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Catching Projectiles (50J+)

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DerAlbi

Sun Apr 26 2015, 10:06PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

What energies did you have?

DerAlbi

Fri May 01 2015, 03:58AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Just as an update: i am currently implementing "ConKbot of Doom"s i decided to give it a try. I might actually work. I have now a 60cm long tube with d=2.5cm.
What i noticed so far is that the water does not wash out the projectile from the tube which kind of sucks. but i dont want to decrease the tube diameter further and water pressure (in sensable amounts) doesnt help either.
hmmmh.

Dr. Slack

Fri May 01 2015, 05:47AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

I see as you're trying out wet methods, you might be interested in the details of the magic water window to use as the projectile entry point to the water brake. I've have a quick look at Mr Doom's picture, the water is sort of discouraged from exitting by the right hand fitting, and by having only a partially filled main tube, so no real pressure drop across the entry point.

This method borrows from the technology of jet entrainment pumps

, of which the laboratory water-jet filter pump is probably the best known. These pumps tend to use a single high pressure jet, but there is also a configuration that uses multiple jets arranged in an annulus, for pumping things that may contain solids. Descriptions of these pumps tend to emphasize the venturi, which is essential for efficiency. However as this pump needs to generate very little head, and zero flow, that can be sacrificed for simplicity. The solid in this case is the projectile that has to pass through the middle of the pump.

This is an illustration of the latter type. Two of the several jets arranged in a ring around the entry pipe are shown. A hydraulic jump exists in the left hand end of the tube tube, where the fluid in the jets slows to a stop and the pressure increases. It is this pressure increase that holds back the pressure in the tank or pipe.

Doing a momentum balance at the jump, it can be shown that the product of the total jet area times its pressure must exceed the window area times its pressure head. As the for this application the jet area could easily be 10% of the window area, and the window needs hold back only 10mbar (10cm head), very little pressure is needed at the jets. For initial experiments, a flow from the tap would be sufficient, though a recirculating pump would be less wasteful for continuous operation. A central heating pump or pond fountain pump would provide more than enough pressure.

DerAlbi

Fri May 01 2015, 06:28AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Hehe, i currently have some success simply by adding foam to the water stream... by the means of an additional tube. The foam-size is so that the foam+projectile blocks the tube, pressure builds up and actually has some surface area to work against. thats soo easy

Thats like flushing the toilet with paper to get a spider down the drain

However the success is limited since the foam-pieces sometimes block the pipe too well..

I am currently thinking about a sponge that sits directly at the tube-entry limiting the water flow. the sponge would get hit and pushed along the pipe. if the foam is fixed by a thread it wouldnt fall out, but would relieably push the projectile out again. the question is, if the projectile would destroy the sponge over time or even penetrate it. As i see it since the sponge is 90% water, it should not suffer so much.. but i bet it will... this never follows intuition here -.-
hmmmh.

DerAlbi

Sat May 02 2015, 11:59PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

I am back with some progress! but the only thing i can say: the water brake is a failure....
the pipe with d=2.5cm and length=60cm is waaaaaaay too short. Even 3 Joule shoots right through... more joules splash back water even with all my counter-measures. Thats unsafe and impractical. The water pipe is either too thick (but if thinner, hard to aim at) or too short (but if longer too big).

Water is flowing with ~8L/min.

clearly does not apply since the gets pushed flows aside. It might be that if the velocity increases the stopping distance still stays nearly the same. Still: i cant say that this is nearly safe enough. Given the size of the apparatus and all the work it needs to keep working...

So far: failure.

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