I had a look at my v2.2 of Twilight: 2000, and it didn't use Coolness Under Fire at all. It uses Initiative.
It does use initiative for both types of combat - a character gets an initiative score through chargen. For a combat round, it starts at initiative 6 [or higher if needed] and everyone with a 6 acts, players choice of melee or ranged. Then everyone with a 5, 4, etc.
If your initiative is above 5 [I think] your character acts at the initiative level and also again at half level. Initiative is reduced through wound levels, so a high-initiative character could lose that second action.
There was nothing about coolness under fire in the T2K index.
CUF is, however, a stat in 2300. I didn't look up how it was used - from what I remember, combat in 2300 was crunchier than T2K, and if you're going to have a T2K game, you're miles ahead using that system.
Well, I haven't decided what system to use. The list of possible options is as long as my hand*, and than some

.
To wit: Twilight 2000, CT, Traveller 2320 AD, T5, Feng Shui 2, Fates Worse Than Death/Hoodoo Blues, Silhouette, Cyberpunk 2020/Interlock Unlimited, Blue Planet 1, Blue Planet 2 (they're NOT the same system by a mile), 7th Sea 1st edition (suitably grittified), StarCluster 4, Kuro, and a few others...
Which is to say, Twilight 2000 sounds like it would work. But I haven't decided yet, just considering the options.
So, 2.2 basically has tick-based initiative, with an attack being 3 ticks long

? Thank you, I've got a game that works exactly like that already. Make that 2 games, not sure how many "rounds" (effectively ticks) Blue Planet 1 used for an action.
*If you print it one word at a line, use double distance between the lines, and 72 pcts.
Twilight: 2000 1E certainly did have "coolness under fire" - it is on page 5/6 of the Play Manual.
CUF is determined by dividing the PC's number of months in combat by 10, and dropping fractions. 1D6 is rolled, and that number is added to the previously-determined number. That total is subtracted from 10 - this is the PC's CUF. Lower is better, as explained next.
The effect of CUF in combat is explained on page 18 of the Play Manual... it is the same for all types of combat - melee, body, and fire - indeed, the action(s) being taken is/are irrelevant. Every PC must spend as many rounds (5 seconds each) of the 6 rounds in a combat turn (30 seconds) "hesitating" as his/her CUF/2 (rounded up).
A hesitation round is one where the PC can do nothing... however, if the same action is being performed for the entire turn (crawling along a ditch, firing at a target, driving a vehicle under fire), then no hesitation need be taken. If the chain of action is broken (running out of ammo, tripping while running, etc) then the hesitation rounds must be taken at that time.
Otherwise, hesitation rounds may be taken whenever the PC's player designates - all together or spread out during the turn - and whenever during the turn the player desires.
If the PC is surprised*, or is knocked down, there is a chance of panicking. 1d10 is rolled - if the result is less than or equal to the PC's CUF, he/she panics, and must immediately hesitate for CUF/2 rounds (rounded up) - all at once, and in addition to any hesitation rounds already taken that turn. The first panic round is shock paralysis - the PC cannot move. The next panic round (if there is one) the PC may drop prone or dive into cover that is within 2 meters**, and remain stationary until all hesitations are used (both the panic ones and any unused normal hesitation rounds for that turn).
Being charged by a vehicle (other than a bicycle, etc) or a large running animal forces a panic roll. If the PC panics, running away is automatic (all panic hesitation rounds are spent running). If the charging threat can probably be stopped by a weapon the PC has ready to use, then the roll for panic is reduced by 2.
* Attacked from an unexpected direction, otherwise see the "encounters" chapter of the Referee's Manual.
** I add the option of running away from whatever surprised the PC for the remainder of the panic rounds.
Thank you for the detailed explanation! Depending on the action allowed during "hesitation", this might make CUF the deciding factor between well-matched opponents... or even between unevenly-matched ones

.
I like that:devil:! Can you even defend actively during "hesitation"?
Ahem, is that number the total number of months spent in combat? And does low-intensity combat count?
What if a PC had spent 8 months in active deployment, then went back for 6 months R&D, then spent an year in urban combat against groups using suicide tactics and blending with the population? Assuming an average roll of 3, would the CUF be 5, 6, or 7

?
Either way, sounds like a good way to show how experience improves a PC's abilities!
Sadly, 2.2 doesn't. Well, not exactly. Initiative serves some of CUF's purpose, plus several of its own.
AFAICT from the descriptions in this thread, Initiative allows more attacks, just like having a lower CUF, and I guess it also serves to determine who acts first. Anything I'm missing

?