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Water Rocket
Problem Page
From time to time, I get asked various questions
to some problems and sometimes, they have very
similar answers although the answers are really too
long to go on a FAQ.
On this page, I aim to provide the answers to some
of these questions in full.
Covered here are:
See also the
Frequently Asked Questions.
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What is
the best way to get the pop bottle label
adhesive off of the bottle? |
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The glue
is sticky because of localised static
electric charge - the glue molecules are
designed to have ths property. On the TV a
while ago, there was a programme with some US
weapons that were designed to incapacitate
but not to injure and one of them squirted a
foamy glue at the target which was then
disabled like a fly on fly paper. One of the
journalists covering the event noticed that
when the military personnel got some of this
stuff on them, they used a "special
liquid" to get it off. On being asked
what it was, they said they couldn't divulge
that information. However, someone spotted a
bottle with the label still on it - it was
baby oil. The oil provides an insulating
layer that shields things from the local
electrostatic field. To get
the oil off a pop bottle, I warm it up by
filling it with water at around 50 degrees
centrigrade (this softens the glue), right up
to the top and then screw the top on. This
makes the warm bottle reasonably rigid. Then
I use a knife (the sort of blunt knife you
would eat at the table with - you don't want
to scratch the plastic too much) to remove
most of the glue. Then, I put some oil on the
glue and rub at the edges to get the oil
between the edge of the glue and the plastic
bottle. Eventually, it all comes off.
Once the glue is off, you need to
get the oil off. I use neat washing up
liquid, making an emulsion out of the oil and
once all of the oil has come away from the
surface, I then wash it off with water.
If you need to have the bottle clean
enough to glue things on such as fins, you
can use a solvent (such as alcohol) to remove
any residual oil from the surface of the
bottle. If you are going to use
gaffer/duct/duck tape, I have found that this
last stage is not necessary.
Once the bottle is clean, dry it if
necessary, and then take the top off.
Carefully pour the water out - remember that
the bottle is warm and therefore is quite
flexible - otherwise it will collapse under
even a slight vacuum and crease. Once it is
empty, put some cold water in it to make sure
it is hard enough to use.
You may wish to find a source of
bottles that have not got labels on them :-)
It is possible to dissolve all of
the glue in isopropyl alcohol but if you
haven't got unlimited access to this
chemical, using oil and washing up liquid is
probably better.
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I am
currently doing
a water bottle rocket. I have given it 5
small fins and a cone for the nose. Is there
anytihing else I can do to improve its flying
quality. |
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Make sure
that the Cp is between one and two rocket
diameters behind the Cg.OR
in English . . .
Cp - - -
The Cp is the centre of pressure. It is the
effective centre of the area of the rocket as
viewed from the side. One way of finding the
Cp is to make a cardboard profile of the
rocket and find where it balances.
To make the cardboard profile, you
can either draw the outline from
measurements, do it by eye or put it on the
floor beneath a light in a room and draw
around the edge of the shadow (a bare bulb
works best). Once you have the outline on the
card, carefully cut it out and then find the
point where it balances.
You can find the point where it
balances by sticking a pin in it with a piece
of thread tied to the pin with a weight on
the other end of the thread so that the
thread is pulled down across the face of the
cardboard. Draw a line with a felt tipped pen
on the cardboard along the line of the thread
and then take the pin out and stick it
somewhere else on the cardboard, repeating
the drawing exercise. This should give you
two lines that cross. Repeat a third time and
the third line should cross where the other
two do. This point represents the Centre of
Pressure and adding weight should not change
its position (unless the weight is bulky).
Finally, put a mark on the rocket in
the position that corresponds to the mark on
the card. You now have your Cp marked on your
rocket.
Cg - - -
To find the Cg, or Centre of Gravity,
(sometimes called COG), hold the rocket on
its side and find the point where it
balances. The diameter of the rocket is just
the diameter of the body of the rocket and
does not include the fins (they are usually
not bulky enough). The balance point of the
rocket, the Cg, should be between one and two
rocket diameters in front of the Cp for the
rocket to be stable. If your rocket balances
behind the point that is one rocket diameter
in front of the Cp, then add some weight to
the nose to bring the Cg forward. Once the Cg
is at least one rocket diameter in front of
the Cp, the rocket should be stable.
