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A water rocket is a type of model rocket using water as its reaction mass. The pressure vessel (the engine of the rocket) is constructed from thin plastic or other non metallic materials (usually a used plastic soft drink bottle) weighing 1,500 grams or less. The water is forced out by compressed air. It is an example of Newton's third law of motion.

At my wit's end figuring out o-ring seal on launch tube

Discussion about Compressors, hose, pipes, fittings, launchers, release mechanisms, and launch tubes.
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anachronist
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Onboard video: why?

Post by anachronist » Wed May 24, 2017 7:00 pm

I'm curious why an onboard video camera is required for competitions. What value does this add? I mean, those onboard videos are interesting and all, but the real proof is the ground-based video that shows the altimeter after recovery.

I'm concerned about the potential loss of equipment due to trees and other hazards that make the rocket un-recoverable. It's bad enough to lose an expensive altimeter. It would be nice to avoid the potential loss of an onboard camera too.



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Re: Onboard video: why?

Post by WRA2 » Wed May 24, 2017 9:46 pm

There are multiple reasons for the requirement of the onboard camera.

1. The onboard camera is guaranteed to capture the entire flight. (even the best ground camera operator may lose sight of the rocket)
2. It has been a requirement for almost 15 years (it would not be fair to previous competitors to have a weight advantage that would be obtained by omitting the camera).
3. The cost of these cameras has decreased significantly over the last 10 years and they only cost a fraction of what they did back in 2003.
4. Adds to the "technical challenge"

Hope that helps.


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Re: Onboard video: why?

Post by anachronist » Wed May 24, 2017 10:20 pm

Thanks for the explanation.

I thought the reason (at least for class D) might be to record the descent time to verify descent rate when one has a peak altimeter, but that reason is negated by having a logging altimeter. The reasons you listed don't include that, but I understand now, thanks.

As for weight advantage of having no camera, I find in my simulations that being too light is a disadvantage, as is being too heavy. Eliminating the camera might actually create a disadvantage depending on what else the rocket carries.

My background is physics. I just spent the last month or so of my spare time creating a spreadsheet simulation of a water rocket flight that accounts for launch tube, adiabatic expansion effects on pressure as the air expands, humidity effects, wind resistance, choked airflow of the final burst of residual air, and a number of other things. There's definitely a sweet spot for the amount of ballast you have -- and the right amount of ballast is just about in the range of electronics available today.



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At my wit's end figuring out o-ring seal on launch tube

Post by anachronist » Fri Jun 02, 2017 1:02 pm

THIS IS THE FIRST POST OF THIS THREAD. THE POSTS ABOVE ARE FROM ANOTHER THREAD THAT MERGED WITH THIS ONE AFTER A DATABASE PROBLEM AT WRA2.

I've seen the tutorials and watched the videos about installing an o-ring onto a launch tube. They boil down to:
  • Manually carve a groove into the tube
  • Join two tube
s with another tube that fits inside[/list]
The first method results in an unreliable seal, and weakens the tube.

The second method provides a better seal, but it's impossible.

Why? Because I cannot find ANYTHING that fits snugly inside a 1/2" diameter schedule 40 PVC pipe!

One tutorial on this site suggests using a 3/8" PVC pipe. NOBODY carries this product. The big-box hardware stores in my area (Home Depot and Lowes) don't sell any 3/8" PVC. The Orchard Supply and True Value hardware stores don't sell it either. A local plumbing supply doesn't have it. I can order it online in huge quantities for a huge shipping cost... and I still don't even know if it will fit. I need to see it in person.

1/2" copper pipe is a bit too big to fit. I found a 3/8" copper sleeve that looked promising, it turned out to be pretty loose inside the 1/2" PVC pipe.

None of these stores carry the required 1/8" thick O-rings of 7/8" outer diameter either, but these are easy to find on eBay or Amazon. I bought a pack of 25 for about $3.

So, how did anyone else here build a launch tube?

-Alex



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Re: At my wit's end figuring out o-ring seal on launch tube

Post by anachronist » Tue Jun 06, 2017 3:26 pm

Hmm, since the database got messed up and then restored, it looks like an old thread got prepended to this one.

Anyway, I'm going to buy a 5/8" countersink and see if I can use it as a reamer to ream out a 1/2" PVC pipe an additional 0.003" to fit a 1/2" copper tube, which has an outer diameter of 5/8". That may provide a good seat for the o-ring.

I am curious how everyone else is doing it. The available tutorials may have worked in the past, but they are not possible to follow now.



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Re: At my wit's end figuring out o-ring seal on launch tube

Post by anachronist » Wed Jun 07, 2017 9:31 pm

The 5/8" countersink did the trick.

