Water Rockets  1 litre Power Rocket
The PET

Initial
state of the bottle
blanks that make the bottles that we use for water
rockets are also used for some soft drink cordials,
squash bottles and so on. These bottles are not meant
to hold any pressure and to stop them collapsing when
the purchaser picks up the bottle, they have
corrugations in the cylindrical part of the bottle.
They seem to be made from the blanks that 2 litre
bottles are made from and the plastic is noticably

With the
bottom blown out and the sides shrunk
thick. With them being fairly small in diameter, they
are ideal for higher pressure launches. I wanted a
rocket that I could launch at 140 psi for altitude
and for downrange distance  this looked ideal.
The first thing that I did once I had emptied
and cleaned the bottle was to blow out the concave
bottom. I did this by pressurising the bottle, using the
cap with the oneway valve in the bottom that is
described previously and carefully heated up the bottle
over the gas stove. The bottom blew out reasonably well
so I thought that it might be a good idea to see if I
could straighten out some of the corrugations as well.
The intention was to make the bottoms of each
groove the same height as the material on either side but
the whole thing went smaller instead. If anything, the
bottle ended up stronger as the plastic was thicker and,
the diameter was smaller for a larger proportion of its
length.
I then cut some fins from the
side of a bottle like so . . . 







Cut a
straightsided cylinder, 

cut up one side, 

fold it in half
and 

mark out and cut
the fins. 
I stuck the three fins on the rocket with tape and then
weighed it full and empty. This gave me the weight of the
rocket empty (effectively the minimum weight of the
rocket on the simulator) and the capacity of the rocket
body. Any weights are taped onto the nose of the rocket and
tape is used to make the rocket roughly aerodynamic.
Here are the vital statistics . . .
Mass of empty rocket 
100 
g 

Capacity of Pressure Vessel 
900 
cm^{3} 

Rocket Diameter 
6.5 
cm 

Rocket Coeff of Drag 
0.42 

* 
Nozzle Diameter 
21.75 
mm 

Constant K for nozzle 
0.22 

* 
Duration of air impulse 
20 
ms 
* 
*
guess based upon figures 
I pressure tested it to 140psi and there was no
sign of any damage so this seemed to be a reasonable
pressure to launch at. Note that to pressure test a
bottle, you should fill it completely with water so that
there is little or no air inside to store the energy that
would do the damage should the bottle burst.
The following is the table for maximum height and
downrange distance at various pressures using the rocket
that I made.
H e i g h t 
Pressure (psi) 
50 
75 
100 
125 
150 
Actual or Ideal 
A 
I 
A 
I 
A 
I 
A 
I 
A 
I 
Water (g) 
285 
210 
285 
240 
285 
240 
285 
240 
270 
255 
Rocket (g) 
100 
40 
100 
55 
100 
65 
100 
70 
100 
80 
Height (feet) 
105 
135 
155 
180 
200 
220 
245 
255 
280 
285 
D o w n r a n g
e D i s t a n c e 
Pressure (psi) 
50 
75 
100 
125 
150 
Actual or Ideal 
A 
I 
A 
I 
A 
I 
A 
I 
A/I 
Water (g) 
290 
245 
295 
265 
280 
270 
270 
270 
285 
Rocket (g) 
100 
55 
100 
70 
100 
85 
100 
95 
110 
Angle (degrees) 
47 
41 
43 
40 
41 
40 
39 
39 
39 
Distance (m) 
55 
65 
80 
85 
100 
100 
115 
115 
130 
I have fired this rocket and tested the
downrange capabilities of it and it works fine. The
calculations are spot on. I have also fored it stright up
and the amount of time that it is in the air gives it the
wow factor as far as newcomers are concerned. On the
pictures page, you can see the results of this little
beast hitting the football pitch which, while wet, was
sufficiently solid to walk on.
To
the pictures . . .
Back to the Real Rockets Index
Back to the Water Rocket Index
