Poseidon- High Powered Modified H2O Rocket
12 x 1.25L Soda bottles spliced with external sleeves using Sikaflex 11FC.
The Poseidon's booster module is not yet complete, it has another 7L of capacity to be added. The booster will then be combined with several other modules before Poseidon is ready for launch.
You may also like to note the quick attach/release fin system. They are saw toothed sign holders normally found in stationary shops. They allow the fins not only to be attached perpendicular to the body, but also provide a reproducible fixing method.
Although this is not my preferred method of splicing, Aircommand.com has put together an excellent tutorial which can show you the principal. My method (which I DID NOT create) involves shrinking the inner bottle with our curled lip. Air command in that tutorial reduces the outside sheath from a larger capacity bottle to suit. The method I use does not call for the use of different sized bottles or a blow torch, just boiling water. Air Commands Splicing tutorial