Bottle Profiles
What are they for?
When you build a water rocket, you need
to attach fins to the bottle near the neck which is almost always. For
stability, you need fins and as a rule of thumb, you need them as far back
as you can easily and practically get them. This means swept back fins
that extend beyond the neck of the bottle where the bottle lid was.
This also means unless you are using extension spars, you need to attach
them to the portion of the bottle that is tapering down to the neck from the
main part. This area is seldom straight so cutting fins to mate to
that curving surface is usually a trial and error, trim-fit-trim-fit,
process and that process is rarely quick and easy nor does it usually result
in a nice, full-contact attach area.
With the profile of the bottle laid out
on a grid, you can easily design your fins so that when you cut them out,
they match the bottle profile perfectly the first time with no trimming
unless you didn't follow the cutout lines closely.
You can also cut out the contour of the
bottle and use the outside of what you cut to check bottle contours to see
if they match a bottle not listed before making fins. There seems to
be one very common profile for 2-liter bottles because Pepsi, Squirt, 7-Up,
and Safeway are all the same bottle profile. Other times, profiles
will be very close but not identical.
How are bottle profiles
obtained?
People have tried all kinds of ways to
get profiles such as using right triangles and a flat surface, taking
circumference measurements at intervals, dividing them bi pi, and plotting
them on a grid,
etc. I have seen it suggested broadly to just use a light and trace
the shadow. This is almost right except that you must use the sun
as your light source unless you have a powerful light a
long ways away or can focus a light to parallel beams (no easy method).
Light sources within reasonable
distances of your bottle will fan out in all directions from the filament of
the incandescent bulb or the tube or bulb surface from florescent and
similar light sources causing distortion of the projected image. The
sun is so far away that the light rays reaching earth are parallel for all
practical purposes and so project an accurate image as demonstrated below.
So the best way is to set the bottle on
a board with a paper attached to the board, align the board so it is exactly
perpendicular to the sun's rays, and then you have a perfect shadow that can
be traced. If the board is not perpendicular then the profile will
also be distorted. A good way to
insure the board is exactly perpendicular to the sun is to drill a
hole in the board with a drill press so that it is exactly
perpendicular to the board and then press a dowel into the whole.
Then you can prop up the board and adjust it so there is no shadow.
A block of wood with exactly perpendicular sides will also work.
You just position the board so the block casts no shadow on any side
of the block. Bottle
Profile Fixture
Getting the initial bottle profile is
pretty quick and once you have them, you don't need to do them again so the
total usage time of a fixture to get profiles will be fairly short.
Since it is a use once and forget (almost), it doesn't make sense to take a
lot of effort making a fixture.
First Attempt
was just a board with a dowel pin and I tried to use
push pins and a paper strap to hold the bottle down. The push
pins were also too hard to push into the wood and kept bending plus
the paper strap would tear at the pins.
It didn't work very well because the bottle was
not secured enough and would move around. Also, grass is not a
good place to try to prop up the fixture, and getting a profile with
the label still on does not give a very accurate profile of that area.
Second Fixture
So okay, maybe just a little more effort was in
order!
I used some scraps on a board--a block with some
foam to hold the mouth of the bottle, and a screw through a piece of
angle iron with a taped hole on the other end. I used blocks of
various sizes between the screw and the bottom of the bottle, 1) so
the screw wouldn't cut into the bottle and 2) so I could used
different blocks for different lengths (heights) of bottles.
This held the bottle a lot more securely--enough to easily take the
profile without it moving. I taped the paper down this time
instead of using push pins. Bricks were used to prop up the
fixture so it could be adjusted perpendicular to the sun and this was
done on concrete. The blanket in the picture was because this
was done in the summer and the concrete was both hot and hard on bare
knees (while wearing short pants). This time the fixture and
process worked very well and profiles were obtained for all my
bottles. Note the label was properly removed this time for all
profiles.
The sun moves across the sky and so your shadow
will move with it. It doesn't move so fast that you can't trace
a bottle outline without realigning your fixture but it does have to
be realigned after a few minutes. Also, it is easiest to trace
when the sun is directly overhead plus the shadow doesn't change
nearly as fast as when at low elevations. So the best time is
closer to noon standard time and if you really want to get particular,
do it on the fall or spring equinox at noon. Just kidding, you
can do it at any time but the middle part of the day is better.
Guess what, this doesn't work if the sun is hiding behind a cloud
either. Even thin clouds diffuse the light and make a fuzzy
shadow.
Final Contour Processing
During the process I was also able to determine
which bottles had the same profiles because I could line them up
perfectly with previously taken profiles.
