[RonBee]

propellant

• Both valves in each disconnect pair are opened to permit propellant flow between the orbiter and the external tank.
  
  The closure of both valves in each disconnect pair prevents propellant discharge from the external tank or orbiter at external tank separation.
  
  Inadvertent closure of either valve in a 17-inch disconnect during main engine thrusting would stop propellant flow from the external tank to all three main engines.
  
  All aspects of the existing SRM were assessed, and design changes were required in the field joint, case-to-nozzle joint, nozzle, factory joint, propellant grain shape, ignition system and ground support equipment.
  
  No changes were made in the propellant, liner or castable inhibitor formulations.
  
  Flight-configuration aft and center segments were fabricated, loaded with live propellant, and used for assembly test article stacking demonstration tests at Kennedy Space Center.
  
  In a long-term stack test, a full-scale casting segment, with live propellant, has been mated vertically with a J-seal insulation segment and is undergoing temperature cycling.
  
  http://science.ksc.nasa.gov/shuttle/...f/sts_asm.html

And:

• An important consideration in designing a rocket is whether to use liquid or solid propellant.