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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:

youtube.com/watch?v=YZUEC1ayOeY

youtube.com/watch?v=B5wtksl__Qg

youtube.com/watch?v=8h920whj4KA

youtube.com/watch?v=VcoiFQwV7u4

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!!!