Home ] workshop ] how does it work ] History of Astronautics page index ] [ Rocket launches ] useful sites ] version francaise ] Contact us ]



Rocket launches

updated :14/10/06












Creation of water jet stream and droplet spray cloud burst 

(photographs taken at 0.04 second interval)


Launch data :Pe :7b ; Tjet : 0,056s ; hjet : 2.078m

parameters to be found in the rocket performance table


Green-streak (8.9.02)









In order to scale these images the reference height of the ladder with pole extension = 2.8 m

This we use to calculate initial velocity and approximate height of water jet.


 Use 'Virtualdub' to simply determine the height achieved by the rocket after a known time tsecs following the launch. 

  • Replay the video using the image by image  review  function to identify the part  of the launch video when the rocket leaves the 'pas de tir'/launch pad.

  • Look at the change in video time (found at the bottom righthand of the virtualdub screen border) between launch and the image at which you calculated the height .

Then simply divide the distance(height) in meters by flight time in seconds to obtain a value of the average  velocity of the rocket after a time interval t .  


Vrm  = Distance  or height h in meters divided by time sec


Vrm = h /t    (meters/sec)


Distance (m)  = Mean or average velocity ( m/s ) *  by time in secs

       s  = Vm * t

Rocket: Papillon

Launch system: Pressure Plug release.

Video: 'Green Streak' 11.22am 8th September 2002.

 Because you can just see a faint vertical green streak in the  second image just to the left of the black pole. The rocket has already left the top of the image!



Launch and flight analysis using video recordings. 


Because of the high velocities involved. Analysis of the short event times at launch and during the flight  requires the process to be filmed and then played back image by image.


How to use video editing software like 'VirtualDub' 

If you have never used  'VirtualDub' before here are some simple instructions.


Note: This technique can be also be used for recording and slowing down other experimental images. So you can discover for yourself .


1.Load a version of virtualdub from the address given in the 'useful sites' page.


2.Select virtualdub.exe file .

vdubimage1.jpg (57460 bytes)

3.Select 'File' in the top menu bar of the Virtualdub screen that opens by  clicking on the tab named 'File'.


4. Now load the video file you want to analyse. For example the 'Green Streak' video used above.

vdubimage2.jpg (62003 bytes)

5.Now select 'Edit' in the top menu by clicking on it.


6.Finally select  'Begining' and the first image of the video selected should appear on the screen.


vdubimage3.jpg (65841 bytes)



7. To find the part of the video which you want to analyse.

    Select and drag the time tab marker found in the long slot below the screen. 

To the point of the video where for example the rocket is just about to leave the launch pad.


8.To  analyse the film image by image click on the small yellow key image to be found in the lower screen  menu bar. 

Click on the right hand yellow key for forward lefthand key symbol for back.


vdubimage4.jpg (47475 bytes)




9.Look at the ellapsed time refered to in the lower menu to sychronise the image with time. This should change as you move from image to image.


10.To view the film at normal speed just click on the start arrow. As used for playing a musicCD.

11.Looking for the point at which the rocket reaches apogee.

This can be made easier by using the image magnification function.

Just click on the image. A menu pops up offering the choice.

Normal, 2 times  or 4 times original image size.

The tiny black speck in the middle of the screen is our rocket 'Papillon '

enlarged 2 times.


Launch with 10m image frame height.


Launch Log 29th June 2003 20.57hrs

St Baudiere

Image128              3,2,1, Top


Image number129

Height h 4.2m

t = 0.04

 Velocity Vam = 4.2 / 0.04

Vam = 105 m/s


Image number130

Height h is in excess of 9.7m

t = 0.08s

Vam = 9.7 / 0.08

Vam = 121m/s


Velocity Vr between 4.2 and 9.7m

Vrm = 5.5 /0.04

Vrm = 137.5 m/s

Real value of velocity Vr will be in excess of 137.5m/s as the rocket has left the top of the image 





71.jpg (66418 bytes)



72.jpg (64616 bytes)



74.jpg (64604 bytes)




The water spike in the third image is 60 mm dia at the base and 40mm at a height of 1.1 m 

this confirms a predicted water jet height of 2.08 m in the performance parameter model.


Note:Refer to attached performance spreadsheet found in how does it work.

Here an Expansion nozzle was used.

Pressure plug launch










  1. Shows the lower spray disc  reflected from the top of the launcher tube beginning to fall and form a 'jupe'(Skirt). Created by the jetstream leaving the rocket nozzle a solid central water spike is frozen instantaniously supporting the rocket at its tip. The helix form of the atomised spray burst cloud in the middle is possibly the signature of mixed fliud expansion from the nozzle. Height of rocket nozzle above launcher is 2.51m at this point. So it is possible that the pure water jet has finished and the mixed fluid/air burst has either started or occurred. Forming the atomised water droplet spray burst. Even though the jet stream is projected downwards there is a relative momentum  given to the spray cloud by the rocket accelerating upwards. That effectively pulls the  spray  pattern into a stretched elipse form before it decays downwards under gravity.

  1. The water spike has started to decay falling under the effect of gravity. Leaving a central water core surrounded by radially expanding cloud of finely atomised water droplets. The base spray disc has expanded to a visible radius of approximately 2m.

  1. The head 'spray burst' cloud is now a stretched eliptical  form , with a reduced minor axis and stretched major axis,surrounding the  falling water its center. That is whats left of the initial water spike as it decays. Some of the lighter  and smaller water spray droplets taking longer to fall back to earth possibly due to them having either a radial or upward velocity component. Appearing to be momentarily frozen in time as they are retarded before falling back to earth. There is also  a cross wind blowing from right to left that is introducing some assymmetry to the spray burst.

Launch pressure Pe 10 bar. Time interval between images 0.04s

Tvol 52s   dist 320m  ( Rocket Korolov 1) v1445



Visualisation of a water droplet flow dynamics /movement of fine water droplets created in a typical atomised  garden water jet. Why does the energy decay in this way/pattern? Why does nature work in this way?

 Could this symmetry, flow and decay  of small droplets be defined by some form of dynamic energy 'Fractal' relating the energy phase difference of each particle, the external ambiant  conditions to which it is exposed and the influence of millions of microparticles in the immediate proximity . Defining the macro energy fractal /pattern of the resulting spray cloud.

The energy decay system  conforms to some form of optimal mixing determined by the restraints of fluid boundary condition.


Note: Refer to 'Constructal Theory'   Optimisation of engineering design by looking at the geometry and design found in nature  Dr.Adrian Bejan.


Remember this is taking place in real time whilst the water rocket jet stream (water spike) is completed after only 0.04sec.

So the fluid flow dynamics need to be analysed with the type of high speed camera we don't have. 

Our films have been shot using a Sony Cybershot DSC F707 5Mp. So if there is anybody out there from Sony  we would like some help. We needs to film at a higher image rate per second.


We use Virtualdub   to analyse the launch videos in slow motion.


1.This shadow-graph image of of a supersonic bullet illustrates well the turbulent wake created by a projectile.

So how does the air flow passing over the rocket effect the creation of the water jet and mixed fluid phase?


2.Does this wake have fractal properties.


3.Does the creation of a subsonic bow wave on our rocket .have and effect on  the aerodynamic efficiency of the rocket.


4.Does the fluid boundary layer behave as a compressible or incompressible fliud.



Visit the Aerodynamics page


What are fractals?


An introduction to fractals and fractal models.



This site was created on the 15th April 2003

ŠJohn 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"