The changes from the Mk4a are that the plywood fins are now coroplast (obtainable free as road-kill estate agents' signs), held on with tape and aligned with cuttings of bent bicycle spoke, and the bodies of the bottles are now reinforced by sleeving them with cuttings of bottle, the same as are used to splint the joints. Although I usually launch at 140 psi, this arrangement can be used at 160 psi. Although the bottles themselves seem happy enough at this pressure it is getting close to the limit of friction in the joints, as demonstrated at the end of the NPL competition when the top bottle blew out during pressurisation for our post competition flight.

The launcher is largely unchanged except for the tube having been sanded. I have reduced the diameter of the rod for most of its length to reduce friction but left the bottom, where the rocket sits until launch, untouched to retain the good seal. For the competition it was removed from the trike and a second launch tube added as we had hoped to fly two rockets.

Simulation

Input Figures

Rocket Volume  -  8 l
Diameter  -  100 mm
Coefficient of drag (guesstimate)  -  0.7
Mass of ejected water  -  2 kg
Launch pressure  -  140 psi
Exhaust diameter  -  21.5 mm
Exhaust loss coefficient (guesstimate)  -  0.3
Spent weight  -  0.7 kg
Parachute  -  round, 4'f flat, timer release (4 sec)

Results

Maximum acceleration  -  40 G
Apogee  -  390 feet
Burnout speed  -  70 knots
Burnout altitude  -  9 feet
Touchdown speed (no parachute)  -  5.5 (65) knots
Total flight time (no parachute)  -  35 (10) sec