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When you release the rocket, water is
expelled in order to accelerate the rocket.
If you use a launch tube, that is to say, a
tube that fits into the nozzle of the rocket
and is fixed to the launcher, you can get a
prolonged period of thrust. If the launch
tube is almost a tight fit, this acceleration
will not allow a significant amount of water
to be lost, only a slight loss of pressure as
the volume of air increases in order to take
up the new volume created by the withdrawal of
the launch tube.
A launch tube can also be hollow so that
it acts as a second (although small)
reservoir of air so that the pressure drop
during launch is not so great.
If you still want to use a restricted bore
nozzle as well as a launch tube, you can
mount this on the end of the launch tube so
that it slots in place and seals as the
rocket reaches the end of the launch tube.
This smaller nozzle is called a T-nozzle. If
you have mounted the T-nozzle on the end of
the launch tube, you must make some holes in
the side of the launch tube so that you can
pressurise the rocket without blowing the
T-nozzle off the end of the launch tube.
However, if you do this, the extra reservoir
of air afforded by the hollow launch tube
will be lost to the rocket earlier on in its
travel up the launch tube.
Check the examples at the bottom of this
page if you have any doubts.
If you checked the Launch
Tube in use checkbox in the Rocket
section, you will be able to edit the
following values (Parameters  marked *
do not appear in the Novice version) . . .
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Length (cm)
The length of the launch tube is the
distance the rocket has to travel
before the end of its nozzle either
clears the end of the launch tube or
the t-nozzle engages - the T-nozzle
is the restricted bore nozzle that
seals into the neck of the bottle,
pictured on the right, sitting on the
end of the launch tube. Without a
T-nozzle fitted it is the distance g-k
(end of launch tube to end of
nozzle). With a T-nozzle, it is the
distance f-j seat of
t-nozzle to restriction in the neck
of the bottle.
Throughout this travel, the launch
tube is the same diameter and the
thrust to the rocket consists of: the
small amount of water escaping
between the nozzle and the launch
tube; and, the push on the end of the
tube by the pressure in the rocket.
Therefore, with a snugly fitting
launch tube, thrust can be generated
with little loss of water and only a
slight loss of pressure in the
rocket.
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External
Diameter (mm) This is
the diameter of the launch tube,
distance a-e. It
determines the amount of water that
escapes and the amount of thrust
provided by the launch tube. Needless
to say that it cannot be greater than
the internal diameter of the rocket's
nozzle. |
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* [X] Hollow
Launch Tube Selecting
a hollow launch tube will allow the
model to take into consideration the
quantity of air inside the launch
tube. This air effectively adds to
the volume of air inside the rocket
whilst it is on the launch tube
therefore the pressure reduction is
less than for a solid launch tube
with the same rocket and water. |
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* Wall
Thickness (mm) If
your launch tube is hollow, you can
tell the model the thickness of the
walls of the tubing that the launch
tube is made from. On the diagram on
the right, this is length d-e. |
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* Length of
Tube Empty (cm) The
model also needs the length of tubing
that is empty (length h-l)
so that it can calculate the volume
of air that is inside the launch
tube. As it is likely that your
launch tube will be filled partly
with water, the best that you can do
is to estimate this length. Note that
in an open ended launch tube, g
and h are in the
same place.
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* Distance of
Vent from End (cm) In
the case where you do not have a
T-nozzle and the end of the launch
tube is open to the inside of the
rocket, this distance is zero. If
the end of the launch tube is blocked
off and you have holes in the side,
it will depend upon whether or not
you are using a T-nozzle.
With a T-nozzle, the distance that
the rocket has to travel between the
holes being blocked off and the end
of the effective launch tube is
length f-i.
Without a t-nozzle, it is g-i
+ j-k. g-i
is the distance between the holes and
the end of the launch tube and j-k
is the distance the nozzle still has
to travel to the end of the launch
tube once the holes have been
blocked.
Note that in the diagram, the
length of the launch tube is quite
short. This is to make the drawing
clear without taking up too much
room. In reality, you would have a
longer launch tube but you would
position the holes as close to the
end as possible which is why it is
this distance that is specified in
order to make calculations involving
changing the length of the launch
tube more realistic.
One other thing to note is that
these distances (Length; Length of
Tube Empty; and, Distance of Vent
from End) need only be measured to
the nearest centimetre as the effects
of other, out of control variables
will have a greater effect on the end
result than a few millimetres error
in launch tube length.
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* [X] T-Nozzle
in use A T-nozzle
allows you to get the best out of
both worlds by having the highly
efficient initial acceleration from
the launch tube but also allowing you
to have a smaller, more efficient
nozzle. As you can see in the
diagram, the T-nozzle sits on top of
the launch tube ready to seal off the
wide-bore nozzle as soon as the
rocket gets to the end of the launch
tube. Once this has happened, the
smaller nozzle allows the water to
escape over a longer period which is
more efficient.
You will see how to find the
optimum diameter for the T-nozzle
using 3
Dimensional Optimisation
on the model.
Select T-Nozzle in use if you are
using a T-nozzle.
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* T-Nozzle
Diameter (mm) The
value you put into the T-nozzle
Diameter is distance b-c
and represents the diameter of the
nozzle the rocket changes to
automatically at the end of the
launch tube. |
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