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#143 - Whalan Reserve

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#117 - Water and Pyro

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#1 to #115 (Updates)

 

HOW IT WORKS
The section explains the basic concepts behind water rocket features.

Drop Away Boosters - Passive separation

Theory

The idea behind the drop away boosters is to increase the launch velocity of the main rocket to achieve a greater altitude. When the stored energy in the boosters is spent they automatically separate from the main stage and take away their dead weight and drag.

This rocket configuration is not considered a 2 stage rocket as the main stage and boosters all fire at the same time at launch. The technique presented here is one we have used with success on numerous flights.

Basic Concept

The boosters are loosely attached to the main stage and in such a way that they can only move backwards. On launch the boosters' greater thrust allows them to remain in place until they burn out and drag pushes them backwards separating them from the main stage. This way the boosters individually fall off at exactly the right time, irrespective of what the other boosters are doing.

 

Practical Implementation

Several key aspects must be maintained if this system is to work effectively.

  1. The boosters must produce more thrust than the main stage. This can be achieved with either bigger nozzles, or higher pressure than the main stage. (Use of foam can also be used to reduce and prolong the thrust of the main stage.)
  2. The boosters and main stage need to be released simultaneously.
  3. Each boosters' thrust profile must match the thrust of the other boosters throughout their flight. This means the pressure, water volume and capacity has to be equal for each booster.

Booster Capacity

The booster capacity can be made equal by using identical bottles and construction techniques to make them.

Simultaneous Release

Different mechanical configurations can be used to simultaneously release all the nozzles. The nozzles of both the main stage and boosters can be held individually and released at the same time, or only the main stage nozzle needs to be held down, with the boosters held down by the main stage.

Equal Thrust

Equal amounts of water are easily achieved with the use of a measuring jug.

To equalize the pressure between the boosters is a little more tricky. Consider the case where a manifold simply connects the boosters together to a common air supply. What happens is that an uneven amount of air will flow into each booster. This uneven pressure then starts forcing the water from one booster into the other through the manifold in order to equalize. (We found this out the hard way :) )

You could potentially rectify this situation with use of non-return valves for each booster, but you still may end up with a pressure difference between the boosters. This is because the non-return valves may not close at exactly the same pressure. The method presented here connects the air chambers of the boosters together through an open air manifold that only transfers air and not water.

While it is feasible to have small hoses connecting together the air chambers in the tops of the boosters the boosters would remain connected when they drop off. The hoses could catch on the fins of the main stage, as well as preventing individual boosters from falling off when they stop producing thrust. One could make an air manifold on the rocket where the hoses separate, but this adds unnecessary complexity and weight to the rocket. Any separating mechanism has to be able to withstand the full pressure while on the pad.

For this reason we suggest building a simple air manifold into the launcher. In order to achieve this, each nozzle seat needs an air fill tube that goes through the nozzle and emerges above the booster's water level.

How It Works

For illustrative purposes the diagrams below have been simplified to include only the relevant components. Click on the diagrams for a larger version.

In this example the boosters and main stage are made from spliced bottles so that they have an opening at the bottom for the nozzle and an opening at the top for filling with water.

The tubes on the side of the main stage are glued to the surface of the bottle with PL premium glue.

The pins on the boosters are also glued to the surface of the boosters with PL premium glue.

In this example the booster nozzles are larger than the main stage nozzle.

The boosters are placed on the launcher first. These slide over the air fill tubes and seal at the bottom with an o-ring. For illustration the booster nozzles here are just the full bore bottle neck.

Also not shown is the stop that prevents the booster dropping too low.

Note that the boosters are not locked down in any way, they are free to move up and down on the air fill tube.

Next the main stage is placed on the launcher carefully aligning the tubes on the main stage with the pins on the boosters.

The main stage nozzle is locked into the central release mechanism such as a Gardena release head.

 

The boosters and main stage are filled with water from the top.

Water is poured into the boosters so that it does not go down the air fill tubes. Equal quantities of water go in each booster.

The tops of the air fill tubes can be closed with a small hole drilled in the side of the tube above the water line. This prevents water from going into the manifold while filling from the top.

The non-return valve in the main stage air supply line prevents water from entering back into the air manifold.

The entire rocket is then pressurised. The open channel through the air manifold allows the pressure to equalize between all the boosters.

When pressurised the boosters are already trying to pull the main stage upwards. But with the pins hooked into the tubes on the main stage the boosters are held down.

Launch is achieved by releasing the main stage nozzle from the launcher. This has the benefit of simultaneously launching everything together.

The boosters having larger thrust than the main stage will always try to "pull" the main stage with it keeping the pins inside the tubes.

The top tubes on the main stage are there to keep the boosters pointing in line with the rocket.

As soon as the rocket starts moving air drag starts acting on the top of the rocket.

The air fill tubes also act as regular launch tubes in this case giving the boosters a boost.

The booster thrust is greater than the main stage thrust and hence the boosters are prevented from falling out backwards.

In flight, the booster pressure and thrust start to drop more rapidly than the main stage.

The rocket continues to accelerate the induced drag also increases.

At burnout the boosters stop producing thrust, and now the net force acting on the boosters is just drag and gravity.

The main stage continues to accelerate.

The air pressure from the drag simply forces the boosters backwards out of the tubes and they fall away.

The main stage continues to accelerate upwards as long as it is producing greater thrust than the drag.

Notes

  1. The rocket should be designed so that the thrust phase is much longer for the main stage compared to the boosters. This can be achieved by having the main stage contain more volume and/or having a smaller nozzle. Adding foam to the main stage also prolongs the thrust.
  2. The tubes and pins should not be too short as there will be some amount of vertical movement between the booster and main stage. If they are too short the boosters may separate early due to vibration or buffeting.
  3. The pins, tubes and main stage nozzle need to be made from very strong materials as a lot of force is transferred through these.
  4. Consider using 2 or more pins on the bottom next to each other to help spread the load.
  5. We use smaller nozzles rather than the full bore nozzles on the boosters as it helps to keep the acceleration down reducing the stress on the tubes and pins.
  6. As the rocket is pressurised there will be a small amount of vertical movement of the boosters as things flex under pressure. This needs to be considered in the design so the o-rings continue to provide a good seal.
  7. Another consideration needs to be given to the increase in diameter of the main stage and boosters as they are pressurised. If the booster bottle is hard up against the main stage bottle then they will tend to want to push apart. If  a certain amount of give is not incorporated into the launcher nozzle seats, the nozzles may wedge.

Videos

Details of the actual launcher:

Flights of the rocket with boosters:

 

References

A similar drop away booster concept has been used before on experimental pyro rockets. The implementation on water rocket shown here was inspired by Trevor's flight of his Green Ant Shuttle:

Ground footage: http://vids.myspace.com/index.cfm?fuseaction=vids.individual&VideoID=16262128

Inflight video: http://vids.myspace.com/index.cfm?fuseaction=vids.individual&videoid=16260924

Here is an old photo of a huge Japanese water rocket with side boosters, but I don't have details as to the booster concept: http://dogrocket.home.mindspring.com/WaterRockets/tripreport.html

If the reader is aware of other water rocket booster references (not 2-stage systems) please contact us and we will add it to the list.

 

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