Space-A travel is an opportunity for U.S. military personnel, their dependents, and retired military personnel to travel on military aircraft for free or for steeply discounted prices--often $27 or less.
Space-A travel was implemented by the U.S. Army for the purpose of casually transporting persons and supplies overseas on a space-available basis. The concept is somewhat ironic, since Space-A travel is statutorily recognized as "sea transportation," even though virtually all travel has been and currently is facilitated by aircraft.
Access to space-available flight is determined by your place in a six-category hierarchy: Members of the U.S. military travelling on emergency leave have the greatest priority, followed by a scant number of individuals on so-called Environmental and Morale Leave (EML)--an ameliorative provision offered to those who have been stationed in undesirable locations for a long time. Next comes regular active-duty military personnel on official leave, which comprises the largest proportion of the six categories. Further down are family members of those on EML and some teachers employed by the department of defense; lower still are certain students and military persons on temporary assignments. Reservists, retired military members, and dependents are at the bottom of the totem pole; moreover, reservists are only permitted to fly within the continental U.S. (CONUS) and U.S. possessions. If you need to travel abroad, you must be in active duty status or show orders to report to your base, if it is located outside CONUS.
Those with flexible schedules are most likely to realize success when utilizing Space-Available travel.
Your chance of securing a seat is a strong function of origination and destination point (large, busy Air Force bases will have a greater selection of flights), season (avoid holidays and on-peak travel periods to your destination), and your place in line (early birds have priority over those who sign up later within the same category). These are especially important considerations if you are in the bottom category, as getting seats around Thanksgiving, Christmas, or the busy summer season can be near impossible. Your reward for having a few days of slack time around your departure dates, travelling off-season, signing up early, and packing light, is a unique flight experience for the price of a cheap dinner for two.
Don't think you can escape baggage restrictions by flying Space-A! The restrictions are considerably more permssive than commercial airliners, and you won't be charged a fee if your bags are too heavy or exceed maximum dimensions. You may, however, suffer some disappointment when you discover that weight and dimensional requirements vary across aircraft, and that your super-long umbrella or 60-pound rock collection is not allowed on the aircraft you're scheduled to board.
Travelers using Space-A will need a passport, appropriate visas, and appropriate vaccinations for their destination country. Travelers will also need to abide by the custom regulations for the destination country.
Government aircraft fully span the hospitability spectrum: some aircraft have all the comforts of home, like high-pitch reclining seats, on-board catering and entertainment, and well-maintained lavatories. At other times, you will be flying in a cargo plane, which can be a two-engine turbo-prop with a minimal interior and maximal response to turbulance, poor weather, and outside temperatures.
Dress in layers before boarding. You may not know what kind of aircraft is waiting for you at the gate, and temperatures can be very uninviting (< 0 C) inside the cabin of some of the more primitive airplanes at cruising altitude.