As the U.S. Air Force considers when it will have a first flight test in the development of the LGM-35A Sentinel ICBM by Northrop Grumman [NOC], Lockheed Martin [LMT] has received a nearly $454 million contract add-on from the Air Force Nuclear Weapon Center at Hill AFB, Utah, for Sentinel's Mk21A re-entry vehicle. The contract add-on, awarded this month, brings Lockheed Martin's total award under Mk21A to nearly $1.5 billion. The company is to do the contract work through September…