Space - A travel Space - A travel is a means by which members of United States Uniformed Services ( United States Military , reservists and retirees , United States Department of Defense civilian personnel ( under certain circumstances ) , and these groups ' family members ) are permitted to travel on aircraft under the jurisdiction of the United States Department of Defense when excess capability allows . Background . Space available travel is a privilege that derives , in part , from United States Code , title 10 , section 4744 , which states , " officers and members of the Military Departments , and their families , when space is available , may be transported on vessels operated by any military transport agency of the Department of Defense " . Space available travel is defined as " travel aboard DoD owned or controlled aircraft and occurs when aircraft are not fully booked with passengers traveling under orders " . It is a privilege offered to United States Uniformed Services members . Retired members are given the privilege in recognition of their career and because they are eligible for recall to active duty . The criteria for extending the privilege to other categories of passengers is their support to the mission being performed by Uniformed Services members and to the enhancement of active duty Service members ' quality of life . Space - A procedures . Guidelines . There are rules and guidelines which apply to such travel . Uniformed personnel may only travel Space - A while on leave or pass for the full duration of their Space - A trip , and Space - A travel can not be used in conjunction with travel required by the service . Space A travel may not be used for personal financial gain or in connection with business enterprises or employment . Other nations ' laws and policies , as well as U . S . foreign policy , may limit the ability to travel using Space - A . Aside from members of the United States Marine Corps , travelers do not have to be in uniform for their flights . Sign - up process . Passengers wanting to travel using DoD Space - A travel are required to sign up with a military installation and are then placed on a locally - managed Space - A register . The registration process varies depending on the installation , but most installations allow signups via electronic mail , fax , or postal mail . Some installations have created their own web - based sign - up forms . Each installation ' s passenger service center maintains the Space - A register . Each person signing up is placed on this register using two factors : category of travel , and signup date . Based on status ( active duty military , retired military , emergency traveler , etc . ) , Space - A travel applicants are assigned a category of travel from 1 to 6 , which categorizes their priority of movement , 1 being the highest priority . Thus , an applicant with priority 1 will gain a place on an available aircraft over an applicant with priority 4 , for example . Just prior to time - of - flight , the numbers of available seats is determined . After sorting the signup register by priority of travel and signup date , the passenger terminal personnel follow a selection procedure . If there is sufficient seating for everyone desiring a seat , then everyone boards ; otherwise , a cutoff point is determined . Eligibility . The branches of service eligible for Space - A travel are : Space - required versus space - available . Space - A travel is not without its pitfalls . Unlike traditional commercial air traffic , military flights are not always assigned predictable takeoff times . Many factors go into planning a military flight , with space - required cargo and passengers forming the basis of planning . There is no consideration given to potential Space - A travelers during the planning process . Aircraft generally allowing for Space - A travel . The majority of flights that passengers take occur on : C - 5 , C - 17 , C - 130 , KC - 10 , and KC - 135 aircraft . Non - governmental support for Space - A travel . Space - A travelers might meet abrupt , sometimes even in - flight , changes in travel . This need for pre - planning has given rise to a small industry surrounding such travel . Non - governmental enterprises ( for the most part , publishers ) produce products , primarily books and maps , which provide travelers with information regarding Space - A travel .