I am a Cadet in ROTC. Am I permitted to refer to my superiors by their rank (LT, CPT, MAJ, etc.) in email/in person or should I stick to sir?

For now; rank and last name in correspondence and sir or ma’am in person.

Recommend linking rank with last name at beginning of email or conversation. For example: Captain Smith, what time is your special presentation on Iraq?

Sir or ma’m works as well, but in the world of uncertain pronouns rank may be safer.

Just an opinion: “Sir” and “Ma’am” should be the way to address seniors. Enlisted members should be addressed by Rank LastName. (You’re not that salty yet.) Best to let them tell you when it’s OK to be more familiar.