Early Problems with the Solid Rocket Boosters
Problems with the fi eld-joint design had been recognized long before the launch of the Challenger. When the rocket is ignited, the internal pressure causes the booster wall to expand outward, putting pressure on the fi eld joint. This pressure causes the joint to open slightly, a process called “joint rotation,” illustrated in Figure 1.1b . The joint was designed so that the internal pressure pushes on the putty, displacing the primary O-ring into this gap, helping to seal it. During testing of the boosters in 1977, Thiokol became aware that this joint-rotation problem was more severe than on the Titan and discussed it with NASA. Design changes were made, including an increase in the thickness of the O-ring, to try to control this problem.
Further testing revealed problems with the secondary seal, and more changes were initiated to correct that problem. In November of 1981, after the second shut- tle fl ight, a postlaunch examination of the booster fi eld joints indicated that the
Figure 1.1 (a) A schematic drawing of a tang and clevis joint like the one on the Challenger solid rocket boosters. (b) The same joint as in Figure 1.1a , but with the effects of joint rotation exaggerated. Note that the O-rings no longer seal the joint.
Pin
Clevis
Inside of booster
O-rings
Putty
Pin
Clevis
O-rings
Putty
Tang
Chapter 1 Introduction 9
O-rings were being eroded by hot gases during the launch. Although there was no failure of the joint, there was some concern about this situation, and Thiokol looked into the use of different types of putty and alternative methods for applying it to solve the problem. Despite these efforts, approximately half of the shuttle fl ights before the Challenger accident had experienced some degree of O-ring erosion. Of course, this type of testing and redesign is not unusual in engineering. Seldom do things work correctly the fi rst time, and modifi cations to the original design are often required.
It should be pointed out that erosion of the O-rings is not necessarily a bad thing. Since the solid rocket boosters are only used for the fi rst few minutes of the fl ight, it might be perfectly acceptable to design a joint in which O-rings erode in a controlled manner. As long as the O-rings don’t completely burn through before the solid boosters run out of fuel and are jettisoned, this design should be fi ne. However, this was not the way the space shuttle was designed, and O-ring erosion was one of the problems that the Thiokol engineers were addressing.
The fi rst documented joint failure came after the launch on January 24, 1985, which occurred during very cold weather. The postfl ight examination of the boost- ers revealed black soot and grease on the outside of the booster, which indicated that hot gases from the booster had blown by the O-ring seals. This observation gave rise to concern about the resiliency of the O-ring materials at reduced tem- peratures. Thiokol performed tests of the ability of the O-rings to compress to fi ll the joints and found that they were inadequate. In July of 1985, Thiokol engineers redesigned the fi eld joints without O-rings. Instead, they used steel billets, which should have been better able to withstand the hot gases. Unfortunately, the new design was not ready in time for the Challenger fl ight in early 1986 [ Elliot et al., 1990 ].
Place Your Order Here!Early Problems with the Solid Rocket Boosters
