WELDING crews began sealing cracks in the main engines of NASA鈥檚 entire shuttle fleet this week. Repairs to the first, Atlantis, could be completed by the weekend.
NASA grounded its fleet of shuttles in June after a routine inspection of Atlantis revealed small cracks in liners that channel liquid hydrogen fuel into the main engine (快猫短视频, 6 July, p 5). The thin tubular liners, made from a nickel alloy called Inconel 718, are fixed inside steel fuel pipes to smooth the flow of propellant into the engine. Each liner is perforated with holes to ensure that the propellant doesn鈥檛 cause a build-up of pressure inside.
Subsequent checks on the other three shuttles, Endeavour, Discovery and Columbia found similar cracks in the same components.
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After nearly two months of investigation, NASA scientists believe the hairline cracks, which all spread out from the holes in the liners, were probably caused by a combination of vibration and extreme thermal cycling during routine operation. When liquid hydrogen passes through them, the fuel liners chill to 鈥�240掳C.
The decision to weld the cracks shut was made last Friday. NASA had considered doing nothing, but feared that the cracks might grow during future missions, causing metal fragments to fall into the engine with potentially catastrophic results.
After welding the cracks shut, engineers will polish the welds and smooth any rough edges and notches in the holes punched into the liners, where cracks are likely to form.
If all the repairs go to plan, Atlantis will be first off the launch pad, on an assembly mission to the International Space Station on 28 September, more than two months later than planned. Endeavour will follow at the beginning of November. Launch dates for the remaining shuttles remain uncertain.
Gene Mathers, a British welding engineer at The Welding Institute in Cambridge, says the welding procedure should be straightforward. However, he warned that it could store up problems for the future. 鈥淲elding fundamentally changes fatigue life and the result will not be as strong,鈥� he says.