Testing without launching -
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To check for stability without launching it,
tie a piece of string around the rocket at
the Cg (note that this test may not be
practical if the fins come this far forward)
and swing it slowly around your head.
Starting from any direction, if it is stable,
if should soon end up pointing in the right
direction.
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We are building a rocket for
Science Olympiad and the competition is in
three days. We made 10 launches today and
never were able to get the parachute to
separate from the nose cone. Is there some
secret we have missed? What do you think
about adding weight to the nose cone to make
is pull away from the rocket. The nose cone
is free falling and does not have a chute of
its own. Any ideas are welcome at this point.
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I think
that the reason why you are not getting
deployment is because the nose cone has no
real reason to come off the chute, ie there
is nothing pulling it off.Solutions
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You suggest
adding some weight to the nose cone. This
would pull the cone off the parachute but it
would have several detrimental effects as
well:
The extra weight would mean
that the water in the rocket would
have more work to do to attain the
same height therefore the rocket
would experience a loss of height
(because of loss of performance) and
therefore the flight would be of a
shorter duration;Extra weight would also mean
that the parachute would be pulled
down through the air quicker and
therefore the flight would be of
shorter duration - remember that in
the SO rules (if it is like previous
years) there will be a condition that
all of the parts of the rocket stay
joined together so your nose cone
needs to be connected to the rest of
the rocket with a piece of cord of
some sort that is packed so that it
does not tangle with the chute during
or after deployment;Extra weight in the nose cone
would, after the water part of the
thrust, cause the nose cone to
separate early. Effectively the
heavier nose cone would be pulling
the rocket through the air and as
there is nothing holding it onto the
rocket, you would get separation
fairly soon after the end of water
thrust instead of between three and
four seconds into the flight at
apogee where you need the chute to
deploy; and,Adding weight would mean that
you would stand a greater chance of
going over the weight limit of the
Science Olympiad rules. If you are no
where near this limit and the
computer model says that you can
increase the weight of the rocket,
you could make the chute bigger but
remember that a larger chute can
cause problems deploying.
I suggest
that you pack a drogue chute (about a foot in
diameter) that is connected to the nose cone.
This has several effects:
At apogee, the nose starts to
separate and the drogue pops out into
the air stream. This then pulls the
nose cone off the main chute. The
drogue should be connected to the
main chute (possibly to _one_ of the
cords using a loop so that it can
slide to the top of the cord or down
to the bottom where all of the cords
are grouped - this way if the
drogue/nose-cone combination has a
slower rate of decent than then
main-chute/rocket combination, it
will slide up and not get in the way
of the main chute whereas if it is
faster, it will slide down to the
bottom and not get in the way) so
that it remains as one piece; and,If the drogue/nose-cone
combination has a slower rate of
decent than the main-chute/rocket
combination, this will slow the
decent slightly as the drogue will go
to the top of the cords on the main
chute.
One point
to note is that there is usually a limit on
the total length of the rocket-parachute-etc.
You should make sure that the drogue at the
end of its travel along the main chute cord
does not allow this limit to be exceeded.
This
could be done by
having:
A
small slip loop and a knot in the
main chute cord alghough the
disadvantage of this is that the main
chute may stand a better chance of
tangling as a result of the knot; or;Having
the loop on the drogue go around the
single main chute cord but also
extend to the main chute eyelet
(where all of the main chute cords
go) so that the total length cannot
exceed the limit. One disadvantage of
this is that you could run into
problems with main chute deployment
so you could solve that by packing
the chutes carefully. Remember to
make the line linking the nose cone
tot he main cute line longer than the
length of the main chute so that it
all pulls out (or pack it so that it
comes out in a lump that then
unfurls. The combinations are
endless.
