SpudFiles

I thought about building a new launcher and thought back to Technician's QDV marshmallow cannon. I already have several changes in mind for the trigger system so the cord will not be used but a rod will be. This doesn't use a propane tank so there isn't a need to weld or braze which helps a lot for those without the necessary equipment (like me). Also, steel pipe is rated for higher pressure so there is access to more power. The piston will not be hollow either. 2'' is the largest pipe I can find locally sadly so it will have to do.

Knob of some kind

Why is the piston hollow?

I said I wasn't going to have it hollow. I thought about it thoroughly. I'm basically doing what Technician did except with steel pipe instead of an empty propane tank.

Mr.Tallahassee wrote:I said I wasn't going to have it hollow. I thought about it thoroughly.

Wow, my criteria for tl;dr are shrinking by the day

Why so much piston travel? 0.25" or so is enough.

That was the initial rendering prior to thinking about minimizing everything. I got lazy and didn't feel like changing the whole diagram. I plan on moving the ports to the front and putting in the piece of PVC for a stop like Tech used. The original point of this was to up the max pressure from the disposable propane tanks tested rating of 375PSI. I'll probably shoot empty 12 gram CO2 cartridges since I have endless piles of them.

Looks like a promising design, i'm sure power would also be impressive.
considering the fact you will be operating at high pressure.
Tweak around with GGDT, I had a reading of Tech's 2.5" QDV with the information he gave me.... energy and muzzle velocity was very impressive using a 1lb projectile...... 12k+ ft/l, velocity of mach 1

it would be awesome to see another QDV...

Please view my video on how the the piston is free to slide on the rod in my marshmallow cannon. A fixed rod will greatly slow the valve. It is in my youtube channel under my username.

The drawing shows huge ports. On mine the port length is about 1/2 the height of the barrel diameter. For example the 2 inch QDV has ports 1 inch long but 2 inches wide each. The sketch is showing ports taller than the barrel diameter. This is not needed and requires a longer heavier piston to bridge the gap resulting in a slower piston.

I do have one with the valve in the back like the drawing. The ABS cannon is built that way.

The valve in the front is for cannons with screw on barrels to minimize the dead space between the valve and a projectile in a larger diameter barrel.

I actually watched that video. I had that idea in mind. I just need to figure out a way to do it with what I have at hand.

If you had a way of quickly producing cheap and lightweight sabots, or had some method of recovering them, this would be an attractive variation on the theme.

:violent1: Nice idea but making a new sabot for each shot is not likely to happen. I would recommend a QDV valve with a sabotted round instead. There is nothing like taking a bag of spuds to the field and be unable to use the launcher because you ran out of pistons. A permanent valve makes much better sense. This enables the use of much lighter sabots resulting in higher speed launches.