Hello Folks,
I'm coming into this thread a bit late, and I apologize if I'm pointing stuff out that has already been mentioned, but I figured I'd point a few things out before I tried to wade through another 7 pages worth of stuff
1) any highly defended site defending against a high speed incoming attack force has a few benefits to consider:
a) any ship that wants to land on its surface has to come in fast, flip over, and decelerate or burn up in the atmosphere. This makes for a reasonably predictable course and a set of parameters that HELP the defender.
b) any high speed missiles inbound from out of range of a planet needs to contend with that really nasty spectre of high speed collisions with smaller objects. A few sand canisters in the way of these high speed missiles will do wonders for destroying them.
c) when the target point is known, any movements that are directed at said target point MUST come within a relatively easily predicted cone of probable transit. This makes aiming at the high speed targets relatively easier than if you didn't know their probable point of inpact.
d) we're talking about a defense in depth here. That means that the invading force must co-ordinate its staging zone, manuever to such a point where they can launch attacks. This means they have to undergo other defensive fires before they can even stage an attack on their points of interest. If you are using HG rules - all you need to get for a mission kill is fuel tanks ruptured. Without fuel for the reactors, the ship becomes unpowered.
e) ranges for Meson Guns *are* in fact listed by Canon rules: specifically in MAYDAY. There, it lists ranges for close range and out of range. Close range is up to 5 light seconds, long range is beyond 5 light seconds up to 15 light seconds. Beyond 15 light seconds, nothing in High Guard may hit (extreme long range).
f) High population worlds have large numbers of system defense boats along with other various weaponry. It would not be difficult to create automated system defense "bunkers" that are not ships, but orbital floating missle bays on automated platforms. Since this is non-canonical (ie no warbots etc) I'm not sure if these automated defense systems would be something anyone would permit in their traveller universes. Needless to say however, SDB do not have to be a mere 400 tons, 200 tons or what have you. They can be 1000 ton ships and such.
g) each of these worlds are considered to be strategically important as specified by the original poster. This means then, that these high tech high population worlds are going to be more important than 25 worlds combined within their sector. This also means they will have a VERY large fleet presense hanging about. In reading all I can from Fifth Frontier war along with the Rebellion Source book etc - DEPOT type systems are NASTY. THey have the manpower and material to force a major problem against any invading force.
h) the biggest problem here is the fact that people have neglected to mention one thing at the beginning of this "thread" and I didn't see mentioned here at the end of the thread. I'm assuming someone brought it up in the middle. If not, then... Fuel Giants are going to be one of the most heavily guarded/protected pieces of stellar estate possible. The Defenders *want* to defend the gas Giants because it is a natural choke point that the attackers (unless they brought along tankers!) *must* go to. Furthermore, the attackers generally tend to use refueling craft to get their fuel rather than refuel directly. This implies then, that you need only mess with the gas nozzle instead of the armored fighting vehicle in order to render such a weapon system immobile (using hydrogen as gasoline analogy and the fuel shuttles as gas nozzle at a gas pump analogy).
I) and finally: when your enemy has massed large numbers of ships in a small region, it becomes easier to nail an enemy ship. While the analogy isn't perfect - that is how muskets killed during Napoleon's time. They weren't aiming at individual soldiers - they were aiming at formations.
In summation? Head on assaults generally tend to require far more manpower than the defenders have. Even assuming that the attacker manages to gain a 2:1 favorable exchange rate against fixed defenses versus mobile attack units - are you willing to presume that the attacker can afford to lose those "mobile" units for such a time in the future when they will wish they had the lost ships instead of sitting over a world they've captured (intact?) and will have to defend *without* the benefit of the original layered defense the defenders had spent years creating? If you've ever played Star Fleet battles - invasion scenario: even winning a sector produces felt attrition like casualties. You have to garrison those areas successfully captured in order to keep what was taken.
As for myself? I'd like to see a "team" of players get together for both offense and Defense. Select one game system ruleset such as High Guard or GURPS TRAVELLER (yes, I'm letting my bias show <g>) and have at it. Design the weapon systems. Design the stellar system. Set up the defenses in depth and put out the pickets and such. Determine how well the invading fleet manages to stage before launching its attack. Then, have the big granddaddy of a donnybrook and see what happens. Here the victory conditions won't just hang on the exchange ratios of defender ships/weapon systems versus offensive units - but on mission accomplishments.