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My current rocket in design / under construction
The 20L record breaking water rocket
I want my next rocket to go high, ..realy high.
Goal is set above current world record, (I want to beat all the guys with their reinforced FTC tubes)
And I want to do this with the (bulky) standard PET bottles.
Impossible? => I'll need a big/ long pressure vessel for shure!. 		Project status overview 1-Jan-2005
Body: Pressure vessel (2 joints) tested @ 240 psi ok, external sleeve type .
Release mech.: Mechanical timer release mechanism.
Fins: 4 Size and shape to be determined
Nozzle: 22 mm nozzle (no nozzle restriction needed for opt. height according to simulator)


The design shown here is the second in order. The first design setup, with the interconnections on the inside (a smoother outer wall and less air resistance) was left. During pressure testing it was found that this type of construction / joints is to sensitive for leakage between inner en outer wall. This leakage will cause the section to breakup and the rocket will explode during pressurization. It was also found that the Spa type bottle could handle more pressure compared with Pepsi bottle. Pepsi bottles (and others) have a variable wall thickness (thick bottom , thin top). The weight / max pressure ratio is favorable for the Spa type bottles because a pressure vessel is just as strong as its weakest part, the thicker bottom just adds to the weight and not to the maximum pressure. In the current design setup Spa bottles parts are used for the main body sections. The Pepsi parts are used for sleeve / interconnecting parts.
Lets do some calculations:

Project name: 20L rocket (still need some inspiration for a catchy name)


Desired capability's:
Apogee: 500m => 500/0.3048=1640ft.

Design starting points:
Pressure vessel designed max. Pressure, must be minimal 20 bar.
Pressure vessel volume: 20L
Base construction material standard PET bottles available in the Netherlands.

two bottle shrunk-fit interleave construction
20l basic sizes and setup
Pressure vessel construction type:
Two bottles shrunk fitted and glued in each other
Double pet layers (2*0.6 mm) => 1.2 mm vessel wall thickness.
If needed for max. pressure, supported with external support wires (fishing wire)

Body Length info:
Top / end section: 2*28 cm= 56 cm
Middle section: 15 pc*20.5 cm= 307 cm
Nose / chute section = 25 cm
Total length 388 cm

Volume:
Top and end section 2*1.35 dm^3 2.7 dm^3
Middle 15 sections of 1.13 17.01 dm^3
Total pressure tank volume 19.7dm^3


Weight budget:
Nose chute / release mech. 150 gram
Top / bottom section 75+75= 150 gram
15 spa sections 663 gram
14 Pepsi sleeves 444 gram
glue 300 gram
Fins / finholder 150 gram
Total weight 1857 gram

Fins: 4
Size / shape t.b.d.

Nozzle:
Nozzle: Standard bottle 22 mm nozzle, simulation showed that a reduced nozzle will give little extra height up to pressures from about 18 bar. With pressures above 18 bar the extra height is limited.

Release mech: Timer release mechanism.

Lets see if this basic design setup is capable to reach the 500 m apogee, enter the parameters in roketsim simulator, for CW 0.8 is used. Seems fair for the size.

Simulation:
Rocketsim apogee 20L: 391m @ 14 bar / 3.51 liter and 543 meter @ 20 bar 3.72 liter

And some simulation graphs from waterrocketsim

simulation numerical data Launch pressure is 20 bar, nozzle size 22 mm, standard bottle nozzle

velocity graph



OK!, looks good, with 20 bar and standard nozzle the +500m must be in reach!!
T-nozzle gives a little extra height with 20 bar, this difference increases at higher pressures. But this will not be worth the effort or extra strain on the pressure vessel as it picks up the nozzle at the end of the launchtube.

Now lets see if the pressure vessel construction is able to hold the pressures used in the simulator.
Continued on next page