Water Rockets - 2 Stage Rocket

The 12 litre rocket was originally intended to be a 2 stage rocket but I couldn't find the parts that I needed for the stage mechanism. After over six months of looking around, have come up with a working alternative (in size - not design) to Bruce Berggren's staging mechanism. The rest of the rocket is pretty much the same as for the 12 litre rocket.

Just to make this rocket a little different to the others, I decided to make it entirely from green bottles (I am not a heavy drinker by any means and getting the 3 litre bottles (only 3 litre cider bottles of a particular brand were suitable at the time) took some time to collect)


Putting bottles bottom to bottom is the same as with the 6 and 12 litre rockets which I will not repeat here (see the 6 litre rocket for details).

I made the fins for the sustainer in the same way that the fins for the 12 litre rocket are made, only making them slighly narrower. Each one has a slight twist at the bottom so that a spin of one to two turns per second is induced so that any error in the angle of the nozzle cancels out during the 'burn' (around 3 seconds for the water part of thust).

Each bottle 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. Unlike the 12 litre rocket, the top section just sits on the bottom section and is free to separate. I taped all of the skirt joints that needed taping but there is no reason why you should not use glue instead.

Note. Before you cut the inter-stage skirts to length, you should first make the Stage Mechanism

Once you have done this, cut the length of the 2 litre bottle diameter skirt so that the nozzle of the sustainer goes into the booster part of the mechanism far enough for it to be able to grip but with a sufficient gap for the one way valve to operate without being jammed against the end of the sustainer nozzle. Start by making it slighty too long and then trimming it as this cannot be done if you make it to short. If you do make it slightly too short, you can increase the gap again by adding extra Blu - Tack to the seal in the sustainer nozzle.


Booster: As the primary objective of the booster is to get the sustainer moving as fast as possible and then to fall away thus allowing the sustainer to continue on its ascent, I decided to have a go at making a booster without fins. The booster only affects the flight of the sustaner while it is thrusting and during all of this, the thrust is axial. Fins on the booster would not have any significant effect on the sustainer. Without fins and without the sustainer, the centre of mass (COG) of the booster is roughly in the same place as the centre of pressure (Cp) and therefore there is no reason why, once it has finished its thrust and jetisonned the sustainer, it should point in any particular direction. If it is pointing sideways, it will fly a lot slower than end on so the booster should just tumble to Earth.

Sustainer: If the sustainer was to hit the ground after falling from 400 - 500 feet, it would do serious damage to itself, therefore it needs some type of recovery to reduce this velocity. With such a long duration of ascent - using a restricted nozzle for a long burn - the rocket has a good chance of going too fast for an NSA deployment method to deploy. One thing is for certain, however, and that is that if the rocket is stable, it will be pointing downwards during the descent.

With the recovery method chosen here, the water that makes the rocket stable during ascent, pours out of the rocket when it tips over and, as the rocket loses weight at the front, the centre of gravity moves rearwards, soon making the COG and the Cp fall within 1 rocket diameter and therefore become aerodynamically unstable and, hopefully start to tumble, hitting the ground a lot slower than it would without (24m/s for straight descent and a lot slower for a tumbled descent).

I managed to make the sustainer have the COG and Cp in the same place (coincidence I assure you). The recovery is simply the top 5" (12.5cm) of a bottle with the neck cut off with a soldering iron. This is taped to the top of the sustainer and water is added when it is in posisition at the launch. The rocket empty (apart from a small amount of water in the top bottle, representing what would be left after a launch) weighed around 200g. to move the COG forward by 1 body diameter, I found that I needed aproximately 75g of water making a sustainer weight of 275g.


Pressurising to 6.5 BarG (95 psig) the amount of water required is approximately 2.25 litres in the booster and 1.15 litres in the sustainer. This should get it (with a 5mm nozzle) to around 450 feet. The water runs out at around 180 feet and it gets to apogee just before 6 seconds. If the recovery fails, it hits the ground at least 6 seconds later at a speed of 32 m/s or less (the deviation from these figures depends on how much weight it has lost by shedding the nose weight). Staying in the air for such a long time (over 12 seconds without tumbling or weight loss means that it has to be launched on a very calm day or great distances may have to be travelled.

On the way up, during acceleration, the water is forced away from the opening in the nose. During decelleration during ascent and during descent, the water goes towards the opening but during ascent, (hopefully) the water will tend to stay inside, only emptying when the rocket is inverted.

This is what was planned. Have a look at the pictures to see what actually happened (I think I'm beginning to get old :-)

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12 litre rocket Stage release mechanism