From the Army HPSP manual:
"4-17. SPACE AVAILABLE (SPACE A) TRAVEL ON MILITARY AIRCRAFT. Travel by Space A is available within the United States and is not permitted for family members. Space A travel can be inexpensive, but flights may be canceled without notice. Submit a written request to: Commander, AHRC, ATTN: AHRC-HST (HPSP), 1 Reserve Way, St. Louis, MO, 63132-5200, for authorization for space available travel. For information regarding requirements, contact the AHRC Incentives Team at (800) 325-4729, menu option 5, then and menu option 2. Students must be in uniform (verify correct type, class A or B, etc. before departing), have a valid ID Card and written authorization (travel orders) to travel."
Have a safe, fun trip.