With such a large rocket, I needed something a little larger than a two litre bottle at the bottom so I used 2 three litre bottles. Construction Putting bottles bottom to bottom is the same as with the 3 x 2 litre rocket which I will not repeat here. The top to top mounting it quite different though. For this, I cut a length of 21.5mm PVC pipe (the grey stuff) and measured and cut out a skirt. Then, I covered the inside of one of the necks with epoxy resin and pushed the tube inside - the expoy acts like a vicous lubricant that then sets. Before it set, I made sure that there was a complete seal all of the way around the neck and there was no epoxy further up the outer part of the tube that would get in the way of the other neck when I did that one. Next, I put some extra epoxy around the outside of the join just to make sure that it was strong and air tight. I used 10 minute epoxy and once the first joint had set, I slid the skirt over, and repeated the process with the other side of the joint, making sure that the skirt was held in place nice and tight before the glue set. The nozzles are aligned pretty accurately and there is no significant straigtening required. As the epoxy joins are permenant, the order of construction becomes important as getting to the brass fittings to tighten them up becomes impossible. First, I made up the two sets of bottom to bottom bottles, a 2 x 3 litre and a 2 x 2 litre. Then, I added a 2 litre bottle to the 2 x 2 litre set and then, I joined the two 6 litre sections together. Each join uses a skirt and with the exception of the 2 - 3 litre join, they are all made from the side of an appropriately sized bottle. The join between the 2 litre and 3 litre bottles was made from the side of a 2 litre bottle and the top of a 3 litre bottle. I marked out and cut the hole in the top of the 3 litre bottle and cut it to length as in the diagram on the left. The idea behind the double support like this is that the 2 litre skirt supports the weight of the top 6 litre section, taped (or glued) at the skirt's top (being the same diameter), and resting on the shoulder of the lower 6 litre section; and the 3 litre skirt, is taped to the lower 6 litre section (being the same diameter) and supports the side of the 2 litre skirt, hopefully reducing any tendency of the rocket to bend. Once finished, make sure that the rocket sections are straight and tape them. Then you need to make the fins. The fins on this rocket (there are three of them) are each made from the side of a bottle, cut out and laid flat, folded in two and then spot-welded with a soldering iron in a square pattern, roughly every 2". I made a plan on a sheet of paper, including the shape of the fin and the positions to spot-weld. After spot-welding, the resulting sheet was fairly stiff but flexible enough to survive a hard landing. I turned up the edges to make a flat surface for taping to the rocket and positoned them at 120º intervals. Once I had done that, I added a parachute nose cone and the rocket was ready. Water Pressurising to between 4 and 6 BarG (between 60 and 90 psig) the amount of water required is approximately 2.3 litres. It turned out that this rocket is a little heavy and tall for the launcher H-Base (not the launcher, that took it quite well) so I ended up flying the rocket at around 40 psi using a circular nylon chute. It was also a little tall for me - standing at around 8 feet high on the launcher - I had trouble removing the tapes that held the NSA nose in place. Also, pumping into a 10 litre headspace was quite exhausting (40 litres of air just to get it to 4 BarG). It flew well - being close to such a large rocket it when it launched, with water coming from the nozzle at 25 m/s, it reminded me of those slow motion pictures of Saturn V rockets taken from the gantry - something that a rocket of this size really needs.
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Not stopping
at 6 litres, I had to go and double that to 12 litres. I
also had a go at using a (for me) new technique for
joining bottles together, neck to neck which, although
probably weighing about the same as using a brass
fitting, allows quicker air flow between the pairs of
bottles coupled so. 
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