# rubber to shoot an object which weighs around .5kg



## astmonster3000

hi there,

i am new to this forum and i am looking for some help.

i never had a good slingshot and i honestly never thought about building one either. therefore i have absolutely no clue which rubber is strong or how to build a slingshot thats safe and wont try to kill me while i am trying to shoot it.

i have the chance to build a multicopter which should aid the firedepartment by shooting special canisters through windows into burning houses. these canisters should then be able to extinguish a small fire. i know, it sounds weird and it definitely is... so, therefore the "slingshot" needs to be able to shoot a canister which weighs about .5kg (1lb). the canister should be in a tube which acts as a barrel and the rubber should also be inside this barrel. the length of the tube / barrel cannot exceed 80cm (30in). the canister needs to fly about 4-5m.

well... i dont know if it is even possible to build something like this. therefore i am asking you. do you think its possible? which rubber should i use for that thing?

cheers


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## Jaximus

Thought of using compressed air instead of a slingshot?


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## astmonster3000

nope, not yet... i believe that compressed air might be too heavy, thought about pyrotechnics but they might light something else on fire and then there would be two burning buildings instead of one


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## Lacumo

Sounds like you're trying to design a product that demands the ability to fling a heavy (and probably awkwardly-shaped) object with serious accuracy -- and likely for a considerable distance &/or height. This isn't some amateur or home-made slingshot project. That's a demanding professional-level bill to fill. Just a guess, but I suspect compressed air will most likely be necessary.


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## Deano 1

You may be better talking to Joerg Sprave, he's brilliant at designing things like this. You can message him through his youtube page.

http://www.youtube.com/user/JoergSprave?feature=g-subs-u

I'm sure he will help you.


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## Sunchierefram

What about Joerg Sprave's slingshot cannon?


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## astmonster3000

hi, thanks for your help, i thought about contacting him since he seems to be the kind of guy who can make something that crazy ...

well, there will be some serious testing necessary but if i buy stuff and it wont even fire the canister i might as well just flush my money down the toilet. therefore i wanted to ask the more experieced people here.

what do you want to do with compressed air ? the barrel would need to be quite tight to get the thrust needed and i would need to have some kind of air tank on the copter, which would make it very heavy, wouldnt it ?


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## Hrawk

You definitely want to use compressed air for this.

A small well made QEV valve will weigh less than all the rubber and release mechanism to 'slingshot' it in there.

Build it well enough and you can use 12g CO2 cartridges to power it.


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## Hrawk

astmonster3000 said:


> hi, thanks for your help, i thought about contacting him since he seems to be the kind of guy who can make something that crazy ...
> 
> well, there will be some serious testing necessary but if i buy stuff and it wont even fire the canister i might as well just flush my money down the toilet. therefore i wanted to ask the more experieced people here.
> 
> what do you want to do with compressed air ? the barrel would need to be quite tight to get the thrust needed and i would need to have some kind of air tank on the copter, which would make it very heavy, wouldnt it ?


No, the barrel does not need to be super tight. Simply make a disintegrating foam sabot for your projectile.

CA is much more reliable, tunable, reusable and safer.

A quality air tank is much lighter rhan you think. Check out some of the carbon fibre ones used in the paintball industry.


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## benzidrine

I would agree with the you want compressed air for this or you could go the traditional process of using chemical explosives.

I'd say a slow burning gunpowder could launch these projectiles from disposable barrels and will be the lightest possible setup.


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## astmonster3000

it may be the lightest way but i dont know if the canisters would survive a launch with gunpoder. in addition to that, the barrel will be part of the multicopter frame, so i cant use a disposable barrel.

any idea how much air pressure would be needet to lauch that thing ? the tanks go up to about 300bar, which might be overkill, all QEV valves i could find go up to about 10bar, which might not be enough.


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## Jaximus

I would try some experiments with the 12g CO2 cartridges. They're inexpensive and very lightweight. They make larger sizes as well.


