Water Rocket 2-Stage Mechanism

This mechanism was developed to provide automatic staging of multi-stage water rockets. This mechanism will release a sustainer with a standard 9mm Gardena nozzle when the booster stops producing thrust.

The following tutorial describes how to build the staging mechanism. It should be used more as a guide rather than an exact step-by-step procedure as it is likely you will not have access to the identical parts used here. Feel free to substitute components as you see fit.

NOTE: Read the tutorial carefully including the notes at the end as there are important bits of information that you will need.

This stager is designed to work with a standard 9mm Gardena nozzle. If you don't have one already, you will need to make the nozzle first. Here's how: Making a Gardena nozzle.

Here are the materials you will need.

Materials
- 5mm Clear vinyl tubing (or similar) :
http://www.clarkrubber.com.au/hosing/food-and-chemical-grade-hosing.html#topsite
- 1 x Gardena quick connector
http://www.amazon.com/Gardena-36917-Garden-Quick-Connector/dp/B00004SDYI/ref=pd_sim_ol_3
- 3 x springs from 3 other Gardena quick connectors
- 2 x plastic pen tips
- 2 x pins
- 1 x bottle cap
- length of wire
- length of T8 FTC tubing (70mm & 35mm) (or similar size tube)
Example: (https://www.1000bulbs.com/product/242/PLAS-100269.html )
- Electrical tape
- Epoxy glue

Tools
- Drill and 5mm drill bit (drill bit needs to match tubing OD)
- Sandpaper
- Scissors
- Craft knife
- Pliers

Make sure that the Gardena quick connector you are using will not slide into the FTC. There is a flange on the quick connector just behind the thread. The flange diameter should be bigger than the FTC diameter. Some cheaper brand quick connectors are slightly narrower and could potentially slip inside the FTC.

Cut two lengths of the T8 FTC. One should be ~70mm long and the other ~35mm.

When you cut these out, you need to make sure that the edges are as square as possible. Use a piece of paper wrapped around the outside and mark along the edge. Carefully cut along the lines. If the edges are not square then the sustainer will not sit straight on the booster. You can use scissors to cut the FTC along the line.

Roughly cut the FTC piece off with a craft knife, and then trim to the marked line with scissors.

Cut the smaller FTC tube length ways with a pair of scissors.

Remove the orange collar from a Gardena quick connector by inserting 3 strips of PET plastic underneath the collar's locking tabs, and then pulling back on the collar.

Remove the spring and re-assemble the collar making sure you have the white locking tabs in place. You will need 4 of these springs altogether so you can remove these from old quick connectors. Make sure the springs are identical in size.

At this point test to see if the sustainer nozzle locks all the way into the quick connector. If it doesn't, you can modify the quick connector as shown here: Launcher Fix

Sand the internal hole of the Gardena quick connector to allow the epoxy to grip better.

Put a piece of Blu-Tack, plasticine or modeling clay on the workbench and stand the Gardena connector on it's end (orange collar pointing up) so that the Blu-Tack blocks the central hole.

Mix up some epoxy and carefully pour it through the open end. Pour enough epoxy in so that it is about 5-10mm deep. It's better to use the stronger epoxy rather than the 5 minute stuff.

Let the glue fully cure for 24 hours.

If the cap is the kind with a removable seal then remove it from the bottle cap and cut a hole in it as shown. The bottle cap may not have a removable seal or may be one of the type C caps then you don't need to do this step.

Assuming the outside diameter of your tubing is exactly 5mm, drill a 5mm hole straight through the top of the cap.  It does not need to be exactly on center.

NOTE: The tube needs to be a snug fit in this hole. You should not be able to slide the cap freely on the tube. If the hole is too big, get a new cap and drill a smaller hole.

Now use the same drill bit and drill a hole in the epoxy glue of the Gardena quick connector.

Cut about a 220-230mm length of the clear tubing with a pair of scissors.

Find an old plastic ball point pen that has a conical pen tip. You may need to look through a number of them, as some have small ridges on the inside. You may also find other small plastic cones as part of other components. As long as it is small enough to fit in the end of the tubing then it should be good. The conical section needs to have smooth internal walls to allow the valve to seal properly.

