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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.

In Search of Stability

Discussion about Compressors, hose, pipes, fittings, launchers, release mechanisms, and launch tubes.
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Pere Ubu
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In Search of Stability

Post by Pere Ubu » Mon Sep 18, 2017 7:43 pm

No, not for my rockets (an entirely different post), for my launcher. Using inspiration from a YouTube video, I've built a mobile launcher that fits in a backpack (I'm actually carrying it in a shopping bag right now) and has a threaded elbow connection for use with both a Gardena launch head and a Clark-cable launch head. Problem is, though, that the Gardena demands a heavier pull on the launch cord to release, and I'm stuck trying to figure out how to make the base stable enough to stand the stress while still remaining easily portable.

The base is a squared-off "W" shape with the launch tube at the center and the two legs about 2 times longer, as such:

| |
|_|_|

I have four tent pegs I bought, and I can use them to anchor the base, but is it best to anchor near the center, where the launch tube is, or on the far ends?


Why your rocket is unstable = CN⍺1/2⍴v2⍺Ar

Northeast Pataphysical Institute, Aerospace Division

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anachronist
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Re: In Search of Stability

Post by anachronist » Mon Sep 18, 2017 10:49 pm

Pere Ubu wrote:Problem is, though, that the Gardena demands a heavier pull on the launch cord to release, and I'm stuck trying to figure out how to make the base stable enough to stand the stress while still remaining easily portable.
...
I have four tent pegs I bought, and I can use them to anchor the base, but is it best to anchor near the center, where the launch tube is, or on the far ends?
I also use tent pegs. I just use 2 of them, on on each side of the launch tube, anywhere from 6" to 1 foot apart. I find it doesn't really matter where they are as long as they don't interfere with your pull string.

I don't use a gardena nozzle (my simulations show it makes no difference, in fact the optimum nozzle size is the diameter of your rocket) but I have to pull about 25-30 pounds on the string before the restraining collar pulls down. It's easy to launch at 40 psi, but at 100 psi it requires a huge amount of force. I don't know how to solve that problem. My string (nylon braid) stretches about 3 feet before it pulls down the collar.



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Re: In Search of Stability

Post by Pere Ubu » Tue Sep 19, 2017 3:21 pm

Looking at the Gardena head, it would seem more efficient to have a sort of lever pushing down on it, but I'm not sure how that would fit with "portability".

This obviously demands more thought. HA HRM.


Why your rocket is unstable = CN⍺1/2⍴v2⍺Ar

Northeast Pataphysical Institute, Aerospace Division

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Re: In Search of Stability

Post by Pere Ubu » Fri Sep 22, 2017 1:45 pm

Here is a pic of my launcher, ready for duty.

As well, a closeup of one of the issues I had today; the Schrader valve I bought is just snugged over the polyethylene pressurizing tube, which I thought would work just fine but actually blows out at about 60 psi. Good for testing rockets in the yard, bad for actual flights. I'll either cut the valve down to bare metal or use a hose barb and put a ring clamp over it to avoid the problem in the future.
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Why your rocket is unstable = CN⍺1/2⍴v2⍺Ar

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Re: In Search of Stability

Post by retrotec29 » Fri Sep 22, 2017 3:23 pm

I found a 1/4" npt to schrader fitting in the air line section at the local Ace. It is used most often as a test fitting in plumbing. That with a 1/4" npt to hose barb fitting should get you there. Just add a clamp or zip tie. Here is one on the end of my inline pressure manifold.
IMG_0535.JPG
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Re: In Search of Stability

Post by Pere Ubu » Fri Sep 22, 2017 9:54 pm

BTW, you know what's really good for holding a launcher down?

The water jug you brought to fill rockets with!


Why your rocket is unstable = CN⍺1/2⍴v2⍺Ar

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Re: In Search of Stability

Post by anachronist » Sat Sep 23, 2017 3:23 am

Pere Ubu wrote:Here is a pic of my launcher, ready for duty...
Ummm. I recommend you don't use tire valves or hose barbs. Use a hose with NPT threaded ends, industrial compressed air fittings, and an air tank valve (which is the same size as a tire valve for hooking up a tire pump, but it has NPT threads on the other end). See my post here for pictures of what I mean. Compressed air fittings aren't expensive, a lot less fuss, and withstand way more pressure than you'd ever put into a soda bottle.



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Re: In Search of Stability

Post by Pere Ubu » Sat Sep 23, 2017 9:12 am

anachronist wrote:
Pere Ubu wrote:Here is a pic of my launcher, ready for duty...
Ummm. I recommend you don't use tire valves or hose barbs. Use a hose with NPT threaded ends, industrial compressed air fittings, and an air tank valve (which is the same size as a tire valve for hooking up a tire pump, but it has NPT threads on the other end). See my post here for pictures of what I mean. Compressed air fittings aren't expensive, a lot less fuss, and withstand way more pressure than you'd ever put into a soda bottle.
I'll have to write that stuff down for my next shopping trip to Home Depot. I've been looking at the compressor and air-power equipment, but I have no idea what is what and what goes with what. I cut what I now think were HPT fittings off the polyethylene hose, but they make a set of replacements. (BTW, for any doubters, he's not kidding about inexpensive - the set of brass pressure connectors I bought at Home Depot was a whole $4, only a little more than the pair of tubeless Schrader valves I bought at Wal-Mart.)

