My earlier post dealt with classifying ships as belly landers and tail landers. On reflection and discussion with my posse (well Ray McVay) I decided that there was a place for ships designed to not land on planets at all: the modular hull.
The advantage is cheaper construction. This only works if operating costs are cheap as well. The major cost for any ship is fuel. However we're talking civilian ships here and only the military, Scouts or crazy pilots use unrefined fuel. The benefit of a belly lander or tail lander is that on e a planet without orbital ports you can still land to refuel. A modular hull will be forced to either hunt down ice-steroids or use ship's boats to scoop fuel.
A modular hull is built on a very long central boom. The engines are placed at one end and the cabins and living areas placed at the other end. This results in a lot less radiation shielding. A small shadow shield screens the rest of the ship, made even less bulky by distance. The inverse square law is your friend this time. It also makes pirates very reluctant to board your ship via EVA.
Various parts are bolted onto the boom at different places. the stuff you don't need to to worry about being irradiated is closer to the fusion plant.
Modular hulls are not designed to ever land on a planet bigger than size 1-2 or with an atmosphere greater than 1. This is due to the large ungainly heat radiators that replace the teeny standard radiators you can take into an atmosphere. Their acceleration compensators only work in the direction they thrust. Lateral accelerations or landing a ship on a decent sized planet will cause the boom to warp. Finally the fusion generator is only shielded at one of its six faces so an atmosphere will cause radiation scatter you definitely don't want
Modular hulls are HALF the cost of a regular hull. In addition they will accept modules that come in 10, 20, 50 and 100 ton sizes. Each module requires a hardpoint equal to .1 times its tonnage for structural support. In addition a modular hull may only count its non-modular tonnage for turret hardpoints. In other words if your 500 ton modular merchant has 300 tons of modules it will mount at most two turrets. A hardpoint costs 1,000 credits per ton of module it can support.
They are also fragile. On any hull hit roll a die. On a '6' a module (chosen randomly) is severed from the ship.
On the other hand they are really easy to unload. Just detach the cargo mod you came with and dock with the cargo mod you're picking up.
Docking and maneuvering modules takes a number of turns equal to their tonnage divided by 10. That's if you're taking it easy, moving in at centimeters per second and under no stress. At the referee's discretion docking can be a lot faster and hilarity and die rolls may ensue.
If you're going with modular designs you might want to put together some 'standard' modules. For example a 20 ton mod could be a high passage module: four staterooms and cargo space for the standard one ton of baggage per high passenger. If you are using the Book 2 drives making the drive a part of the main ship is an excellent sanity saving measure.
With their fragile structure, lower capacity for damage and limited refueling options you might think a modular ship would have little military use. You would also probably change your mind when you saw a 1000 ton hull swap its 500 tons of cargo modules for a hangar holding fifty ten tone starfighters. Modular ships make fine carriers or even battle tenders and may be switched from civilian to military roles in a matter of hours. This also lets your space navy help pay for itself to a degree. If your borders are secure put your ships to work hauling freight or for speculation. It also cuts down on attacks on your merchant fleet. Most pirates won't attack a ship that might unload a wave of fighters.
Modular ships can also make things easier for pirates. At the first sign of trouble disconnect your cargo modules and go on your way. The pirates go over the loot at their leisure. You leave without being boarded. Just make sure you have something good in the modules left behind (like a black market nuke).