Since we live in a three dimensional world, it is necessary to control
the attitude or orientation of a flying rocket in all three
In flight, any rocket will rotate about its
center of gravity,
a point which is the average location of the mass of the rocket.
We can define a three dimensional coordinate system through the
center of gravity with each axis of this coordinate system perpendicular
to the other two axes.
We can then define the orientation, or attitude of the rocket by
the amount of
rotation of the parts of the rocket along these principal axes.
Most rockets are symmetric about a line from the tip of the nose to the
center of the nozzle exit. We will call this line the
roll axis and motion about this axis is called a
Because the rocket is symmmetric about the roll axis, engineers call
this configuration axisymmteric.
The center of gravity lies along the roll axis.
To define the other principle axes, we pick some distinguishing
characteristic of the design, like a fin placement, or a window location,
and place the yaw axis perpendicular to the roll axis and
through the center of gravity. On the figure, we have drawn a yellow
line on the body of the rocket that lies in the plane formed by the roll
and yaw axes. The yellow line passes through the fin on the "top" of the
rocket. Motions about the yaw axis are called
and result in the nose of the rocket moving side to side.
The pitch axis is perpendicular to the yaw and roll axes
origin at the center of gravity. A
is an up or down movement of the nose of the rocket.
A rocket can be maneuvered in several different ways.
In flight, the fins of the rocket produce
These forces are applied at the
center of pressure
of the rocket which is
some distance from the rocket cg and produce
torques (or moments)
about the principal axes. The torques
the rocket to rotate.
Most full scale rockets produce pitch or yaw motions by
or rotating, the exhaust nozzle. If the
is not alligned with the roll axis, it produces a torque
about the center of gravity.
Forces, Torques and Motion:
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