This FAQ
(Frequently Asked Questions) is built up from
questions that people ask me or the AWARS group
from time to time. See also the Problems Page.
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| What
is the Nozzle Constant? |
 |
The nozzle constant is a little
like the drag factor only this applies to the
inside of the nozzle. It takes into account the
flow concentration and the shape of the nozzle
inlet and so on. Again, examples are given. |
| How
does changing the shape of the bottle help the
rocket fly better? |
|
Normally, the rocket separates
the air as it flies through it and then the air
joins together again as the rocket clears the
air. It appears, from wind tunnel experiments,
that the optimum shape for the nose of the rocket
is slightly squashed or extended hemisphere.
However, the air that joins up at the rear of the
rocket should be allowed to do this fairly slowly
- any sudden constriction causing an increase in
drag. Shapes such as a hollow skirt - forming a
sudden change from full diameter to zero diameter
- will have an adverse effect on the coefficient
of drag. Making this transition smoother by
having a long taper will reduce this considerably
- too long a taper will increase drag caused by
the increased surface area although it does have
to be quite long for this. For an idea of a good
shape, imagine the cross-section of a bird's
wing, straightened out and symmetrical. |
| What
is the difference between speed and velocity? |
|
Speed is measured in any
direction so if you travel at 2m/s forwards, your
speed is 2m/s and if you travel at 2m/s
backwards, your speed is still 2m/s. Velocity
depends on direction and is called a vector
quantity. If you define forwards as positive then
if you travel at 2m/s forwards, your speed is
2m/s and if you travel at 2m/s backwards, you
speed is -2m/s. If, in a system where we are
looking at a water rocket, we define upwards as
positive, as the rocket travels upwards, drag
tries to slow it down - effectively, in terms of
the rocket, pulling downwards - and so does
gravity. On the way down, gravity is still
pulling down but the effect of drag is, in terms
of the viewpoint of the rocket, pulling upwards.
Gravity always pulls downwards and drag always
pulls in the opposite direction to travel. By
using velocity, it makes the maths easier. |
| What
is a Launch Tube? |
|
A launch tube is a tube that
fits inside the nozzle and is attached to the
launcher. It allows the rocket to gain some speed
without using up a lot of water. In this way, the
rocket gains initial stability (the rocket gets
to go towards a speed where the fins can have
some effect) with only a minimal use of water. As
a result of this extra speed, the rocket can go
higher. |
| What
is a T-Nozzle? |
|
The diameter of a water rocket
is such that drag has a significant effect at the
sort of velocities that they travel at. A smaller
nozzle gives better altitude as the rocket does
not move so fast that its speed is used up as
drag - instead, the smaller nozzle allows a
longer "burn" and achieve a higher
altitude. One drawback with smaller nozzles is
that they take a long time to get to a speed
whereby the fins make them stable and this
initial low velocity can cause such a rocket to
travel in virtually any direction before it goes
fast enough to be stable. One way to get around
this is to use a launcher that has a guide that
the rocket travels along but a more effective way
is to use the launch tube to do this. The
thrust from the launch tube is only half that of
the water thrust so using a small diameter launch
tube will not work as the rocket will not be
travelling very fast by the time it gets to the
end of the launch tube (if it gets that far). The
solution to this is to have a small-diameter
nozzle (the T-Nozzle) that fits inside the rocket
and sits on the end of the launch tube before
launch. At the time of launch, the launch tube
provides the initial, full diameter launch tube
thrust until it gets to the end of the launch
tube. When it does get to the end of the launch
tube, the T-Nozzle seals off the neck of the
rocket, leaving the smaller diameter nozzle of
the T-Nozzle for the water to go through. Now,
the rocket can travel more efficiently on the
smaller bore nozzle whilst retaining the
stability gained from the launch tube. In short,
the rocket has the best of both worlds.
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| Why
use a launcher? |
|
Without a launcher, you have to
use: a bottle top (with a one way valve and an
adapter in it) to pressurise the rocket and then
unscrew the top so that the rocket can fly; or,
a cork or rubber bung that blows out when the
right pressure is reached. The first
will get you very wet and there is a risk of
damage in the event of pressure vessel failure.
The second will blow out at a low pressure and
it will do it spontaneously. A launcher will allow
you to pressurise the rocket to the pressure you
want it to be at and launch it from a
safe distance without getting wet. |
| What
is a sustainer? |
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A sustainer is the second stage
of a two stage rocket. |
| Why
do I need to use a recovery system? |
|
When your rockets start to
attain a certain altitude, they will crumple or
otherwise damage themselves on impact. You will
need to have a recovery mechanism to prevent this
type of damage. Recovery mechanisms include
parachutes, helicopter blades, tennis ball noses
and so on - anything that prevents or reduces the
amount of damage suffered by the rocket on
impact. |
| Which
type of parachute should I make? |
|
This depends upon your use. If
you want a sturdy chute that you can use over and
over again and you know that you are only going
to launch the rocket in places where there are no
trees or other obstructions that will prevent you
from retreiving the chute, you may like to take
the time to make a nylon chute. There is a large
time investment in a nylon chute but it is worth
while. If, on the other hand, you want a
lightweight, disposable chute or one that is only
going to be used a few times (such as in a
Science Olympiad) then a bin-liner chute will be
better suited. |
| What
type of cord should I use for the parachute
strings? |
|
There are essentially three
types of cord: monofilament, twisted and woven.
The monofilament is strong in twist and will form
loops that can hook and snag the line as it is
released thus making the release less reliable.
The twisted cord is made from a number of threads
twisted together and any tension will tend to
increase or decrease the twist that is stable
thus resulting in a tangling set of strings.
Woven cord is made up of lines that are woven
together and there is no residual twist in them.
When these are put under tension, they will not
try to twist one way and they will not form loops
that are stable under tension either. Woven cord
is therefore the best although some people use
waxed dental floss instead with good results. |
| Is
it worth spending a lot of time making a
parachute? |
|
If you want a good quality
parachute that you can use over and over again,
yes it is worth the effort. However, a nylon
chute will weigh more than a bin-liner chute and
therefore you will tend not to get such good hang
times (although this effect may be small compared
to the variability in hang times when the air has
a little turbulence in it). If there are a lot of
trees or other obstructions around, or you only
need a chute for a few launches (such as in a
Science Olympiad) then a bin-liner chute is your
best bet. |
| Why
does the chute need to be released at apogee (the
highest point in the flight)? |
|
Strictly speaking, it depends
upon what you are using your chute for. If you
want to get the greatest hang times then you need
to release it close to the highest point in the
flight but if you just want to stop the rocket
from taking a soil sample, then it only needs to
have opened early enough so that it has deployed
fully and the rocket has reached its new terminal
velocity (the fastest it will go with only
gravity pulling it down) before it hits the
ground (lands). |
| What
is a Tomy timer? |
|
A Tomy timer is the name given
to the small clockwork motors that you find in
children's toys. These can be used to perform
automatic timed events on rockets such as
parachute release or camera shutter release. |
Back to the Water Rocket Index
