Limited pressure drop design

[aEx155]

Originally Posted by C-A_99

For stopping the piston, the method mentioned by aEx is a bit cumbersome and could be unreliable and/or easy to break, and isn't good for the pipe since it creates weak points. Perhaps it would be possible to use a coupler and place a washer between the 2 pieces of pipe. I'm not exactly sure what Ben means by the brushing though. Either way, the method should maintain air flow and be reliable.

Well, I put my idea in there anyway...

By bushing, I believe Ben means he's going to put a coupler and a reducing bushing on the pipe end. Because the reducer bushing basically is flat on the inside, it will stop the piston and won't mess up the seal. Although, I wonder what the ridge of the coupler might do. Then again, there are other ways top get a reducer bushing connected to a piece of pipe.

What size pipe were you planning on using for the main PC, Ben? Was it going to be the 2" like you were talking about, or is it going to be something larger?

[Silence]

A reducing bushing is a reducer that has one socket port and one plug port. The plug is the same OD as regular tubing, so it fits into any fitting of the larger diameter.

aEx, the large coupler that the bushing fits in probably won't matter much. The ridge will be practically flush with the face of the bushing, and at any rate, that ridge is probably not as thick as the actual pipe so it won't protrude out from the walls.

Ben, you could also use a large washer-like piece (obviously not a real washer, which would be too big) between the 4" pipe and a 4" elbow, eliminating the need for any reducers, just like what you use to stop pumps from disappearing inside the pump tube. Unless you need 2" pipe for other purposes, like a fill gauge.

[Ben]

aEx is right about my bushing stopper idea. The ridge of the coupler in my experience doesn't make a different. Remember, I'm using two piston seals. If one goes over the ridge and is no longer sealed the other one should still be sealed.

Looking at the bushings I have I might have to do something different like a washer as SilentGuy said though. The bushings don't end flat. I could add some simple PVC sheeting to make them end flat. I'll probably do that because I already have those parts and I'm not sure if I could find a 2 3/8 inch diameter washer.

What size pipe were you planning on using for the main PC, Ben? Was it going to be the 2" like you were talking about, or is it going to be something larger?

2 inches, yes. The entire PC will be made from 2 inch pipe because I have a lot of it and most of the necessary fittings already. I'm going to have 3 air chambers and one water chamber for a 3:1 ratio. The chambers will be organized into a square like formation. That's the plan at the moment.

[aEx155]

Originally Posted by Ben

2 inches, yes. The entire PC will be made from 2 inch pipe because I have a lot of it and most of the necessary fittings already. I'm going to have 3 air chambers and one water chamber for a 3:1 ratio. The chambers will be organized into a square like formation. That's the plan at the moment.

Just a suggestion, but have you considered using 1L or 2L bottles as air chambers? They would help keep weight down and they're already measured, making it easy to try out different ratios.

[Ben]

Originally Posted by aEx155

Just a suggestion, but have you considered using 1L or 2L bottles as air chambers? They would help keep weight down and they're already measured, making it easy to try out different ratios.

Nope, I hadn't considered them. Doing the math though, even with a 2L bottle (122 ci) I'd need two to get a ratio in the 3:1 to 4:1 range. The ratio would be 3.2:1 for a 24 inch long 2 inch diameter pressure chamber and 3.9:1 for a 20 inch long 2 inch diameter pressure chamber. Two liter bottles are reasonable, but either tall or wide. I'd prefer length over width so I probably won't go with them but it's enticing.

[Ben]

I've started building the water gun for the contest that will use the LPD design. It has a 4:1 ratio and a little over a liter of water in the pressure chamber.

A picture of the layout I've decided to use is attached. It's basically just everything attached to a board of wood in the middle. So far I like the setup, which is good because I had no expectations for it!

[Silence]

Is it just me, or are there four main tubes? That's not a big problem since the pipe is of low diameter, but it's an interesting design choice. I'm looking forward to seeing the water gun once all the hose is attached.

[Ben]

Yeah, there's 4 tubes. It was the best use of available parts and it fits the layout. Seems a little weird but it works.

If I had a machine shop or something I'd make a big boxy air chamber that goes over top of a pipe, but I don't.

[C-A_99]

Hmm... I don't see enough of the layout to really make suggestions on it since theres a lot of stuff, like the tube connections, that I can't really see.

For the pump, are the O-rings 5/8" OD? I've always had trouble getting those to fit without being too tight. For the nozzle, what size of firing valve will be used? Will I have to add a torque arm to it?

I gotta say though, the design I had in mind was more along the lines of the Supercannon II, with 3" pipe instead of 2". I wonder how this will turn out. The method of stopping the piston is pretty simple and something I thought of but didn't like because of the lower diameter of the air transitions. How will those transferings affect performance?

