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Mass of
Rocket Empty (g) or
Weight
of Rocket Empty (g)
This is the weight of the rocket that
you will have once the water is used
up (it is actually once the air
is used up but there is no need to
bother with that at this stage).
The model needs to know this to work
out the acceleration of the rocket
during flight. With large rockets
(such as the space shuttle), we would
need to know the weight of the rocket
in a vacuum and calculate its
buoyancy as it travels up through
progressively less dense air but
water rockets are small and don't
travel into the vacuum of space, so
weighing a rocket on your kitchen
scales is good enough for this
purpose.
Weigh the finished rocket dry, on
a kitchen balance or postal scales.
You don't need to be particularly
accurate - to the nearest 5g is quite
adequate for lightweight rockets.
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Capacity of
Pressure Vessel (cm3)
This is the actual capacity of the
pressure vessel part of the rocket.
The model needs to know this to work
out how much pressure is left in the
bottle during the water stage of
thrust and how much extra push the
air in the rocket can give after the
water has gone. A two litre bottle
will have a capacity of more than 2
litres. There needs to be enough room
for over 2 litres of liquid (the
nominal 2 litres plus an extra amount
that represents three standard
deviations of the filling process)
plus enough ullage (the space above
the liquid) to allow for expansion in
hot weather and allow for bubbling
during opening the bottle for the
first time.
If you are going to mess around
with the bottle by softening it and
then blowing out the bottom to make a
hemispherical end (for strength and
aerodynamic quilities) and shrink the
neck end to make a conical back end
(to the rocket in order to assist
with the flow of water on the
inside), any assumption that it will
be even close to 2 litres can be
discarded.
The best way that I have found to
measure accurately the capacity of a
pressure vessel for a water rocket is
to weigh the rocket dry, weigh it
again but full of water and the
difference will give you the capacity
(a density of 0.998 kg/litre will not
be significantly different to a
density of 1.000 kg/litre for these
purposes).
One problem with this method is
encountered when you get to large
rockets that go beyond the limits of
your scales or the weight goes beyond
the strength of the joins in the
rocket (especially with narrow
couplings stiffened only with a
skirt).
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Rocket
Diameter (cm) The
model needs to know the diameter of
the rocket so that it can work out
the drag force during flight. This
force is also dependent upon the Drag
Factor below. You need to measure
the largest diameter of your rocket
(solid diameter as seen from the nose
excluding the fins of course). You
can do this directly by:
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Making a set of
giant callipers from card and a ruler;
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Putting the rocket
between two, movable flat surfaces or
edges and then measuring the distance
between them when the rocket has been
taken away; or, |
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You can put a
flexible tape measure around the
widest part of the rocket, measure
the circumference and divide the
result by Pi (3.142). |
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* Rocket Coeff
of Drag The Rocket's
Coefficient of Drag (Cd) is little
more than a fiddle factor to convert
the frontal area presented by the
rocket into a force when multiplied
by the speed squared and the density
of the air. Simple though this is, it
works reasonably well for the speeds
that water rockets go at. The drag
force increases proportionally to the
area presented by the rocket
(therefore proportionally to the
square of the diameter) and
proportionally to the square of the
speed of the rocket. Unless you
are equipped with a wind tunnel and
sensitive measuring devices, you are
not going to be able to measure the
Cd for your rocket. There are two
ways around this:
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Measure it yourself
by timing the rocket's fall from a
known height and putting the figures
into the computer model, adjusting
the Cd on the computer model until it
gives the correct time (not
particularly practical as the heights
involved can be quite great and you
could damage your rocket by using
this method unless you had it landing
in a pool); or, |
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Make an educated
guess from the figures that are
supplied with the computer model (in
the top right window) when you edit
the Cd value. |
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Nozzle
Diameter (mm) The
computer needs to know the nozzle
diameter so that it can work out haw
fast the water is ejected from this
nozzle during the water part thrust.
Measuring this can be quite difficult
as the minimum diameter needs to be
as close to the end of the nozzle as
possible but rarely is. With open
nozzles, the diameter is often
slightly larger than 21.5mm but
sometimes is less. With reduced
nozzles, you should take care to
measure them accurately as a
difference of 0.5mm in a nozzle of
3mm diameter can make a significant
difference. However, putting a ruler
across the end will usually suffice.
A Woods Schrader adapter diameter is
approximately 4.5mm.
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* Constant K
for nozzle This
constant is analogous to the
coefficient of Drag for the outside of
the rocket only this applies to the
flow of liquid through the nozzle.
The higher the value, the more
pressure is required to get the same
flow of liquid through the nozzle. Its
value is dependent upon the way that
the liquid flows into the nozzle - if
is has to turn tight corners or
contract sharply, the constant is
higher than if the flow concentration
is more gradual such as in a conical
neck entrance.
Again, it is virtually impossible
to measure this unless you have a
special timed set-up with nozzles and
flow measuring equipment. Bruce
Berggren did some measurements and,
for a normal 2 litre pop bottle, the
constant is 0.16. For different types
of nozzle inlet, there are suggested
values on the computer model that you
can use to make an educated guess.
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[ X ] Launch
Tube in use If you
use a launch tube, you should check
this box so that you can enter the
relevant values in the Launch Tube
frame. |
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* Duration of
air impulse (ms) Once
the water has been expelled, the
compressed air inside the rocket
pressure vessel starts to flow
through the nozzle. It flows quicker
than the water and, a two litre
bottle, pressurised to 5 Bar Gauge (5
BarG or around 75 psig) takes around
50ms to escape. In reality, this
escape takes quite a long time - the
temperature of the compressed air in
the rocket falls as the water flow
out and then, as the air flows out,
it fall even further, to around -100
Celsius. At this temperature, the
bottle warms up the gas and it
expands even more (although this does
not provide any thrust as such).
So as to make the graphs more
meaningful, an approximation is made
of how long the gas takes to escape
so that the thrusting part of the gas
phase of the thrust is taken as the
time for the air impulse. The
computer model calculates the air
impulse from data such as the vessel
capacity filled with air, its
temperature, density, compressibility,
the pressure of the air outside and
so on.
If you have a large rocket, or a
rocket with a reduced nozzle, you can
estimate the air impulse time by
pumping the rocket up to the pressure
that it will be when the water has
run out, securing the rocket
physically and then releasing the air
whilst timing it. Good look with this
one :-)
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[ X ]
Parachute in use If
you use a parachute, you should check
this box so that you can enter the
relevant values in the Parachute
frame. |
