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Interesting GGDT phenomena...

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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Interesting GGDT phenomena...

Unread postAuthor: jeepkahn » Sat Oct 31, 2009 9:26 am

I've been playing with GGDT a lot here lately in an attempt to refine some designs I've been working on, so I've been looking at the acceleration curves as opposed to just the velocity curves...

I'm guessing that what I'm getting ready to describe is occuring because of gas cooling during expansion and then compression heating, because the projectile is "outrunning the pressure and then the pressure catches up"....

What GGDT is showing(decimater and a superball) is that at firing the acceleration is 16,000g's, and at 14" it has dropped to 5,000g's, but after that it starts increasing acceleration so that at 24" it's 6,000g's, at 40" it's 8,000g's, and at 60" it's accelerating at 10,000gs....


What does this mean in relation to internal ballistics??? I thought it was a strange phenomena, so I figured I'd throw it out there for therocket scientists....

I've also noticed that with 750psi and superballs GGDT predicts better performance with co2 than air, but with heavier balls it predicts air as faster....
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Unread postAuthor: Ragnarok » Sat Oct 31, 2009 10:22 am

Not entirely sure what phenomena causes GGDT to do this, because Apocalypse doesn't replicate it. (Then again, I accept that it may be my modelling that's in error. I do get things wrong at times.)

For some reason, at around Mach in the gas, the acceleration stops falling, and starts rising again.

Whatever causes it, I think it's probably part of why GGDT has a habit of giving questionably generous velocities (relative to real life figures) around and above the SOS in the gas.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: D_Hall » Sat Oct 31, 2009 11:51 am

I'll be the first to agree that GGDT has issues above the SOS. There are some routines there that I know aren't entirely accurate from a physics perspective but where attempts to model shock heating and the like. I need to take them out, but I've just not gotten around to it.

That said, Jeep, can you post some screen shots? 'Tis hard to make any sort of statements with only the data presented.
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Simulation geek (GGDT / HGDT) and designer of Vera.
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Unread postAuthor: jeepkahn » Sat Oct 31, 2009 12:28 pm

I can email the gdt files, but I'm on an old crappy computor that doesn't take screenshots... or does SF allow gdt file transfers...
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Unread postAuthor: D_Hall » Sat Oct 31, 2009 3:00 pm

Well, you could copy/paste the contents of the .gdt file to a post (they're just text files, after all).
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Unread postAuthor: jeepkahn » Sat Oct 31, 2009 3:21 pm

Anything special about the design? Make a note of it here!
Time(ms) ProAcc(Gs) ProVel(ft/s) ProPos(in) ResPres(psi) BarPres(psi) BarTemp(F) ValPos
0 0 0 0.75 750 0 80 0
0.24 0 0 0.75 749.992 0.57812 80 0.00171937
0.49 113.376 0.311897 0.750251 749.948 4.89574 79.9909 0.00772882
0.74 500.237 2.58641 0.754088 749.812 18.5465 79.7132 0.0218647
0.99 1480.46 10.1531 0.772122 749.457 52.9489 78.0066 0.0558496
1.24 3667.21 30.164 0.830566 748.572 129.348 72.524 0.138795
1.49 7693.75 75.2526 0.98592 746.418 269.66 60.5418 0.33373
1.74 13120 159.518 1.33718 741.447 463.465 42.4806 0.752748
1.99 16101.7 282.561 2.00904 733.072 587.886 19.653 1
2.24 13902.9 404.197 3.05843 724.708 550.403 -3.52477 1
2.49 11297.1 504.958 4.43949 716.004 493.599 -17.7486 1
2.74 9230.11 586.879 6.09121 707.134 445.222 -25.2648 1
2.99 7682.19 654.417 7.96435 698.256 406.875 -28.789 1
3.24 6517.24 711.182 10.0219 689.447 376.502 -30.0064 1
3.49 5622.04 759.76 12.2359 680.739 352.062 -29.8796 1
3.74 4918.29 801.974 14.585 672.139 332.05 -28.9526 1
3.99 5149.05 842.462 17.056 663.646 314.656 -28.4009 1
4.24 5428.08 885.069 19.6519 655.257 298.532 -29.0071 1
4.49 5738.78 930.056 22.3793 646.972 283.328 -30.6221 1
4.74 6079.74 977.673 25.2459 638.79 268.814 -33.1587 1
4.99 6449.6 1028.16 28.26 630.708 254.83 -36.5717 1
5.24 6846.61 1081.73 31.4305 622.727 241.264 -40.844 1
5.49 7268.04 1138.61 34.767 614.844 228.034 -45.9788 1
5.74 7709.39 1198.97 38.2797 607.059 215.081 -51.9921 1
5.99 8163.46 1262.94 41.9793 599.37 202.366 -58.9057 1
6.24 8618.94 1330.59 45.8768 591.777 189.869 -66.7387 1
6.49 9058.72 1401.84 49.9831 584.277 177.586 -75.4968 1
6.74 9457.73 1476.49 54.3088 576.87 165.538 -85.156 1
6.99 9780.68 1554.07 58.8633 569.554 153.77 -95.6414 1
7.24 9980.38 1633.76 63.6542 562.329 142.366 -106.8 1
7.27 9989.34 1640.19 64.0478 561.755 141.472 -107.714 1
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