The base system assumes that each subsystem is 1/20th the total ship mass. However it is fairly easy addition to have modules of different sizes. Below are working rules for adding smaller or larger subsystems to ships.
Not all systems can be different sized. Note that the SM of subsystems does not indicate the SM of the system, instead it indicates the size of the craft where that system is “normal sized” and thus 1/20 of the ship mass.
Note: A similar, but more restricted system has since been published in Space ships 7.
You can fit 3 modules of one less SM, or 10 modules of 2 less SM into the space of one module. You could really mount even smaller systems at the same progression, but the design tends to get messy that way, so it is good normally to limit the systems to minimum of -2 SM from normal.
Anyone ignoring the above suggested limits: you can fit 30 modules of -3 SM, 100 modules of -4 SM, 300 modules of -5 SM, 1000 modules of -6 SM and so on.
It also makes sense that you could mount any suitable RM 6 ship module as habitat module as each such space is 5 tons too.
Each such subsystems has the characteristics of the system of one less SM, except what is noted below. Not all subsystems can be installed in smaller than normal size.
If the system is a high energy system a -1SM system uses 1/3 power point and a -2SM system 1/10 power point. Cosmic power systems use same 1/3 and 1/10 cosmic power point.
Adding smaller systems to small ships: note the minimum sizes for many systems, for SM 4 systems see space ships 4
You can mount one system of one larger SM by using 3 module slots. And a system of two larger SM by using 10 module slots. All such slots must normally be adjacent to each other, either in the same hull section or two bordering hull sections.
Many modules like armor and habitat do not need to be larger systems, instead you should just add several such systems. The main benefits come from systems that get extra capability from being larger, like sensors.
If the system is a high energy system a +1SM system uses 3 power points and a +2SM system 10 power points. Cosmic power systems use same 3 and 10 cosmic power points.
For example the front section of a ship with both large and small modules could look like this:
When hitting a ship with multiple module sizes you have two options:
on hitting, roll module hit normally (ss:61) and just have all submodules disabled/destroyed as normal. For large things disabling/destroying one part disables/destroys all three
roll hit module normally, but then look if it has submodules. If it does, determine submodule hit and compare the damage done to the submodule against 10% and 50% of a ship that the module is the right size for, not the ship it is installed in. For any part of a large module hit, compare the same way.
Damage example: lets assume that the ship above is hit in front and that it has 100 dHP for being SM 9. If the module hit roll is 1 or 6, use the rules on page 61 normally. If 2-4 is hit, compare the penetrating damage against 10% and 50% of 150 HP(SM 10 ship HP) for damaging module purposes. on damage/destroy, all three locations are damaged/destroyed. On hit module 5, reroll a 1d, on 1-2 hit submodule 1 and so on. Next compare the penetrating damage against 10% and 50% of 70 HP(SM 8 ship HP) for damaging module purposes. on damage/destroy, that submodule is damaged/destroyed.
(ss:11-13) A smaller armor system does not use any information from the lower SM except cost. Larger Armor systems are not used, instead add several armor systems.
A -1 SM armor system protects 1/3 the full armor system for right SM, round down.
A -2 SM armor system protects 1/10 the full armor system for right SM, round down.
for even less protection if you want smaller tham -2 SM systems:
A -3 SM armor system protects 1/30 the full armor system for right SM, round down.
A -4 SM armor system protects 1/100 the full armor system for right SM, round down.
and so on
Example: unstreamlined SM 10 ship would have following effect from smaller light alloy armor(ss:11).
|-1 SM(SM 9)||3||$500k|
|-2 SM(SM 8)||1||$150k|
(ss:13) Smaller cargo holds are fully possible, use the capacity of the smaller SM hold. All the options are also possible. Larger Cargo holds are not used, instead add several systems. But if you have 3 cargo systems adjacent to each other they can be counted as +1 SM for size of door, and 10 systems for +2 SM.
(ss:13) Not normally possible in smaller or larger sizes, unless all the cloaking devices in the setting are smaller or larger than 1/20 of ship mass.
(ss:14) Contragravity lifters may or may not be possible in smaller sizes. If they are a -1 SM lifter will negate up to 3G gravity and a -2SM lifter will negate up to 1G. Larger Contragravity lifters are not normally used, instead add several systems.
(ss:14) Cannot be installed in larger size. Though you can install seating and add +$50k each to turn it to a control station if you need more control stations.
You can install a one step smaller system if your ship is simple: Maximum of six modules total of: drives,power plants,factories, cloaking devices,ecm,force screens,contragrav lifters, weapon systems, mining refinery.