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## Hrawk

astmonster3000 said:


> it may be the lightest way but i dont know if the canisters would survive a launch with gunpoder. in addition to that, the barrel will be part of the multicopter frame, so i cant use a disposable barrel.
> 
> any idea how much air pressure would be needet to lauch that thing ? the tanks go up to about 300bar, which might be overkill, all QEV valves i could find go up to about 10bar, which might not be enough.


Can you give me approximate dimensions on the canister and I'll crunch the numbers.

Don't worry, it wont be over 10 bar, especially if the canister is around 50mm or bigger.


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## astmonster3000

Dimension: 200 mm x ø 60 mm
Combined weight: 600 g
Volume: grenade contains 500 ml of liquid Bonpet in plastic ampoule
these are the dimensions of the canister


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## Imperial

ever think of just making or using an air cannon like the ones used to launch t-shirts into the crowd .


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## astmonster3000

Imperial said:


> ever think of just making or using an air cannon like the ones used to launch t-shirts into the crowd .


we dont have something like that in germany, i have never seen one here


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## Hrawk

Imperial said:


> ever think of just making or using an air cannon like the ones used to launch t-shirts into the crowd .


That's exactly what I'm referring to.

A barrel, an air chamber, a valve in between. Simple, reliable, effective.


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## astmonster3000

it should work... i am really looking forward to seeing the copter accelerate backwards whilst the container comes out at the front

do you have any good site where i can find the equations needed to calculate how much pressure i will need ?


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## Hrawk

Ok I've run the numbers and this is super easy to do with only 100psi of air. No need for co2 or anything, and small air compressor will do.

What you have is whats commonly known as a Coxial Air Cannon with Piston valve. These can be made with easy to get schedule 80 PVC plumbing pipe and a few air fittings.









Basically how it works is air enters from behind the piston, pushing it forward and sealing against the barrel creating a chamber around the outside. The pressure differential will keep the piston in place and chamber sealed.

To fire, pressure is vented from behind the piston allowing the piston to travel backwards and dumping all the air from the chamber into the barrel.

Looking at the calculator below, a 80cm barrel with 60cm of chamber for a total length of 1 meter, will happily send your 60mm / 550 gram canister @ 57.4 M/s.


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## Hrawk

Here's a decent video showing a easy how to.


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## Hrawk

Oh and one thing I have to add.

Safety. Safety. More safety.

If you look at the muzzle energy of the above setup, you will notice that it is more than a .45 ACP.

BE CAREFUL!


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## Imperial

astmonster3000 said:


> Imperial said:
> 
> 
> 
> ever think of just making or using an air cannon like the ones used to launch t-shirts into the crowd .
> 
> 
> 
> we dont have something like that in germany, i have never seen one here
Click to expand...

http://tshirtgun.com/

i drool at the gatling gun. :imslow:


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## Hrawk

Meh, you should check out some of the build by user D Hall over at spudfiles.com


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## ash

Hrawk said:


> Looking at the calculator below, a 80cm barrel with 60cm of chamber for a total length of 1 meter, will happily send your 60mm / 550 gram canister @ 57.4 M/s.


And now for Mr Newton's input...

How much does the multicopter weigh?

Looking around, I see some that weigh 1kg and can carry another 1kg or more - canister and cannon or slingshot.

What happens when you shoot 500g at 57m/s? You get an equal and opposite reaction on the multicopter. Expect your multicopter to recoil at something like 25+ m/s!! Even if it was possible to aim with such dramatic recoil, you can kiss goodbye to perhaps half of your canister ground speed. Would the 'copter even survive?

It doesn't sound practical to me.

Your canister launcher would need to have the motive power on board the canister. A rocket motor of some kind, pyrotechnic or pneumatic, to avoid recoil problems.

A simpler option would be to suspend the canister on a thread below the multicopter and fly it into the window or opening from above, relying on its momentum to break into the fire scene.


