4 Liter Water Rocket Joining Question

[Lasheyda]

I am making a 4 liter water rocket and I have question about joining them:

Can you (like in a robinson coupling) glue (PL Premium) the bottles together so the "feet" interlock then drill a hole through it and use it just like that, without adding a robinson coupling? Will the glue hold the bottles together without the coupling?

The pressure would be around 80-120psi.

[Lasheyda]

Oh! I forgot to specify, I will be trying to join two 2 liter bottles.

[Nick B]

Well,


I dunno how you will drill a hole in them after gluing them together without an expensive and long drill bit,


That aside, I doubt PL will handle the pressure in that manner, being pulled by the bottle expansion and pressure trying to separate them. When peep use PL for cutting and joining bottles, they have a large overlap with a tight fit, plus the adhesive to cope with the shear.

All you can do is try.


I would try for you, but my family does not use beverages with 2 liter bottles often, so they are hard to come by for me and I have no current spares.

No harm in trying, but I'd sand the plastic areas before gluing,............



oops...








and drill the holes first.

[Lasheyda]

Thanks so much for taking the time to reply!

I happen to have a long expensive drill bit...

Well, thanks for your input it is appreciated. I will try it out and see if it works, will post results in a few days.(Would be sooner, but PL Premium takes so long to dry...)

[poriet]

I dont know whether PL Premium will hold 2 bottles together or not, but here ( in no particular order )
are some things I've learned which may be of interest.
Theres no advantage to holding 2 bottles together as you describe, as opposed to a coupling. What gives the 'spliced' bottles their drive is
removal of the restriction where the bottles join, isnt it?
Are you using a parachute? If so, then OK, but, if you rely on a soft landing like I do ( with a crumple-zone to help )
then the impact will maybe split the joint. I used to use a rigid coupler which simply broke when it hit the ground. Now I use a flexible link
instead.
I'm in the UK and know nothing of PL Premium. I've found a good PET glue: a polyurethane adhesive marketed as wood-glue! It seems to be functionally identical to some adhesive I bought in a small bottle marketed as Loctite SUMO. This dries very fast, although, to be fair, all my joints have had access to air as I've never done a splice. Perhaps these 2 adhesives are the same, and it would also take 2-3 days to dry inside a splice. If someone wants to send me a tube of PL Premium I'll see if they really are the same. I'll happily pay, but it all depends on the cost of carriage.
bruce

[Lasheyda]

I dont know whether PL Premium will hold 2 bottles together or not, but here ( in no particular order )
are some things I've learned which may be of interest.
Theres no advantage to holding 2 bottles together as you describe, as opposed to a coupling. What gives the 'spliced' bottles their drive is
removal of the restriction where the bottles join, isnt it?
Are you using a parachute? If so, then OK, but, if you rely on a soft landing like I do ( with a crumple-zone to help )
then the impact will maybe split the joint. I used to use a rigid coupler which simply broke when it hit the ground. Now I use a flexible link
instead.
I'm in the UK and know nothing of PL Premium. I've found a good PET glue: a polyurethane adhesive marketed as wood-glue! It seems to be functionally identical to some adhesive I bought in a small bottle marketed as Loctite SUMO. This dries very fast, although, to be fair, all my joints have had access to air as I've never done a splice. Perhaps these 2 adhesives are the same, and it would also take 2-3 days to dry inside a splice. If someone wants to send me a tube of PL Premium I'll see if they really are the same. I'll happily pay, but it all depends on the cost of carriage.
bruce

Thanks!

I use a parachute.

I was wondering because I don't have easy access to the materials for a robinson coupling.

A flexible link? Like to join two bottles together bottom to bottom (like a coupling?) If so, i would be interested in knowing how you made that. (Actually, i would like to know either way )

Ok, so here is the bottom line, I need a easy way to join two 2 liter bottles together (without a splice) to form a pressure chamber. Any ideas?

[poriet]

Without a diagram, this may be tricky to follow, but I join two 2L bottles either bottom to bottom or bottom to top.
I have tried various method and the one I favour is this:
You pull the brass valve from discarded bicycle inner tubes and these must be threaded PRESTA valves.
You need 2; remove the valve, discard the rubber tube.
Remove the threaded brass tube from the rubber inner-tube by violence. A lot of rubber will remain stuck to the end. Burn it off with a tiny
blow-torch. You will now observe that the threaded tube has a slightly wider diameter where it was vulcanised into the rubber. This is important.
Assume you are connecting the bottle bottoms together. There seem to be 2 types of PET bottle. At the middle of the base, the plastic will
be slightly curved. Some bottles are convex, some concave. You want the sort ( IMO ) with the curve facing the top of the bottle. This may be a minor point.
Drill a hole 5.5mm dia in the base. You will probably need to remove swarf. I have done this by using a flat wood-bit and a very long rod.

You now screw the brass valve into the hole. The valve is 6mm and so it needs force. Keep it straight! It cuts its own thread, which helps, but isnt air-tight, unfortunately.
I now put a rubber washer, a metal washer and a nut on either side of the bottle. Using lots of silicone sealant I tighten down. You now have 2 bottles with hopefully air-tight tubes in the base.
The two metal ends are joined by a small piece of rubber tubing. This gives you a flexible connection which wont break if the wrocket crashes.

