NASA could soon be facing renewed calls to replace its ageing space shuttle fleet. Its engineers have been trying to find out why some explosive charges in the bolts that release the shuttle from its launch pad failed to fire last month during Atlantis鈥檚 launch. If the problem looks as if it might recur, it could delay next week鈥檚 launch of Endeavour, and is likely to spark another bout of concern over the veteran fleet鈥檚 reliability.
The set of 10 bolts are fixed underneath the shuttle鈥檚 solid-rocket boosters, keeping the shuttle firmly attached to the launch pad to stop it moving around in high winds. Each bolt contains two explosive charges 鈥� primary and secondary 鈥� that are designed to detonate moments before lift-off. The shuttle can launch safely if only one set of charges detonates, but both should operate together.
NASA has never tested what might happen if both sets of charges failed to go off, but the result would probably be catastrophic. 鈥淲e won鈥檛 speculate on loss of the spacecraft,鈥� says a spokesman.
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NASA says the failure of the entire set of Atlantis鈥檚 secondary charges points to a problem in either generating or transmitting the firing command to the bolts. Such problems have occurred before. Two years ago, shuttle launches were delayed after a similar problem with an explosive bolt holding one of the solid-fuel boosters to Endeavour鈥檚 giant external fuel tank. Engineers traced that failure to worn electrical cables that would not transmit the firing signal (New 杏吧原创, 23 December 2000, p 6).
The risk that both sets of charges might fail to fire led NASA to test every cable in its inventory. It found four faulty cables. But the detonation commands for the bolts come from the shuttle itself, so NASA is analysing the control system as well as the cabling. But by early this week, the engineers had not reached any conclusions.
Charles Vick, a space policy analyst based in Fredericksburg, Virginia, says that the shuttle鈥檚 explosive bolts have fail-safe notches cut into them, so the bolts should snap when subjected to launch forces. But, Vick says,if they don鈥檛 break, the vehicle, which could be free on one side yet bolted to the launch pad on the other, 鈥渂ecomes for all practical purposes a suicide vehicle.鈥