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Vacuum Crossbow - No limbs, no rubber.


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Old 07-21-2014, 01:10 PM   #1
BoyntonStu
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Lightbulb Vacuum Crossbow - No limbs, no rubber.

Clever idea. Looks very fast.



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Old 07-21-2014, 01:43 PM   #2
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That's pretty impressive, fairly tight grouping, obviously powerful, and makes a cool noise, I want one!
Are you going to try to make one Stu? And how in the world does it work?
I saw a break action barrel presumably pulling a piston to create a vacuum, was the piston somehow connected to the bow string?
It's a very interesting piece that's one step away from no longer being a crossbow, thanks for sharing I hope you decide to do something with it.


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Old 07-21-2014, 02:22 PM   #3
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Quote:
Originally Posted by kohlqez View Post
That's pretty impressive, fairly tight grouping, obviously powerful, and makes a cool noise, I want one!
Are you going to try to make one Stu? And how in the world does it work?
I saw a break action barrel presumably pulling a piston to create a vacuum, was the piston somehow connected to the bow string?
It's a very interesting piece that's one step away from no longer being a crossbow, thanks for sharing I hope you decide to do something with it.
Here is my take on it:


The vacuum chamber appears to be about 3" in diameter.

That is about 7 square inches in area.

Atmospheric pressure is about 14.7 lb/square inch.


7 x 14.7 = 102.9 lb of initial force.

Notice the 'crack' sound which appears to indicate the power.

The concept is easier than an air rifle because it is easier to create the chamber of vacuum than to pump it to 100 psi.

I hope that this helps.
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Old 07-21-2014, 05:37 PM   #4
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I've seen this crossbow,in another video, where it didn't look as powerful. Damn 2mm mild steel sheet is more than what the most,medieval crossbows could do,and they were 1000lb draw weights also,that couldn't do this. Because they had like 6" of acceleration length compared to 25" or how much this one have.

Nice thing about this design,is that you can have a very long draw length. And I bet,you could have close to no "arms" to stick out,as they just need to hold the rollers. I think they just made some arms to look more like a crossbow.. I think this one should be much less affected by outside temperature than rubber or steel. Although if you go high up in the mountains, it'll be less powerful,while most powerful at sea level.
I've been tempted to build something like this before, now it tickled my fantasy..
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Old 07-21-2014, 05:46 PM   #5
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Quote:
Originally Posted by jlasud View Post
I've seen this crossbow,in another video, where it didn't look as powerful. Damn 2mm mild steel sheet is more than what the most,medieval crossbows could do,and they were 1000lb draw weights also,that couldn't do this. Because they had like 6" of acceleration length compared to 25" or how much this one have.

Nice thing about this design,is that you can have a very long draw length. And I bet,you could have close to no "arms" to stick out,as they just need to hold the rollers. I think they just made some arms to look more like a crossbow.. I think this one should be much less affected by outside temperature than rubber or steel. Although if you go high up in the mountains, it'll be less powerful,while most powerful at sea level.
I've been tempted to build something like this before, now it tickled my fantasy..

Altitude has some effect, besides, what are you going to shoot at 2 miles high?


Vacuum Crossbow - No limbs, no rubber. - Crazy contraptions
Vacuum Crossbow - No limbs, no rubber. - Crazy contraptions

Last edited by BoyntonStu; 07-21-2014 at 05:54 PM.
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Old 07-21-2014, 06:28 PM   #6
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Hmm even at 1.000 m you would loose quite a lot of power..about 40% if i'm right.
I just found some info about it specs:http://survincity.com/2011/08/vacuum-crossbow/

Last edited by jlasud; 07-21-2014 at 06:34 PM.
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Old 07-21-2014, 07:38 PM   #7
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Quote:
Originally Posted by jlasud View Post
Hmm even at 1.000 m you would loose quite a lot of power..about 40% if i'm right.
I just found some info about it specs:http://survincity.com/2011/08/vacuum-crossbow/

Thanks, good info. I wish that they had posted the arrow speed.

Last edited by BoyntonStu; 07-21-2014 at 07:46 PM.
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Old 07-21-2014, 07:49 PM   #8
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Got a silly idea,that might even work. i like simple and effective launchers. Less stuff to go wrong is always good.
So this would use air pressure,but instead of pumping,using a spring powered piston,it would just need a tube, and a flechless arrow,and a release mechanism. The arrow itself would be the piston,so it would have to be stiff to resist very high compression forces, and not to bend, although the tube itself would keep it from doing that, and the pressure would get higher as more of the arrow is inside the tube already, less arrow sticking out,less chance to bend. Because I was unable to figure out a way to keep a small piston in place from outside the cilynder, without air pressure to escape,I thought to block the arrow itself. Here's a picture to clear all this nonsense.. It's totally not to scale, it's just to show everything in a child's drawing style :P
Loading would be like the Spetsnaz ballistic knife. Put a piece plank or something on the ground ,and use your whole body weight to push the piston(arrow) inside the cilynder, until the retention mechanism clicks in,and holds the arrow until the trigger is pulled.
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File Type: bmp Piston launcher.bmp (346.2 KB, 342 views)
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Old 07-21-2014, 07:57 PM   #9
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P.S. I have one of those car back door? opener pistons. If you compress it and let it go,it kicks like it possibly could break ones jaw. And those, aren't even very long, or the max. that one could push. And diameter wise, much smaller than the vacuum crossbows cilynder. Also it should be much more efficient,because it has much less friction, not having 6 pulleys, cords. The arrow could be made so it has the least contact area with the cilynders inside.
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Old 07-21-2014, 08:44 PM   #10
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Default Bullsh!t removed

This is awesome!

5 cents about pressure (metric..)

1 [bar] is 1kg per square centimeter.

Under water, the "column" of water pressing on one square
centimeter at a depth of 10 meters is ten times one hundred
times one cubic centimeter; which is one liter of volume - or
one kilogram..

One cubic meter of air weighs 40 grams, if I´m right.
(I´m not: it´s 1293grams!!!!!!!!)

Normal atmospheric pressure is 1 bar (1kg/cm²)

- a column of air, 1000 x 100 (cm) x 1cm³ of air
(which is 1/10th of a cubic meter) should weigh

100.000 times 1,293kg/1.000.000 which is 0,1293kg- or 129 grams

(which simply is 1293 grams divided by ten..)

- so an altitude of 1km is reducing power by rd 13% I guess.

This is what Wikipedia says:

Troposphäre from sea level up to 7 km (Polar region) and 17 km (Tropen), begrenzt durch die Tropopause
Stratosphäre bis zur Stratopause in 50 km Höhe
Mesosphäre bis zur Mesopause in 80 bis 85 km Höhe

starting from 1bar=1kg/cm² - which is the pressure of that air "lake"
we live in, an atmosphere of linear density would have to be
1000g/1293g= 773 meters..


Bull**** Alert! EDIT:
1m³ of air weighs 1,293 kg/m3; to stack that up a column 1x1cm of
7,733km is needed.. - sorry, woke up today remembering the feel
sth was wrong with these calculations..

But in reality it´s this:

Vacuum Crossbow - No limbs, no rubber. - Crazy contraptions

reality is pretty interesting.. thanks for making me think about this!


kind regards,

Be


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Last edited by BeMahoney; 07-22-2014 at 08:24 AM.
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