Mind forwarding this high speed data to me?

Of course, I'd only be too happy to help.
I haven't recorded the exact data in the past, but next time I'm out with the chrono, I'll start making records of it for you.
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Re: GGDT v4.5

Unread postAuthor: clide » Sat Apr 19, 2008 10:00 pm

D_Hall wrote:4 - Improved accuracy for long barreled systems. Not perfect, of course, but it should be better.


Just out of curiosity, do you have any more details about the nature of this change?

Thanks for the update. The rescale should come in handy.
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Re: GGDT v4.5

Unread postAuthor: D_Hall » Sat Apr 19, 2008 10:24 pm

clide wrote:Just out of curiosity, do you have any more details about the nature of this change?

Previously, GGDT made allowances for the pressure exerted on the projectile by air in the barrel as a fucntion of Mach Number (I'm talking about air in front of the projectile). But working with HGDT made me realize something... Those methodologies made allowances for *steady state* situations. The methodologies did not account for the fact that the mass of air had to be accelerated. So I tacked on the mass of the air for the acceleration equations. Mind you, I treat the mass as a single mass and do not account for leading shocks and such, but air mass is a second order effect to begin with. The stuff I'm ignoring is probably third order.
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Unread postAuthor: JDP12 » Sun Apr 20, 2008 4:11 pm

woot! very nice job D hall.. i like it a bunch thanks for all your work on this
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