The US Military makes its equipment -- tanks, ships and the like -- available to filmmakers at affordable prices if the film casts a flattering light on the military.  If a film presents the military as inglorious, they will have to rent from private collectors or foreign militaries.  Apocalypse Now was blackballed for featuring a soldier ordered to kill a superior as a major plot point and had to rent helicopters from the Philippine Air Force, who repeatedly recalled them to battle communists during the course of filming.  Top Gun, on the other hand, had the full support of the Navy, who even set-up recruitment tables outside some theaters, despite the fact that it is a terrible, terrible movie.