Water Rockets - 1 litre Power Rocket

The PET 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 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 one-way 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 straight-sided 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 cm3 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 . . .