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.
even use a cannula for otherwise inaccessible places,
but probably only once.
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
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
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!
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.
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.
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.
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.
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.
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.