Stage Mechanism

This is an adaptation of Bruce Berggren's Crushing Sleave Staging Mechanism which can be found at http://www.geocities.com/wrgarage/stage.htm .

I have adapted it so that it can be made with the materials that I could find in the UK. There seems to be a lack of materials that are suitable for the making of water rockets in the UK so here is what I came up with (after searching the shops and everywhere else for over six months).

For computer modelling purposes, it has a Stage Mechanism Diameter of 15mm and a release differential pressure (with one hose band) of around 45 psi or roughly 3 Bar. The latter can be increased by using extra or wider bands.

Materials

To make one of these, you will need:

Two bottle tops - the type with a thin, plastic sealer in them; around 1" of 21.5mm PVC Pipe (just normal overflow pipe); around 5" of 15mm PVC Pipe (this stuff is pretty neat (it is made from three layers - picture on the right) but it appears to be standard. It is marked with the following - "Speedfit 15mm B-PEX to BS7291 : Part 3:1990 Class S 12 BAR 20ºC - 4 BAR 82ºC - 3 BAR 92ºC 9927/0609/279"); around 4" of 20mm o/d transparent, flexible PVC Hose (garden irrigation type - it fits snuggly inside the 21.5mm o/d PVC tubing); around 1/2" reinforced Garden Hose cut into two pieces around 4mm wide; 22mm Tube Inserts (Speedfit J158 for polybutylene pipe (see15mm pipe above)); 15mm Tube Inserts (Speedfit J157 for polybutylene pipe (see15mm pipe above)); two Paper Clips; a Plastic Bead; some Blu Tack or similar - epoxy resin may do this instead for the bead - see construction details below; some PET plastic from the side of a bottle; and, some 4mm to 5mm i/d plastic pipe (keep your eye open for this stuff - lolly handles, child's balloon sticks (a balloon on an adaptor on a hollow stick - the type that they get given as a store promotion when you are out shopping and they quickly become a liability as the balloon bursts and the little dear wants another one:-).

Construction

Nozzle

1. Make good the end of the 15mm pipe using a pipe cutter. This pipe will cut okay but once you have got through to the blue plastic (which is very ductile and will stretch instead of cut) you have probably weakened the inner part of white plastic enough to finish the job off with a sharp knife. Make sure that the end is flat but DO NOT get rid of the burr on the inside of this end.
2. Using the same cutting technique, cut the other end giving you a tube around 11cm long. Remove the burr at this end (the water will flow into the pipe through here so it needs to be reasonably smooth).
3. Get one of the 22mm Tube Inserts and ram the inlet end of the 15mm pipe into the end without a flange (as per the picture on the right). Use a small hammer but don't hit it too hard or the 22m Tube Insert will split (it may not do this straight away so if you have hit it too hard, you may not know until it is too late). It is tight enough when you can't pull it off the tubing using a similar force to that of the pressure in the rocket during the flight.
4. Cut a hole in the centre of a bottle top so that the Tube Insert just fits through if and then a hole in the centre of the bottle top seal that is just smaller than the Tube Insert. Put the seal into the top and slide them onto the pipe.
5. Get one of the 15mm Tube Inserts and roll up a strip of bottle side PET (around 20mm wide x 20cm long). Pull the PET strip over a sharp edge to make it curl and then roll it up so that the hole in the middle is as small as you can make it. Trim the PET strip so that it will roll up small enough to get into the flange end of the 15mm Tube Insert (you may have to trim it so that the external diameter is small enough to do this) and then tap it home with a small hammer.
6. Cut approximately 25mm of the 4.5mm i/d tubing, making sure that the ends have no burrs. You may find that this will fit snuggly into the hole in the rolled PET but I found that there was enough room to insert another layer of PET. Roll the PET up (you want enough to go around the outside of the 4.5mm i/d tubing - the PET that is already there is a spiral so get the outer end of the new piece to butt upto the inner end of the outer piece thus forming a tight continuation of the spiral. I found that one layer was enough), put the 4.5mm i/d tubing in it and carefully tap that into place.
7. Using a sharp knife, cut a smooth, tapered inlet to the 15mm Tube Insert nozzle that you have just made. The water has to flow into the nozzle throught this so it should be smooth and conical (get that k nozzle factor as low as possible).
8. Push the tube insert nozzle into the end of the 15mm tubing - this should be a tight fit and the burr should make it more so. Be careful not to damage the end of the 4.5mm i/d tubing. Using a sharp knife, take off any burrs that have formed - a burr on one side will push eht jet over to one side and make your rocket loop in the air instead of flying straight.
9. Take some Blu Tack and role it into a small cylinder (around 6mm diameter) and cut off around 5cm of this. Roll around the 22mm Tube Insert below the flange, forming it into a seal, so that it is slightly narrower than the internal diameter of the bottle neck.
10. To fit into the bottle, push the flange with the Blu Tack seal into the neck of the bottle so that the seal is below the end of the nozzle. Screw on the bottle top and then pull then pull the nozzle so that the flange slides towards the top, squashing the seal and making it it seal. The flange and the hole in the bottle top will keep the nozzle aligned - this is why it is important to get the hole central. This is effectively a removable, watertight seal.
    

