The F-35 Joint Strike Fighter (JSF), also called the Lighting II, is a new strike fighter being procured in different versions by the Air Force, Marine Corps, and Navy. The F-35 program is the Department of Defense's (DOD's) largest weapon procurement program in terms of total estimated acquisition cost. Current DOD plans call for acquiring a total of 2,456 JSFs for the Air Force, Marine Corps, and Navy at an estimated total acquisition cost (as of December 31, 2007) of about $246 billion in constant (i.e., inflation-adjusted) FY2009 dollars. Procurement of F-35s began in FY2007. Hundreds of additional F- 35s are to be purchased by several U.S. allies. The administration's proposed FY2010 defense budget requests a total of about $10.4 billion in Air Force and Navy research and development funding and procurement funding for the F-35 program, including about $3.6 billion in Air Force and Navy research and development funding and about $6.8 billion in Air Force and Navy procurement funding. The proposed FY2010 budget would fund the procurement of 10 F-35As for the Air Force, 16 F-35Bs for the Marine Corps, and four F-35Cs for the Navy. (Development and procurement of Marine Corps aircraft are funded through the Navy's budget.) The administration's proposed FY2010 defense budget also proposes to terminate the F-35 alternate engine program, which is intended to develop the General Electric/Rolls-Royce F136 engine as an alternative to the Pratt and Whitney F135 engine that currently powers the F-35. The George W. Bush administration proposed terminating the alternate engine program in FY2007, FY2008, and FY2009, but Congress rejected these proposals and each year provided funding for the program's continuation.