If your ship is very simple(maximally 2 such modules) you can use -2 SM control.
Note that at sm -1 you get -1 to control rolls and at sm -2 you get -2. These penalties also count if you use a smaller ship as tug or upper stage to control a larger load.
(ss:14) Defensive ECM cannot be installed in wrong size. Though a GM may rule that a -1 SM system will give -1 penalty and that such systems do not stack with other systems.
(ss:15) Engine rooms cannot be installed in wrong size.
(ss:15) You can normally mount a larger or smaller sensor. They will use all the statistics of the different size sensor. Though a +2 SM sensor will likely require a very weird hull structure. Note that a -2 SM sensor has the same array level that the control room sensor has, so does not give any better capability except in case of special sensors.
(ss:16) Different size factory modules are normally available. Use the statistics of the module of the size you want to use. Note the lower limit of SM 6 for factory module size.
Note: Use the SM 6 factory module instead of minifacs.
(ss:17) A fuel tank can be smaller and then will have 1/3 the delta-v of full size for -1SM and 1/10 delta-v for -2 SM tank. Note that the Delta-V increase table is only for full size tanks. Do not use larger than normal fuel tanks, instead add several fuel tank modules. Smaller fueltanks used for powerplants have less fuel 1/3 fuel for -1SM and 1/10 fuel for -2SM, cutting the use time unless being used for smaller powerplants.
(ss:18) Smaller Habitats holds are fully possible, use the capacity of the smaller SM habitat. All the options are also possible. Larger Habitats are not used, instead add several systems.
Note that SM +7 is the smallest full size habitat. SM +6 vehicles can use 1.5 modules for one stateroom, SM +5 can use 5 modules for one and Sm +4 can use 15 modules.
(ss:18) Spaceships allready containts rules for larger hangerbays. It is fully possible to have a smaller hangar, use all the statistics of the smaller one in such case.
(ss:19) A ship can mount smaller Jet engines than normal. For larger buy several modules. Small modules give 1/3G acceleration for -1SM module and use 1/3 fuel, and 1/10G acceleration and fuel for -2SM module.
(ss:19) Not normally possible in smaller or larger sizes, unless all the star drives in the setting are smaller or larger than 1/20 of ship mass or the jumpgate is external use and only sends other ships. In external jumpgates, use the capacity of the installed module.
(ss:19) Different size Mining and Refinery modules are normally available. Use the statistics of the module of the size you want to use.
(ss:19) This can normally be any size module.
(ss:20) Smaller passenger seating is fully possible, use the capacity of the smaller SM seating. Larger passenger seating are not used, instead add several systems.
(ss:20) Normally fully possible is smaller sizes(some technologies have a minimum size). For larger add several modules. -1SM modules give 1/3 power point each and use 1/3 the normal ammout of fuel. -2SM modules give 1/10 power point each and use 1/10 the normal ammout of fuel.
(ss:21) If you install a smaller module, it only provides fuel for corresponding size engine. For larger, just install several.
(ss:21-24) A ship can mount smaller Reaction/reactionless engines than normal. For larger buy several modules. Small modules give 1/3 acceleration for -1SM module and use 1/3 fuel for the types that use such and 1/10 acceleration and fuel for -2SM module.
(ss:24) These can equally likely be any made of size modules. For real complexity you can allow the submodules and their sizes to change…
(ss:24) Cannot be installed in wrong size.
(ss:25) Normally fully possible is smaller sizes. For larger add several modules. -1SM modules give 1/3 power point each. -2SM modules give 1/10 power point each. For more realistic effects of solar panels see fixes section.
(ss:25) A ship can mount smaller Space sails than normal. For larger buy several modules, but see fixes section on the maximum numbers. Small space sails give 1/3 thrust for -1SM module and 1/10 thrust for -2SM module
(ss:25) Not normally possible in smaller or larger sizes, unless all the star drives in the setting are smaller or larger than 1/20 of ship mass.
(ss:26) Not normally possible in smaller or larger sizes, unless all the stasis webs in the setting are smaller or larger than 1/20 of ship mass.
(ss:26) The only alternative upper stage that makes sense is a -2 SM upper stage that just uses the first two 2 module slots from the front section.
(ss:26-29) Weapons already are in smaller/larger SM sizes. The spinal battery is a SM+1 weapon, Major battery is a right SM weapon, Medium 3 SM -1 weapons, Secondary battery is 10 SM -2 weapons, Tertiary is 30 SM -2 weapons. So you can use individual weapons from above as such subsize modules.