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## Hrawk

While I agree with everything you say Ash, I stick my thumbs in both ears and blow a raspberry at you :neener:

You're totally right though but given a larger quad, hex or octa copter, it could be managed. You need not use the speeds / pressures I've mentioned above to throw it 10 m through a window. Much less would still suffice. Heck, with the performance you can get out of these launchers, just fire it from the ground. Half a kilometer away.

That being said, it does sound like a pretty fun project to work on.


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## JohnKrakatoa

In an episode of Almost human series s01e03 they had exactly this. a quadcopter drone that shot canisters into building windows up high, maybe check it out.
I think they used comp. air and i deffinitely think C.A. is the way to go with what u want.


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## Hrawk

This topic has my mind racing a bit right now.

If you're into quads then I'm sure you've seen the camera gimble systems now available. Specifically the high speed brush less ones.

Mount your cannon via one of these and even though the quad is going to get shoved back, at least your aim will remain true!


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## Jaximus

Install a hydraulic damping system?


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## astmonster3000

hi,

sorry to keep you waiting here... i just crunched the numbers since i did not get to a hardware store. the cannons inner tube alone would weigh about 1.5kg... thats way too heavy. that means its back to catapult or pyrotechnics


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## TimR

Jaximus said:


> Install a hydraulic damping system?


Or maybe shoot another mass backwards at the same momentum. Recoilless rifle artillery does this. I used to own a competition air pistol that cancelled recoil by having two heavy pistons move in opposite directions.

In this case, you could let a water balloon shoot backwards as your cannister moved forwards, and the copter would remain unaffected.


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## astmonster3000

dampening the force will just be an issue when there is a force to dampen, at the moment it looks like the air cannon would weigh about 5kg and the copter will not be able to lift it, at least the flying time will be miserable with it , in addition to that i am not allowed to start a copter with a mass over 5kg in germany without permission, that means basically that my copter with battery and firing system can only weigh about 4.4kg.


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## Hrawk

I think you're ever estimating the weight a little bit. I just had another play with GGDT (Gas gun design tool) and you can get adequate performance from a 40cm long unit. Going of figgures from the Engineers Toolbox for SCH40 PVC, your all up weight would be about 1.2kg.

That's cool though, you're going for lightweight, I can understand that.

Look at using a self propelled projectile then. Much simpler and lighter mechanism.


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## Hrawk




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## astmonster3000

i am forced to go lightweight

just found some videos with air cannons made with an air tank, a sprinkler valve and a barrel, that should be quite light, since the diameter of the canister is about 60mm i would need about a 3" barrel though and i do not know if a 1" valve would do the job there.

your self propelled system looks a lot like a gun, doesnt it ?


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## Hrawk

Sprinkler valves are ok, but heavy and not all that efficient.

A simple piston valve is much lighter and more efficient.

I just weighted the sprinkler valve sitting on my bench here (1"), 312 grams, and that is with the solenoid removed and no pilot valve attached. A well ,made piston is only about 40-60 grams. Lighter pistons = better performance.



astmonster3000 said:


> your self propelled system looks a lot like a gun, doesnt it ?


Why reinvent the wheel ?


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## astmonster3000

well, okay, reinventing anything isnt really useful here but this unfortunately wont work with german law, you have to get permission for everything here, especially for weapons. since we want to sell those quads it might be useful to do it legally

the piston valve was the first idea, wasnt it? i just looked around online and they had the weights of different tubes, looking at these i cant imagine how you would get the weight down to 1.2kg

by the way, in case you are interested in stuff i built already, this is from the hobbyking beerlift competition, unfortunately we only came in fifth but we didnt build the model for the competition and therefore i think it was quite a good job. the model was sold to dubai


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## Hrawk

Nice, I do like the beerlift comp. Hobby king, great price, ok product, lousy service.

I got me rough weight by looking at the weights for SCH40 tubing as given by The Engineers Toolbox based on a 2" 30cm barrel & 3" 40cm chamber.

