The solid-propellant motor is the oldest and
simplest of all forms of rocketry, dating back to the ancient Chinese. It's
simply a casing, usually steel, filled with a mixture of solid-form chemicals
(fuel and oxidizer) which burn at a rapid rate, expelling hot gases from a
nozzle to achieve thrust.
Solids require no turbopumps or complex
propellant-feed systems. A simple squib device at the top of the motor directs a
high-temperature flame along the surface of the propellant grain, igniting it
Solid propellants are stable and easily storable.
Unlike liquid-propellant engines, though, a solid- propellant motor cannot be
shut down. Once ignited, it will burn until all the propellant is exhausted.
Solids have a variety of uses for space
operations. Small solids often power the final stage of a launch vehicle, or
attach to payload elements to boost satellites and spacecraft to higher orbits.
Medium solids such as the Payload Assist Module
(PAM) and the Inertial Upper Stage (IUS) provide the added boost to place
satellites into geosynchronous orbit or on planetary trajectories.
The PAM-DII provides a boost for Delta and Space
Shuttle payloads. The IUS goes on the Space Shuttle and the Titan III and Titan
IV class of launch vehicles.
Only one of the nation's launch vehicles, the
Scout, uses solids exclusively. This four-stage rocket launches small satellites
Titan, Delta and Space Shuttle vehicles depend on
solid rockets to provide added thrust at liftoff.
The Space Shuttle uses the largest solid rocket
motors ever built and flown. Each reusable booster contains 1.1 million pounds
(453,600 kilograms) of propellant, in the form of a hard, rubbery substance with
a consistency like that of the eraser on a pencil. The four center segments are
the ones containing propellant. The uppermost one has a star-shaped, hollow
channel in the center, extending from the top to about two thirds of the way
down, where it gradually rounds out until the channel assumes the form of a
cylinder. This opening connects to a similar cylindrical hole through the center
of the second through fourth segments. When ignited, the propellant burns on all
exposed surfaces, from top to bottom of all four segments. Since the star-shaped
channel provides more exposed surface than the simple cylinder in the lower
three segments, the total thrust is greatest at liftoff, and gradually decreases
as the points of the star burn away, until that channel also becomes cylindrical
in shape. The propellant in the star-shaped segment is also thicker than that in
the other three.
A solid propellant always contains its own oxygen
supply. The oxidizer in the Shuttle solids is ammonium perchlorate, which forms
69.93 percent of the mixture. The fuel is a form of powdered aluminum (16
percent), with an iron oxidizer powder (0.07) as a catalyst. The binder that
holds the mixture together is polybutadiene acrylic acid acrylonitrile (12.04
percent). In addition, the mixture contains an epoxy-curing agent (1.96
percent). The binder and epoxy also burn as fuel, adding thrust.
The specific impulse of the Space Shuttle solid
rocket booster propellant is 242 seconds at sea level and 268.6 seconds in a