Tools & Tips for Water Rocketeers

Applying PU glue with a syringe

The tip on my PU glue container PUR-Leim 501 from Kleiberit is intended for construction use and therefore coarse. To apply it I use either matches (remnants of match operation on the photo, left) or, better, a syringe. I suck the honey-like substance into syringe, applying it where necessary. After use I cover the syringe opening with kitchen foil. The glue remains usable several weeks.

If a little bit hardens in the tip, it can be shoved into the syringe with a nail and does no further harm to the rest of the glue. As always, use one way gloves while working with polyurethane glue. It's a nasty thing on your table, carpet, skin or whatever.
One can even use a cannula for otherwise inaccessible places, but probably only once.

Cut marks on PET bottles

To make accurate cut marks, use a felt-pen marker taped to a clamp at a vertical board of a closet. Just turn the bottle, holding the bottom of it flat on the surface (works great!)

Cutting of PET bottles

can be done freehanded, but it needs care and gives not always good results.

With simple means one can work a lot easier:
A cutting knife is clamped to a vertical board, cutting side up. A centimeter rule (or anything else of that size) is jammed under it to keep it position.

The bottle is kept with both hands. Turn bottle against the cutting edge. If you marked the cutting line properly, cutting accuracy can be brought to less than 1 mm without much effort.

Safety: retract cutting edge into knife immediately after EVERY use.
Testimony (untested): These kind of knifes, as cheap as they may be, cut your skin and muscles (maybe not the bones) much better than PET!

Cut marks on FTCs

FTCs, the Fluorescent Tube Covers that are not obtainable in Germany and are so very apt to make lean water rockets from, can be marked and cut very simple: I simply open the top drawer (covered with white paper here for visibility) of my desk and put the FTC on it. The FTC touches the grey table top behind and the wall on the left. The left hand rests non-moving on the drawer, felt pen on marking position. Right hand rotates the FTC.

Cutting of FTCs

The FTC I got a hold of via list member Tom Lanigan (thanks!) is quite strong and difficult to cut with a knife. I therefore use a Dremel type of tool with a flexible shaft. It holds a very thin (0.4 mm, 22 mm Ø) circular blade saw. Holding the FTC between right arm and body, the left hand turns the FTC. A little exercise helps you make nice straight cuts. The secret is to push the blade only just below the surface - going deeper produces mostly undesirable heat.
Safety: these blade saws are terribly dangerous: they cut flesh (tried) and bones (untried) easier than a hot knife butter. Since my first close acqaintance with it I use it with utmost attention and never without Kevlar gloves.

Guppied nosecones (explained)

The form of the nosecone is most important if you want to yield high altitudes with your rockets. On Paul Grosse's site I saw the guppy principle for the first time. My first experiments were interesting but left room for improvement (see photo). By just heating the bottle end, I felt that I have no adequate control of the final form of the nosecone; often I ended up with a bottle end consisting of one hump in the middle surrounded with 5 humps (the remnants from the original bottle form) - aerodynamically better than the bare bottle, but far from the optimum.

To begin, I made this valve-in-a-bottlecap with a Woods bicycle valve. Presta or Schrader valves will do as well (look here for a pic of  Presta and the other valves). Screw this cap with the attached valve onto the bottle to be guppied. Then pressurize to 3 ... 4.5 bar.
Safety: as long as the bottle is pressurized, do wear eye- and ear protection and leave windows open. (Why?)

For uniform rotation, I use this cordless drilldriver with slow speed. Lefthand the pressurized bottle.

A hot air gun with a reducing nozzle for good control of the heat distribution.

The process: After a discussion in the WR list Clifford Heath sent me this detailed description:

"The most important is to heat the outer part of the bottle base, not the centre. I hold the bottle tilted over between 45 degrees and horizontal. The aim is not to heat the centre of the base much at all - it starts growing between the humps first. To get a feel for it, wave your hand over the heat source until you know exactly where it's hottest and what the heat spread is, or you can't tell which part of the bottle you'll be heating. Touch the bottle base to feel which parts are hotter - you'll get a feel for it.
Next, use long slow expansion. Get the bottle base hot enough to start expanding, and don't heat it more until it stops. It takes two or three minutes to do it properly, as the plastic is very viscous - so it takes time to form the shape of least stress."

Thanks, Clifford, for this description - it makes a big difference! With your technique, almost hemispherical nose cones can be obtained. However, not every single bottle behaves the same, some practice is necessary. I found it good to do several bottles in a row to accumulate experience and 'the feel for it'. A bowl of cold water nearby doesn't hurt, in case you want to stop the expanding process quickly.