1 - 7 of 7 Posts

#### Dan_the_DJ

·
##### Registered
Joined
·
2 Posts
Discussion Starter
Hi everyone!

I had this idea in my head for a long time now. Recently I found out that is indeed plausible to make something that will actually project and with some speed at that!

Here are the links to a few videos of a russian guy and his vacuum powered crossbow:

All Ive got on this build is that his piston is about 7.5cm or 3in in diameter with a stroke length of 35cm or 13.78in and a draw length of 50cm or 19.68in...
I came up with a formula to calculate the force that the piston creates:

m[kg]=0.08*p*D^2

F- force in newtons
p- pressure at a given height, in N/cm^2, which I got from here: altitude.org/air_pressure.php
D- diameter in cm
0.08- is an approximation of Pi/4g, where g is the gravitational acceleration of 9.806 m/s^2

If you want to get the reading in lbs, you simply multiply m[kg] with 2.20462:

m[lbs]=2.20462*m[kg]

And if you want newtons, you just multiply m[kg] by g:

F[N]=m[kg]*g

I sincerely hope that my calculations are correct, but youre free to check them yourselves.

Anyways, for an altitude of 300-400m, where Im at, and for a piston with 12.5cm or 4.92in diameter I got the staggering amount of 267.31 lbs!

Using the same formula Ive calculated that the bow in the videos, with assumed diameter of 3in at altitude of 300-400m pulls about 96.23 lbs. And in theory that force is constant all the way through the cylinder cycle.

I wanted to calculate the speed of the arrow, but his pulley system is confusing me completely. I dont know jack about pulleys and compound bows, but I doubt this qualifies as one?

Anyways, how would one go about calculating the speed and basically improving the design?

Could someone make a drawing or explain how to reduce the pulling force, to make the string easier to bring to the trigger and at the same time multiply arrow speed, something like a more traditional compound bow, but only powered with the said cylinder?
Maybe even a flipper design, I dont know.

Please help guys, I really like the idea. Heck, it worked, it was manufactured for sale, but failed due to its high cost, 75000 russian rubles at that time...
Why is that, I am not sure, it seems like a cheap and easy product to make...

I also want to be able to calculate arrow speed if I know the force of my powersource, how do I go about that?

Thank you guys for your help, cheers!!!

#### TechStuf

·
##### Registered
Joined
·
495 Posts
The design relies upon the seal integrity of the vacuum piston, and uses solid bolts that rely less on speed and more upon their mass to effect penetration. This is likely because the speed of the piston with lighter, hollow bolts over comes the integrity of the seal, or damages the internals. It is certainly interesting, but a bit of a strange animal that would likely neither hold up as long, nor perform as well, as modern crossbows using flexed glass/composite energy storage.

Anyways, how would one go about calculating the speed...?
If the stated distance of 50 meters is accurate, then it is a simple matter to closely approximate average bolt speed by using soundforge, audacity, or any of the other free or low cost sound apps to calculate the time of flight between sound of launch and sound of impact, factoring of course, for the 50-51 meter distance from sound of impact from the camera and applying the speed of sound at the elevation of location there in Serbia.

Good hunting and good journeys

P.S. Speaking of strange animals....if you really want to radically break with tradition, make your own EMP rail-gun crossbow. One can still put decorative "limbs" on it, to pluck any vestigial heart strings attached to crossbow hunting legacy, but your high density lithium battery and ultra fast dump graphene capacitor array in the stock will produce a pulse that could send an aluminum flechette down range to partially cauterize the entry and exit holes on any big game....?

#### Bloodsport*

·
##### Registered
Joined
·
20 Posts
Steam bow has an interesting concept also

·
Joined
·
22 Posts
Scary

#### aemal1

·
##### Registered
Joined
·
22 Posts
Will be interesting to use one.

#### kfl0004

·
##### Registered
Joined
·
23 Posts
Interesting concept, not sure if there is a practical use for that though

#### DancerDon

·
##### Registered
Joined
·
21 Posts
I am wondering why you would you use a vacuum, instead of pressurized air. The vacuum is working on the difference between ambient air pressure (14.7 lbs/in^2) and the vacuum (close to 0). Whereas with pressurized air you have air guns that go to 3000 psi. That's why the tank is so large on it. You could use a much smaller tank with pressurized air as long as you can set it up to push with air rather than pull with a vacuum.

1 - 7 of 7 Posts