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launching heavy projectiles....gravity

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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launching heavy projectiles....gravity

Unread postAuthor: trollhameran » Tue Jul 15, 2008 2:04 am

If I was going to launch a heavy projectile straight up in the air and ggdt says it will have a fps of under 32, would it still work? because I am quite confused by the gravity aspect of it, and I am under the impression that unless an object is moving faster than 32fps it will not overcome gravity, but I have a feeling I am completely wrong about this.
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Unread postAuthor: rednecktatertosser » Tue Jul 15, 2008 2:07 am

it may just kinda pop outa the barrel and fall to the ground... I may be wrong, but thats just my thoughts.
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Unread postAuthor: SpudFarm » Tue Jul 15, 2008 2:31 am

remember that you can throw things. i have shot lead bullets that was 1kg at 3x pre ignition pressure from UPGBH10x and it went about 20 feet before it touched the ground so the dirt flew everywhere then bounced sky high
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Unread postAuthor: jackssmirkingrevenge » Tue Jul 15, 2008 2:54 am

It might not even leave the barrel. What exactly are you planning on launching?
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Unread postAuthor: SpudFarm » Tue Jul 15, 2008 3:21 am

why don't you just try? i would recomend to set the gun on the ground while doing it, if not the gun will go backwards in your hands (and you will follow) and the bullet stay still.
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Re: launching heavy projectiles....gravity

Unread postAuthor: Ragnarok » Tue Jul 15, 2008 6:39 am

trollhameran wrote:and I am under the impression that unless an object is moving faster than 32fps it will not overcome gravity, but I have a feeling I am completely wrong about this.

Although it's nice to see some application of thought, your logic isn't quite right.

The launch velocity is 32 feet per second (although as GGDT assumes a horizontal launch, it will actually be less)
Gravity's acceleration is 32 feet per second per second.

Note the two "per seconds" in acceleration. That means to change the velocity by 32 fps, it takes one second for gravity to do so. To change it by another 32 fps takes another second.

What that means is that assuming a 32 fps launch, ignoring drag (although on a very heavy projectile travelling that fast, it will be negligible) is that it will take one second for gravity to strip away all of the 32 fps launch velocity - so it'll go about 16 feet up, and will then come crashing back down about 2 seconds after the launch.

For a quick experiment, pick up a pencil, rubik's cube, whatever you have to hand. Throw it up very lightly - assuming you're throwing lightly, you're not throwing it at 32 fps... but you can still throw it up, can't you?.

It's quite common for people to mentally mix up acceleration and velocity, and there are enough people who think that if you drop something, it falls at a constant 32 fps.
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Unread postAuthor: trollhameran » Tue Jul 15, 2008 12:42 pm

so say for example the projectile had a velocity of 10.66r fps it would take 1/3 of a second for gravity to strip away all the velocity and make it fall back down? which would presumably give a maximum projectile height of approximately 3.55 feet? or would it be half of that again? If that is correct then I understand now.

I was thinking about the hydraulic kits people put on there cars and how they can make the cars "hop" up to 6 feet off the ground, which got me thinking about whether it would be feasible to make a compressed air "cannon" that would have a lightweight projectile the length of the barrels which would be used as a sort of ram to push against the ground and push a person into the air around 6-10 feet. Having read a topic on here a while ago about making a co2 jet pack is what got me thinking about this, and it seems to me that a ram like i described above would be an effective way of making use of all the energy that the co2 can provide. So I had a bit of a play around with ggdt to see if 115psi from a compressor could produce enough energy to propel my weight at 32fps without the cannon being huge and impossible to carry.
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Unread postAuthor: Ragnarok » Tue Jul 15, 2008 1:05 pm

trollhameran wrote:Which would presumably give a maximum projectile height of approximately 3.55 feet? or would it be half of that again? If that is correct then I understand now.

It would be half again, ignoring any air resistance. It seems you've grasped it pretty well.
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Unread postAuthor: trollhameran » Tue Jul 15, 2008 1:42 pm

Ok yes i understand now, thank you very much for explaining to me.
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Unread postAuthor: trollhameran » Tue Jul 15, 2008 2:16 pm

Sorry for doubly post but I have just written a formula in excel which hopefully works out the correct height a projectile would go without any air resistance, do you think you could have a look and see if it looks right to you? All you have to do is type your projectiles fps in the correct box and it will tell you the height it will travel to. Theres is a formula hidden in A4 which is needed to work out the correct height.

EXCELFILE
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Unread postAuthor: Ragnarok » Tue Jul 15, 2008 2:47 pm

trollhameran wrote:Do you think you could have a look and see if it looks right to you?

Seems OK to me.

Of course, if you know how to do it, you can get an estimate of your projectile's ballistic co-efficent by firing it into the air and measuring the time it takes to return to ground level, then comparing that with the calculated figure without air resistance, then working out the drag that would be needed to make the difference.
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Unread postAuthor: trollhameran » Tue Jul 15, 2008 3:00 pm

Cool, thats great that I managed to work that out.

That sounds quite complicated but also it would be a good challenge to do it and compare it to my formula
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Unread postAuthor: starman » Tue Jul 15, 2008 3:35 pm

trollhameran wrote:Sorry for doubly post but I have just written a formula in excel which hopefully works out the correct height a projectile would go without any air resistance, do you think you could have a look and see if it looks right to you? All you have to do is type your projectiles fps in the correct box and it will tell you the height it will travel to. Theres is a formula hidden in A4 which is needed to work out the correct height.

EXCELFILE


Your link loaded me down with pop-ups and what should be an unnecessary request to type in graphic characters. This is somtimes a ruse to unwittingly agree to also load spy or adware.

Anyway can you find another location to save your file?
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Last edited by starman on Tue Jul 15, 2008 6:05 pm, edited 1 time in total.
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Unread postAuthor: trollhameran » Tue Jul 15, 2008 4:08 pm

Sorry about that starman, Is megaupload a good place to upload it?
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Unread postAuthor: trollhameran » Tue Jul 15, 2008 4:13 pm

ive put it in a zip file and uploaded it straight to spudfiles...hopefully
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