This blog covers the day to day progress of water rocket development by the Air Command Water Rockets team. It is also a facility for people to provide feedback and ask questions.

Wednesday, October 14, 2009

New Splices

We have been using the same symmetrical splice technique on our 2L bottles for a couple of years now, but I have never quite been happy with it. We have had quite a few failures along the way, and the no-leak yield hasn't been as high as we would like. This meant a lot of wasted bottles and wasted time. The other issue has been the relatively low pressures the spliced pairs could hold. 130psi has been the limit we could practically push them to.

We used to use 2.25L bottles as the sleeve since they fit well over the 2L bottles and no shrinking was required. But recently we have found that some of the 2.25L bottles don't fit as well as they used to so it has been difficult to use them as well. To try to stop the leaks we have been using a different glue to do the sealing in the joint and PL to hold the splice together but this made the process more complex and we still ended up with leaks although less often.

So we are currently trialing a different technique that will hopefully yield better results. The technique is not new but a combination of several different techniques used together. We are using just PL glue for this splice and only the one type of bottle. I'll post the full technique on the main site, but here is a quick run down:

We first heat shrink about 2-3 cm of one of the bottles using hot water so that it fits inside another bottle of the same type. Then curl the edge on a pan. This has the effect of giving a tight seal where the heat shrink section ends and meets the other bottle since they are the same diameter at this point. We sand and glue these sections together. We then make up a sleeve about 4 cm wide made out of the same type of bottle. Because the diameter is the same we simply cut the sleeve to turn it into a strip. We then use another section of sleeve about 4 x 4cm and glue half of it to the sleeve strip to cover the gap. We let the glue dry for a couple of days. Then we glue the sleeve strip over the splice to provide further strength to the splice.

The drawback is that inserting one bottle into the other means that the volume is smaller, but not by too much.

We have had to previously reinforce the necks of the bottles with rings made from other bottles, but it only strengthened the necks not the rest of the bottle. This time we are going to use Richard Wayman's bottle-on-bottle technique to not only strengthen the neck but a significant portion of the rest of the bottle.

Hopefully this technique will yield better sealing results and will be able to take higher pressures.

The glue is curing at the moment so tests will be carried out at the next opportunity.



Steve [rockets-in-brighton] said...

George, is there some reason you think that a symmetric splice where you shrink both bottles into a central sleeve made from a third bottle of the same size would not solve your problem? I'm sure I read on the old newsgroup that the asymmetric splice that the pioneers tried first just didn't hold as much pressure as the symmetric that was developed later.

George Katz said...

Hi Steve,

That is a good question. Our biggest problem has been getting a good seal on the splices. When we assemble the regular symmetrical splice the place where the two bottles meet almost never have a good seal because of the shape of the curled edges against each other. The seal then relies on rest of the glue in the splice.
With the symmetrical splice you then have to worry about ensuring the seal at both ends of the sleeve.

We find that PL is quite porous and does not seal real well. This could have a lot to do with the fact that where we live there is high humidity.

With the single overlap splice you only have to seal the one join. The outer sleeve is there to assist in holding the bottles together rather than necessarily provide a good seal.

In the next web page update I am doing a full writeup on the splice tests with diagrams and our reasoning for the changes.


Steve [rockets-in-brighton] said...

Yes, my splices have the same problem of leaking because of the poor seal at the internal edges. I'm experimenting with using a line of silicone sealant at the meeting point before gluing the second bottle. Early days yet... Good luck with your splices.