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#1 to #160 (Updates)



Each flight log entry usually represents a launch or test day, and describes the events that took place.
Click on an image to view a larger image, and click the browser's BACK button to return back to the page.

Day 51 - Katz Stager
The main components of the Katz Stager. The piston is made from aluminium, and the nozzle from plastic.
A better view showing the location of the nozzle exit holes on the nozzle component. Included here are two regular bottle caps that hold the components to the bottles.
Here the bottle caps are fitted to the stager components.
The piston inserted into the nozzle.
A reverse view of the inserted piston.
Stager components fitted to the sustainer (on left) and the booster (on right)
The booster mated to the sustainer.

NOTE: no spring is fitted in this picture.

Date: 6th January 2008
Team Members at Event: GK and PK

This week I thought I'd share one of the things we have been working on in the background - the Katz Stager. It's a staging mechanism based on a different principle to friction types such as a crushing sleeve, or the locking types such as a Gardena mechanism.

Katz Stager

The key to this system is trapping atmospheric pressure in a part of the mechanism to ensure that there is no net force acting on the mechanism to separate the stages. A spring is used to separate the stages after burnout.

Main Components

The staging mechanism consists of the following components:
(Click on images to see more detail)

  • Piston - The main component in the booster providing the air supply to the sustainer.
  • Sustainer Nozzle - The main component in the sustainer, traps atmospheric pressure and acts as a nozzle.
  • Plug with non-return valve - screwed into the base of the Piston. It prevents air and water from coming back into the booster.
  • Caps - Regular bottle caps used to hold the nozzle and piston in place.
  • Spring - Used to provide the separation force between the sustainer and booster just after burnout.

How It Works

1. The sustainer is partially filled with water and the nozzle is screwed on.
Note: Water has been omitted from the diagrams for clarity. (It also made it easier to make the diagrams)

2. The booster is filled with water and the piston is screwed on. The booster and sustainer are turned upside down and fitted together. This is so that water does not enter chamber A in the sustainer. (A little bit of water is okay). When the top o-ring of the piston moves past the nozzle exit holes, atmospheric pressure is trapped in chamber A.

3. When the combination is turned back upright, the weight of the water and the sustainer keeps the spring compressed and prevents the spring from pushing the booster and sustainer apart.

4. The piston o-rings prevent water/air from leaking into chamber A and also out past the piston.

5. The piston has a small hole that emerges from between these o-rings and lines up with one of the nozzle exit holes. This small hole continues through the center of the piston to the plug that contains a non-return valve made from a bent pin, much like you might find in a crushing sleeve mechanism.

6. When the booster is pressurized, the higher pressure in the booster opens the non-return valve and air is allowed to flow into the sustainer through the piston.


7. Now as the pressure grows inside the sustainer, the forces on the piston are only circumferential and cancel each other out (Red arrows at top). There is no net force on the piston pushing it out of the sustainer. This is because the only area of the piston that can exert a force to push it out is the top of the piston and that remains at atmospheric pressure. This is the key to the Katz stager. This means that pressure inside the sustainer can be very large, yet the piston can remain free moving as long as the pressure in chamber A remains at atmospheric levels. There is no need for friction or locking mechanism to keep the sustainer from flying off.

8. During launch the inertia of the sustainer, working against the thrust of the booster, keeps the spring even more compressed preventing the mechanism from staging.


9. As the pressure drops inside the booster the non-return valve in the piston keeps the pressure from escaping from the sustainer.

10. As the thrust from the booster drops to zero, the sustainer and booster are essentially in free fall and the booster is no longer providing an opposing force to the sustainer. (As the ground was providing while still on the launcher, and the thrust was providing during the boost phase) The spring is then free to start forcing the booster and sustainer apart.

11. As the piston starts to move out of the nozzle and the top o-ring moves past the nozzle exit holes, the sustainer pressure enters chamber A and now acts in full force on top of the piston and forces it out of the nozzle. Similar to the way a launch tube works.

12. When the piston is forced completely out of the nozzle staging has occurred and the sustainer can go about its business. The combined cross-sectional area of the nozzle exit holes is much larger than the nozzle cross sectional area so there is minimal restriction of flow of water and air. (In real life the nozzle exit holes are a lot lower than in the diagram so water does not collect around the base of the nozzle.)

You are probably asking yourself why such a complex mechanism is needed, when a crushing sleeve is so much simpler? We wanted to bring another design idea to the table that may help spark other ideas based on this concept. Here are some advantages and disadvantages.


  • The sustainer pressure can be very large.
  • Other than the spring there are no moving parts.
  • Relatively lightweight and compact design.
  • A long thin nozzle is not necessary as with a crushing sleeve.
  • The nozzle diameter can be quite large. Perhaps as much as 20mm. Large diameter crushing sleeve mechanisms are difficult to build.
  • There is potential to use a De Laval nozzle with this design.


  • Components need to be fabricated - cannot be made from household items.
  • The spring tension needs to be chosen appropriately for the sustainer.

Current Status

We have built a prototype of the nozzle and piston and have fired it 3 times horizontally with air only. This was to test to see if Chamber A would retain the atmospheric pressure. We used our hand to push the sustainer and booster apart in the tests. The piston was a little more stiff to push out than what we would have liked, but softer o-rings and a little more silicon grease should help that out.

What's Next ?

We need to find the right spring with the correct force to separate the two and then test fly the mechanism. 

Concept Extensions

This mechanism could equally be used as a launch mechanism on a launcher. The rocket could be pressurized to high pressure without the need for a strong retaining mechanism. A small lever would replace the spring and would serve to push the two apart in order to launch the rocket.

With the described mechanism, you could not use a launch tube, however, the design could be easily modified to place the chamber at the top of the rocket (such as in the neck of the upper bottle) and the launch tube would work as the piston, trapping the atmospheric pressure in the neck of the top most bottle. 

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