To ream out the inner diameter of a 1/2" PVC pipe to 5/8" so that it fits a 1/2" copper pipe insert, first cut your PVC pipe perfectly perpendicular to the length. I use one of thos disc cutters for copper pipe, which you rotate around and around, tightening as you go, until the pipe is cut.

Then get a cup of water. You'll want to dip the end of the pipe and the countersink bit into the water every few seconds.

Wet the pipe and the countersink. Use a HIGH SPEED electric drill. If you try it with slow speed, it will just bind and get stuck.

Put the countersink bit into the end of the pipe and give it a burst of power. Just half a second, or enough to bore down 3 millimeters or so. Pull the bit out while it is still spinning. If you let it come to a stop inside the pipe, it can get stuck.

Use your finger to clean out the burrs in the pipe. Wet the pipe and countersink again, and repeat until the countersink bit goes about 2-3 mm below the end of the pipe.

Avoid the temptation to go too fast or too long in each burst. If you do, the bit will get stuck in the pipe!

After you're done, you can cut a piece of 1/2" copper pipe to the length you need to fit into the bored holes. The fit will be easy, but snug enough that PL adhesive should create a good seal. Remember to lightly sand the pipe where you are going to glue it, and squirt some water on the PL adhesive (because it cures with water) before you insert the copper pipe into the PVC pipe.



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Re: At my wit's end figuring out o-ring seal on launch tube

Post by onefromb5 » Mon Jul 03, 2017 4:59 pm

I used a chapstick tube for the interior pipe. Just had to disassemble the chapstick tube, rough it up with some sand paper and then use the pvc glue to cement it together. wait till it dries and then cand the excess glue from the cavity and outer pipe. roll the oring down and there u have your oring seal.

see my other post on the proper oring size.



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Re: At my wit's end figuring out o-ring seal on launch tube

Post by anachronist » Wed Jul 05, 2017 3:49 am

onefromb5 wrote:I used a chapstick tube for the interior pipe. Just had to disassemble the chapstick tube, rough it up with some sand paper and then use the pvc glue to cement it together.
Hey, that's clever... although the chapstick tubes I have here are a bit larger in diameter than the inner diameter of a schedule 40 PVC pipe, so I still need to ream it out a bit. Thanks though! Much cheaper than buying a copper pipe.



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Re: At my wit's end figuring out o-ring seal on launch tube

Post by Pere Ubu » Mon Sep 18, 2017 7:27 pm

Air Command suggests cutting a sheet of PVC from a bottle and rolling it into a cylinder for the connecting tube. I think when I finally get up to Home Depot and get some o-rings that's the route I'm going to take.

onefromb5: let us know how the chapstick tube worked. I'd try that, but I don't want to shell out for a chapstick I'd never use and I'd have doubts about its holding up to pressure - I'm willing to be proven wrong on that, though.


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Re: At my wit's end figuring out o-ring seal on launch tube

Post by anachronist » Mon Sep 18, 2017 10:44 pm

Cutting a sheet of plastic from a bottle and rolling it into a cylinder won't give you a good seal. Not only do you have a seam, but you also have a step AT the seam for the o-ring to cross. I'd spend a dollar on a tube of Chapstick before dealing with the headaches associated with a rolled up tube. Also, Home Depot doesn't carry the correct size o-rings (1/8" thick and 5/8" inner diameter). I had to order some from Ebay. Now I have about 90 more o-rings than I need. As I wrote above, I ended up using a 5/8" countersink drill bit to ream out the inside of the pipe to fit a piece of 1/2" copper pipe (which is 5/8" outer diameter).



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Re: At my wit's end figuring out o-ring seal on launch tube

Post by retrotec29 » Fri Sep 22, 2017 10:43 am

I think what you are looking for is some 3/8" CPVC. The CPVC is much more common in the 3/8" size. It's true that CPVC and regular PVC do not play nice with the regular plumbing cement. But they will work just fine with a good marine epoxy. The 3/8" OD of the CPVC is a little big for the schedule 40 1/2" ID. But that can be fixed with a little sanding. I chucked a tape covered dowle in my drill press and friction fit the CPVC on it. After about 5 minutes with some 80 grit the fit started to happen. Then I cleaned up the center to a 400 grit finish. The inside of the 1/2" PVC was prepped with 80 grit as well. After cleaning everything with alcohol I used a generous amount of epoxy on bolth sides of the joint. A pice of 1/8" ply makes a nice spacer for the O-ring gap. After cleaning away the extra epoxy I taped it up on a pice of aluminum L to align everything. A door jam works well to. I have several launch tubes built this way and have had no problems.


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