SpudFiles

Alright guys, heres the deal.
I came up with a fairly ingenious way of hanging a large banner from the roof of a building (guerrilla) involving shooting a tennisball with a pulley attached to fishing line, onto the roof of the building.
I am now trying to scheme up the best way of launching the tennisball. I don't yet know if a tennisball is going to be best, because they may not be heavy enough, so I might want to fill them with water, sand, or something like that.
I am definately headed towards something like http://www.spudfiles.com/forums/v-a-l-p ... 14702.html in the general theme of things.
My biggest debate is whether I should go for pneumatic or combustion.
The pros of a combustion, are that I can suppress it much more easily, due to the hot gases (an absolute necessity, due to the fact that this is not my building I am planning on bannering, and so stealth is a requirement). The downside with combustions is that there is that there is a flash from the flame, and they are a little finicky to get the mix just right.
Pneumatics have the advantage of an easily adjustable pressure, to exact the distance fired, though compressors are noisy, and since the gases aren't hot, it is harder to quiet. I could invest in a CO2 tank, but that adds extra expense.

Not sure exactly what it is I am asking for, or anything, but comments, questions, suggestions, etc. are greatly welcome.

I'd go for either a pneumatic with a hand pump or 12 gram CO2 cartridges, or a small metered propane combustion gun. For added stealth and efficiency, go for a low C:B ratio (long barrel).

its actually not too hard to make a supressor for a pneumatic gun, they just arent as effective as they are on combustions, but pneumatics are quiter then combustions to start with, it really depends on how quiet it needs to be. also, a good way to weight tennis balls that ive done is filling them with water and freezing them, or filling them with lead shot, but you need some serious firepower to shoot that, and is either going to be huge or really expensive (made out of metal most likely) depending on how far you need to shoot the ball and how compact/concealable you need the gun to be, you could make something like this
http://www.spudfiles.com/forums/mini-te ... 17404.html
that shoots about 100 feet according to the builder

Good point on the low CB ratio.
Making a silencer is not the hard part. I have read tons and tons on that, so I don't need any special advice on that. I am going to be making it shroud the entire barrel, and extend out a foot or so, with loads of baffles, and steel wool.
I was thinking that maybe with burst disks, I could get better accuracy and adjustable range with a combustion. I could set up a little table of muzzle velocities vs layers of Al foil, so that I could calculate on the spot how many layers I need.

I am expecting that I will be able to make measurements in the day, beforehand, so as long as I can do the calculations at home, I should be good.

I completely forgot to mention size. It will definitely need to be quite portable, preferably able to sling over one's back, for more easy movement. Most def we will be based out of a car, but we can't launch it from too near the car, for secrecy's sake.

I will need to mount a level and protractor, so that I can launch it at a predetermined angle.

I am so excited, I can tell this is going to be a fun, interesting project. I may also be able to couple this with my end of year physics project, so double awesome.

In my mind, it's really all about a portability issue. Both pneumatic and combustion designs could be easily successfully at this application.

The other issue to consider is the desired range...how tall a building are you looking at bannering? This will dictate the performance characteristics of the design. For instance, if you are limiting yourself to 2 or 3 story buildings, the requirements will be quite small. The TransAmerica building in downtown SF, another story altogether...obviously.

If you go pneumatic, I think you're into a CO2 or HPA situation for all practical purposes. A combustion gun's portability is built-in.... just getting the c:b optimized (lowwww) with the mass of the ammo can go a long way toward a quiet launch.

If you need to adjust the muzzle velocity, then pneumatic is probably better, as you can simply adjust the pressure used.

I am not quite shooting for the transamerica building, for sure, but I do intend on being able to get a good 100'+ feet up. Also, I will be going for higher than 2-3 stories, because at that range it would be better just to throw it up. Probably somewhere in the 50-100' range.

Maybe I should do a little GGDT and HGDT playing around.
I figure that just by adjusting launch angle I can get fairly precise about how high it gets and how far it goes. I do want it to be a little on the overpowerfull side, because it would just suck so much to build the entire thing, and then be 10' short of getting it up on the building I want...

Thanks for all the input!

Another thought: the suppressor will be some huge casing around the entire thing, maybe painted some harmless color or put into a golf bag, or something like that, to not arouse suspicion.

