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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.

My launcher without gluing metal to plastic

Discussion about Compressors, hose, pipes, fittings, launchers, release mechanisms, and launch tubes.
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anachronist
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My launcher without gluing metal to plastic

Post by anachronist » Mon Jul 17, 2017 8:10 pm

The tutorials and videos I've found online about building a water rocket launcher all include instructions about bonding metal to PVC -- a very bad idea in my opinion. Common examples are bonding a hose barb to PVC, or bonding a tire valve stem to a PVC end cap.

Nonsense!

Why do that when you can accomplish the same thing for a lot less work using actual air compressor fittings? Why use a tire valve stem when you can get an air tank valve stem that's exactly the same size? And with compressor fittings, you can detach the hose from your launcher for easy storage.

So here's what I got:
1 - 50 foot coil of 200 psi compressor hose with 1/4" NPT threaded ends ($15 from Amazon)
2 - brass 1/4" NPT industrial spring-collar couplers, femaie 1/4" NPT end (about $6 total)
1 - brass industrial plug to mate with coupler, 14" NPT male end ($2)
1 - brass industrial plug to mate with coupler, 1/4" NPT female end ($2)
1 - tank valve stem (came in a package of 2 from Amazon) ($6)
1 - brass tee connector, 1/4" NPT female-male-female ($2.50)
1 - highly accurate (1%) pressure gauge (I lucked out when the price was briefly $5, now about $30, but you can get 2% accurate gauges for less than $10)
1 - brass pipe adapter, 1/4" female NPT to 1/2" male NPT (surprisingly expensive, about $8) - this screws into the PVC adapter, below.
1 - PVC elbow with glue-able slip socket on one side and 1/2" NPT female thread on the other - this goes on the launcher (less than $1)
1 - roll of teflon tape for thread sealing ($1)

Some of the items came from Amazon, everything else came from Home Depot. The tee connector and pressure gauge are optional; you can get a female tank valve stem that screws directly onto the male end of the pressure hose. But my tire pump has a very inaccurate pressure gauge, and since I'm interested in competition, I want it to be honest and accurate, so I bought an accurate pressure gauge.

The attached images show all the parts put together, connected for rocket flight, and disconnected for easy storage.
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Re: My launcher without gluing metal to plastic

Post by addstogether2 » Sat Jul 22, 2017 12:34 am

Looks good.

My question is why use plastic PVC at all in the launcher.
Metal all the way I say.

However each to their own, 15mm steel pipe and fittings from your local hardware wont cost much more.

Regards Al


Life is like riding a bicycle. To keep your balance you must keep moving.
"Albert Einstein"

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Re: My launcher without gluing metal to plastic

Post by anachronist » Sun Jul 23, 2017 7:15 pm

addstogether2 wrote:My question is why use plastic PVC at all in the launcher.
Reasons to use plastic:
  • PVC is easy to work with. Metal plumbing requires either soldering (in case of copper) or threading (in case of steel).
  • PVC is cheap. If you make a mistake, it doesn't cost much to do it over. And it isn't much effort. Not so with metal.
  • Metal is just more weight to lug around. I want my small child to be able to carry everything if needed.
  • A PVC launch tube is (to me) desirable over metal. A steel pipe isn't smooth, and a copper pipe is more subject to dents.
  • Corrosion may not be a big deal, but steel does corrode, and makes a mess of things it touches, although this is a minor consideration.
  • We live in a condo. I don't have a garage or a shop. My wife would object to me using a propane torch to solder copper together in the living room. :) Lay down a newspaper, open a window, and working with PVC is pretty clean.
Admittedly some of those points are debatable, but those were my reasons.



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Re: My launcher without gluing metal to plastic

Post by addstogether2 » Mon Jul 24, 2017 7:33 am

Hi as I said each to their own.

If you are not doing high pressure then should be fine.

I have a 3.5 x 3.5m shed / man cave to work in so I am spoilt.

I use metal as I use it with cub scouts and it has to comply with safety standards and all the red tape and hoops I have to jump through.


Regards Al


Life is like riding a bicycle. To keep your balance you must keep moving.
"Albert Einstein"

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Re: My launcher without gluing metal to plastic

Post by anachronist » Mon Jul 24, 2017 11:43 pm

addstogether2 wrote:If you are not doing high pressure then should be fine.
I think my weakest component is the hose, rated at 200psi. Everything else can withstand much higher pressure. But I don't expect to go past 120psi using soda bottles. I may try to burst a few bottles of various brands and sizes just to see what their limits are, but that should still be well within the limits of the hose. And my fancy new super-accurate pressure gauge only goes up to 160psi anyway. This setup will work well for me and my son for quite a while, I imagine.

I guess another thing influencing my launcher design are BIG disincentives here in Silicon Valley to try for high-pressure high-altitude rockets because (a) it's very difficult to find enough open space, (b) the wind is always blowing, (c) all the air traffic flight paths require notifying the government of any high-altitude flights. Point (c) isn't a problem for me, but otherwise I'm unwilling to risk losing a fully-instrumented rocket in a tree or on top of a building. The biggest nearby open space is San Francisco Bay or the Pacific Ocean, but there's no way I'd let my fully-instrumented rocket land in sea water!

