The Pentagon, looking to save money, has accelerated spending on unmanned systems since the Sept. 11, 2001, terrorist attacks. This year, it allocated $2 billion for unmanned aircraft and millions more in the supplemental budget, compared with $363 million in 2001. The figure is projected to reach more than $3 billion by the end of the decade. What has resulted is a hodgepodge of unmanned vehicles, such as small, bomb-seeking robots that can be carried in a backpack, and airplanes that provide surveillance for days at a time. The systems have become bigger and more expensive in recent years, such as the Predator, built by General Atomics Aeronautical Systems Inc., and the Global Hawk, which has a 134-foot wingspan, comparable to the Boeing 737.