Space Shuttle Challenger disaster
On January 28, 1986, Space Shuttle Challenger broke apart 73 seconds into its flight, killing all seven crew members. The spacecraft disintegrated above the Atlantic Ocean, off the coast of Cape Canaveral, Florida, at 16:39:13UTC. It was the first fatal accident involving an American spacecraft while in flight.
The mission, designated STS-51-L, was the 10th flight for the orbiter and the 25th flight of the Space Shuttle fleet. The crew was scheduled to deploy a commercial communications satellite and study Halley's Comet while they were in orbit, in addition to taking schoolteacher Christa McAuliffe into space under the Teacher in Space Project. The latter task resulted in a higher-than-usual media interest in and coverage of the mission; the launch and subsequent disaster were seen live in many schools across the United States.
The cause of the disaster was the failure of the primary and secondary O-ring seals in a joint in the right Space Shuttle Solid Rocket Booster. The record-low temperatures on the morning of the launch had stiffened the rubber O-rings, reducing their ability to seal the joints. Shortly after liftoff, the seals were breached, and hot pressurized gas from within the SRB leaked through the joint and burned through the aft attachment strut connecting it to the external propellant tank, then into the tank itself. The collapse of the ET's internal structures and the rotation of the SRB that followed propelled the shuttle stack, traveling at a speed of Mach 1.92, into a direction that allowed aerodynamic forces to tear the orbiter apart. Both SRBs detached from the now-destroyed ET and continued to fly uncontrollably until the range safety officer destroyed them.
The crew compartment, containing human remains, and many other fragments from the shuttle were recovered from the ocean floor after a three-month search and recovery operation. The exact timing of the deaths of the crew is unknown, but several crew members are thought to have survived the initial breakup of the spacecraft. The orbiter had no escape system, and the impact of the crew compartment at terminal velocity with the ocean surface was too violent to be survivable.
The disaster resulted in a 32-month hiatus in the Space Shuttle program. President Ronald Reagan created the Rogers Commission to investigate the accident. The commission criticized NASA's organizational culture and decision-making processes that had contributed to the accident. Test data since 1977 had demonstrated a potentially catastrophic flaw in the SRBs' O-rings, but neither NASA nor SRB manufacturer Morton Thiokol had addressed this known defect. NASA managers also disregarded engineers' warnings about the dangers of launching in low temperatures and did not report these technical concerns to their superiors.
As a result of this disaster, NASA established the Office of Safety, Reliability, and Quality Assurance, and arranged for deployment of commercial satellites from expendable launch vehicles rather than from a crewed orbiter. To replace Challenger, the construction of a new Space Shuttle orbiter,, was approved in 1987, and the new orbiter first flew in 1992. Subsequent missions were launched with redesigned SRBs and their crews wore pressurized suits during ascent and reentry. In February 2003, the Space Shuttle Columbia disintegrated during reentry; the Columbia Accident Investigation Board concluded that NASA had failed to learn many lessons from the Challenger disaster, which resulted in the second disaster.
Background
Space Shuttle
The Space Shuttle was a partially reusable spacecraft operated by the US National Aeronautics and Space Administration. It flew for the first time in April 1981, and was used to conduct in-orbit research, and deploy commercial, military, and scientific payloads. At launch, it consisted of the orbiter, which contained the crew and payload, the external tank, and the two solid rocket boosters. The orbiter was a reusable, winged vehicle that launched vertically and landed as a glider. Five orbiters were built during the Space Shuttle program. Challenger was the second orbiter constructed after its conversion from a structural test article. The orbiter contained the crew compartment, where the crew predominantly lived and worked throughout a mission. Three Space Shuttle main engines were mounted at the aft end of the orbiter and provided thrust during launch. Once in space, the crew maneuvered using the two smaller, aft-mounted Orbital Maneuvering System engines.When it launched, the orbiter was connected to the ET, which held the fuel for the SSMEs. The ET consisted of a larger tank for liquid hydrogen and a smaller tank for liquid oxygen, both of which were required for the SSMEs to operate. After its fuel had been expended, the ET separated from the orbiter and reentered the atmosphere, where it would break apart during reentry and its pieces would land in the Indian or Pacific Ocean.
