|
In general, you would want your water rocket to fly as high or as far as possible and return
to the ground in a safe manner. This performance is governed by many factors and balancing
all these is one of the difficult, but also one of the most interesting sides of water
rocketry, if you ask me.
Getting it all right is impossible - let me get that point down first!
Some factors counters other factors so finding a balance is needed. So - what are these factors
anyway? I've listed some of the most common factors, and how they affect the design of the rocket:
- Large volume
- Good: Greater volume means more energy can be stored
- Bad: Larger size leads to more drag and more weight
- Small mass
- Good: A lighter rocket accelerates faster than a heavy rocket
- Bad: The rocket can get too fragile
- Small diameter
- Good: Lower drag
- Bad: Lower volume for the same length
- Large nozzle
- Good: Greater speed, better energy-efficiency
- Bad: Launcher has to hold a greater force, rocket must be capable of converting all this energy to speed
- Aerodynamic stability
- Good: Makes the rocket fly straight
- Bad: Over-stable rockets can "weathercock" and causes more drag
- High launch-pressure
- Good: More power
- Bad: Safety - a high-pressure explosion is a very dangerous risk!
|
|
Any child will know that a rocket has a pointed nose and a long slender shape. But why is it
so?
A rocket travels at high speeds through the air, and drag is a major force to overcome at
high speeds. Any object in the atmosphere displaces an amount of air due to its volume. When
it moves, this displacement is also moving and that creates problems. The air in front of the
displacement has to move out of the way and air behind it has to refill the void that is caused
by the displacement. This creates a zone of higher pressure air in front of the rocket and lower
pressure behind the rocket. A force pushing from the high-pressure zone towards the low-pressure
zone is the result, and it tends to resist forward movement.
This is called pressure-drag.
To minimize the pressure-drag, we must reduce the size and the magnitude of both the high- and
the low-pressure zones. One way to do this is to make the rocket with a smaller diameter, and to
streamline its shape.
The magnitude of the low-pressure zone can be reduced by making the transision from rocket body
to air as gentle as possible by tapering the rear fuselage.
There is not much to do about the high-pressure zone in front of the rocket. Air is being pushed
out of the way, either by the rocket itself, or by the air in front of the rocket. We can however
ensure that the rocket does not push aside any more air than neccesary. We do this, by making a
smooth transition from one side of the rocket to the other - a rounding...
|
|
This figure sketches the airflow around three "rocket" shapes. Green areas are high-pressure zones,
red areas are low-pressure zones.
1 is cylindrical; the flat front deflects the airflow aside and the flat rear causes the
flow to detach from the side and a void to form behind the rocket.
2 has tapered rear; this reduces both the size and the magnitude of the low-pressure zone.
3 has rounded nose; this minimizes the size of the high-pressure zone and allows a tighter
airflow around the rocket.
|
It is important to note that the shape of the rocket's nose does not have large saying. It is important
that the nose is flush with the sides of the rocket, and that it does not accelerate the airflow too much
outwards. A good shape would be blunt-tipped and curve rearwards at a decreasing rate untill it is parallel
with the rocket body.
A mathematical parabola does just that. The sides get steeper and steeper at a constant rate, as we move out
from the center. BUT the sides are never going to be parallel. Therefore we chose the shape that closely
resembles a parabola, but has parallel sides: an ellipse ( or rather a semi-ellipsoid).
My rationale for this, is that a parabola can in fact be seen as less than one half of an ellipse, that is
infinitely long.
But how should this semi-ellipsoid be shaped? Ellipses has a long and a short axis, and one of them must be
along the top of our rocket fuselage. The other axis is along the flight-direction of our rocket and can be
any size. If it is short, we get a blunt nose; long, and our nose gets more pointed.
Most likely, the ideal is the one with the lowest surface area to volume: a hemisphere!
This hemispherical nose, however looks a little blunt, so i usually chose a slightly longer semi-ellipsoid
for my nosecones. The one in the construction-section is 1.5 times as long as it is wide. This is more pleasing
to the eye and gives the rockets some "wow-factor" while suffering virtually no drag-penalty.
|
|
|
The hemisphere - probably the best shape for a water rocket nose.
|
The semi-ellipseoid - looks better :-)
|
The shape of the rear taper is in part the same, but in this case, the taper has to come to a point
to avoid a "bubble" of low pressure behind the rocket.
We can't make this, because a water rocket has to has a nozzle for the water to exit through, or it
may need the flange of the bottle exposed for a launcher to grab. So we must grab what we can, and
make the rear end of our rockets as narrow as the circumstances allow. The resulting shape is often
called a "boat-tail"
Here, also, i it important to get the transition from the side to the taper as smooth as possible. If
your bottle has a smoothly tapering neck, you can use it as it is, or cut it off and use it to make a
replacable fin-set as described in the Construction section.
|
