two SRBs provide the main thrust to lift the space shuttle off the
pad and up to an altitude of about 150,000 feet, or 24 nautical
miles (28 statute miles). In addition, the two SRBs carry the entire
weight of the external tank and orbiter and transmit the weight
load through their structure to the mobile launcher platform. Each
booster has a thrust (sea level) of approximately 3,300,000 pounds
at launch. They are ignited after the three space shuttle main engines'
thrust level is verified. The two SRBs provide 71.4 percent of the
thrust at lift- off and during first-stage ascent. Seventy- five
seconds after SRB separation, SRB apogee occurs at an altitude of
approximately 220,000 feet, or 35 nautical miles (41 statute miles).
SRB impact occurs in the ocean approximately 122 nautical miles
(141 statute miles) downrange.
The SRBs are the largest
solid-propellant motors ever flown and the first designed for
reuse. Each is 149.16 feet long and 12.17 feet in diameter.
Each SRB weighs approximately
1,300,000 pounds at launch. The propellant for each solid rocket
motor weighs approximately 1,100,000 pounds. The inert weight
of each SRB is approximately 192,000 pounds.
Primary elements of each
booster are the motor (including case, propellant, igniter and
nozzle), structure, separation systems, operational flight instrumentation,
recovery avionics, pyrotechnics, deceleration system, thrust vector
control system and range safety destruct system.
Each booster is attached
to the external tank at the SRB's aft frame by two lateral sway
braces and a diagonal attachment. The forward end of each SRB
is attached to the external tank at the forward end of the SRB's
forward skirt. On the launch pad, each booster also is attached
to the mobile launcher platform at the aft skirt by four bolts
and nuts that are severed by small explosives at lift-off.
During the downtime following
the Challenger accident, detailed structural analyses were performed
on critical structural elements of the SRB. Analyses were primarily
focused in areas where anomalies had been noted during postflight
inspection of recovered hardware.
One of the areas was
the attach ring where the SRBs are connected to the external tank.
Areas of distress were noted in some of the fasteners where the
ring attaches to the SRB motor case. This situation was attributed
to the high loads encountered during water impact. To correct
the situation and ensure higher strength margins during ascent,
the attach ring was redesigned to encircle the motor case completely
(360 degrees). Previously, the attach ring formed a C and encircled
the motor case 270 degrees.
structural tests were performed on the aft skirt. During this
test program, an anomaly occurred in a critical weld between the
hold-down post and skin of the skirt. A redesign was implemented
to add reinforcement brackets and fittings in the aft ring of
These two modifications
added approximately 450 pounds to the weight of each SRB.
The propellant mixture
in each SRB motor consists of an ammonium perchlorate (oxidizer,
69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide
(a catalyst, 0.4 percent), a polymer (a binder that holds the
mixture together, 12.04 percent), and an epoxy curing agent (1.96
percent). The propellant is an 11-point star- shaped perforation
in the forward motor segment and a double- truncated- cone perforation
in each of the aft segments and aft closure. This configuration
provides high thrust at ignition and then reduces the thrust by
approximately a third 50 seconds after lift-off to prevent overstressing
the vehicle during maximum dynamic pressure.
The SRBs are used as
matched pairs and each is made up of four solid rocket motor segments.
The pairs are matched by loading each of the four motor segments
in pairs from the same batches of propellant ingredients to minimize
any thrust imbalance. The segmented-casing design assures maximum
flexibility in fabrication and ease of transportation and handling.
Each segment is shipped to the launch site on a heavy- duty rail
car with a specially built cover.
The nozzle expansion
ratio of each booster beginning with the STS-8 mission is 7-to-79.
The nozzle is gimbaled for thrust vector (direction) control.
Each SRB has its own redundant auxiliary power units and hydraulic
pumps. The all-axis gimbaling capability is 8 degrees. Each nozzle
has a carbon cloth liner that erodes and chars during firing.
The nozzle is a convergent- divergent, movable design in which
an aft pivot- point flexible bearing is the gimbal mechanism.
The cone-shaped aft skirt
reacts the aft loads between the SRB and the mobile launcher platform.
The four aft separation motors are mounted on the skirt. The aft
section contains avionics, a thrust vector control system that
consists of two auxiliary power units and hydraulic pumps, hydraulic
systems and a nozzle extension jettison system.
The forward section of
each booster contains avionics, a sequencer, forward separation
motors, a nose cone separation system, drogue and main parachutes,
a recovery beacon, a recovery light, a parachute camera on selected
flights and a range safety system.
Each SRB has two integrated
electronic assemblies, one forward and one aft. After burnout,
the forward assembly initiates the release of the nose cap and
frustum and turns on the recovery aids. The aft assembly, mounted
in the external tank/SRB attach ring, connects with the forward
assembly and the orbiter avionics systems for SRB ignition commands
and nozzle thrust vector control. Each integrated electronic assembly
has a multiplexer/ demultiplexer, which sends or receives more
than one message, signal or unit of information on a single communication
Eight booster separation
motors (four in the nose frustum and four in the aft skirt) of
each SRB thrust for 1.02 seconds at SRB separation from the external
tank. Each solid rocket separation motor is 31.1 inches long and
12.8 inches in diameter.
Location aids are provided
for each SRB, frustum/ drogue chutes and main parachutes. These
include a transmitter, antenna, strobe/ converter, battery and
salt water switch electronics. The location aids are designed
for a minimum operating life of 72 hours and when refurbished
are considered usable up to 20 times. The flashing light is an
exception. It has an operating life of 280 hours. The battery
is used only once.
The SRB nose caps and
nozzle extensions are not recovered.
The recovery crew retrieves
the SRBs, frustum/ drogue chutes, and main parachutes. The nozzles
are plugged, the solid rocket motors are dewatered, and the SRBs
are towed back to the launch site. Each booster is removed from
the water, and its components are disassembled and washed with
fresh and deionized water to limit salt water corrosion. The motor
segments, igniter and nozzle are shipped back to Thiokol for refurbishment.
Each SRB incorporates
a range safety system that includes a battery power source, receiver/decoder,
antennas and ordnance.