Solid Rocket Boosters
By Marty McDowell/NASA
The two SRBs (Solid Rocket Boosters) 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
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
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
Additionally, special 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 the skirt.
These two modifications added approximately 450 pounds to the weight of
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
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 channel.
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
Each SRB incorporates a range safety system that includes a battery
power source, receiver/decoder, antennas and ordnance.