Not quite.
Classic Traveller had a bunch of essentially modular substitutions...
For personal combat... ,
... CT Book 1 has the corebook combat system, and it's table heavy. CT Book 4: Mercenary adds a bunch more weapons, but doesn't change the system.
... Snapshot was a boardgame adaptation of Book 1. and includes the Book 4 weapons.
... Azhanti High Lightning (AHL) was a different combat system based upon Armor Penetration capability vs Armor Value.
... Striker was a minis wargame based upon the AHL .
... Book 4 also has an abstract combat system for resolving mercenary actions without actually playing out the combats.
For Craft Design...
... Book 2 (in the core) has a table-heavy ship design system which has a very age of sail feel ... no armor, only 3 types of weapon (Missiles, Lasers, and the defensive Sandcaster). The difference between pulse and beam lasers, except for cost, isn't included... (Pulse fire rolls for hits twice, but at -2, per Starter Traveller) It caps at "5000 Tons"... the tech level effects are actually in Bk 3... It also doesn't design small craft
... Book 5 (1979) includes a design and combat system for much bigger ships (up to 1 million {1e6} Tons.)
... Book 5 (1980) includes a very similar design system, but different combat system, and includes small craft design
... Striker has ground, air, and gravitic vehicle design systems
For ship combat...
... Book 2 is a tabletop minis game, essentially, with character tie-ins.
... Mayday is a wargame that is very closely derived from Book 2, and uses a hex grid.
... Book 5 is a fairly abstracted combat system designed to handle fleet on fleet.
... later printings of Mayday include rules for using its movement with Book 5 (1980) combat
... the boardgames Fifth Frontier War and Invasion Earth include fleet vs Fleet combat mechanics, but no details nor conversions rules.
For Trade:
... Book 2 1977 has a trade system that includes labeled speculative cargo lots, and unspecified freight lots to be carried for hire.
... Book 2 1981 has a number of small differences (especially passenger numbers), but is very much like the 1977 version.
... Book 7: Merchant Prince has a different trade system, with very abstract speculative cargos, but many more...
For System Generation
... Book 2 has a mainworld only system generation - no star types, no list of moons, only the presence or absence of gas giants.
... Book 6 has an expansion to book 2, that generates all the planets in the system, as well as the stars, and some of the local data.
Basic vs Advanced Character gen
Book 1 resolves in 4 year terms. Has Army, Navy, Marine, Scout, Merchant, and "Other"
Supplement 4 adds 12 more (mostly civilian) careers.
Book 4 adds advanced char gen for Army and Marines... year by year. Bk 4 characters have a lower Char Gen survival, but often have twice as many skills as basic gen.
Book 5 adds advanced gen for Navy, and differentiates system, Subsector, and Imperial Navies. Thiss starts to actually define elements of the OTU, as well
Book 6 adds advanced gen for Scouts. It also includes a differentiation between rankless Field Scouts, and ranked Administration scouts
Book 7 adds advanced gen for Merchants, including free traders, and a couple levels of lines.
Later materials (most after 1983) fully integrate the OTU, including Books 6-8, and all the large format "modules" - all 8 alien modules, Atlas of the Imperium, Spinward Marches Campaign (which advances the timeline from the early 1101-1106 of early CT, to the 1107-1111 Fifth Frontier War era), Alien Realms (a bunch of Alien Module needed adventures)... all presume the OTU is in use.
MegaTraveller is written by Digest Group, under contract, for GDW.
They took Book 1 char gen, and expanded it, and added "Special Duty" and "Bonus Skills" to make basic characters capable of the smae number of skills as the various advanced gen. They also included the Supplement 4 additional basic careers; they also deleted "Other" with "Rogue." They made a lot more skills on the tables cascade skills (a cascade is a category - you pick one of the contained skills). They also added anagathics rules. The weapons list was not reduced, but the skills were... there went from individual weapon types to Large Blade, Small Blade, Polearm, Axe/Mace, Pistol, Rifle, Combat Rifleman, Laser Weapons, High Energy Weapons, and Neural Weapons.
They also included all 4 Advanced Gen options... mostly changing to use more cascades.
They opted to make a variant of Striker/AHL combat.
They hybridized Striker and High Guard (1980) craft design for ground, gravitic, and space vehicles all to be designed under one cohesive system; it did not include fixed wing nor rotary wing aircraft; those were in an expansion (COACC).
They merged and updated for better accuracy the Book2 and Book 6 system generation.
They used Book 7 Trade, but added detailed labels and chances for hazardous and/or perishable goods.
They included all the Library data from Supplements 8 & 11, and made a few additions.
