The U.S. Air Force is considering "modular" missile silos to house the service's future LGM-35A Sentinel ICBM by Northrop Grumman [NOC]. Such modular silos would use pre-cast concrete and standardized components to reduce construction cost and time, but the approach may also mean less silo hardening and present logistical challenges in transporting the large modules, which would be dropped into place at existing sites under the 90th Wing at F.E. Warren AFB, Wyo., the 91st Wing at Minot AFB, N.D.,…