Traveller My 3I Imperial Military

Imperial Navy

also see Fleet Composition


Core Assumptions

(1) Alpha systems (class A star port & high pop & high tech) have the capacity to create large fleets of large ships.

(2) Alpha clusters, an alpha system and its hinterland generally represents 95%+ of the wealth of a sub-sector.

(3) Alpha clusters are the core of the 3I and they and the routes between them are what the fleets protect.

(4) Refueling large fleets of large ships is a major pain.



Under Traveller rules large fleets of large ships require huge amounts of fuel.

For example a squadron of 500k dton battleships would use 800k dton of fuel for a single J2 jump and at 500Cr per dton of refined fuel that would be 400 million Cr.

Obviously the navy would have its own refueling depots but it gives an indication of the scale of infrastructure needed to support the huge tonnage of the main battle fleets.

The refueling issue keeps the main battle fleets in their home system in peace time leaving peace time patrolling to smaller ships

  • silk road trade routes to cruisers
  • hinterland cluster patrols to the much smaller colonial cruisers
  • backwater systems to the Scouts and the occasional colonial corvette.

Fuel sources like gas giants are often guarded by an orbital station or SDB.

Systems acting as refueling stops along the long distance “Silk Road” trade routes between alpha clusters have naval orbital stations providing fuel for naval patrols and fighter escorts for merchants.


Sector Fleet

The IN is organized by sector.

There is a sector fleet admiral for day to day operations but imtu the sector Duke is the CinC and technically can take direct command although this is rare.

The sector fleet is divided into colonial fleets and reserve fleets.

Each sub-sector collects a navy tax of which half goes to the sub-sector Duke for the colonial fleet and half goes to the sector Duke for the sector fleet.

Defense is based on the hedgehog principle where each sub-sector’s colonial fleet acts as a strong point defending its own sub-sector (although in reality colonial fleets tend to focus on the sub-sector’s alpha cluster).

The reserve fleets provide the mobile component that maneuvers around the strong points.


Colonial Fleets

There is one named colonial fleet for each sub-sector.

The colonial fleet has a commanding admiral but imtu the sub-sector Duke is CinC and can take direct command although this is rare.

Technically colonial fleets can be ordered out of their sub-sector by sector fleet but rarely are.

The colonial fleet includes fixed defenses like orbital stations and planetary bases, SDBs and trade lane patrols as well as the mobile colonial fleet.

The size and composition of colonial fleets varies greatly.

There is generally at least one alpha system in each sub-sector and these systems will build the colonial fleet unless the TL is low in which case some of the ships will be bought from shipyards elsewhere.

Weak sub-sectors will have a minimal colonial fleet and need direct backup from the 3I.

The colonial fleets are mostly recruited from the sub-sector.


Reserve Fleets

This is the name given to the official IN. The name comes from the colonial fleets providing the forward line of defense with the IN fleets held in reserve ready to be massed against any intruder.

The reserve fleets are numbered.

IN reserve fleets tend to follow set organisational patterns.

Some of the IN fleet types are: battle, assault, interdiction, strike, penetration, depot.

Reserve fleets come under the direct command of the sector fleet not the command of whichever sub-sector they are based in – although they can be assigned to a sub-sector command by sector fleet.

The reserve fleets are largely constructed in and recruited from the core.


System Defenses

Important systems can have substantial non jump capable defenses also – orbital stations, ground installation and SDBs.

Gas giants and other refueling sources are often very well guarded in this way.

Alpha clusters provide these for their own clusters.

The IN maintains the equivalent of a roman road type system with static defenses around the refueling points along the long distance trade routes between clusters.



Imperial Army

Generally IMTU the Imperial arms are the Navy and Marines with the army being a planetary thing.

The doctrine is generally marines attack, army defends but there is a special case when the Imperium are on the offensive where arny units come in directly after ground has been taken by the marines. This is where the Imperial army comes in. The sub-sector capitals maintain a planetary force but also Imperial Army units who are designated as priary backup to the marines. These units are also used in police action and rebellion situations where the marines would be overkill.

The IA is maintained at TL12 for consistency.

The core unit of the IA are regiments and sub-sector capitals will generally maintain divisions of these troops containing four regiments.


IA Regiment

Squad: Grav + 2 x squads of 4

Platoon: 4 x Squads, 1 x Platoon HQ

Company: 4 x Platoons, 1 x Company HQ

Battalion: 4 x Companies, 1 x Battalion HQ

Regiment: 4 x Battalions, 1 x Regiment HQ, 1 x Training Battalion

c. 6,250+ men and 400 gravs

nb the training battalion remains at base.


