For Army, (I know you said AF), we have heard the only people getting an ED are biology majors going to Med school and it is very competitive. Besides that it would be needs of the Army (or AF). While I see your need to get your masters, I don't know if the AF would have a need. Something to ask your CoC about specifically for physics. You might have to be open to going to grad school after your service. I got my masters when I was 40, there is no time limit!
I know there was is someone on these forums from the USAFA that was able to get his PhD (I believe in Physics) straight out of the academy with little or no additional military commitment. But, that was out of the SA, not ROTC. Best bet, like others have suggested is to talk to your CoC and see what, if any, options you have.