Tech. Sgt. Guillermo Gutierrez, 81st Logistics Readiness Squadron fleet management and analysis NCO in charge, fills a propane tank Oct. 13, 2017, on Keesler Air Force Base, Mississippi. Thirteen vehicles were selected to have propane kits installed as part of a year-long Defense Department study to determine the environmental effects of using propane instead of gasoline in vehicles. (U.S. Air Force photo by Airman 1st Class Suzanna Plotnikov)