The wider end of the valve-tube helps the link. The rubber tubing slides over the end and can be held just by a plastic cable tie. Neat, huh!

To be fair, I havnt launched it yet, The downside is that the bottle-bottle connection is very small ( a hole only 2mm dia ). Whether this a terribly important, we have yet to discover.

HTH
Bruce

[Lasheyda]

Thanks so much for describing it! You described it well.

Sounds interesting, I will have to try it soon.

Any ideas on how to easily connect two 2 liters bottles together? (No splice please.)

[Nick B]

Any ideas on how to easily connect two 2 liters bottles together? (No splice please.)

Drill n tap for 1" NPT

Add 1" poly nipple

Apply adhesive to inner side of nipple/bottle unions



Launch in a really big open area.

[Lasheyda]

Thanks a bunch! I have a couple of brass and plastic (irrigation PVC) nipples lying around, would love to put them to use!

Have you ever tried this yourself?

Thanks again!

[Nick B]

Thanks a bunch! I have a couple of brass and plastic (irrigation PVC) nipples lying around, would love to put them to use!

Have you ever tried this yourself?

Thanks again!

I would avoid the brass ones. They will unsettle the balance of the rockets and add unneeded weight.

Joining PET bottles is a huge advantage due to best air flow and minimal weight addition, but if you must use robinson coupling methods, stick to largest openings you can effectively apply. Pressure and water the same, the joined rocket should always beat out the coupled rocket, So keep that in mind.


Yes, I use irrigation nipples.

At this time, I like to keep my rockets modular, with interchangeable components.

I need to add a few comments on the process:

Careful how deep you tap, you want the thread as tight as possible.
If possible, tighten the bottles as an assy. while under some pressure, to include expansion and to set the bottoms firmly against each other.
Remember to apply your preferred adhesive to the inner side of the bottle.
(normal robinson process uses a seal between bottles.)
You may need to apply adhesive more than once.

There are no nuts used.

Done properly, you should be good to 110/120 psi

[Lasheyda]

Thanks for the comments!

I tried with the brass nipples and two 1 liter bottles (just for a pressure test), with the brass ones if you drill the right sized hole they tap themselves! It pressure tested (with no adhesive, just the brass nipple threaded into the bottle) to 40 PSI! Right now it is curing (PL Premium) pretty psyched about it.

The opening on the plastic irrigation nipples is just shy of 3/4 of an inch, I'm going to need a new drill bit (None of mine are big enough.)

I see what you are saying about the splice, more air flow/less weight, but it takes (for me anyway) a long time to do a splice (probably just my technique.)

Anyway, thanks again for the info, will post results soon...

[U.S. Water Rockets1]

I am making a 4 liter water rocket and I have question about joining them:

Can you (like in a robinson coupling) glue (PL Premium) the bottles together so the "feet" interlock then drill a hole through it and use it just like that, without adding a robinson coupling? Will the glue hold the bottles together without the coupling?

The pressure would be around 80-120psi.

This method of coupling bottles together is not likely to be very reliable. We have seen that when you use an adhesive on PET bottles the PET material can peel away very easily. The strength of the plastic bottle to resist peeling is the only thing holding your bottles together. The glue joint should fail at a lower pressure than a spliced overlapping joint. In a properly engineered splice joint, the only force applied to the joint is a shearing force, and the glue is most resistant to shearing. Simply butting the bottom of the bottles together makes a joint only peeling forces, which is the weakest type of joint.

[Lasheyda]

I am making a 4 liter water rocket and I have question about joining them:

Can you (like in a robinson coupling) glue (PL Premium) the bottles together so the "feet" interlock then drill a hole through it and use it just like that, without adding a robinson coupling? Will the glue hold the bottles together without the coupling?

The pressure would be around 80-120psi.

This method of coupling bottles together is not likely to be very reliable. We have seen that when you use an adhesive on PET bottles the PET material can peel away very easily. The strength of the plastic bottle to resist peeling is the only thing holding your bottles together. The glue joint should fail at a lower pressure than a spliced overlapping joint. In a properly engineered splice joint, the only force applied to the joint is a shearing force, and the glue is most resistant to shearing. Simply butting the bottom of the bottles together makes a joint only peeling forces, which is the weakest type of joint.

Very interesting...

Thanks for the advice/information I never knew that. Very cool.

Well, i will see how this "coupling" works and on the actual rocket I will probably end up doing a splice...

[Lasheyda]

Well, I tested the coupling (of sorts) and went up to 80PSI no problem! (I need at least 80 for my crushing-sleeve staging mechanism (one-way valve from a bicycle tire) )

So, I might end up doing a this coupling on the main rocket...(A two stage-4 liter booster (two 2 liters) and 2 liter sustainer (two 1 liter) )

My only concern will be the extra weight from this method...

Anyway, thanks a ton for all the advice! I will post results of my rocket when it is finished ie... height, max pressure, video and so on and so forth...

Thanks again you guys!