Dr. David E. Walker, center, visited an Air Force Research Laboratory rocket propulsion facility at Edwards Air Force Base, Calif., Sept. 23, 2015. At the time, Walker served as deputy assistant secretary of the Air Force for science, technology and engineering in the Office of the Assistant Secretary of the Air Force for Acquisition. As a senior executive for the Air Force, Walker was responsible for preparing policy, guidance and advocacy for the Air Force's science and technology program, which was then estimated at an annual $2 billion.(U. S. Air Force photo / Kenji Thuloweit)