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#1 to #150 (Updates)

 

Nozzles

Which Nozzle?

This article describes the various aspects of water rocket nozzles. Choosing the right nozzle for your water rocket depends on a number of factors, and may change depending on what you are trying to achieve with a particular flight. When choosing a nozzle for any water rocket you need to consider the following:

  • Desired flight profile. For example slow vs. fast launch.
  • Minimum lift-off thrust requirement depending on the weight of the rocket.
  • Launcher type available as not all nozzles fit all launchers.

There are three important aspects to any nozzle:

1. Nozzle Size

The nozzle size in water rockets is measured by the narrowest internal diameter (throat) in the nozzle and is often given in mm. The internal diameter is important because it directly relates to the mass flow rate out of the nozzle. The larger the nozzle the higher the thrust for a given pressure. Note that the length of the nozzle typically isn't important although there will be slight performance differences with variations in the length. Internally nozzles are typically cylindrical because water is incompressible and hence does not benefit from a DeLaval nozzle the same way combustion rockets do. Discussion of DeLaval or Convergent-Divergent (CD) nozzles in water rockets is a topic of its own and is left for a separate article.

2. Retention Method

In general most nozzles are also used to hold down the rocket on the pad. Different types of nozzles have different retention mechanisms and as a result each will use a particular launcher design. Some nozzles lend themselves to several different launcher release head designs. Refer to the Launcher article for more information on launcher release mechanisms.

The arrows in the image show where the nozzle is held down by the launcher. 

3. Seals

A nozzle may have 1 or 2 seals depending on the nozzle type. The first seal goes between the bottle and nozzle while the second seal is between the nozzle and the launcher. When using the bottle neck as the nozzle then you only need one seal and that is between the nozzle and the launcher.

Seals prevent water and air escaping through the nozzle-launcher interface. There are various seal designs depending on the nozzle type and release head combination.

What nozzles are available?

There are several different types of nozzles commonly in use:

Image Type Description
Full-bore This is the standard neck opening of a PET bottle used for most water rockets.

The diameter is ~22mm. Sometimes these nozzles are attached to other rockets such as those made from FTC. These are by far the simplest nozzles to make and use.

 

Gardena Gardena nozzles are made from garden hose quick connectors. The garden industry typically refers to their measurement by their external diameter of 12mm whereas the water rocket community refers to the same quick connectors by their internal diameter of 9mm.

Normal tap quick connector adaptors are unsuitable for use directly on most PET bottles as the threads don't match. Some work is required to make a Gardena nozzle.

See the Gardena Nozzle tutorial for more details.

Maxi-Flo This nozzle is similar to the Gardena quick connector, but with a larger diameter. The internal diameter is 15mm. The garden industry refers to these again by their external diameter of 18mm.
Cap This nozzle is simply a standard bottle cap with a hole drilled in it. AntiGravity Research uses these with their unique launcher.

 

Sport-cap These nozzles are made from the cap on sports drinks. These are not commonly in use.

The tutorial on how to make the sport cap nozzle is here:

Custom Custom nozzles can be made from any material, any profile and any size, however, these are less common and typically designed for specific rockets and launchers.

Here are some other examples of custom nozzles:

 

T-nozzle T-nozzles are a special nozzle that changes the internal diameter. The nozzle has a larger diameter while it is on the launch tube to maximize the piston effect, but once it leaves the launch tube it reduces the diameter for a longer sustained burn. The T-Nozzles concept can be used with various nozzles diameters.

Here are some examples of how to make t-nozzles:

Large Nozzles vs. Small Nozzles

Many people ask whether a larger or a smaller nozzle is better, however, nozzle size must be chosen in combination with launch pressure and lift-off weight. The rocket must achieve at least a minimum acceleration to reach a high enough velocity by the time it leaves the guide rail or launch tube so the fins can help stabilise the rocket.

For small light rockets you have a choice between small or large nozzles, while for large or heavy rockets you typically need proportionally larger nozzles.

Regardless of nozzle size, accelerations well below 3G are likely to result in the rocket tipping over soon after launch especially during cross wind conditions. As a rule-of-thumb you should try to aim for accelerations of 3G or higher for most launches.

