Bottle Splicing

A very important technique for building bigger and better rockets is splicing. It is generally quite difficult to find bottles larger than 2L (although there are a few 3L bottles around) and so for volumes larger than that you must use a method to join bottles together. Besides, larger volume bottles get wider rather than taller (I would guess this is to ensure that they can still fit inside a fridge), and since drag is a function of the cross-sectional area of the rocket, which in turn is a function of the square of the radius, larger bottles generate much more air resistance. For example, a bottle with diameter double that of another bottle will generate four times as much drag.

Before I talk about splicing, I should probably mention the Robinson Coupling (Dave Johnson's website), another method used to join bottles. This joint consists of a threaded lamp rod, two rubber gaskets, two washers and two nuts. You can join bottles neck-to-bottom, neck-to-neck or bottom-to-bottom using this method. You drill holes through the two ends you want to join, insert the threaded lamp rod through the holes and then tighten the nuts onto the lamp rod (with the gaskets and washers of course). This method is easier to get right than splicing, but in the long run it's definitely worth perfecting the splicing technique for a number of reasons. First of all, a Robinson coupling is heavier than a splice because of the metal parts involved - splicing only requires some glue which is relatively light. Secondly, the Robinson coupling only provides a very narrow 'throat' between the joined sections, meaning that it takes much longer for the air to pass through the coupling all the way down to the nozzle. This means less thrust (although the 'burn' time is longer) and so the rocket won't go as high as it would if it were spliced instead. For more information about this method, click on the link above.