Complex sentence: KC-X is the name of the United States Air Force program to procure its next-generation aerial refueling tanker aircraft to replace some of the older Boeing KC-135 Stratotankers.
Split into simpler sentences: KC-X is the name of the United States Air Force program to procure its next-generation aerial refueling tanker aircraft. This aircraft is intended to replace some of the older Boeing KC-135 Stratotankers.