Home ] workshop ] how does it work ] Rocket launches ] Space History ] useful sites ] version francaise ] Contact us ]

Launching systems










Pressure plug release.

Very simple in concept. Comprising a plug that fits into the neck of the bottle that has a one way none return valve forming the center axis of the plug seal.

The valve is to allow compressed air to be pumped into the bottle and not permitted to escape.

This is where most people start. Usually making a pressure plug from a champagne/cider cork that is drilled to fit a bicycle or car inner tube valve at its centre. 

To check the release pressure of the plug. Fill the bottle/rocket completely with water and pressurize with a good tyre pump that is fitted with a pressure gauge.

Release pressures range of 2 to 5 bar are ideal to start with.

Note : This will also give you an indication at what point the rocket will launch.

Never return to a rocket that is pressurised. Keep pumping until it launches.

Basic launch procedure:

Health and Safety come first in everything so please take attention to the following.

Depending on the age and experience of children involved. Initially it is advised to use a high launch platform that prevents the risk of any inquisitive children ever placing a head above the rocket.

This can be achieved by launching from a levelled garden table or concrete slab mounted on a suitably high wall.

Obviously parents should assume responsibility to clear a zone around the rocket launch site and launch the rocket.

That said a water rocket launch on a hot summer’s day can be both exciting and refreshing. Providing the safety precautions are followed.

Launch procedure.

  • Level launch platform.

  • Meteo Report > Never launch in strong winds.

Note the direction of the wind. Take some dry grass and see which way it falls. 

  • Fuel rocket with water to no more than one third of its volume.

  • Insert the pressure plug being careful not to damage the seal for future launches.

  • Attach the flexible extension to the valve and turn upright before carefully placing on the launch pad.

  • Install a polystyrene nose cone taping it to the top of the rocket.

  • Clear the zone around the rocket launch site.

Spectators to be 5m upwind from the launch site.

  • Attach the tyre pump.

  • Introduce compressed air into the rocket without pulling on the connecting tube.

  • Launch is achieved when the compressed air charge overcomes the plug fit friction. Get the ground crew to observe where the rocket goes downwind.

Time the flight.

  • Send out the recovery crew.

  • One of the ground crew should be made responsible for recording the flight time, rocket specification and weather details. This can be the start of a flight log. Data base

  • Hold a flight debriefing to discuss and record events, rocket damage, future flight worthiness (failures, success, and improvements). What they thought.

High pressure plug design.

A development of the pressure plug idea can be seen in photographs above.

Where the valve stem of either a bicycle inner tube or car tyre is used with a piston made up from either rubber grommets or drilled out rubber tap washers of the correct diameter.

The rubber grommets can be subsequently fitted with neoprene ‘O’ rings to improve the piston seal. That can be expanded once the plug has been inserted into the bottle by tightening the valve-clamping nut.


Advantages :

  • Simple and easy to manufacture.

  • Internal clamping gives the possibility to test expansion nozzles fitted to the exterior of the bottle neck


  •  Time taken to insert plug before flight.

  •  Predicting launch point during pumping phase is difficult to predict.

  • So taking launch photographs or videos is difficult to synchronize.

  •  Damaging the plug seal and wear progressively alter launch repeatability.






Dsc00873.jpg (60261 bytes)



Dsc00914.jpg (59238 bytes)




Dsc02402.jpg (60083 bytes)


1995 Launch Alex 5 and Gabriel 2.5



Dsc00836Paillion with nose mass and chute20902.jpg (61801 bytes)



Dsc01469.jpg (63412 bytes)



Dsc01468.jpg (60966 bytes)



Dsc01501.jpg (60918 bytes)




Dsc01831.jpg (62308 bytes)

Guillotine or sprung gate, flange lock release

    This type of launcher uses a sprung gate that locks above the external bottle neck flange.


    Launch Procedure:

  •  Fuel rocket with water and insert into sealed release head locking the gate above the nozzle flange.

  •  Introduce air pressure to the rocket using either a tyre pump or a remote compressor. Controlling and monitoring the maximum pressure.

  •  Secure area adjacent to launch site.

  •  Release gate lock. This is usually achieved remotely by pulling a cord.

  •  Refer to remote launching section.



  • You control exactly when the rocket will be launched.

  • Remote control of launch possible.



  • Speed of gate release can cause a problem with initial launch stability.

  • If the gate is released too slowly then the entire rocket flange will not be released at the same instant. Causing the rocket to be released at an angle away from that initially set.

  • Launch event times are very short Tjet 0.04s. So someone taking say 1sec to fully release the gate mechanism could seriously reduce flight performance.



spgatel1.jpg (61274 bytes)

Launch tube release. 

    Launcher tubes use a coaxial sliding collar to clamp above the bottleneck flange using a ring of flexible radial fingers that retract when the sliding tube is lowered.

