If you commission in to the Air Force as a non-rated officer (not a pilot, navigator, air battle manager) you'll be required to serve 4 years active duty. Pilots are required to serve 10 years. Basically it would be 8 years active (2ish years for flight school, 6 years actually flying) then 2 years inactive........
Stumbling block. For other uses, see Stumbling block (disambiguation). A stumbling block or scandal in the Bible is a metaphor for a behavior or attitude that leads another to sin.
Finally the barrier to all of this happening is not the scramjet engine itself. It is already in advanced development and could be ready for military and commercial uses any time from now on. However the stumbling block at this time is the new barrier to flight.