I added a line exactly down the center of the raw
profile with marks at exactly 6" or 8" depending on the bottle size.
This is so the final contour could be scaled in a pdf file properly.
The raw contour was scanned directly into Microsoft Image
Composer which was bundled with FrontPage 2000 (unfortunately not
later versions) at double normal size for more accurate curve plotting.
Using a point plotting function (shapes-polygon), I plotted points around
the image so that it would create a nice dark "smooth" curve accurately
depicting the raw image. I created lines for the scale lines on
the scanned image. The curves and lines were separate from the
scanned image and the original image was not used after this point.
The profile was reduced back to near scale size,
saved as a full bottle profile in gif format, then cropped and saved
as a side profile in gif format.
A quarter inch grid was created in MS Word (for
easy conversion to a pdf file) and saved as a template.
A generated profile was inserted into the grid
document, resized to the final accurate scale to match the grid, and
saved as a pdf file.
Technically, you only need to take a profile from
one side of the bottle but to obtain a centerline, I did both sides.
The centerline is used to align the bottle sides with the grid so fins
can have parallel and perpendicular sides and then was also saved in
case someone wants the full profile.
Bottle Contours
Here is what the 2-liter coke bottle profile with
grid looks like (not
to scale) with a couple of sketch fins:
And here are those fins cut out:
The picture below shows how well a fin fits when
a bottle profile is used. Note the grain of the fin is parallel
to the leading edge. Also note heavier glue (this is hot melt)
at the top and bottom of the fin where the most stress is.
The following are links to bottle
profiles that I have in pdf format. Just click on the links and
you can open them directly in your browser (if you have installed the
pdf viewer) or you can download them to your computer. The pdf
files can be printed full size and will be accurate. You can
then sketch your fins, cut them out, lay them on your fin material,
trace around them, cut them out, and glue them to your bottle.
If you create other bottle profiles in a gif
format with a scale line on them, you can send them to me at
gary@waterrocketmanual.com
and I will add them to the list and give you credit for them.
You can also send fin patterns and I will post them here.
Usually all products from the same company and
same size have the same profile, especially in the 2-liter size.
Exceptions to the rule are special bottle designs such as coke and
sprite. The diet versions and the standard versions have the
same bottle profile such as coke and diet coke. It helps to know
what company makes what product. Here is a partial list, not all
necessarily come in bottles or bottles suitable for water rockets :
Pepsi: Aquafina, Brisk, Dole, Jazz, Lipton
Tea, Mirinda, Mountain Dew, Mug Root Beer & Cream, Pepsi, Sierra Mist,
SoBe, Starbucks, Tropicana,
Coke: Barqs, Citra, Coke, Dasani, Enviga,
Fanta, Fresca, Fruitopia, Full Throttle, Mellow Yellow, Minute Made,
Nestea, Pibb, Sprite, Tab, Vault
Cadbury-Shweppes: 7-Up, A&W, Canada Dry,
Country Time, Crush, Deja Blue, Diet Rite, Dr. Pepper, Hawaiian Punch,
Hires, RC Cola, Snapple, Squirt, Sunkist,
The table below is arranged by the size first,
then alphabetically. For each bottle, their are two links -- side
and full. The side link is the normal one you
would use. The full link gives you the full bottle
profile top-to-bottom, side-to-side centered on the grid in case you
want more of the bottle profile or want to see what your rocket would
look like with fins on both sides. The first entry is the
grid with no profiles. If you need more area, you can print
the grid without a profile and tape it to the grid with the profile.
For the full version of a bottle, you would probably need to tape
another grid sheet to both sides and maybe more on the bottom since
the bottle, especially for the 2-liter bottles, takes up most of a
sheet of paper.
These profiles are not perfect even though all
efforts were made to make them as accurate as possible; they are
accurate enough to give you a good fit and your adhesive will fill in
any slight voids. If you find any profile that doesn't seem to
fit, make sure you have the right profile for the right bottle and if
you do,
contact me and I will check the problem.
One final note on accuracy: The fins go on the
neck end of a bottle and that is where the contour will be most
accurate. The bottom of bottles are composed of five lobes, an
odd number, so when the bottle is set up for tracing, a full lobe
contour will be on one side and the other side will be between lobes
so the two sides will be different. No effort was made to make
the bottoms in a specific and consistent orientation.
|
IMPORTANT
I cannot guarantee your printer will print to
scale. Check your settings before printing, then check your
grid spacing with a ruler. The full grid is 7.5" X 10",
major grid lines are .5", minor .25". |
The files are all to scale and even if your printer isn't precisely to
scale, it will probably print the profile close enough so fins cut
from it will be a good enough fit on your bottle profile.
|