The Administrator of the National Aeronautics and Space Administration (hereinafter referred to as the “Administrator”) and the Secretary of Defense are authorized and directed jointly to develop a unitary plan for the construction of transsonic, supersonic, and hypersonic wind-tunnel facilities for the solution of research, development, and evaluation problems in aeronautics, including the construction of facilities at educational institutions within the continental limits of the United States for training and research in aeronautics, and to revise the uncompleted portions of the unitary plan from time to time to accord with changes in national defense requirements and scientific and technical advances. The Administrator and the Secretaries of the Army, the Navy, and the Air Force are authorized to proceed with the construction and equipment of facilities in implementation of the unitary plan to the extent permitted by appropriations pursuant to existing authority and the authority contained in this chapter. Any further implementation of the unitary plan shall be subject to such additional authorizations as may be approved by Congress.