This was only intended as a short term development step, not a serious competition rocket.

The body of the rocket consists of three 2 litre bottles on top of a single three litre bottle. At launch the upper bottles will contain air with the water only in the bottom, 3 litre, bottle. The reason for this is that I wanted more than 2 litres of water. The glue used by rocketeers in the USA is not available in the UK so bottles must be 'plumbed' together. If the water is split over more than one bottle, the 'pipe' linking the two bottles must have a bigger bore than the exhaust so as not to waste energy forcing the water through needlessly small holes. In the Mk3 I bypassed this by using a three litre bottle.

The upper section is similar to the body of the Mk2b but rather than using wooden stringers to stop the rocket bending, the joints are sleeved with cuttings of other bottles. The other difference is that the top and middle bottles have been turned round so that their necks face aft to reduce the amount of water that will be trapped in the rocket. This section is plumbed into the 3 l bottle in the same manner as the other bottles but the sleeve over the joint involves a lot more sticky tape.

The nose cone on this rocket is shaped out of polystyrene and held in place with a sleeve like those covering the joints.

The drilled plywood fins have been replaced with coroplast (formerly the packaging of an optical table). These are attached with sticky tape.

This was not a good rocket at all! Lessons? Sticky tape might work on some small rockets but is no good here. The 3 l bottle was nowhere near well enough held in place, so the body bent in flight and the rocket went off in the wrong direction. The loading on the fins was much too great for the sticky tape. I am unsure whether or not the coroplast itself is stiff enough. I should really have looked at these numbers first.

Simulation

Input Figures

Rocket Volume  -  9 l
Diameter  -  130 mm
Coefficient of drag (guesstimate)  -  0.7
Mass of ejected water  -  3 kg
Launch pressure  -  100 psi
Exhaust diameter  -  21.5 mm
Exhaust loss coefficient (guesstimate)  -  0.3
Spent weight  -  0.4 kg

Results

Maximum acceleration  -  37 G
Apogee  -  290 feet
Burnout speed  -  86 knots
Burnout altitude  -  19 feet
Touchdown speed  -  45 knots
Total flight time  -  9 sec