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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.


Hello Everyone. It is that time of year again. Time to approve changes to the competition rules for the new year.

You Can vote at:
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AYY GUYS I'M FREAKING BACK

Discussions about rockets, construction materials, adhesives, nozzles, nosecones and fin design.
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Alien Space Agency
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AYY GUYS I'M FREAKING BACK

Post by Alien Space Agency » Fri Jul 21, 2017 10:20 pm

Sorry for a year of hiatus. I lost motivation for water rockets and switched to SimplePlanes (tell meh if u have SP).

So, I am hoping to launch again this year and have a better chance of reliability in my rockets. Here will be the most important changes:

1. OD SEALING LAUNCHERS

These things kill two birds in one stone. Firstly it has better sealing and higher ejection force, and secondly it has better structural stability (the bottle lip sits square in the 32mm PVC coupling. Did I mention that I won't be using Gardenas anymore?)


2. KISS

Keep it simple, stupid. Berggren's crushing sleeve and Fischer's friction nozzles will be the only stuff I'll ever use. That way I can cut down on weight and fly higher.



Before I took a break from WRs last year I tested Elmer's™ Glue-All MAX. And it is freaking awesome. Dries in 8 hours, ready to use in three days. It holds up to pressures very well. Unless it crashes. Then it'll leak. But I think it won't explode.


Max pressure I'll ever use is 120psi, lowest will be 70.


Nothing can stop a curious mind from inventing- Anonymous

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Re: AYY GUYS I'M FREAKING BACK

Post by anachronist » Sun Jul 23, 2017 7:24 pm

Cool, welcome back. I joined a few months ago, and I feel like I'm the only person active here lately, except for the WRA2 admin.

What is a "32mm PVC coupling"? As far as I can tell, that looks like a metric size not available in the United States (or at least at none of the shops in my area). The sizes in my local hardware stores are for 1/2", 1.25", 1.5", and 2" PVC pipe. I have a 1/2" PVC coupling in front of me. Its inner diameter would have to be bored out to fit around the lip of a PET soda bottle.

I've been building my own launcher, using the standard inner o-ring seal. I was dismayed by the information I found online, which had two big problems:
  • There isn't really anything that slips snugly into the inside of a 1/2" schedule 40 PVC pipe. Supposedly 5/8" pipe does, but it isn't easily available. I found out a 1/2" copper tube is 5/8" diameter, so I used a 5/8" countersink bit to bore out the inside of a pipe to fit the copper tube. It took me several tries to get the bore centered properly. That copper tube becomes the seat of the o-ring. Someone else pointed out to me (in this thread) that a plastic Chapstick container also works well as an o-ring seat, but also requires the PVC pipe to be bored out.
  • I saw recommendations about bonding metal parts to plastic parts (such as a tire valve stem or hose barb to a PVC end cap), which I thought was a bad idea. So I made a launcher using only threaded NPC fittings. I put pictures in this thread.



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Re: AYY GUYS I'M FREAKING BACK

Post by Alien Space Agency » Sun Jul 23, 2017 10:13 pm

32mm - 1 1/2.

I'm not sure if this size is still available here (in the Philippines) but if I don't get it i will look for alternatives.

One problem with this is that, the coupling is thick (6mm) and it may require me to use big zipties just so it can lock the bottle.

I've been into ID sealing launchers, and I hate it. I just don't have the materials to keep it from bending. With OD sealing launchers, it has the advantage of keeping it steady (as long as the coupling, bottle lip and ties are hard up against each other).


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Re: AYY GUYS I'M FREAKING BACK

Post by anachronist » Mon Jul 24, 2017 12:23 am

Well... except a 1-1/2" PVC pipe coupler in the US has an inner diameter of 1.9" or 48mm, which is way too big.

The outside of a bottle lip is 25.07mm according to the international specs at https://www.bevtech.org/assets/Committe ... 253-19.pdf - I don't see any US-sized coupler that will work. A 3/4" coupler has an inner diameter of 26.67mm, which would have a clearance all around of 0.8mm, which would fit 1/16" thick o-ring, but not tight enough for me to feel comfortable about the seal. One other option would be to use a bigger o-ring and not try to seal around the lip, but seal the end of the lip with the o-ring seated on the ridge inside the coupler. If that's what you're doing, then as you said, you'd need big strong zip ties to hold that.

I don't have a problem with the launch tube bending at my ID o-ring seal, but that may be because I'm sleeving a short copper pipe into the PVC pipe to provide a seat for the o-ring. Glued with a generous amount of PL adhesive, it's pretty stiff.



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Re: AYY GUYS I'M FREAKING BACK

Post by Alien Space Agency » Wed Jul 26, 2017 4:19 am

Also I just realized... yeah they dont fit. I remember when I first used 32mm couplers it can fit the cap inside... loosely.

Oh no, I may be going back to the good old ID seali~

Oh wait, how about a hybrid?

It seals from the inside, but is held tight by the lip and ties.


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Re: AYY GUYS I'M FREAKING BACK

Post by anachronist » Thu Jul 27, 2017 1:51 am

If you have a 3D printer that prints PVC, you can fabricate your own launcher part using both an inner and outer o-ring. Although my feeling is, an inner o-ring is so simple, and there's plenty of room in the neck of a bottle for more than one o-ring if you need extra sealing. Also if you can find silicone grease (my local hardware store has it in the plumbing department), it improves the seal even more, and doesn't degrade the rubber of the o-ring like petroleum-based grease.



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Re: AYY GUYS I'M FREAKING BACK

Post by anachronist » Thu Jul 27, 2017 4:37 pm

Hey Alien Space Agency, how about a launcher that you screw the bottle into, so that it's held by the threads and not the collar, with the end of the bottle sealing against an o-ring, and then the thread grasping mechanism splits open to release the rocket?

It's been done, or at least a concept. See this page on the "Poppy launcher" - http://www.et.byu.edu/~wheeler/benchtop/poppy.php - looks like a good idea to me.

One could grind the collet parts by hand, or 3D print them. They don't have to be particularly strong because they've got multiple threads taking up the load. They just need to be held in place by a collar (like a PVC coupler). The threads and the o-ring at the bottom do all the work.



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