They define (in Referee's Companion and Rebellion Sourcebook) system forces in terms of the values in FFW and IE... which are based upon infantry battalions and some level of squadrons. They alter slightly the nature of local fleets, deleting the "Subsector Navies" and replacing them with "Reserve Fleets"
They advance the OTU timeline from 1111 to 1117... and introduce the rebellion metaplot
It adds a consistent task mechanic, one which first appears in their CT supplements. After 1984, they were apparently the only 3rd party publisher for CT supplments to retain their license...
In an expansion, they added Flyer Advanced gen and aircraft design.
It's basically a consolidation of CT rules, and a particular subset of the modules chosen. Despite claims by many, it's not the first Traveller edition to include the OTU... that's CT's The Traveller Book. It's the first to totally focus on a metaplot in the OTU.
Ships designed in HG 1980 and MegaTraveller have compatible ratings, but not in the same format of presentation. And one can use either HG-80 or MT-HG combat rules - they'e a close variant of HG-80.
Characters are directly compatible.
Essentially, MT is not so much "advanced Traveller" as CT 3rd ed... but many of the choices were suboptimal. Many CT refs adopted parts of MT, and some MT Refs discovered certain parts of CT, and so hybrid use is common.
As far as carry forward, T:TNE largely is a rework of MT design systems, but using the Twilight 2000 2.2 rules for characters and combat, expanded for the future-tech.
Other Editions
The setting materials from MT carry forward into all later editions except T4... but T4 uses the same design as T:TNE, but with different ratings. T4 is also set in year 0 to 100, rather than 1100-1115 of CT, 1116 to 1130 of MT, 1200 to 1300 of T:TNE, 990-1050 of T20...
MGT is set in the same time as CT. MGT is not, however, a mechanical compatible with any GDW/IG version. It's derived from, and for those not system-focused, close enough for convert-on-the-fly... but the ships aren't ratings nor design compatible.
T5 is conceptually T4 on steroids... but that's a whole rant I'm not goign to make.
T20, like MT, has ratings and design compatibility with CT Bk5 High Guard (1980) - which means it's also ratings compatible with MT and CT Bk 5. Some designs loose a bit of capability in T20. The combat mechanics, tho', are quite different. (They can be backported to CT or MT). The character rules in T20 are D20 system, built off the D&D 3.0 SRD. The T20 Trade Rules are Book2 on Steroids, plus provide some additional options not allowed in CT, MT, T:TNE, nor T4.
GT keeps the OTU, but diverges at an unidentified point well before 1116: The rebellion never happens, and since the planning alone for launching it would take the characters some 2+ years to happen, it's assuredly well before 1115. (Probably 1100 or earlier)
GT:IW is set before the OTU Third Imperium... it's the distant past.
Both GT and GTIW use GURPS rules, and GURPS Vehicles designs. They also seem to assume a lot more ships than other editions.
HT was only on sale for a VERY short time.
Also... Traveller:2300 is a different setting, and a related ruleset to MT... it predates it shortly (under a year), and is essentially a wholly new game and setting with similar first principles, and using a 1d10 success & 3d6 time variant of DGP Task System (which uses 2d6 success & 3d6 time roll). It is not attribute compatible with any other editions. It's second edition was renamed 2300AD. It's 3rd edition was for MGT 1e, and I believe it's been updated for MGT2e...
(MGT=Mongoose Traveller.)
An excellent summary, but I expect that from Aramis.
I started on the original release of some of the 1977 stuff and then got the Deluxe boxed set when it first came out and became a completeist (everything from GDW, Judges Guild, Comstar, Imperium Games, GURPS Traveller, Mongoose 1st edition, Paranoia Press, Seeker, and many others). I stopped being a completist in that costs of some products went up while my wealth went down and I didn't get T5, MgT 2 (because although I liked some of the changes, MgT 1 was good enough), and I've only been able to get into one of the big Kickstaters lately. I remember dogfights on EBay to get copies of Arrival Vengeance and some of the other rare supplements from 3rd parties. And I E-bayed 5th Frontier War, Invasion Earth, Tarsus, Beltstrike, Belter, and some other boxed sets. ACQ from BITS, Striker, Striker II, Fire Fusion and Steel, Snapshot, Azhanti High Lightning boxed set, and on and on. And I bought the minis - ships, 25mm figs. My Traveller books fill multiple shelves in a big bookcase and the boxed games and minis take a lot of space elsewhere. Deck plans span multiple bins. Got a lot of the software too - including Heaven and Earth and Traveller Universe (best software product, just wish the new version would reach completion with a different underlying DB). I've run events at conventions in Canada and the US, played in Travellers miniature wargames and ran a few.