Two solid rocket boosters, built by Morton Thiokol at the time of the disaster, provided the majority of thrust at liftoff. They were connected to the external tank, and burned for the first two minutes of flight. The SRBs separated from the orbiter once they had expended their fuel and fell into the Atlantic Ocean under a parachute. NASA retrieval teams recovered the SRBs and returned them to the Kennedy Space Center, where they were disassembled and their components were reused on future flights.
Each SRB was constructed in four main sections at the factory in Utah and transported to KSC, then assembled in the Vehicle Assembly Building at KSC with three tang-and-clevis field joints, each joint consisting of a tang from the upper segment fitting into the clevis of the lower segment. Each field joint was sealed with two diameter Viton-rubber O-rings around the circumference of the SRB and had a cross-section diameter of. The O-rings were required to contain the hot, high-pressure gases produced by the burning solid propellant and allowed for the SRBs to be rated for crewed missions. The two O-rings were configured to create a double bore seal, and the gap between segments was filled with putty. When the motor was running, this configuration was designed to compress air in the gap against the upper O-ring, pressing it against the sealing surfaces of its seat. On the SRB Critical Items List, the O-rings were listed as Criticality 1R, which indicated that an O-ring failure could result in the destruction of the vehicle and loss of life, and that it was considered a redundant system due to the secondary O-ring.
O-ring concerns
Evaluations of the proposed SRB design in the early 1970s and field joint testing showed that the wide tolerances between the mated parts allowed the O-rings to be extruded from their seats rather than compressed. This extrusion was judged to be acceptable by NASA and Morton Thiokol despite concerns of NASA's engineers. A 1977 test showed that up to of joint rotation occurred during the simulated internal pressure of a launch. Joint rotation, which occurred when the tang and clevis bent away from each other, reduced the pressure on the O-rings, which weakened their seals and made it possible for combustion gases to erode the O-rings. NASA engineers suggested that the field joints should be redesigned to include shims around the O-rings, but they received no response. In 1980, the NASA Verification/Certification Committee requested further tests on joint integrity to include testing in the temperature range of and with only a single O-ring installed. The NASA program managers decided that their current level of testing was sufficient and further testing was not required. In December1982, the Critical Items List was updated to indicate that the secondary O-ring could not provide a backup to the primary O-ring, as it would not necessarily form a seal in the event of joint rotation. The O-rings were redesignated as Criticality1, removing the "R" to indicate it was no longer considered a redundant system.The first occurrence of in-flight O-ring erosion occurred on the right SRB on in November1981. In August1984, a post-flight inspection of the left SRB on revealed that soot had blown past the primary O-ring and was found in between the O-rings. Although there was no damage to the secondary O-ring, this indicated that the primary O-ring was not creating a reliable seal and was allowing hot gas to pass. The amount of O-ring erosion was insufficient to prevent the O-ring from sealing, and investigators concluded that the soot between the O-rings resulted from non-uniform pressure at the time of ignition. The January1985 launch of was the coldest Space Shuttle launch to date. The air temperature was at the time of launch, and the calculated O-ring temperature was. Post-flight analysis revealed erosion in primary O-rings in both SRBs. Morton Thiokol engineers determined that the cold temperatures caused a loss of flexibility in the O-rings that decreased their ability to seal the field joints, which allowed hot gas and soot to flow past the primary O-ring. O-ring erosion occurred on all but one of the Space Shuttle flights in 1985, and erosion of both the primary and secondary O-rings occurred on.
To correct the issues with O-ring erosion, engineers at Morton Thiokol, led by Allan McDonald and Roger Boisjoly, proposed a redesigned field joint that introduced a metal lip to limit movement in the joint. They also recommended adding a spacer to provide additional thermal protection and using an O-ring with a larger cross section. In July1985, Morton Thiokol ordered redesigned SRB casings, with the intention of using already-manufactured casings for the upcoming launches until the redesigned cases were available the following year.
File:Challenger flight 51-l crew.jpg|thumb|STS-51-L crew: Onizuka, McAuliffe, Jarvis, Resnik;
Smith, Scobee, McNair.|alt=Picture of the seven crew members in flight suits and holding their helmets