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.
- 5mm Clear vinyl tubing (or similar) :
- 1 x Gardena quick connector
- 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
- Drill and 5mm drill bit (drill bit needs to match tubing OD)
- Craft knife
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
Alternatively you can use something like a round glass bead with a hole in it and push it into the end of the tubing.
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.
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)
Step 28: Measuring Spring Tension
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)
Step 29: Notes
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.
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.
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.
Step 30: How It Works & Video Tutorial
If you wish to find out how this staging mechanism works, then full details are available here:
This tutorial is also available as a video tutorial:
Full flight report with video and photos is here: