From an excellent Space-A blog good for all, but geared towards retirees. This is the best explanation I've seen about how the Space-A system works.
Military Space-A flights were a major element of our strategy for saving money during our year of travel. We flew Space-A to Europe, Hawaii, Japan, and many places within the continental U.S. We estimate that we saved more than $15,000 by taking military flights. Overall, we had great experiences, and whenever it’s a viable option, Space-A is my preferred way to fly.
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https://www.poppinsmoke.com/space-a-flights/
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