I was reading through the The Journal of the Travellers' Aid Society looking for ideas. I rarely come away from such reading empty handed and did not this time. In particular I was reading Special Supplement 3: Missiles in Traveller (JTAS #21.)
Without recounting every detail of the missile design system three features stand out. First, the missiles weigh in at 50 kilograms and use reaction drives. An average missile warhead weighs 10 to 20 kilos. Thirty odd kilos of fuel allows these beasts to thrust at about 3 gees for 20 to 30 minutes.
This tells us a lot about rocketry in Traveller. First it's a bit better than present TL 7-8 but not as good as maneuver drives. We can build a rocket that thrusts around four gees for 15 minutes or so. We need to to make it to orbit.
Second, maneuver drives can't be built for vehicles less than a few tons (at least not until TL 13 introduces the grav belt.) It's odd that missiles don't experience a huge leap in effectiveness by TL 13. Grav belts are expensive and not used on cheap munitions normally.
Book 2 says that vacc suits can make a three gee burn for a standard turn. The fuel and engine for the thruster would weigh 20 kilograms or more. Not something you want to carry around in normal gravity. Don't give me any of that guff about it being part of the worn clothing allowance either.
Most importantly, the third point is that missile warheads are about 10 kg. (maybe 20 kilograms.) A field Howitzer in Mercenary fires shells weighing 15 kilograms and it isn't awe inspiring. If the missile warheads lose force in space without an atmosphere to conduct the blast spacecraft must have relatively thin hulls. Classic Traveller doesn't say how tough ships are compared to vehicles or mercenary style weapons.
I don't think infantry with ACRs can down a Free Trader. But we tend to think starships are flying tanks immune to most air defense and ground force weapons. The 1981 revision of Traveller said a 'ton' of starship displacement was 13.5 cubic meters. That volume of liquid hydrogen weighs about 1 ton (or 71 kilos per cubic meter.) Interesting that they picked that volume. It seems the original intention was to have a 100 ton Scout /mass/ 100 tons. If not then it's a hell of a coincidence. What a strange number to pick otherwise.
Atomic Rockets has the following densities:
modern commercial a jet- 250 kg/cubic meter
fighter jets 350 kg/cubic meter
wet navy vessels 500 to 600 kg.cubic meter.
Most SF roleplaying games assume a density of 100 to 200 kg/cubic meter. That doesn't sound like a flying tank. Even with improvements in material technology 71 kg. per cubic meter is pretty light.
Ships already defy gravity, side step relativity and operate in all environments. Having them shrug off artillery is asking a lot. A starship might be able to hold off a grav tank if it had a smart gun crew and a good pilot. I doubt a Scout ship crew, for example, would enjoy the experience or feel smug about the outcome. A RAM HEAP grenade or tac missile could likely damage a ship.
Classic Traveller treated starships more like big aircraft than battleships. When GDW put out High Guard the ships became tremendous and seemed more like wet navy warships. Wet navy battleships are very well armored. But they do not have to make orbit.
Striker also came out in '81 and stated starships had armor like a tank. That would mean they weigh in at a lot more than 1 ton/cubic meter. GDW forgot the old implied relationship of volume to weight.
If we have low mass ships firearms would also be better at putting holes in interior walls, equipment and conduits. Boarding actions become more dangerous for the defenders. Marine cutlasses stop looking silly in a boarding action. Combat becomes more interesting (thoughI doubt a revolver fired into the main reactor would blow it up.)
Maybe I'm over thinking this but take a look at Mercenary. There are mercs swarming over planets in the Imperium. Why don't more beleaguered rulers just hire an armed free trader to hover over the opposing forces and laser and missile them into nonexistence? Why even have an army at TL 9 or higher? I tend to believe that starships and their weapons are not immediate game changers -not the kind of starships a sane referee will let his group acquire.
Even if you have something like a maneuver drive, the mass of a ship will affect its acceleration which affects trip time. To a merchant time is money. To a navy time may mean victory. Economics and strategy alone will make designers build ships as lightly as possible. In space ships have natural armor, it's called distance. Even at 60,000 km a radar wave will take .2 seconds to reach a target and and .2 seconds to return to the sender. The laser the attacker fires also takes .2 seconds to reach the target. In .6 seconds a scout ship can accelerate by 12 meters or 2/3 of its length. That's a hard target to hit at 60 kiloklicks. It gets even worse if the pilot has enough brains to point the Scout at you or away. The beam of a Scout is only 12 meters.
If you use Story Vehicles or Vehicles for Stories? to run a fight between a starship and some vehicles I'd treat the ship as paramilitary (Battledress +4.) Lasers do 1D hits to vehicles and starships at such close ranges. Missiles, artillery and rockets do 2D hits (because they explode on contact.) Ship hulls ignore small arms doing less than 6 dice of damage. Weapons doing 7-8D of damage do one hit to a ship. Weapons doing 10D or more do two hits. Weapons doing 3D or more will puncture interior walls of a ship. Use a cutlass or body pistol. Really.