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Rocket construction: PET fins

Building fins is the most time consuming process for the average water rocketeer. So, since long I am searching for an easier way of building them.
In the WR mailing list Russell McMahon described a simple way of manufacturing UNBREAKABLE fins from PET bottle material.

RuMMa2, Rocket with PET fins This is the rocket "RuMMa2" that I built using Russell's technique. I  particularly liked the idea of the exchangeable skirt part for the fins.  Did I mention that they are UNBREAKABLE?

As a nose cone and some sort of a retrieving system I used a regular tennis ball.

I named it after the author of the description below. The "2" stands for the 2 liter bottle that was used.

To read about flight characteristics, see the launch report here and here with videos.

Here is how to build a rocket with PET fins. From Russell McMahon.

"While the overall result obtained with the fin skirt and foldable fins is
different from any that I have seen so far elsewhere, I don't claim any
originality for the various parts of this design. As in most things, one
gleans an idea here and a concept there. If you are the originator of any of
these ideas that I have picked up along the way let me know and we'll give
you due credit.
I personally find that the folded PET fins are indeed very nearly ideal.
I take about a 1/3 of a cylinder cut from a PET bottle (you now have a
curved sheet).
Bend in the middle and fold flat so the two ends meet. I have tried bending
against and with the curve and both work. Arguably against the curve is
better. You now have a double thickness plastic sheet that is trying to
spring apart.
Squash it flat and on the open ends fold out a small tab which will sit
against the bottle.
    x    <- tab folded out
    xxxxxxxxxxxxxxxxxxxxxxxxxxxxx\   <-- fold
    x    <- tab folded out
Cut a fin of appropriate shape.
x = tab
y = fin
x  y
x    y
x     y
x      y
x      y
Attach to fins skirt with tape or glue (Cyanoacrylate works).
Note that as this is NOT attached to the pressure vessel the glue does not
affect pressure vessel integrity.
To stop the two parts springing apart either tape the leading and trailing
edges (I use thin flexible horticultural plant tape) or, if very keen, place
a few drops of super-glue between the two halves and squash in a press (a
few heavy books?) for a few minutes. You get a very thin fairly stiff but
flexible fin.
Side cross section
s        fin skirt (Pepsi smooth cylinder)
b        bottle
n        nozzle
x        fin flap attaching to skirt
y        fin proper
c        is an added Pepsi neck as a nose cone
             c    c
           c  bbbb  c
          cb         bc
          b           b
          b           b
          b           b
          b           b
          b           b
          b           b
        xsb           bs
       yxsb           bsxy
     y  xsb           bsx  y
   y    xsb           bsx    y
  y     xs b         b sx     y
 y      xs   b     b   sx      y
 y      xs     nn      sx      y
 yyyyyyyxs     nn      sxyyyyyyy
In my case it is a vertical cylindrical fin skirt of the exact same
diameter as the bottle. The smooth cylindrical skirt helps extend the centre
of pressure well back, mounts the fins so their bottom is in line with the 
nozzle exit and looks good.
Nose cone is sacrificial and really needs to be the way these things dive
(70 kph odd impact from 100 metres up).
Fin skirt is lightly taped in place and can be transferred to another bottle
in seconds.
The fins are by far the most time consuming part of this design.
Fin cylinder is just a smooth cylinder.
Nose cone is just a bottle end.
If you mount these squarely they perform superbly.
They are very thin (2 x PET bottle thickness, very light and NEVER break
(although you can tear them off of you try hard enough).
On my much mentioned Pepsi Rocket, on a vertical launch they produce a
beautifully straight vertical flight without twitch or deviation - not even
any rotation if you mount them squarely. The rocket goes to apogee, hangs
stationary in the air then flips over immediately and dives straight down.
Once having tried them I would be surprised if most people did not adopt
them for all except the most pretty of models."

Thank you, Russell, for this article !



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