I have been working with assorted variants of this beastie for some time. The basic design is not that far removed from the Mk2b but the stack is now four bottles rather than three allowing me to use two litres of water. The plywood fins are now coroplast (obtainable free as road-kill estate agents' signs). These are held on with tape and aligned with bend bicycle spokes. The joints between the bottles are 'splinted' with cuttings of bottle as on the upper part of the Mk3 but the bodies of the bottles are now reinforced in the same way. Although I usually launch at 120 psi, this arrangement should be happy enough at 160 psi.

The launcher has also changed; this is now a clarke cable launcher based on a length of 21.5 mmf overflow pipe. This both makes an effective launch rod and gives a good seal with the neck of the bottle which makes pumping a great deal easier and makes the higher pressures practical. It is actually a tight enough fit to give blow-off pressures of around 40 psi for an unrestrained bottle.

The picture (right) show one varient on the launcher (yes, it is built onto the back of a tricycle). The strings hanging by the side of the rocket are part of the parachute lines. Far right is a detail of the nose. From top down: the nose cone is polystyrene and is designed to fall off at apogee and the draft should then be plenty to pull the canopy, below, clear of the body. The parachute is rigged so as to suspend the rocket horizontally so that the camera, below, just above the top of the pressure vessel (black), will be pointing at the ground.

The camera has a two second timer which is tripped by the parachute lines so that the rocket has something of a chance to stabilise before the shutter is triggered. The catch is that I have yet to get a parachute system to work reliably; this is bad news for the camera (although I haven't written one off totally yet) so to date I have just been using used disposable cameras. These have the added benifits of being free, if you ask nicely, reloadable (if you know how) and very light weight even including the flash (useful for indicating when the shutter fires).

It is likely that it will be a version of this that will be used for the M.R.F. entry for the NPL Water Rocket Challenge on 26th June.

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