The study of rockets is an excellent way for students
to learn the basics of forces and
the response of an object to external forces. The
motion
of an object in response to an
external force was first accurately described
over 300 years ago by Sir Isaac Newton,
using his three
laws of motion.
Engineers still use Newton's laws to design and predict the flight of
full scale rockets.
Forces are
vector quantities
having both a magnitude and a direction. When describing the
action of forces, one must
account for
both the magnitude and the direction.
In flight, a rocket is
subjected to four forces;
weight,
thrust,
and the
aerodynamic forces,
lift and drag.
The magnitude of the weight
depends
on the mass of all of the
parts of the rocket. The weight force is always directed towards
the center of the earth and acts through the
center of gravity, the yellow dot on the figure.
The magnitude of the thrust
depends
on the mass flow rate through the engine and the velocity and pressure
at the exit of the nozzle.
The thrust force normally acts along the longitudinal axis of the rocket
and therefore acts through the center of gravity. Some full scale rockets
can move, or
gimbal,
their nozzles to produce a force which is not aligned with the center of gravity.
The resulting
torque
about the center of gravity can be used to maneuver the rocket.
The magnitude of the aerodynamic forces
depends
on the shape, size, and velocity of the rocket and on
properties
of the atmosphere.
The aerodynamic forces act through the
center of pressure, the black and yellow dot on the figure.
Aerodynamic forces are very important for model rockets,
but may not be as important for
full scale
rockets, depending on the mission of the rocket.
Full scale boosters
usually
spend only a short amount of
time in the atmosphere.
In flight the magnitude, and sometimes the direction, of the four forces
is constantly changing.
The response of the rocket depends on the relative magnitude and direction
of the forces, much like the motion of the rope in a "tug-of-war"
contest. If we add up the forces, being careful to
account
for the direction, we obtain a net external force on the rocket. The
resulting motion
of the rocket is described by Newton's laws of motion.
Although the same four forces act on a rocket as on an
airplane, there are some important
differences in the application of the forces:
-
On an airplane, the lift force (the
aerodynamic force perpendicular to the flight direction) is
used to overcome the weight.
On a rocket, thrust is used in
opposition to weight. On many rockets,
lift is used to stabilize
and control the direction of flight.
-
On an airplane, most of the aerodynamic forces are generated
by the wings and the tail surfaces. For a rocket, the
aerodynamic forces are generated by the fins, nose cone, and body tube. For
both airplane and rocket, the aerodynamic forces
act through the center of pressure
(the yellow dot with the black center on the figure) while the
weight acts through the
center of gravity
(the yellow dot on the figure).
-
While most airplanes have a high lift to drag ratio,
the drag of a rocket is usually much greater than the lift.
-
While the magnitude and direction of the forces remain fairly
constant for an airplane, the magnitude and direction of the
forces acting on a rocket change dramatically during a
typical flight.
Activities:
Guided Tours
-
Forces, Torques and Motion:
-
Forces on a Rocket:
-
Vectors:
Related Sites:
Rocket Index
Rocket Home
Beginner's Guide Home
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