The kite flying field near the Navy base here is just about right for recovering a 100psi bottle rocket in a light wind, so I designed a launcher for that.

The purpose of my initial post was to point out the compressor quick-release fittings, which I haven't seen on other launchers. They turned out to be convenient and not costly.



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Re: My launcher without gluing metal to plastic

Post by anachronist » Tue Aug 01, 2017 1:06 am

Hmm. This weekend my son and I pressure tested the launcher with an empty bottle, and it has a slow leak. Now I have to figure out where it is.... Removing the tire pump helped a bit (the tire pump has a leak for sure) but the pressure was still going down slowly, more than one would expect from the air temperature elevated by compression cooling in the bottle. Also, with four air launches, we noticed the cable ties creeping up, suggesting we need additional hose clamps.

The release collar is HARD to pull down. I wonder what the secret is to making it easier. For now, I've bought a couple of tent pegs to drive into the ground in front of the launcher to hold it in place while the string is pulled.

This is my first-ever water rocket launcher, and it's a learning experience.

If the leak is in any of the threaded fittings, that's easy to unscrew, re-seal, and re-fit. If it's in a PVC joint, I can cut it off and glue on new fittings. If it's the o-ring seal, I'm not sure what to do about that. It's very tight in the bottle, but I'm more concerned about the seal between the launch tube and the copper o-ring seat - I glued that in with PL adhesive. If the leak is in the hose connections, well, the hose came that way so I guess I'd live with it. We shall see....



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Re: My launcher without gluing metal to plastic

Post by addstogether2 » Sun Aug 06, 2017 5:33 pm

An easy way to check for leaks is either full submersion if possible while low pressured or soapy water over all joints while pressurized.


Life is like riding a bicycle. To keep your balance you must keep moving.
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Re: My launcher without gluing metal to plastic

Post by anachronist » Mon Aug 07, 2017 1:43 pm

addstogether2 wrote:An easy way to check for leaks is either full submersion if possible while low pressured or soapy water over all joints while pressurized.
That's what I had planned, but it turned out to be unnecessary. The leak was evidently due to the cable tie restraints slipping slowly, leaving the o-ring seal right at the lip of the bottle, not making a good seal. When I repositioned the cable ties and secured them with 2 extra hose clamps (I was using only one), I pumped it up to 80 psi to see what would happen. It held pressure very well, losing just 2 psi over 30 minutes. Good enough. And for all I know it may still be the o-ring seal.

I did get this new foot pump because it's the only foot pump I could find that goes above 100 psi, and my 35-year-old hand pump's internal valve has begun failing around 75 psi. The new foot pump has a gauge that goes to 180 psi, but the gauge is way off, reads 20% too high when compared to the calibrated gauge in my photo in this thread. Otherwise it's a decent foot pump.



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Re: My launcher without gluing metal to plastic

Post by addstogether2 » Mon Aug 07, 2017 6:21 pm

Nice looking twin barrel pump.

I just purchased a $15 job from k-mart

http://www.kmart.com.au/product/steel-f ... ed/1602312


I haven't calibrated it yet though.

Al


Life is like riding a bicycle. To keep your balance you must keep moving.
"Albert Einstein"

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Re: My launcher without gluing metal to plastic

Post by anachronist » Tue Aug 08, 2017 2:42 pm

addstogether2 wrote:I just purchased a $15 job from k-mart
http://www.kmart.com.au/product/steel-f ... ed/1602312
I haven't calibrated it yet though.
I got the foot pump because my 8-year-old couldn't manage the hand pump (it was as tall as him when the handle was raised). He can more easily put his weight into the foot pump, which becomes important at higher pressures. I got the twin barrel to speed up inflation. Even so, I'm still likely going to have to help my son after 50 psi or so.

Those built-in pressure gauges are notoriously inaccurate. As I said, mine reads 20% too high.... and if it reads too low (like the gauge on my older hand pump), it's a safety risk, you could burst your rocket if you, say, pump it to 120psi and it's actually 150 psi.

For that reason, and also if you're going to compete, I highly recommend buying a real pressure gauge with an accuracy rating, and hook it up inline with your pressure hose (in my picture above I did it with a T-connector and a tank valve stem). I lucked out when I got a nice 1% accurate gauge for less than $5 on Amazon before the price went up. Mine is this one: https://www.amazon.com/dp/B0087UAYTW
There are plenty of inexpensive 2%-3% accurate gauges for under $10, like this one: https://www.amazon.com/dp/B016MAOHRI
If you do buy a separate gauge, just be sure it has a published accuracy rating.



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Re: My launcher without gluing metal to plastic

Post by addstogether2 » Fri Aug 11, 2017 7:40 am

Thanks for that. I was looking at putting an inline pressure gauge and now definitely are going to. I already have the t connector along with what seems like half the plumbing section at bunnings hardware. The man cave is bursting at the seams.

Most of the time the working pressure I use is 115psi from my 25lts compressor connected to a generator. So knowing the accurate inline pressure will be handy.

Al


Life is like riding a bicycle. To keep your balance you must keep moving.
"Albert Einstein"

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