Thanks for the help, gents.
The winner is...ground presure and serious vibration. Larger ships require very thick, reinforced concrete aprons to land on. These aprons require maintenance.
As a practical matter, this limits frontier landings to 200 ton ships or smaller (for typical worlds of size 5+). A world may have flat expanses of bedrock that can support a ship, but these are seldom close enough to anything to make this a useful option. The airports of most TL8+ worlds can support smaller starships as well (though without cargo handling).
Regional airports can accomodate 400 ton ships. International caliber airports can accomodate 600 ton ships.
In addition, starports have extensive cargo handling machinery which make cargo far easier to debark.
The military has experimented with "dump-ships" -- larger ships that can land anywhere, but can't takeoff. These have not impressed anyone (yet).
In my Commonwealth campaign, the archetypal orbital invasion campaign goes something like this:
1. Jump troops sieze the starport.
2. Medium weight marine forces reinforce and secure the bridgehead.
3. Heavy armor and mechanized forces build up inside the bridgehead.
4. Forces in the bridgehead break out.
Sometimes, marine landing forces replace jump troops. Jump troops may be used instead for diversions, raids on critical targets, etc. Sometimes, marines aren't available and Army light infantry units are used in step 2.
Note that while starports are key objectives, this is well-known by defenders. With time to prepare, a starport can be heavily fortified. So many campaigns have *not* featured the early seizure of the starport. Instead, the bridgehead is established in a reasonably defensible area (say, between two rivers). It is then supplied with small craft (200 tons-) until the breakout forces are ready to move. Then, the starport is seized in a normal land campaign. It typically takes a lot longer for this kind of bridgehead to reach step 4, but this can be offset by the fact that defending forces are usually far away. This allows the bridgehead to be fully established before the enemy can launch serious counterattacks. And note that the Commonwealth has a very high percentage of
airmobile forces. A remote bridgehead will not be a serious hindrance for an airmobile force.
The standard insertion ship is the
30 ton D-7 dropship (aka the LBB2 slow boat). 19.9 dton capacity (80 metric tons); has 3 modules. "The passenger module has seating for 39 men. Seats can be folded down to provide up to 9 ad-hoc medical beds (4 seats per bed). The cargo module has cargo fittings for 19.9 tons of cargo or 15 tons of combat loaded cargo. In an emergency, the cargo module can carry about 30 passengers, but it will be a very uncomfortable ride and most will suffer bruises and scrapes. The medical module has medical fittings for 12 casualties. A gunship module with 2 VRF gauss guns and a huge ammunition load has been tested but is not in service."
The D-20 Sparrowhawk DST (dropship, tank), aka the LBB2 95 ton shuttle, carries 71 dtons (285 metric tons) of cargo. Has a passenger module, a cargo module and a mixed passenger/cargo module. Typical armament is a VRF Gauss Gun and 2 plug-in weapons modules. Weapon options are missile launchers, 10cm mass drivers, or disposable MRL pods (400 6cm rockets each).
Passenger module carries 142 men in couches. Seats can be converted into beds at the rate of 4 to 1 for ad-hoc medical transport. Cargo module carries 70 dtons (285 metric tons) of cargo; there's sufficient room for a platoon of 4 main battle tanks. Mixed module has 60 seats for passengers and 40 dtons (160 metric tons) of cargo space.
The D-20 is popular with the Army; it has 3.5 times the capacity of the D-7 at twice the cost. And the crew size is the same, so it's a more efficient transport. The Marines prefer the D-7 because it provides redundancy and can be carried by smaller starships.
