James of Grognardia has a series of posts on miniatures and various RPGs. One of which is about Traveller. In it he talks about the personal combat system and the starship combat system. This brought to mine that like original D&D, Classic Traveller has an alternate combat system. Like D&D's combat system it became the default in MegaTraveller.
The system is spread out in two supplements, Azhanti High Lightning and Striker. A third, Snapshot, modifies the range band system of the original books into a combat system that uses grid. It focuses on combat within a starship. There is a fourth, Mayday, that adapts starship combat to a hex and counter system.
Classic Traveller originally used a base 8+ chance to hit on 2d6. This is modified by your weapon and the armor the target is wearing. You add up the modifiers and that your base chance to hit. This system was continued in Book 4 Mercenary. Judges Guild made things easier by coming up with a referee screen for Traveller that added all the modifiers up. You crossindexed the weapon with the armor type and then pick which of the five ranges you were using. (Two for melee weapons). A similar chart is found in Snapshot.
In Snapshot the actions within a round (15 seconds) is regulated by the expenditure of action points. You get a sum of action points equal to the sum of your dexterity and endurance.
The Azhanti High Lightning/Striker system is somewhat different. It also has an action point system but it works slightly different than the Snapshot system. One major difference is that the original 15 second round has been divided into 5 action phases of roughly 3 seconds each.
The to hit roll is still 8+ and it is modified by range and situation. Instead of damage there is now a penetration roll of 2d6. Higher is better. Each weapons has a penetration modifier which varies by range. Each armor type subtracts off of the penetration roll. You look up the result on the damage chart to see whether you get no damage, a light wounds, a serious wound, or die.
Striker expands on this to the full range of military hardware and includes the ability to design your own vehicles and other military hardware.
The handful of times I ran Traveller I waffled between using the original system and the Azhanti High Lightning system. Most of the time, because of the Judges Guild Referee screen, I used the original system. Not sure what I would do now if I ran Traveller today.