    The launch tube is clamped or supported vertical with the rocket gripped at the top by the flexible fingers. The nozzle of the rocket being a pressure seal fit inside the top diameter of the launch tube

    Water and air feed lines are attached at the base of the launch tube.

    Water is introduced to the bottom of the tube to fill the bottle to the level. required.

    Then the supply valve closed before the feed line is detached.

    Then the water level set to the desired level using the supply valve.

    The trim of the launch tube is checked to guarantee that the rocket is vertical using the water level inside the rocket as a reference.

    Pressurisation commences with the possibility at any instant to decompress the system by opening the water supply valve. That now acts as an emergency dump valve

    Launch pressure is reached and the rocket fired.

    Remote launch can be achieved by pulling a cord or using one of the systems listed below.

    Variations on this type of launcher include the incorporation of an internal air tube into the neck  of the rocket bottle that is sufficiently long to open above the water level in the bottle. This limits the volume of air bubbling through the water at launch.

    This internal tube can also be used as a launch direction guide. Select just the right size diameter of tube so that it has a sliding fit into the neck of the bottle.

    If you are really interested then look at future section.





Dsc01110Blackbird1 on table de tir.jpg (60967 bytes)



Dsc01255 Mark1 and Mark2  JAGlaunchers.jpg (63051 bytes)


Dsc01440.jpg (62408 bytes)

High pressure caps.

                   This is a type of launch tube that allows the compressed air charge to be built up in the 

                   tube volume in the base of the launcher to a preset pressure controlled by a special 

                   pressure cap.

                   Once reached the compressed air charge is released in a small diameter light 

                    weight tube rocket.



Dsc01826.jpg (62374 bytes)

Remote launch systems.


Remote launch capability prevents you from getting wet at each launch and brings a level of sophistication to the launch procedure.

Types of remote launch.

Pulled cord. Is the most common but has limitations with launch consistency/repeatability when being used by different people.


Hydraulic release can be simply achieved using two syringes one acting as an actuator piston triggered by another at the remote launch position. A 2mm bore plastic tube connecting the two is filled with water and all air purged. from the highest point. Depressing the syringe actuating the gate or the sliding collar.

As water is incompressible the delay is nearly instantaneous.



Pneumatic release collar.

A small pneumatic actuator is used to lower the release collar or open the gate clamping the bottle flange.

Pressure or spring load guarantees that the collar will be kept closed during preparation for launch. Refer to photographs.

Has the advantage that it uses the same medium as is required to pressurise the rocket.

So if you have compressed air then this is a simple step to take.


Hydraulic actuator

Hydraulic actuator with a 30mm stroke taken from an old car. Using hydraulic oil and reservoir to prime the master and slave cylinders.







Dsc01442.jpg (63282 bytes)







Dsc01203.jpg (60734 bytes)

Launch site selection and preparation.

  • Good-launch site location is important.

The ideal site is where there is a large open area down wind with 250 to 500m recovery zone.

For early flights that will inevitably be ballistic in nature before you develop a reliable parachute deployment system.

You ideally need an area where there is no people, no pedestrians, traffic or sunbathers.

Usually using areas such as parks, sports fields, agricultural land or beaches out of the peak hours can accommodate this.

Always seek permission first.

  •  A firm flat platform on which to install your launching system is very important.

We use a standard concrete slab 60cm by 60cms. 5cms thick This is set just above ground level so that it drains easily and provides a stable all year round launch capability.

  • With the plug release system you can launch directly from the surface of the slab.

  • When we started to test different expansion nozzles this meant we had to use several packing blocks 70mm*100mm t= 40mm one for each fin.

  •  For our launcher tube design we used a small heavy ‘Black and Decker’ workmate to clamp the base of the launcher tube assembly. Just sat on to the surface of the slab.







Dsc01141.jpg (62055 bytes)



Supply services to the launch site:

  • Water:

From late spring through to the first frost we use a garden water supply using

Gardena water fittings.

During autumn and winter we use warm water to charge both the launcher and the rocket in addition to using another bottle of hot water used like a hot water bottle to keep the pump and valves from freezing up when the temperature are below zero.

We also put the pump on the radiator to warm in through before taking it outside.



  • Compressed air.

We use a good quality air pump with a pressure gauge incorporated that can supply pressures up to and including 10bar. In conjunction with a high pressure flexible adapter tube.

Electric motor driven compressors connected to a reservoir are a good source of compressed air for multiple launches and competitions.

We have recovered an old compressor from an air conditioning plant. Coupled to a pressure regulator and reservoir. Which we used for ‘ExpoScience’ demonstration launches. This guarantees a regulated supply pressure up to 35 bar!



Make sure that the rate of pressure increase is controlled by using a air flow regulator valve.

This should be installed into the air pressure feed line before the pressure gauge that measures the pressure being supplied to the rocket.

This is to control the rate of pressure increase and prevent a step change of pressure that could explode the plastic bottle.

 Start with the valve almost closed and experiment to find at what setting gives a progressive pressure increase to achieve launch pressure over a period of 20 seconds.


