Q is the second most powerful computer in the world. The lab unveiled the first piece of Q in May 2002, saying the machine would eventually be capable of performing 30 trillion operations per second. But the Energy Department has abandoned plans to complete the final phase of the supercomputer. As it stands, it can perform 20 trillion operations per second. Lab officials chalk that decision up to politics and budgets, saying the existing machine works fine. But others see Q as a troubled machine.