Each of the two solid rocket boosters (SRBs) was constructed of seven sections, six of which were permanently joined in pairs at the factory.
For each flight, the four resulting segments were then assembled in the Vehicle Assembly Building at Kennedy Space Center (KSC), with three field joints.
The factory joints were sealed with asbestos-silica insulation applied over the joint, while each field joint was sealed with two rubber O-rings.
After the destruction of Challenger, the number of O-rings per field joint was increased to three.
The seals of all of the SRB joints were required to contain the hot, high-pressure gases produced by the burning solid propellant inside, thus forcing them out of the nozzle at the aft end of each rocket.

During the Space Shuttle design process, a McDonnell Douglas report in September 1971 discussed the safety record of solid rockets.
While a safe abort was possible after most types of failures, one was especially dangerous: a burnthrough by hot gases of the rocket's casing.
The report stated that "if burnthrough occurs adjacent to [liquid hydrogen/oxygen] tank or orbiter, timely sensing may not be feasible and abort not possible", accurately foreshadowing the Challenger accident.
Morton-Thiokol was the contractor responsible for the construction and maintenance of the shuttle's SRBs.
As originally designed by Thiokol, the O-ring joints in the SRBs were supposed to close more tightly due to forces generated at ignition, but a 1977 test showed that when pressurized water was used to simulate the effects of booster combustion, the metal parts bent away from each other, opening a gap through which gases could leak.
This phenomenon, known as "joint rotation", caused a momentary drop in air pressure.
This made it possible for combustion gases to erode the O-rings.
In the event of widespread erosion, a flame path could develop, causing the joint to burst—which would have destroyed the booster and the shuttle.

Engineers at the Marshall Space Flight Center wrote to the manager of the Solid Rocket Booster project, George Hardy, on several occasions suggesting that Thiokol's field joint design was unacceptable.
For example, one engineer suggested that joint rotation would render the secondary O-ring useless, but Hardy did not forward these memos to Thiokol, and the field joints were accepted for flight in 1980.

Evidence of serious O-ring erosion was present as early as the second space shuttle mission, STS-2, which was flown by Columbia.
Contrary to NASA regulations, the Marshall Center did not report this problem to senior management at NASA, but opted to keep the problem within their reporting channels with Thiokol.
Even after the O-rings were redesignated as "Criticality 1" — meaning that their failure would result in the destruction of the Orbiter, no one at Marshall suggested that the shuttles be grounded until the flaw could be fixed.

After the 1984 launch of STS-41-D, flown by Discovery, the first occurrence of hot gas "blow-by" was discovered beyond the primary O-ring.
In the post-flight analysis, Thiokol engineers found that the amount of blow-by was relatively small and had not impinged upon the secondary O-ring, and concluded that for future flights, the damage was an acceptable risk.
However, after the Challenger disaster, Thiokol engineer Brian Russell identified this event as the first "big red flag" regarding O-ring safety.

By 1985, with seven of nine shuttle launches that year using boosters displaying O-ring erosion or hot gas blow-by, Marshall and Thiokol realized that they had a potentially catastrophic problem on their hands.
Perhaps most concerning was the launch of STS-51-B in April 1985, flown by Challenger, in which the worst O-ring damage to date was discovered in post-flight analysis.
The primary O-ring of the left nozzle had been eroded so extensively that it had failed to seal, and for the first time hot gases had eroded the secondary O-ring.
They began the process of redesigning the joint with three inches (76 mm) of additional steel around the tang.
This tang would grip
Each of the two solid rocket boosters (SRBs) was constructed of seven sections, six of which were permanently joined in pairs at the factory. For each flight, the four resulting segments were then assembled in the Vehicle Assembly Building at Kennedy Space Center (KSC), with three field joints. The factory joints were sealed with asbestos-silica insulation applied over the joint, while each field joint was sealed with two rubber O-rings. After the destruction of Challenger, the number of O-rings per field joint was increased to three. The seals of all of the SRB joints were required to contain the hot, high-pressure gases produced by the burning solid propellant inside, thus forcing them out of the nozzle at the aft end of each rocket.
During the Space Shuttle design process, a McDonnell Douglas report in September 1971 discussed the safety record of solid rockets. While a safe abort was possible after most types of failures, one was especially dangerous: a burnthrough by hot gases of the rocket's casing. The report stated that "if burnthrough occurs adjacent to [liquid hydrogen/oxygen] tank or orbiter, timely sensing may not be feasible and abort not possible", accurately foreshadowing the Challenger accident. Morton-Thiokol was the contractor responsible for the construction and maintenance of the shuttle's SRBs. As originally designed by Thiokol, the O-ring joints in the SRBs were supposed to close more tightly due to forces generated at ignition, but a 1977 test showed that when pressurized water was used to simulate the effects of booster combustion, the metal parts bent away from each other, opening a gap through which gases could leak. This phenomenon, known as "joint rotation", caused a momentary drop in air pressure. This made it possible for combustion gases to erode the O-rings. In the event of widespread erosion, a flame path could develop, causing the joint to burst—which would have destroyed the booster and the shuttle.
Engineers at the Marshall Space Flight Center wrote to the manager of the Solid Rocket Booster project, George Hardy, on several occasions suggesting that Thiokol's field joint design was unacceptable. For example, one engineer suggested that joint rotation would render the secondary O-ring useless, but Hardy did not forward these memos to Thiokol, and the field joints were accepted for flight in 1980.
Evidence of serious O-ring erosion was present as early as the second space shuttle mission, STS-2, which was flown by Columbia. Contrary to NASA regulations, the Marshall Center did not report this problem to senior management at NASA, but opted to keep the problem within their reporting channels with Thiokol. Even after the O-rings were redesignated as "Criticality 1" — meaning that their failure would result in the destruction of the Orbiter, no one at Marshall suggested that the shuttles be grounded until the flaw could be fixed.
After the 1984 launch of STS-41-D, flown by Discovery, the first occurrence of hot gas "blow-by" was discovered beyond the primary O-ring. In the post-flight analysis, Thiokol engineers found that the amount of blow-by was relatively small and had not impinged upon the secondary O-ring, and concluded that for future flights, the damage was an acceptable risk. However, after the Challenger disaster, Thiokol engineer Brian Russell identified this event as the first "big red flag" regarding O-ring safety.
By 1985, with seven of nine shuttle launches that year using boosters displaying O-ring erosion or hot gas blow-by, Marshall and Thiokol realized that they had a potentially catastrophic problem on their hands. Perhaps most concerning was the launch of STS-51-B in April 1985, flown by Challenger, in which the worst O-ring damage to date was discovered in post-flight analysis. The primary O-ring of the left nozzle had been eroded so extensively that it had failed to seal, and for the first time hot gases had eroded the secondary O-ring. They began the process of redesigning the joint with three inches (76 mm) of additional steel around the tang. This tang would grip 
Space Shuttle Challenger disaster