30cm of SCH40 2" Pipe - 303 Grams

40cm of SCH40 3" Pipe - 839 Grams

3" - 2" reducer - 80 Grams

3" End cap - 80 Grams

Piston - 50 Grams

Modified blow gun trigger (for pilot valve) 28 grams

PVC cement - 20g

Total weight - 1400 grams.

Meh, first time I took some rough calculations in my head, this time I've spreadsheeted it. Add your cannister to the mix and your coming up with just under 2kg.

Easy enough to build a 3kg quad to lift this. Go for a 3 kg octa and it's a breeze.


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## Hrawk

So we're down to 32.9 M/s however more than enough to lob a canister though a window when you're hovering 5m from it.


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## incubu

astmonster3000 said:


> i am forced to go lightweight
> 
> just found some videos with air cannons made with an air tank, a sprinkler valve and a barrel, that should be quite light, since the diameter of the canister is about 60mm i would need about a 3" barrel though and i do not know if a 1" valve would do the job there.
> 
> your self propelled system looks a lot like a gun, doesnt it ?


If you really are interested in using air pressure, another possible option would be making an air pressure rocket using a florescent tube cover or 2 liter plastic bottle. However, you generally need to add water to increase the thrust, which might push the payload weight higher than you want. http://pnpsoap.com/rocket/FTC.htm










Although I like the idea of using compressed air, I do think a rubber-powered system has the potential to be very light-weight. Keep in mind that you can permanently attach the rubber power band to the capsule/projectile and make a rubber-powered "rocket". There is actually an example of one at the NASA website, which has a how-to on how to build and launch one. Even though the NASA version is intended as a learning project for kids (and therefore has some amusingly annoying sound effects), you could easily adapt the design with heavier rubber bands or rubber tubes and have something that would launch your 600 gram payload on a stable trajectory. With the liquid filled canister at the front of the rubber-band "rocket" the balance should be very stable. If the "rocket" body is rigid, you can anchor the rubber band/s near the back instead of the front and increase the stretch. http://www.jpl.nasa.gov/education/videos/playVideo.cfm?videoID=29










If all else fails you can dispense with the copter altogether and use a water balloon slingshot 










By the way, will the window need to be broken? Is so, you might want a separate projectile to break the window first and clear the way for the canister...


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## astmonster3000

Hrawk said:


> Nice, I do like the beerlift comp. Hobby king, great price, ok product, lousy service.
> 
> I got me rough weight by looking at the weights for SCH40 tubing as given by The Engineers Toolbox based on a 2" 30cm barrel & 3" 40cm chamber.
> 
> 30cm of SCH40 2" Pipe - 303 Grams
> 
> 40cm of SCH40 3" Pipe - 839 Grams
> 
> 3" - 2" reducer - 80 Grams
> 
> 3" End cap - 80 Grams
> 
> Piston - 50 Grams
> 
> Modified blow gun trigger (for pilot valve) 28 grams
> 
> PVC cement - 20g
> 
> Total weight - 1400 grams.
> 
> Meh, first time I took some rough calculations in my head, this time I've spreadsheeted it. Add your cannister to the mix and your coming up with just under 2kg.
> 
> Easy enough to build a 3kg quad to lift this. Go for a 3 kg octa and it's a breeze.


you didnt consider that the canister has a diameter of 60mm, thats about 2.4". it wont fit in the 2" tube, instead i would need to use a 3"tube as a barrel which means that the thing will get a lot heavier.

30 m/s will be plenty enough

building a quad wont be a problem, an octocopter will just make the thing more expensive and more complicated to build, in addition to that the build time will be longer.

with the air cannon someone could shoot the canister from the ground but they want a multicopter to do the job... makes the whole thing a lot more difficult

i just found a rubber calculator on jörg spraves website... it said that for a 600g projectile i lwill need rubber which is 30cm x30cm to get the power... considering that he shoots a lot further i still think that i will at least need haf of that, wich means that it will be a lot of rubber which then will be quite heavy and there will be enormous forces on the quad, which will need to hold the rubber


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