I guess I'm being a little picky here, but I'd just like to check up on the performance impacts of using lower diameter pipe (and tube connections) since I was thinking of something that would perform more like a Supercannon, then have excess air de-pressurized after taking the big shot, so it won't be such a pain pumping water in later. Still, air pressure seems to allow for much flexibility with power but lower diameters seem to place bottlenecks on how well that power can be used. Just wondering...

[cantab]

Air has neglible viscosity, so small pipes with air flowing through shouldn't be an issue.
The most likely way I think for the design to waste power is in overcoming friction between the piston and the cylinder. You want a tight seal, but you also want it to be free moving.
The lower diameter pipe in the water chamber will reduce performance. However, if you want a SCII, then build an SCII! I think what Ben is trying to do is use a similar concept in a smaller, more practical, package.

[Ben]

The piston and cylinder have less friction in the 2-inch pipe than the 4-inch pipe because there is less contact area and the two pistons are similarly tight in the pipe. I'm also only using one piston cup with a plastic straightener so the friction will be half of the two piston cup version.

So with negligible air flow loss there shouldn't be any serious negative performance consequences if there are any performance consequences at all.

For the pump, are the O-rings 5/8" OD? I've always had trouble getting those to fit without being too tight. For the nozzle, what size of firing valve will be used? Will I have to add a torque arm to it?

I've always used 5/8 inch OD O-rings and with wood making them seal has been fairly easy. The fit with the 7/16 inch diameter dowel rod I have is a little tight so a small notch should be cut, but it'll be much less deep than the notch on 1/2 inch. Then I'll epoxy some things on the sides to keep the O-ring on. It should work well.

I'm using a 3/4 inch valve. I'm going to leave enough room for you to cut it off it it's too hard to turn for you and you want to put an easier to turn valve on. I can add a torque arm if one is desired too.

[C-A_99]

Do you use a file to cut those notches? I never had any luck yet getting those to work. For the valve, just stick with the PVC valve (since they're cheaper and lighter) and I'll add a torque arm myself. For the nozzle, don't worry too much about including one, as long as a 3/4" male adapter is used, I can attach several of the nozzles I have. (I have several endcaps and water balloon nozzles which are pretty effective) However, any suggestions for optimum nozzle size would be appreciated, though I'll also look into that myself.

For the backpack, could I get a list of the specific sizes of the parts I need to get? This would make things easier for me when I go and get them, but it seems like the diameter of the shower tubing may vary a bit. Still, I'd like a measurement, even if its just for reference.

Thanks and good luck as the project progresses.

[Ben]

I use a saw blade to cut the notches. Today I used a diamond wire on a hacksaw because it's about as wide as the O-ring. I imagine a lathe would be best.

I'm going to buy a solar shower so don't worry about the backpack. I imagine that the tubing size can vary, yes. The one I last used was 3/8 inch if I remember correctly.

Some more pictures of the progress are below. The piston is done and can be dropped in after being lubricated. Because I didn't use a second piston I have to use something to stop it from moving forward. With two pistons the assembly would stop and still seal, but with one piston it would get stuck and not seal so I need a stopped. The stopper should reduce the pressure rating a bit but not substantially. I'm thinking of trying a different stopper that shouldn't affect pressure rating so that's not final. The pump also should be done but I haven't tried a seal test yet.

The PC pipes also have been cemented. I was a little worried about a short drip of PVC cement in the water chamber that might affect the piston's seal, but after a test I can see it doesn't affect the seal at all. I've sanded it flatter just to be sure. The water chamber is removable too so if I did screw up it won't be hard to replace.

I'm considering taking this to the Washington DC war in a few days to test it. Hopefully that'll get some people interested in water guns...

[Silence]

Oh right...the D.C. war. I'd forgotten about that.

Definitely take one or two homemades, but I can't say it matters which ones you take. If I go I probably won't bother with my APH. Which reminds me...I need to get a soaker working fast. Recently I've been using my friend's Max-D 5000, but its trigger is getting unreliable, too.

[Ben]

It's a good idea to use the board calendar to send you a reminder email.

If this water gun is working I can let you use my original APH there. It's actually the only water gun I have in good working condition at the moment. SuperCPS was unsuccessfully cannibalized to try larger diameter tubing. My other two APHs were sent to some guy in Iraq that I never heard from again.

I have a CPS 3000 sans backpack and a CPS 2100 worth using too. I've been meaning to get rid of the CPS 2100 so if you show up at the war I'll let you keep it because I need to get rid of it. The only problem with it is that I cut some of the case off to try a PC expansion modification. It was then that I realized I hadn't thought ahead enough about how I planned on expanding the PC... whoops. There's no structural problems. It just looks weird.