Push the cone section into the tubing as far as it will go. This is easy to do if you grip the end of a pop rivet or something similar in a vice and then push the pen tip over it.

NOTE: It is important to have the pen tip securely pushed into the tubing. If it is loose, it may get pushed out under pressure.

Cut the head off a pin with a plastic round head, or alternatively find a ball bearing of the right size and make sure it fits inside the pen tip cone section. It should be big enough not to fall through the smaller hole of the cone.

Heat another pin and push it through the plastic walls of the cone so that the round pin head can move up and down, but cannot fall out.

Push the pin through, cut it with a pair of wire cutters a couple of mm from the pen tip, and bend it with pliers to stop it from falling out.

You can wrap a couple of wire loops over the tubing and tighten with pliers.

This is how the non-return valve should look.

Wrap a length of tape around one end of the longer section of FTC. Here we are using glass strapping tape about 10mm wide. Wrap enough of the tape until the outside diameter of the tape is the same as the orange collar diameter.

Cut out two slots as shown in the FTC body. These should be wide enough to let the tubing move freely inside them. Cut them elongated so the tubing has room to move as the spring compresses.

NOTE: The location of these slots will depend on how your springs sit. I assemble the stager first without the tubing and then draw on the outside of the FTC where the holes need to go. The upper hole should be as high as possible and the lower hole should be as low as possible. This allows the tubing to sit more vertically in the cap and the quick connector giving a better seal.

Insert the free end of the tubing through the quick connector and pull it all the way through until the non-return valve is sitting snugly inside the quick connector against the epoxy.

Thread 3 of the springs completely onto the tubing, with the first spring pushed all the way up to the flange of the quick connector and the fourth spring should be only partially threaded. Let the tubing emerge out the side of the 4th spring.

Now insert the springs and tubing into the FTC body, allowing the tubing to emerge from the bottom hole. This is a little fiddly but not difficult. After all the springs are inserted, make a loop in the tubing and rethread it back through the upper hole and back down the center of the FTC body.

Pull the tubing out as far as it will go (the loop will tighten) and insert the tubing in the top of the bottle cap.

You can now push the cap into the FTC body. The caps and FTC we use fit snugly together. Your FTC may be a little bigger or cap smaller, so at this point you can wrap a bit of tape around the cap to give a tight fit. You may need to try a few caps otherwise, until you find one that works well.

For the next step we need to widen the free end of the tubing so it cannot come out of the cap. For that we use the plastic tip from another pen. This is a very common type of pen, with a metal tip fitted in a plastic adaptor with the ink straw inserted from the other end. Pull out the metal tip with pliers, and remove the ink straw. You'll be left with the hollow plastic adaptor. This is a nice size for the tubing, and the tapered and makes it easy to insert into the tubing.

Alternatively you can use something like a round glass bead with a hole in it and push it into the end of the tubing.

You can again use the pop rivet or similar in a vice to hold the pen tip.

Push the tubing over the pen tip until the end of the tubing is level with the end of the pen tip.

Now wrap a couple of loops of wire over the end to prevent the pen tip from being pushed out under pressure.

Push the tubing back through the cap until the pen tip is pushed up snugly against the hole in the cap. The tubing loop will loosen again.

Screw the cap onto the booster bottle and tighten. Now push the FTC body down over the cap until it rests on the bottle flange. This will help support the sustainer during acceleration.

NOTE: There needs to be quite a bit of friction between the cap and the FTC body so that the spring does not push the cap out.

Take the smaller piece of split FTC and slip it over the rolled up tape at the top end of the FTC body. Tape this down with the electrical tape. Make sure you tighten the tape really well and wrap it several times. You also need to pay special attention to the alignment of the split FTC so that it is not at an angle to the FTC body.

NOTE: The next step is very important, and it helps to get someone to help you.

Place the nozzle into the stager and push down the inner part of the quick connector compressing the spring until the flange rests on top of the FTC body.

