The M1A1 Abrams tank to which Pfc. Nicholaus E. Zimmer was assigned was designed with the purpose of preserving the lives of crew members. But Zimmer was killed when the tank came under attack Sunday from rocket-propelled grenades, which should not have been able to penetrate the tank's armor. Details of the attack and how Zimmer, of southern Delaware County, died were sketchy yesterday. "We know it was RPGs and he was assigned to a tank, and the tank came under attack," said Cathy Gramling, a Defense Department spokeswoman. "But we don't know the circumstances." Zimmer could have been struck...