BTW the barb leading into the launcher isn't even cemented in place; it rests in the threaded PVC cap and butts up against the PVC tube forming a nice seal with no glue involved. (The idea was to unscrew one of the threaded caps & pour water out of the launcher; the caps are tougher to unscrew than I thought so I just pull the hose loose and pour water out the barb.)


Why your rocket is unstable = CN⍺1/2⍴v2⍺Ar

Northeast Pataphysical Institute, Aerospace Division

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Re: In Search of Stability

Post by Pere Ubu » Sat Sep 23, 2017 9:16 am

anachronist - I see that the blue hose in your post is the same one I bought and cut up. D'OH!


Why your rocket is unstable = CN⍺1/2⍴v2⍺Ar

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Re: In Search of Stability

Post by Pere Ubu » Sat Sep 23, 2017 9:19 am

retrotec29 wrote:I found a 1/4" npt to schrader fitting in the air line section at the local Ace. It is used most often as a test fitting in plumbing. That with a 1/4" npt to hose barb fitting should get you there. Just add a clamp or zip tie. Here is one on the end of my inline pressure manifold.

IMG_0535.JPG
May I just say I love the tripod base. It's got me thinking how I might do a PVC tube tripod that's collapsible.


Why your rocket is unstable = CN⍺1/2⍴v2⍺Ar

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retrotec29
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Re: In Search of Stability

Post by retrotec29 » Sat Sep 23, 2017 10:57 am

Thanks. The legs are friction fit and the chains have clips on the ends for easy disassembly. I was tying to build a pad with a low center of gravity. That’s why the air line comes out the bottom of the head. There is also 16” long threaded rod in one leg for elevation adjustment from 0-15 degrees. During testing the the entire thing would leap off the ground with a launch of 100 psi or more. Even with schedule 40 3/4” PVC and the chains the problem persisted. So I added the adjuster and sand in the other legs. That did the trick. Not light but it can break down very small and is stable.
A017317F-A944-4D68-A541-950141003C9A.jpeg
9379294B-D12D-410E-BAF2-797CA5977665.jpeg
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Re: In Search of Stability

Post by anachronist » Sat Sep 23, 2017 11:34 am

Pere Ubu wrote:May I just say I love the tripod base. It's got me thinking how I might do a PVC tube tripod that's collapsible.
The tripod wouldn't work for me. I find at 100 psi I have to pull about 30 lb on the string to release the collar. I'm afraid the tripod would probably slide or fall over if I built that. If I could just figure out how to make that PVC collar slip more easily.....

I designed my launcher to be simple, lightweight, and portable. The base is just a "T" laying on the ground, with the launch tube sticking up from the leg of the T.

The launch tube assembly unscrews from the base, but I find that repeated screwing and unscrewing has caused the two pieces to butt up against each other, putting the thread seal at risk, so I just leave it in place. The stabilizer leg slides into the snap-T end cap. My compressor hose fitting is on the opposite end. Two tent pegs, one on either side of each pull string grommet, prevents the launcher from slipping when I pull the string.

It's quite stable and non-moving on the ground. It's easy to level by putting a rock or something under one of the ends. And easy to drain when you're done flying your rocket.

Here's a picture of it with the stabilizing leg removed from the T end cap:
launcher_a.jpg
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Re: In Search of Stability

Post by retrotec29 » Sat Sep 23, 2017 3:05 pm

Wow! 30 lbs on the pull sounds really high. I tried about six different combos of ties and collars before I settled on mine. The GB zip tie and schedule 20 collar I settle on has no play and moves quite freely. In fact I had to remove my little metal keychain clip on the end of the string where it attaches to the loop. The act of picking the string up off the ground with the clip caused the collar to move at 100 psi. Actual pull is probably 5-10 lbs. I will put a fish scale inline next time to get you a solid number.


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Re: In Search of Stability

Post by retrotec29 » Sat Sep 23, 2017 3:06 pm

Wow! 30 lbs on the pull sounds really high. I tried about six different combos of ties and collars before I settled on mine. The GB zip tie and schedule 20 collar I settle on has no play and moves quite freely. In fact I had to remove my little metal keychain clip on the end of the string where it attaches to the loop. The act of picking the string up off the ground with the clip caused the collar to move at 100 psi. Actual pull is probably 5-10 lbs. I will put a fish scale inline next time to get you a solid number.


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Re: In Search of Stability

Post by anachronist » Sat Sep 23, 2017 5:07 pm

For the collar, I can't find schedule 20 anywhere, so I used a schedule 40 1-1/4" PVC pipe reamed out carefully on a lathe until it slipped smoothly over the cable ties. It releases just fine at lower pressures, but it's really tight at 100 psi.



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