You can use one of the on-line water rocket simulators to calculate the predicted acceleration of your rocket for a given nozzle size. 

Once you determine that your rocket will achieve at least the minimum acceleration with a particular nozzle, water fill and pressure, you then often have a choice at how much faster you want the rocket to accelerate.

While keeping the weight and launch pressure the same, you can change the acceleration by changing the size of the nozzle. Small light rockets with large nozzles can easily reach accelerations of 150G+.  For Example: A 2L rocket that weighs 150grams, uses a 22mm full-bore nozzle, filled with 800mL of water and pressurised to 130psi will reach a maximum acceleration of ~155G.

High and low accelerations have their advantages and disadvantages and which one you choose depends on what you are trying to achieve.

Acceleration Advantages Disadvantages
High
  • Allows your rocket to reach a stable speed sooner reducing the need for longer guide rails.
  • The rocket will generally reach a higher altitude but is dependant on a number of factors.
  • Generally the launcher does not require a separate guide rail if a launch tube is used.
  • More stress put on components. Components need to be secured against the acceleration force,
  • Aerodynamics becomes more important as drag is proportional to the square of the velocity.
  • Difficult to film and photograph. The rocket moves very quickly during launch.
Low
  • Less stress put on components during launch.
  • Easy to film and photograph
  • Suitable for upper stages in multi-stage rockets to keep peak velocity down.
  • Generally need a guide rail on the launcher
  • More susceptible to cross wind during early part of flight
  • Single stage rocket may not fly nearly as high as a high acceleration rocket, but again that is dependant on a number of factors.


Practical Tips

  • Removable Nozzles - If you are adding a restricted nozzle to your rocket then it is a good idea to make it removable. That way you can swap it between rockets as needed and you can try flying the same rocket with different nozzles.
  • Nozzle Weight - Keep your nozzle weight to a minimum. Using a heavy nozzle not only means you will achieve lower altitude, but will make your rocket less stable because it moves the center of gravity further back down the rocket.
  • Nozzle Alignment - Make sure your nozzle is well aligned. If you are using a removable nozzle, or are gluing your nozzle permanently, then it is important that you make sure your nozzle directs the thrust down the centerline of your rocket and the thrust vector goes through the rocket's center of gravity. If the nozzle is misaligned you may end up with a rocket that flies in an arc rather than straight up.

    One way test your nozzle alignment is to get a long (~1m) dowel, rod or tube that fits tightly into your nozzle. Stand the rocket up on it's nosecone so the dowel is sticking up. This way the weight of the dowel doesn't affect the nozzle. It should be fairly obvious if the dowel is leaning to one side indicating the nozzle is misaligned.

  • Increase Acceleration with same nozzle - If you can't achieve the minimal acceleration because you cannot change the nozzle size or increase the pressure you can decrease the amount of water in the rocket which reduces the lift-off weight. Although the amount of water may be sub-optimal, you may still achieve a comparable altitude.

    For example: A water rocket with 5L capacity, weighing 500g, having a 9mm nozzle, 90mm diameter, pressurised to 120psi and 1.6L (33%) of water will reach 109m (357') altitude and have a burnout acceleration of 8.3G.
    The same rocket with 1L (20%) of water will reach 106m (347') altitude and have a burnout acceleration of 11.3G.
  • Reducing Acceleration further with the same nozzle - Using a foam in the rocket such as with the Jet foaming technique you can further reduce the average thrust without altering the size of the nozzle or pressure.
  • Enlarging Gardena Nozzles - Gardena nozzles can be drilled out to about 10mm and still work effectively. Beyond 10mm the o-ring groove depth starts becoming a problem and could fail at elevated pressures. The difference between 9mm and 10mm represents ~ 20% increase in cross sectional area resulting in increased thrust.

  • Reducing Gardena nozzles - You can make Gardena nozzles even smaller if desired. You can fill them completely with Epoxy and then drill them out to the desired diameter. Alternatively you can glue in a tube of a particular diameter to make it smaller. Reducing the nozzle diameter this way still allows you to use the standard Gardena launcher.

20 / 10 / 2013

 

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