Crushing sleeve

1. Get the cut-off end of a pop bottle (the nozzle and a few inches of neck) and push through, from the neck end, some 21.5mm o/d plastic overflow pipe so that around 5cm protrudes through the nozzle end.
2. Heat up the very end (around 1cm) using a cooker or a candle - the surface will change its texture - and pull the tubing back into the bottle so that only around 4mm protrudes. Push the end down onto a flat surface, being careful to make sure that the end is square. The aim of this is to flare the end of the tubing so that it is slightly larger than the end of the bottle neck - the bottle neck that you are using will help to keep the PVC tubing in shape. Don't press too hard as you may flare the end unevenly. Once you have done this, it should have around 1mm of PVC pipe that should not go through the bottle neck (that doesn't mean that you have squashed all 4mm into just 1 mm though - it tends to slide as you are pushing it).
3. Cut the pipe using the pipe cutter around 25mm from the flared end. Do not remove the burr - this will help to grip the flexible PVC tubing when the system is not pressurised. You should now have a short length of pipe that will not go completely into a bottle and should be fairly easy to remove from a bottle neck.
4. Cut around 15cm of 20mm o/d clear flexible PVC tubing making sure that both ends are flat and square. Push one end into the length of 21.5mm o/d pipe that you have just made so that the end of the flexible PVC pipe is flush with the flared end of the 21.5mm o/d pipe. The burr on the outer pipe should help to hold the flexible pipe in place.
5. Cut a piece of 15mm o/d pipe around 15mm long, leaving the burrs. Push a 15mm Tube Insert into the pipe leaving around 5mm gap between the flange and the end of the pipe (see bottom of diagram below).
6. Get a paper clip and straighten it out. Bend around 6mm of the end back on itself, using a pair of pliers, so that it is a tight fit in the hole in the plastic bead. Push the straight end of the paper clip through the hole in the bead and pull the bent over end into the hole so that it is a tight fit.
7. Push some Blu-tack into hole to seal it (it is only going to have any serious pressure along it for a fraction of a second) or, as an alternative, fill the hole with epoxy resin, using a flatened piece of Blu-tack as an end to the mould formed by the inside of the bead (when the epoxy is set, remove the Blu-tack).
8. Put the paper clip in the bead through the 15mm Tube Insert and bend the end over so that it won't come out. remember to leave a few millimetres of travel so that it can act as a valve.
9. Push the 15mm Tube Insert into the end of the 20mm o/d clear flexible PVC tubing so that the ends are flush.
10. Cut two rings of garden hose and fit them in the positions shown on the diagram. To get them over the PVC tubing, you will need to stretch them first. I did this by putting the handles of two table knives into the hose rings and twisting them to stretch the rings. After a few minutes the rings shrink to their original size. I found it necessary to put a ring at position 2 (not seen in the original US design) because the flexible tubing I could find in the UK was not a close enough fit - without some initial seal such as that provided by this extra ring, too much air escaped during pressurisation. This works like a dream.
11. Straighten out the other paper clip and wrap it once around the 20mm o/d plastic tubing, twisting the two ends together using pliers. Cut off the excess and bend the twist down so that it is flat to te side of the tubing (remember that this assembly has to pass through the neck of a bottle).
12. Cut a hole in the centre of a bottle top so that the 15mm o/d PVC pipe fits through and then a hole in the centre of the bottle top seal of the same size. Put the seal into the top. Be careful to make sure that the 15mm PVC pipe will fit smoothly by making sure that the hole is central. One way of doing this is to put the flexible sustainer end into the end of a bottle and to make the hole in the bottle top (being careful not to cut the flexible PVC) in situ - passing the 15mm PVC through the hole close to finishing to make sure that it is aligned properly. A tollerance of 0.5mm is okay here as the seal is made by the end of the PVC pushing against the bottle top seal.
13. To fit into the bottle, wet the garden hose rings (they are a tight fit) and push the assembly into the bottle as shown in the diagram. The 21.5mm PVC pipe should stop short of going all of the way in. Screw the top down and the 21.5mm o/d PVC pipe and the 20mm o/d flexible PVC tubing should fit flush to the seal. The pressure in the bottle will push these harder against the seal for an air tight seal in use.