We ran one multi-DM convention game aboard the ISV King Richard (big luxury liner) that had a Starship Titanic aspect (computer issues that were worsening) but the players all had their own backstories and goals (among them an assassin, a wanted criminal impersonating a cop, a scientist looking to get the local Marquis to fund his research, and others). It was a blast!
We've played in multi-table games with each table being a shp in a fleet when everyone just found out about the assassination of Strephon and with each ship having supporters of Lucan, Strephon and Neutrals and Captains on the different ships with different allegiances, there was chaos and a big board that constantly tracked who was targeting who and so on... wild!
I ran Imperial Marines vs. Zhodani Commandos on a airless moon with an alien step pyramid as the objective.
I got to play a 15mm scale assault on a starbase one year, and another year we got to run a bunch of Mercs with a Shuttle from a Broadsword trying to raid a village to capture local insurgents (who were secretly being trained by Sword Worlds Special Forces).
I've run a year long campaign that was very political and with much secret activity, another that was a straight up merchant campaign, and another that was a covert mission to locate a lost Imperial vessel that had a new prototype drive installed.
There are so many good stories that Traveller can encompass that it's hard not to find something to be excited about.
For a long while, until Hunter Gordon passed, I was kaladorn on CTI and on the TML. Guess I still would be, but I haven't been back in a while.
I've run games from the 1977 version of CT up through MT and TNE (which has the BEST ship to ship combat game that unfortunately doesn't have easy ways to backport standard CT ships into because of the system in TNE Brilliant Lances). I've run MgT and home brewed versions and we tried TNE once or twice. Played several campaigns in Traveller: 2300 before it became 2300 A.D. Didn't like T4's are much, but got the books on PDF.
My favourite version:
MegaTraveller (MgT 1 my second choice)
Why?
It wasn't the setting of the shattered Imperium. I generally hewed to a GURPs Traveller timeline with no collapse. And the fact they gave lots of rules including a combat system that claimed to handle a punch to starship gunnery was not necessarily a claim that was fully executed well. And anytime you pickup any MT printing or any of the supplements, go find the Consolidated Errata List for Megatraveller. The editing was horrific and for a lot of the construction systems, the mistakes, omissions, etc. were confounding.
But it did two things that I thought were magnificent:
1. They expanded the skill table to give many different skills for a game that had more vision than pirates with shotguns and cutlasses in space (that's a fun perspective, but it never motivated our gaming groups). And with that, they brought a universal skill system. Instead of trying to figure out what levels of what skills applied (in CT, sometimes you had to look at the rulebook to recall modifiers and when they kicked in because it varied a lot between skills), you had a Task Statement:
To Fix A Broken Speeder, Electronics, INT, Difficult, 10 minutes, Note: Having vehicle manuals drops difficulty to Routine.
That one sentence identifies the task, the applicable skill and/or stat , the difficulty level (3+, 7+, 11+, 15+, Routine being 7, Difficult being 11), told you the time increment (3d6 x time increment equals task duration) and any special notes or mods. And there were tags (Fateful, Hazardous, and others) that added some additional meanings. And the task system included partial success or critical failure or exceptional success. There were rules for being Cautious (lengthening task for reduced difficulty) or Hurried (with faster task execution for increased difficulty).
And with this tool, a GM could play with a referee's screen (for some of the combat tables) and dice and virtually NOTHING ELSE, because you could knock up task statements on the fly that were entirely reasonable. Throw in a space map and really I almost never had to pull out a rulebook other than at character generation.
The BITS task system is also not bad (free from the British Isles Traveller Something)
2. World building with the World Builder's Handbook. Lots of depth, lots of getting to know the world you built and letting the GM make it come to life for the players.
I suppose also the art in the MT books and the 3rd party stuff that showed starship interiors, made wonderful deck plans and exterior looks for the ships, and that gave us a better idea of what the world of the characters looked like was pretty inspiring.
My personal favourite Trav products:
Starship Operators Guide, Vol 1 (MT)
The MT Boxed Set & Ref Screen
TAS for the breadth and interesting peeks at the many worlds of the Third Imperium
The Keith Supplments (Undersea Environment, Arctic Environment, Mountain Environment plus Skyraiders trilogy among others)
Azahanti High Lightning and At Close Quarters (from BITS also) for giving a tactical board game option for combat (Striker too, but it had higher effort to get into)
My own (or Allan Goodall's own) mods to use Stargrunt II from GZG for platoon to company level combat in Traveller
Still looking for a good small ship game that integrates player skills and still lets you fight tactical battles with your characters involved on the ships with ships from 10 to 800 tons (nothing bigger) - Snapshot or Brilliant Lances could have been this, but both had shortcomings.
World Builder's Handbook from the MT era (or the later World Tamer's Handbook from GURPS) - amazing generation systems for complex, interesting worlds
Nice to see people are still finding the game.
