you are talking about 2 different programs. hpsp is for med school and affects your residency choices. FAP is during/after residency.
for HPSP, there is no guarantee you will get ortho. your chances are much higher if you do a few years of flight surgery, but you are not 100%. if you are not currently in the military or hpsp, you can do residency and then apply for FAP, which is basically a loan repayment scholarship from what i understand (don't quote me on that.)
most recently i understand the AF was not allowing you to reapply as an intern if you did not get your specialty of choice as they did when i went through. i don't know if this has changed again, you'd have to ask some current applicants.
cheers
fg