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#1 to #160 (Updates)


The section explains the basic concepts behind water rocket features.

Katz Stager Mk2.

This mechanism was developed to provide automatic staging of muli-stage water rockets. This mechanism will release a sustainer with a standard 9mm Gardena nozzle when the pressure inside the booster drops to near atmospheric pressure at burnout.

Total weight of prototype: 94 grams.

How It Works

For illustrative purposes the diagrams below have been simplified. Click on the diagrams for a larger version.

A photograph showing the actual components.
This diagram shows the main components of the stager. It is based on an off-the-shelf Gardena hose attachment with the hose fitting and spring removed.

Primary construction materials are plastic and aluminium. The majority of threads are not shown in order to simplify the diagrams.

The stager simply screws to the top of a booster.

In the unpressurised state the piston spring keeps the piston and locking pin retracted inside the piston housing. The piston housing cap prevents the piston from moving too far back and allows access to the piston should the spring or o-ring need to be replaced.

The piston housing has a small hole in the side that allows pressurised air to enter above the piston head. The piston head is designed so that it creates a small space near the hole.

With the locking pin retracted the locking arm and collar are free to move downward. Tension provided by the rubber bands ensures positive pressure on the collar and hence locking arm to keep it open.

The air supply connector and adaptor at the bottom of the stager allow a strong sealed mechanical connection to the top of the booster.

Next, the sustainer with water is placed into the stager.

The collar is raised into the locked position manually, and the locking tabs in the Gardena mechanism engage into the sustainer nozzle groove.

Because the locking pin is still retracted, a temporary support pin is placed under the collar to keep it locked.

The non-return valve just under the nozzle prevents any water from leaking into the booster.

When pressurisation starts, the air from the booster enters the stager body. Most of the air flows around the piston housing, through the non-return valve and into the sustainer. The positive pressure difference in the stager body keeps the water from flowing out through the non-return valve. When pressure equalizes the spring in the non-return valve keeps it shut.

Some of the air from the booster also enters the air inlet hole in the piston housing. As this pressure grows, the piston spring is compressed as the other side of the piston only has atmospheric pressure acting on it. As a result of this the locking pin extends out of the piston housing. For our prototype this happened between 20 and 30psi.

When the locking pin is extended sufficiently pressurisation is stopped at that point and the support ring can be removed. The locking arm is now resting on top of the locking pin keeping the Gardena mechanism in the locked position.

When everyone is clear of the rocket, the pressurisation can then continue to the full launch pressure. The pressure in the booster continues to hold the locking pin extended.

After launch the booster pressure starts to drop. As the booster pressure drops to around the 10-20psi mark the locking pin starts to retract because of the spring. (The retraction pressure is lower than the extension pressure on the piston due to frictional effects between the locking pin and the locking arm.

The non-return valve remains closed due to the increasing pressure difference between the sustainer and stager body.

When the locking pin has retracted far enough, the locking arm is free to move downwards under the tension of the rubber bands and pulling the collar with it.

This releases the sustainer.


  1. We used a healthy coating of silicone grease on all moving components.
  2. A number of o-rings were used on different components to provide a good seal. These could be omitted if the sections are glued together. We wanted to be able to pull the mechanism apart as we were developing it.
  3. The air supply connector and adaptor at the bottom of the stager are more complicated than what is required. We designed it this way so that we could connect a hose instead of a bottle to the bottom.


Ground Tests:

Early Flight Tests:


The design of this stager was inspired from Trevor's original piston based TDD. There are many examples of various staging mechanisms developed by water rocketeers over the years. Here are just a few examples of the variety developed:


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