There's no "department head" in the sense of a commissioned officer that has that duty, and chief warrant officers in that community are rare afloat, however most of my research seems to indicate that the minimum manning level for "security chief" is one Chief Petty Officer or Senior Chief (on support vessels and subs), who may be supported by one or more Petty Officers on frigates, destroyers and cruisers, and will have a staff of Petty Officers and rated and unrated Seamen on larger vessels.
I have to correct myself, I have learned that within the last 10 years the organization of nuclear carriers and big amphibs (LHA, LHD) has begun to include a security department, largely of MA rates, with a line officer (LDO, former enlisted) department head. So there you go.
In TOS there wasn't supposed to be that officer/enlisted distinction, at least according to the 1967 writers' guide:
Onscreen, though, there were enough references to non-officer type titles (yeoman third class, technician first class, specialist, chief, engineer grade four), plus Spock's reference to the "captain, officers and crew" of the
Enterprise, that it's questionable how closely that part of the writer's guide was followed.
Of course
The Making of Star Trek flat-out stated that everyone was an officer, but again, onscreen...
Searching the transcripts on chakoteya.net, it seems that the only time "Ensign" was used in Season 1 was in "Court Martial," to refer to the personnel officer on the Enterprise who testifies, and also to Kirk back when he was assigned to the USS Republic.
An Ensign O'Neil was mentioned in "The Galileo Seven."
Which would lead to the question, Why is Kirk's yeoman wearing red? Maybe she is pooled from—or a part of—Security?
If so, it's unfortunate that Zahra wasn't issued a phaser!
BTW, thank you so much for not using the phrase "begs the question" there!
NB: There is a some precedent for both 3 1/2 stripe "captains" (which logically is what Kirk is during TOS and Pike was at retcon'd into via DSC) in the RW, and for a 1/2 stripe, typically given to either cadets/midshipmen, but occasionally very junior officers (roughly corresponding to US ENS).
Before 1912, US Navy midshipmen (also called "naval cadets" 1882-1902) had to serve two years in the fleet after graduation from the Naval Academy but before being commissioned as ensigns. They wore one "half stripe" (quarter inch).
A single quarter-inch/6mm stripe was used for Royal Navy warrant officers 1918-1949, and is still the mark of the RAF pilot officer.
What is a 3 1/2 stripe captain in "the real world"?
Some navies use it for what we would call the commander grade.
Kind of a stretch but interesting. If we really wanted to push it, Pike could be a LT in charge of the then-smaller ship, with Number One having taken such a grade reduction.
It seems too convoluted to be likely. Why all these temporary rank reductions? Why not just assign people of the appropriate grades?
Anyway, in the 20th episode ("Court Martial," 2 Feb 1967) Kirk's, Spock's and McCoy's official ranks are stated. Then you get to deal with the discrepancy of the ranks and the stripes!