I
also suggest that you
line the sides of the nose cone with a PTFE
oven liner (made by DuPont - they are
expensive, around £3.00 (possibly $3 - 5 in
the US) but half a sheet does nicely). Have a
look at http://ourworld.compuserve.com/homepages/pagrosse/h2orrectef1pics.htm
for pictures of
where to put it. If you cannot get hold of
one of these, use plenty of talc but make
sure it is dry because the stuff is like
cement when it is wet (this is where PTFE
sheet comes into its own as it will work wet
and dry - I have tried it ;-)
Also
have a look at the
packing strategies, half way down http://ourworld.compuserve.com/homepages/pagrosse/h2orrecsys1.htm
Also,
you could put a series
of 16 or so holes in the nose cone on the
straight bit, just after the curve finishes
(just above the PTFE liner). There is a low
pressure area here and when the rocket is
moving fast, the air is sucked out of the
nose cone at this point, increasing its
tendency to stay on while the rocket is in
motion. At apogee, there is no motion (unless
the flight was not particularly vertical) and
the nose will separate.
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For
a school project
I have to build a water rocket. I am building
my out of a two litre pop bottle. For extra
credit you can put a parachute that works on
the rocket. I have decided to do that but I
do not know how to have it work Please send
me a . . . message saying how you would
install a parachute on to a two litre bottle
rocket. |
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Chute
Design - - -
The parachute type depends on what you are
after. I would recommend a circular chute
(the parafoil is okay bot is rather fiddly
and can fail to deploy sometimes. Chute
Materials - - -
The materials of construction depend on time
available, materials available, wow factor
required. The most reliable, lightest,
easiest to make and one that requires least
in terms of initial outlay is the bin-liner
chute. These are lighter and pack smaller
than the nylon ones which, I'm affraid has it
for the wow factor (unless you can come up
with brightly coloured, primary coloured bin
liners:-).
Deployment
Method - - -
The NSA (Nose Separates (or falls of) at
Apogee method is reasonably reliable although
some people go for clockwork timers (three or
four seconds and then deploy the chute -
those little timers that you get in some
children's toys) or air speed flap design.
These will be more reliable but require more
in terms of setting them up in the first
place. Perfecting the NSA technique may be
all that is required.
General
- - -
Use plenty of talc (to allow the chute to
slip out of the nose cone).
I aim
to have a go at the speed flap and timer
methods but I suggest that you look at some
other's web sites that have details on these
methods. Depending on the tiem you have
available, I suggest that you either have a
go at building all three or pick the one that
you think will give you best results.
Look
almost half way down the page . . . http://ourworld.compuserve.com/homepages/pagrosse/h2orrecindex.htm . . . for some
links.
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Question
on Momentum:
John mentioned that [zero] mass is not the
optimum for water rocket flight. Does anyone
know how to calculate the ideal mass for
maximum flight? It seems to me that for a
given thrust/resitance, a reduction in mass
will always yield a higher flight. Of course,
dealing with short duration impulse thrust of
the water rockets has thrown me way off in my
understanding of thrust and rocket flight. |
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With
a short duration
inpulse (full open nozzle water rocket), the
ideal weight for height is not zero. If you
had a very light weight rocket (a couple of
grammes), it would very quickly come to a
stop as there was no real weight pulling it
through the air. As you increase the weight
of the rocket, the height increases until you
reach a limit. With longer
duration water rockets, the ideal weight is
very low and with 2 stage it is also low (I
like to think of a 2 stage rocket as a
variant on the launch tube/t-nozzle
combination).
If you
are going for distance, a heavier weight is
more ideal than going for height.
To find
the ideal weight of a rocket, use a computer
model (WRCM). There are a number (PC, Mac,
Online and so on), all but one is for free
download (that one is a commercial version or
the water rockets that are availble for free
download within the group) or no download (an
online version where all the calcs are done
at the server end).
I have
a link to the WRCMs on the following page but
for more general browsing, use the most
complete WR links list on the web at
Clifford's site.
My
WRCM
http://ourworld.compuserve.com/homepages/pagrosse/h2orckt.htm (screen shots
and HTML based help link from this page)
My
site's WRCM links list with details
http://ourworld.compuserve.com/homepages/pagrosse/h2orocketlinksi.htm
Clifford's
links list
http://www.osa.com.au/~cjh/rockets/links.html
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