Alternatively it has been suggested that we can use a regulated air supply from an old scuba diving air tank.


Dsc01450.jpg (60964 bytes)

Dsc01502.jpg (54673 bytes)

  • Clean and dry preparation area

To prepare the parachute nose module and fold the parachute. Test the nose separation angle for subsequent flight clearance.

Check dynamic stability by finding the centre of mass for the unfuelled rocket M2 and reset nose mass.

Set mass centre to be one rocket diameter above aerodynamic centre of drag.



  • Meteo

Wind velocity and direction. Calculate recovery distance.

Air temperature.

Atmospheric pressure.

Measuring wind velocity and direction

One of the simplest methods of measuring wind speed and direction is by using a wind sock.

The velocity is indicated by the angle of the wind sock relative to the vertical. When there is no wind the wind sock does not inflate and hangs vertically.

When there is a high wind velocity the wind sock inflates and is horizontal to the ground. Since the wind sock is pivoted it will also stream out in the direction of the wind. So indicating the velocity and direction of the wind to be found at the height of the wind sock.

Note: There is a restriction when measuring wind speed at ground level.

In that it will not necessarily be the same at higher altitudes.


A more accurate but more expensive method is to use a air flow velocity meter. That measures the wind velocity using air flow driven

fan or propeller.

A unit like this can be used to calibrate the windsock.

By creating a graph of windsock angle to measured air speed.

Knowledge of wind direction is also important for determining the direction of flight of a launched rocket.



When you have none of these available. We frequently take a hand full of grass and release it. Checking which way it falls aided by the wind.

If it falls vertically it is almost calm at ground level.


Look for other signs that indicate  the direction of the wind:

  • Cloud movement.

  • Tree deflection due to the wind.

  • Weather vane direction.

Compass.jpg (48631 bytes)

Sky Mate

wpe4.jpg (11322 bytes)





  • Good earth point. (What on earth do you need one of these for!)?

We have improved our parachute deployment during successive launches by reducing the build up of electro-static charge in the rocket nose cone.

Flight data recording.

Delicate digital electronics for recording flight test data do not like an environment that is heavily charged with static electricity.

We have created a good earth by drilling the ground close to the launch site and installing a good earth strap that we attach to the metal of the launch tube.

Refer to Electrostatic charge.


static.gif (34492 bytes)

Recording Launches.


Photographs and short video clips are very useful due to the launch velocity and speed of flight involved.

Analysis of launch and flight recordings using VirtualDub helps slow down the events and makes it easier to understand what happens. Analysing the events image by image.

Launch velocities and accelerations can be estimated by referring to a fixed height marker next to the launch site. A scaling tower.

I have encouraged Alexandre and Gabriel to use the digital camera to record events so that we can analyse them later.

So our project has been well documented with photographs and short 16s videos.



Launch and rocket flight data recording. Using electronic data recording downloaded after the flight to a portable computer. This can then be used to develop a more representative flight simulation mathematical model or program.




Dsc02434.jpg (61004 bytes)



Flight data.jpg (55885 bytes)

  • Photographic equipment.

We have recorded all our experiments and test launches using a Sony 

Cybershot DSC F707 digital camera with*10 optical zoom.



Pre-launch sequence.

1.Install water and air feed lines then rocket.


2.Install nose cone module and tape in position.


3.Fuel with water to correct level required and adjust rocket to the vertical before locking launcher clamp.



4.Disconnect hose and leave safety valve ready to dump pressure if required

during pressurising or pre launch checks.

Note: Here the pressure valve has been opened to show it in the safety    

         pressure release position.


5. Fold and install parachute and nose cone. Check balance mass.
6. Check electronic flight data logger.
7. Secure launch site with launch crew upwind.

8. Slowly start to pressurise the rocket to the required launch pressure using  

    the hand pump or compressor.

9.Start flight data logger by pulling flag.

10.Give instruction to start filming

11 Launch

12.Observe flight.


13.Recover rocket


!4.Hold flight debrief.


Using an aerodynamic plug nozzle.

Because matching fluid flow dynamics to a fixed nozzle geometry is a bit of a compromise where variable forcing pressure is concerned. Then the use of some form of variable geometry nozzle configuration would seem a good place to start developing new ideas.

Use a true plug type nozzle 

 To improve the consistency of the jet force as the internal pressure drops. Could help the Isp. Specific impulse consistency during Tjet.

  • Optimising Specific impulse during power jet phase.

  • Multi-stage rocket designs  and the development of light high pressure staging mechanism August2003

  • Accurate flight data logging. Jan 2004


back ] menu of this section ] next ]



Dsc01125Sunsetattheendofagoodday09103.jpg (62634 bytes)

10/12/05 This site was created on the 15th April 2003

 ©John Gwynn and sons2003 

You're welcome to reproduce any material on this site for educational or other non commercial purposes

 as long as you give us proper credit (by referring to "The Water-Rocket Explorer"