On January 28, 1986, 1 min and 13 s after liftoff, Space Shuttle Challenger exploded. The crew of seven was lost, but their sacrifice was not in vain, because the lessons we can learn from this event will make the spaceplanes of tomorrow

Fig. 7.3 Ice at the pad: cold temperatures on the morning of STS-51-L's launch contributed to the loss of Challenger (courtesy NASA)

safe enough to transport millions. This is often the path of progress and is no different than the many sacrifices made by the early pioneers of rocketry and aviation. Only by making mistakes can we learn and improve.

What happened to Challenger? Why did it explode? Could it have been prevented? The investigation following the accident revealed that a joint in one of the solid rocket boosters had failed to seal properly. This condition was brought about by a combination of poor design and cold temperatures (Fig. 7.3) preceding launch. A brittle O-ring seal in SRB joint allowed hot combustion gases to escape and act like a blowtorch, aimed directly at the ET. Eventually the tank wall burned through, and the liquid propellants inside were ignited. The result was an immediate fireball, with no chance of escape for the crew.

Solid-propellant rockets work differently than liquid rockets. When a solid rocket is ignited, the propellant grain burns up and down its entire length from the inside toward the outside. The innards of an entire solid rocket case are therefore under continuous high pressure, and the exhaust gases are allowed to escape only through a nozzle at one end, creating the thrust. In the case of Challenger, some of these gases escaped sideways through the faulty seal, eventually igniting the liquid propellants in the adjoining tank.

The short reason for the Challenger tragedy was improperly designed O-ring seals in the joints of the booster rockets, exacerbated by cold temperatures, poor communications, and inadequate NASA management. It was the usual comedy of errors. But was there another reason for this tragedy? Was there an overarching flaw in the entire system? The answer is yes. Recall that the original design for the Shuttle called for a fully reusable two-stage launcher made up of a fly-back booster and a winged orbiter. Both of these components were powered by liquid propel-lants. If the original design had been funded by the Congress, there might never have been any solid rocket boosters, there might never have been any leaky joints, and there might never have been a Challenger-type tragedy.

What lessons for future spaceplane design can we glean from Challenger? The main lesson is that solid rocket boosters should not be used with manned spaceplanes. Clustered launch vehicle "stacks" should be avoided as well. Complete reusability is far preferable to partial reusability, both in terms of safety and operational cost. In the case of Challenger, a fully reusable vehicle would have been far safer and less costly in the long run.

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