Perhaps a propane pressurized pneumatic? A bit cheaper than using CO2, HPA etc. Tank is relatively small so portability is pretty good. Doesn't require a regulator since it can be built to pressurize the the chamber slowly enough that a pressure gauge would be adequate for setting the pressure.

You would be limited a bit in the max chamber pressure.

Ya, the propane pneumatic has always been appealing to me because I have access to practically free, large quantities of it, and I have an adapter to refill canisters. limited chamber pressure won't be that much of an issue, unless I were to use this in winter.
I hadn't really though about it, but the cold would definitely be an argument for a combustion, due to CO2 (not that I would use it at the max pressure, but it would get so crazy cold) and propane decreasing vapor pressure, and pvc embrittlement. The less time it is under pressure, the better, and as long as I have a decent sized propane meter, I won't need much of a pressure coming out of the tank.

akb's site has some useful info about shooting lines. Should help give you some ideas about what kind of setup would work and the range you can get.

http://www.antennalaunchers.com/antlaunching.html

One thing you might want to consider about propane is that people often overreact about things they are unfamiliar with. If you do get spotted by somebody, having a propane bottle attached to your device would probably make an overreaction more likely.

He's got a flamethrower!11!1!

Well I did a little very rough calculations (using the drag coefficient at muzzle velocities for the entire flight, and not accounting for having to pull the string) for launching 150' up, and it seems like I only need 30ftlbs.
For reference, I can get 40ftlb with a chamber 2"x12", a 2.5"x12" barrel, and a 3/4" QEV...

Good point about the propane fears. For that reason I am leaning towards CO2. Thanks for the link. I have seen it before, but now I can see it in a new light.

Quick question: If I have a CO2 bike tire filler, how accurately (and safely) could I fill up a several square inch chamber to around 200psi?
I am trying to see whether I could just use a small section of iron pipe as a chamber, to keep size down.


edit: I played around a bit, and found that with the following setup, I would be getting 60% more energy than I need, and it would be practically silent.

You will notice that when the tennis ball leaves the barrel, there is actually a slight vacuum, which means I should probably shorten the barrel a tad, in case I want to shoot it at a lower pressure.
Do my valve specs look about right for a McMaster 3/4" QEV? I made all of the measurements I could (mine is open at the moment,) but I don't have the slightest clue how to figure out the flow coef, I just went with what was initially filled in.

Sorry bout the double post, but I already edited the previous one too much.


I found this "regulator" which is actually just a puncturer and flow control, but I was expecting that with a pressure gauge I could regulate the flow enough to keep it within the range I need. I also have a little shock pump that is broken, whose pressure relief valve I could add on.
http://www.bikemania.biz/ProductDetails ... CO2-REG_NC

In essence, the cannon will be just a tad larger than a 1' section of 2.5" pvc pipe, because I will only be adding a small section of 3/4" iron pipe, a 3/4" QEV, and some fittings I need to deal with the CO2.
The entire thing should easily fit in a backpack, if not two of them...

Just a thought, but as far as a non-threatening package, why not pick up a cheap briefcase from a thrift shop, and house the whole package in there. All it'd take would be one big hole in the end for the barrel to exit(mount the launcher inside the briefcase w/ the barrel flush w/ the end of the case), and a small hole to run a wire to your triggering blowgun right beneath the handle so you can hold it by the handle when firing and just pull the ring with your trigger finger. Almost like the HK 'operational briefcase'. Just have the line spool in a pocket on the opposite side. Pull the spool out, drop it on the ground and launch away. This setup would give you one convenient shot, and darn near the ultimate in concealment.

In the barrel when not shooting, you could put a plug, made out of the material removed.
Ya, I though about that. I am going to be doing it by night, so I am not so sure that it really matters all that much. I am thinking I will just keep it in a duffel, and pull it out when I am going to use it. Painting it brown would make it not stand out so much.
More important I think is portability and flexibility.
I am thinking I will make the barrel as modular as possible, so that I can screw one in, that has a fishing reel attached, with the ball already loaded. Then I fire it, unscrew the barrel, hand it to someone, and can go straight to launching the next, so I can get two lines, for better stability for the banner, or put up multiple, possibly on multiple buildings, as quick as possible, with as little time of it being out of the bag as possible.