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Mass of Water
(g) or Weight of
Water (g) This is the
amount of water that gets pushed out
of the nozzle during the flight and
is usually the same as the amount of
water that you put in the rocket. The
only time that this will be different
will be if you are using a method of
joining several bottles together and
there are places that water can
collect. The mass of this water is
effectively added to the Weight of
the Rocket in the Rocket
Parameters input field. There
are a number of ways of measuring the
water that goes into the rocket:
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Weigh the rocket
empty and then again with the
required amount of water added. Make
a mark on the side of the rocket (in
pen on a piece of tape rather than
scratching the level onto the
pressure vessel) both for the rocket
in the inverted, filling position (so
that you know when you have put
enough in without having to turn it
over each time) and then the right
way up with a launch tube in the
nozzle (so that you can see on the
launcher if you have lost too much
water for some reason). In the
field, you simply invert the rocket
and pour the water in, up to the
mark. Turn it the right way up on the
launcher and you can see if you still
have enough.
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Measure it in with a
jug. Either make marks on the side as
described above or take the jug into
the field with you. |
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Note
that the more water you put in, the
less volume of air will be available
and the greater the weight the nozzle
has to lift. There comes a point
where, with small nozzles and
increasing amounts of water, that the
initial acceleration is high but then
falls off quickly as the pressure
falls. The rocket can then start to
slow down. Shortly after this, the
weight is reduced sufficiently and
the small amount of thrust left can
produce a `kick' at the end of the
water thrust. If the duration of the
slow part of this process was
sufficiently long, even a near
vertical flight will allow the rocket
to be angled fairly low (although at
quite an altitude) by the time this
`kick' manifests itself. In this
case, the rocket can fly off quite a
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Pressure in
Vessel (BarG) or Pressure in
Vessel (psig) The
more air you pump in, the greater the
force of the liquid that comes out of
the nozzle. Also, once the water has
ran out, the air itself will provide
some extra thrust. However, you
cannot just pump higher and higher
pressures as the water rocket will
fail structurally at some pressure.
Always pressure test the rocket, the
launcher and any hose that you use in
addition to making sure that the
launcher will hold a rocket down at
that pressure.
The pressure value is the pressure
measured on a pressure gauge, that is
to say that 10 psi on the gauge (10
psig) is actually 10 psi +
atmospheric pressure (a total of
around 24 psia) or a pressure of 0.7
Bar on the gauge (0.7BarG) is
actually 0.7 Bar + atmospheric
pressure (a total of around 1.7BarA).
Atmospheric pressure is around 14.6
psi(a) or 1.01325 Bar(A).
The absolute pressure (that
measured against a vacuum) is denoted
psia or BarA whereas the pressure
relative to atmospheric pressure
(that measured with a gauge which has
atmospheric pressure on the outside)
is denoted psig or BarG.
The computer model automatically
adds in atmospheric pressure so you
only have to worry about the pressure
on your gauge.
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Height (m) or Height
(feet) This is the
initial height of the rocket. It only
becomes significant if you are
launching from a great height or are
using large parachutes. Most
launchers will hold the bottom of the
rocket at a height of between 6"
and 1'6" (0.15m and 0.45m). If
you are investigating the potential
of a second stage rocket, you can put
the height that it is released from
the booster in here along with the
speed and angle below.
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Angle of
Elevation (° - degrees)
There is no such thing as perfectly
vertical, most launchers being
somewhere around the 88º or 89º
mark unless you use a plumb bob or
are aiming it away from the launcher
as you would in a competition for
maximum downrange distance. This
parameter allows you to see the
effect on a slight tilt from vertical
which can be very large with small
nozzles and a lot of water. If you
are investigating the potential of a
second stage rocket, you can put the
angle at the point of release, along
with the height that it is released
from the booster (above) and the
speed below.
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Speed at
Angle of Elevation (m/s)
This is the speed that the rocket is
flying at the beginning of the
flight. This will normally be zero
but if you are using a hand held
launcher and move the rocket upwards
while releasing it, you should add
that speed in as well. Moving the
rocket during launch means that the
rocket reaches a stable speed sooner
but care must be taken as irregular
movements can cause waves or
turbulence in the water inside the
rocket which can lead to unstable
flow of the water. If you are
investigating the potential of a
second stage rocket, you can put the
speed that it is released from the
booster in here along with the height
and angle above.
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Temperature
(C) The temperature
of the pressurised air inside the
rocket is considered here as it
affects the total amount of air in
the system and therefore how much
force it will be able to exert on the
water during thrust together with the
amount of extra force produced when
the air escapes out of the nozzle
after the water. When air is
compressed quickly so that there is
little opportunity for heat to escape
(this is adiabatic
compression), the air heats up
considerably. This hot air then
passes along the air line to the
rocket where it then either bubbles
up through the water or it passes up
the launch tube and into the
head-space above the water. It then
stays in the rocket for a few tens of
seconds until you release the rocket
from the launcher.
Between being heated up during the
initial stage of compression to the
time of release, the air has had
plenty of time to cool down and reach
(or at least approach) equilibrium
with its surroundings. If you are
using water at ambient temperature,
the cooling effect of the high
surface area of the air line and the
bubbling through the water will bring
the air temperature towards the
temperature of the water. If you are
launching in the snow and using warm
water, the cooling effect of the snow
on the air line will bring down the
temperature.
Essentially, the temperature of
the air in the rocket is not going to
be greatly different from the
temperature of the water that you are
using.
Copyright
©2000 Paul Grosse. All Rights
Reserved.
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