1.5" Burst Disk Antenna Launcher

[Lentamentalisk]

After only mild success with my previous attempt at a compact portable antenna launcher, I decided to skip the whole big heavy valve concept, and go back to basics. There is nothing lighter and smaller than a burst disk.
At first, I thought "What could be better than Davidvaini's preloaded cartridge idea?" Well, as it turns out, at 1.5", a lot of things could be better. Unfortunately, with a single layer of foil tape, it bursts at closer to 20psi. When I tried using aluminum foil, I could not get the balance right. Either it would burst at 40psi (far too low for my purposes) or it would shoot the fitting off instead.
It seems as though he hit the hot spot with 1.25" 60psi launchers.

For my purposes, it looks as though I will have to go out and buy a threaded union, preferably with an o-ring.
I haven't made any decissions on length. I will work that out in GGDT, but I want to keep it as small and quiet as possible, so we shall see. The plus side of this style of launcher is that it collapses really small.


In terms of toeing a fishing line, for the "antenna launching" side of this, do you have any suggestions on projectiles? I have been using 1.5" mini tennis balls, because I have easy access, but if you have had better experience with other projectiles, let me know. The momentum retention to initial velocity compromise is a tricky one. Discarding sabbots are an option, but I want to avoid metals, as I am going to be launching it in an urban environment. To be specific, I am trying to get it over the building across the courtyard from the one with the big white roof. It is about 50' tall, for a rough estimate.


Comments? Questions?

edit: super brownie points to anyone who knows how this photo was taken. I can tell you, it is quite possibly one of the coolest techniques ever.

[Technician1002]

Burst disk is difficult to adjust down the power. Finding the right disk material for a calibrated shot is difficult. I would stick with an air cannon. I found water bottles work well for projectiles even in an urban environment. If you know the height and width of the building, the length of string can be calculated to land the bottle on the ground in the plazza. Using a partner with a radio a shot over the building will not be difficult.

If you have the room, I found trying to spool out line from the launcher is more trouble than it's worth in many cases. I found spooling out line to the base of the target works well. Tie the end of the line down and then back away to where the line is taunt. Shoot straight up so the line will remain taunt. It will gracefully arc over the tallest treen and return the line donwn the other side. I should do a video of the proceedure sometime. I see more and more posts on towing a line here.

[jackssmirkingrevenge]

To keep things quiet go for as high a pressure as you can manage combined with a tiny C:B ratio, use GGDT to optimise your dimensions. As to a projectile, how about a wooden down with fins, and perhaps a soft ball on the end to keep things safe?

[Lentamentalisk]

@Tech: I really like your idea about attaching the line to the base of the target, assuming the projectile has enough energy. It is particularly good with hitting a small target like a branch (though with an entire building, that shouldn't matter too much.
I am not going to worry about getting it super precise, I just plan on having some excess power, and overshoot it. There is a whole nother courtyard on the other side to land in, so I just need to do some test shots to make sure I have it about right.

@jsr: I like the wooden dowel idea. I can definitely make some sort of finned dart.

As for keeping it quiet, I will of course keep my C:B ratio low, but with burst disks of that size, getting high pressure can be hard. Also, as Tech and I have discussed previously, high pressures can make such a pressure wave in front of the projectile as to make quieting the cannon very difficult.

[jackssmirkingrevenge]

As for keeping it quiet, I will of course keep my C:B ratio low, but with burst disks of that size, getting high pressure can be hard..

Add more/use thicker foil I don't suppose this will be something you will be doing regularly anyway.

Also, as Tech and I have discussed previously, high pressures can make such a pressure wave in front of the projectile as to make quieting the cannon very difficult

I find that given a sufficiently small chamber, you can keep your power up and still keep the noise down. Some kind of suppressor (say 3" DWV pipe around the barrel and a couple of holes drilled in the muzzle) would eat up the muzzle crack without affecting performance or increasing bulk.

[Lentamentalisk]

I don't suppose this will be something you will be doing regularly anyway.

True that. I will shoot this cannon exactly once in its actual intended environment. If I have to fire it a second time, something totally failed, and I probably won't be sticking around...

I find that given a sufficiently small chamber, you can keep your power up and still keep the noise down. Some kind of suppressor (say 3" DWV pipe around the barrel and a couple of holes drilled in the muzzle) would eat up the muzzle crack without affecting performance or increasing bulk.

We shall see. I still need to figure out what I will be using for air. If I am using CO2, I am only limited to the volume of a 12g cartridge, but if I go for some other sort of compressed air, maximum pressure could be the limiting factor.

[Technician1002]

@Tech: I really like your idea about attaching the line to the base of the target, assuming the projectile has enough energy. It is particularly good with hitting a small target like a branch (though with an entire building, that shouldn't matter too much.
I am not going to worry about getting it super precise, I just plan on having some excess power, and overshoot it. There is a whole nother courtyard on the other side to land in, so I just need to do some test shots to make sure I have it about right.

We tested this at a ham outing. We wanted to launch a line over a pair of trees about 150 feet apart. We tried to simply shoot a line over, but it is easy to miss the center of the second tree so the line landed in the lower branches. We changed to tying off the line and then backing out to a taunt line and sighting the two tree trunks so the launch point was in line with them. A shot straight up swung the line over both trees dead center. Works like a charm. It tends to not break the line as it doesn't come tight at high speed which is also good in an urban environment. With triangulation to find the tree heights and pacing the distance between trees, the shot dropped the water bottle right near the base of the second tree with a pre measured line length.

This should work well with a building launch to prevent hitting cars, people, windows, etc.

Where is that building located? It looks familiar.

[Lentamentalisk]

I suppose in theory if I use your method, the line should be attached around the CG of the dart, to encourage stable flight.
Like I said, it sounds like a fantastic idea, safe and everything, I just need to make sure I will have enough power to do that. I have to bring everything in by hand, rather covertly, so I can't get away with large water bottle cannons.

In other news, its 1:30 in the morning, so I will go sleep on this for a while.

[Technician1002]

I have to bring everything in by hand, rather covertly,

If you get caught, I don't know you..

Edit, According to Goggle maps in the Satellite view, the building is about 75 feet across. I thought I had seen it before.

Bonus points on how the photo was taken. Most likely a helium balloon or RC aircraft. Due to the clarity, I would guess balloon.

[rikukiakuchiki777]

super brownie points to anyone who knows how this photo was taken. I can tell you, it is quite possibly one of the coolest techniques ever.

Oooh oh pick me! Was the image taken through the use of kite photography?

[Moonbogg]

The photo was taken with an R/C heli or a balloon I bet.

[Lentamentalisk]

Riku is the winner! KAP, Kite areal photography, from the park across the street.
Anyways, back on topic.

[Moonbogg]

Why not use a large kite to drape the antenna over the building?

[psycix]

Because we are spudgunners Moonbogg!

Why do you have to use a burst disk? I'd go for the convenience and the reliability of a premade valve, like a sprinkler or QEV.
Heck, if you use a hammer valve as pilot, you might close the valve right before the projectile exits, leaving some pressure in the chamber, way more silent launcher.
As for projectiles, I'd use an empty 0,5L water bottle. If it is to light, try again with some water in it. Adjustable weight, and with not too much water, very lightweight.

[kenbo0422]

Oooohhh, I was gonna say the G.I. Joe with the Kung Fu grip in the paratrooper/covert insertion package with working telephoto camera (TM).


Last time I did something like this was in high school, and I used a model rocket. It worked, but there were unintended things that happened with it as well.