While holding down the inner part of the quick connector, slide the orange collar all the way up until the nozzle is locked. Now wrap electrical tape around the orange collar where it overlaps the short piece of FTC. What you are trying to achieve is to attach the orange collar to the split FTC.

Once you tape it down firmly, (use a few wraps) you can release the nozzle, and the inner part of the quick connector should spring out again.

There will be approximately a 10mm gap in the split FTC. This gap cap be used to place a locking tab in the stager. (see below)

(This image was taken from our other stager and is why the lower tape is white in this image)

Because there are so many varieties of springs and quick connectors it is difficult to predict how much force your springs will give.

You will need to measure the spring force that opens your stager:
1. Place a nozzle into the stager and then place the nozzle on a kitchen scale.
2. Zero the scale
3. Press down on the stager until the mechanism locks.
4. Now slowly start raising the stager until the nozzle just starts moving.
5. Take a reading on the scale at that point.

You should get a reading of 800grams or more. If it is much less than that, you may find that the stager fails to open in flight. (We found out the hard way). This means your sustainer needs to weight at least 800 grams to keep the stager locked. This is not unreasonable for a small sustainer considering it may weigh 200 grams with 600mL of water. If the fully fuelled sustainer weighs less than than, you can still use this stager with a locking tab. (See below)

Adjusting the Spring Tension

You may find that you need to adjust the spring tension of the stager. This can be achieved by trimming the bottom of the FTC body and pushing it down against the bottle flange again. This will increase the spring tension.  Trim a couple of mm at a time.

Notes

1. You may find that the dimensions of your FTC are such that the inner part of the quick connector does not move freely. You can apply a coat of silicone grease on the inside of the FTC body and the springs to help it move better. If this does not work you may need to get different FTC from another manufacturer.

2. Though the clear vinyl tubing says it is rated to 50psi, the actual burst pressure is a lot higher. It will easily handle 200psi+.

3. If you find that your nozzle does not lock properly into the stager, you can fix it by modifying the quick connector.

4. What if the sustainer is too light to keep the stager locked? See 'locking tab' below.

5. What if I can't get T8-FTC? - FTC is commonly available and can be ordered on line, but you could try making your own tubing by rolling up and gluing PET plastic together. As long as the dimensions of the tube are correct you don't need to use FTC.

6. In order to use this stager as shown here, you will need to construct the booster in such a way that it has a bottle neck at either end. This can be achieved either by splicing the bottles together or joining them with a Robinson coupling.

7. The sustainer will normally need to be supported with additional bracing to prevent the sustainer bending over and snapping the staging mechanism. This is normally required for most staging mechanisms anyway.

8. The stager could be damaged if the booster lands directly on it. The sustainer support can be used to protect the stager.

9. The springs you used should all be the same diameter so that when they are inside the FTC body they don't thread themselves into one another.

10. If you don't like the idea of using tape to attach the split FTC, you can try gluing it. Tape allows you to pull the stager apart if you need to.

11. If you find there is a tiny leak around the flexible tubing, it will likely stop once pressure is applied.


Using the Stager

To use the stager, simply fill the booster with water and place it on the launch pad. Fill the sustainer with water and push the sustainer nozzle into the stager and let the sustainer's weight lock the stager.

Locking tab

If the sustainer is not heavy enough, you can insert a locking tab in the space between the collar and the inner part of the quick connector. This is in the gap of the shorter piece of FTC. Tie this locking tab with a string to the launch pad. When the rocket launches, the locking tab will get pulled out, but acceleration will keep the sustainer in the locked position until the booster runs out of thrust.

The locking tab is just a small block of plastic or wood that is just thick enough to fit between the collar and the flange on the quick connector.

How It Works

If you wish to find out how this staging mechanism works, then full details are available here:
http://www.aircommandrockets.com/howitworks_4.htm

This tutorial is also available as a video tutorial:
http://www.aircommandrockets.com/mark_3_stager.htm#_vidpart1

Full flight report with video and photos is here:
http://www.aircommandrockets.com/day94.htm