Operation

Filling

1. With the booster end of the Stage Mechanism in place on the top of the booster, fill the booster to the appropriate level and mount on the launcher. Beware of the tendancy for the booster to empty if the one-way valve is not in place (this shouldn't happen but it is as well to be aware of this in case it does).
   An alternative method is to put a normal bottle top on the top of the booster, fill the booster with the appropraite amount of water, place it in position on the launcher and, when it is water tight, take the top off and replace it with the crushing sleeve mechanism.
2. Fill the sustainer (second stage) to the appropriate level.
3. Push the sustainer nozzle in, along with its Blu Tack seal, screw on the top and pull the nozzle up so that the seal fill in the space. If the hole in the top is central, the nozzle will be straight.
4. Put the sustainer onto the booster. The one way valve (the bead) will stop the water form flowing into the top of the booster.

Pressurisation

1. Before pressurisation, water flow into the booster from the sustainer is stopped by the one way valve and the sustainer is held in place by its weight.
2. When pressurisation commences, the pressure at a increases, forcing air past the one-way valve into b. Air does not pass to atmosphere at 2 because of the light pressure seal caused by the garden hose. Note that the pressure from the garden hose is never enough to hold the 15mm pipe in place.
3. As the pressure increases, the pressure in a, forces the flexible tubing at 1 onto the surface of the 15mm PVC tubing (displacing the air that was in this space to atmosphere - c) which provides a sufficient grip to stop the PVC tubing from being released. This force also pushes the tubing against the Cap Seal and enhances its seal. The more the pressure, the more the gripping force, the more the seal.
4. Once pressurisation has been completed, the pressures in a and b are equal. The one-way valve only has the hydrostatic pressure of the water in the sustainer on it as it did before pressurisation commenced and the pressure from the paper clip at 3 is sufficient to hold it in place.

Release

1. When the rocket is released, the water is ejected from the booster and the pressure in the booster decreases. The whole rocket is accelerated to maximum velocity. The one-way valve holds the water and the pressure in the second stage of the rocket and the pressure on the flexible tubing holds the second stage nozzle pipe in place.
2. Once the water has finished, the air escapes from the booster and the pressure carries on falling towards atmospheric pressure. The restraining pressure on the flexible tubing diminishes until the pressure at the end of the second stage nozzle (between 2 and 3) - still at the initial pressurisation pressure - is suffucient to overcome both the reduced grip on the nozzle and the lower acceleration from the booster.
3. At this point, the nozzle, lubricated by a small amount of water from the sustainer, is forced out of the flexible tubing and the second stage is released - still at the initial pressurisation pressure.

During the depressurisation of the booster, two things happen:

• The force holding the sustainer in place reduces as the acceleration falls - trying to force a release when the force that is a result of the pressure on the area of the end of the nozzle (15mm diameter in this case) exceeds the force from accelerating the mass of the sustainer with its water; and,
• The pressure in the booster falls by the amount required to release the grip on the side of the nozzle.

Note that this pressure is the difference between the pressure in the booster and sustainer and is not relative to the pressure between the inside of the booster and the outside air. This is because the pressure on the inside of the tubing is derived from the sustainer via the end of the sustainer nozzle because the one-way valve (the bead) prevents the water from flowing into the booster.

As a result of this, it is possible to measure the release pressure by putting the sustainer nozzle onto a spare bottle (a 1 litre will do) with a little water in it (not to weight too much) and putting the booster release mechanism onto a bottle that can be pressurised (two bottles bottom to bottom like the 4 litre rocket or one with a tyre adapter in the base) and pressurising the base bottle in 5 psi steps, releasing the pressure each time to see if the release mechanism releases. I found that it did this for this mechanism at 45 psi. It is not unreasonable to assume that it will release, dropping from, say, 100 psi, when it gets to 55 psi if the acceleration makes the force on the nozzle from the sustainer fall below the force from the end of the nozzle (100 psi x 0.27 square inches = 27 lbs force or 122 Newtons). The numbers that you put into the computer model for 2 stage optimisation are 45 psi and 15mm.

Back to the 2 Stage Rocket
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