The InterPlanetary Water Rocket Society Notebook                           Rev. 6/8/05

Water Rockets at School
Other Water Rocket Sites on the Web


Dr Pepper Water Rocket line up.

     A comment from my friend Jean Berthelot  has inspired me to post a picture of my rockets.  These rockets are usually Dr Pepper bottles with conical necks.  Some have been shrunken from 500 ml to 400 ml using boiling water.  The two tall bottles on the left are joined bottom to bottom for larger motor volume.  The noses are half tennis balls, attached using hot melt glue and colored electrical tape.  The fins are corrugated plastic sign board friction fitted into report cover spines.  The spines are glued to the bottles with either low temp hot melt glue (faster) or Household Goop (stronger).  Recovery is ballistic, the fins typically pop off without damage.  These rockets fly fast and very high.  They make a very exciting whistling sound as they come home, discouraging us from trying to catch them.
    They do not fly as fast or as high as my fluorescent tube cover rockets fly.

Gliding Calistoga Rocket

Calistoga Water rocket glider image

      This rocket took off vertically at 100PSI and went into a flat stable glide. It was lost some 300 feet from the launch site on the roof of a building. A sketch with approximate dimensions follows. I have seen and heard of behavior similar to this several times since with similar rockets.
     More recent launch successes have involved smaller bottles, tubular fins or corrugated plastic fins inserted in the report cover spines, and a half tennis ball nose.  See Above.

    In 2005, The Alameda Middle School Water Rocket Group built a number of gliders with two liter bottles and typically oversized fins glued directly to the bottles.  We  hot glue tack the fins to the bottle, and then reinforce the joint with PL Premium construction adhesive.  Bottles that glided typically had fins half way up their length.

We also had a great success cutting the neck off a three liter bottle, inserting and gluing (PLP) a two liter bottle neck inside it, and with oversized (by eye) fins, at about 100 psi convinced this one to fly out of sight.  We never did get it back as it sailed off into the neighborhood behind the bleachers.  We were launching from the center of the football field in the school's rocket range.

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Why Tube Launchers?

     They are elegant and effective. During any open nozzle* launch, the water is expelled in a tiny fraction of a second. Acceleration is almost immediate, and the rocket's flight is almost entirely ballistic. This all starts with both the rocket and it's 'fuel' standing still. With a tube launcher, the initial thrust takes place with very little loss of water, and lasts the length of the launch tube. At that point, not only the rocket but also it's considerably more massive fuel are already traveling at speed. Any loss of pressure up to this point is partially offset by air entering the rocket from the launch tube.

Launch tube

*An open nozzle launch uses the original dimensions of the bottle's mouth for the nozzle. This is not the only way to launch these rockets, nor is it going to result in the highest performance. It is probably the easiest. Some people like to fasten other sorts of nozzles inside the bottle mouth. (There is agreement in the water rocket community that an open mouth is a maximum desirable opening.) An intriguing alternative is the so-called T-nozzle, which employs a full size launch tube for initial acceleration, but then a fitting drops into the nozzle to reduce and extend the thrust over time. For something with as little mass and as much drag as a soda pop bottle, this can make a lot of difference.

  Ian Clark's cable tie launch tube design

Cross-Section Key:
WaterMotorLaunch Tube Sleeve'O' Ring
Cable TiesRetainerTape

This drawing portrays my version of Ian Clark's Cable Tie Release. Cable ties are used by electricians to bind wires together. I have only shown one on the left, but I use as many as fit around the launch tube. There is a #94 rubber 'O' ring between the upper and lower parts of the launch tube. The upper part of the launch tube is snug on the inner sleeve, but is removable to exchange with others of lengths appropriate to my motors. (Long motor / long tube.) The ties are held engaged with the collar on the motor by a ring of flourescent tube cover about two inches long. To increase visibility from the pumping station, there is red electrical tape around the ring. The ring is light enough to hold itself up with friction before pumping starts. The pull down strings go to a metal ring. The remote release string is tied to a second ring on the launcher base, it passes up through the upper ring, back down through the lower ring and out horizontally to the launch crew.

Assembly is a bit tricky. I use light rubber bands and masking tape to hold the ties while I position them. I place a bottle on the launch tube to line up the tie heads with the bottle's collar. When everything looks right, I put some cable ties around the rest (Drawing on the left), and tighten them. If everything still looks good, I smear warm epoxy all over this section. (Warm so it flows better.) Then I trim all the ends flush. If one desires to experiment with different bottle placings, one can use hose clamps without the glue, but it results in a clunky looking job and the clamp can get in the way of the retaining ring.                       Top


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