A key element of the IA is maintaining an Electronic Intelligence Battle Grid (EIG). Each squad has intelligence gathering sensors and each tier of HQ has an element dedicated to collating the electronic intelligence from each combat element and also gathering more of their own via radar, ladar, drones etc.

nb C&C = Command & Comms, S&R = supply and repair (includes vehicle salvage), IF = Indirect Fire


Platoon HQ:

1 x Command Squad (C&C / EIG)

Company HQ:

1 x Command Platoon (1 x C&C Squad, 1 x EIG Squad, 1 x S&R squad, 1 x Medical Team)

1 x IF section (2 weapons)

Battalion HQ:

1 x Command Company (1 x C&C Platoon, 1 x  EIG Platoon, 1 x S&R Platoon, 1 x Medical Squad)

1 x IF Battery (4 weapons)

Regiment HQ:

1 x Command Battalion (1 x C&C Company, 1 x EIG Company, 1 x S&R Company, 1 x Medical Platoon

1 x IF Battalion (16 weapons)

1 x Recon Company

1 x Engineer Company

(Recon = grav belt and stealth suit infiltrator unit, Engineers = battle dress bunker assault unit)


Squad Equipment

The gravs

– at TL 10 hover tanks & IFVs

– at TL 11 flying tanks & IFVs

– at TL 12+ more like gunships

A quote from the Traveller forum seems appropriate:

Standard Kit is combat armor, plus a Gauss Rifle and under body RAM GL, one fire team has a PGMP-12 replacement, the other has a Gauss SAW. Two members of each fire team carries a Plasma LAW, the final two carry extra rounds for the Gauss SAW.

Fleet Composition

Having a clear concept of this helps with creating the scenery the players move through.

The naval concept in the OTU is heavily influenced by modern naval carrier task forces and the fighter heavy naval combat of Star Wars and Battlestar Galactica imo.

I prefer the naval concept IMTU to be more like WWI battleships or Napoleonic era ships of the line blasting away at long range.

I want to use as many of the original CT ships as possible.


Principle of Naval Combat

Basically longest range wins so from Napoleonic up to WWI-ish the quest is bigger guns with longer range which then need bigger ships to carry them.

The limit to the range of direct fire weapons is the horizon.

When that limit was breached by planes battleships were replaced by carriers (and maybe at some point carriers will be replaced with drone / missile battleships).

How does this apply in space?


Core Assumptions

In space there is no horizon so if

  • there is no stealth in space
  • sensors can target ships at extreme ranges
  • energy weapons are accurate enough to be able to hit even small targets at extreme ranges
  • energy weapon penetrating power is controlled by the inverse-square law

then if all those assumptions are true – and I don’t know if they are but various people on forums seem to think they are – it seems to me the pendulum might swing back to battleships again because to get minimum penetrating power at the longest range means the biggest weapon possible – big in the sense of the maximum power at source.

Using abstract numbers

  • a weapon with a penetration of 4 at range 1 will have a pen of 1 at range 2
  • a weapon with a penetration of 16 at range 1 will have pen of 4 at range 2 and a pen of 1 at range 4
  • a weapon with a penetration of 64 at range 1 will have pen of 16 at range 2 and a pen of 4 at range 4 and a pen of 1 at range 8

So back to big weapons needing a big power supply carried by a big ship.


At the limit where weapon tech > ship tech there should be single weapons which can take all of the biggest ship’s maximum power output. Even in that case though if there’s physical space for them a battleship could have redundant weapons. For example (again using abstract numbers) a 64 ship with 64 max energy output could have a single 64 spinal and 4x16s and 16x4s and switch power as necessary.


Again, I don’t know enough to know if the assumptions hold but some people seem to and I prefer the idea of monster battleships to monster carriers so that’s that.


Ship Types


Battle Fleet
  • capital ships
  • fleet escorts
  • scouts
  • auxiliaries


Capital Ships (Battleships and Dreadnoughts)

These operate together to form the core of the battle fleet.

Assuming big energy weapons as the core there are two models.

If weapon tech > ship tech such that weapons can be built which use all the ship’s power then capital ships might become dedicated to a single large, long range weapon – I’ll call this the Dreadnought model.

If ship tech > weapon tech such that ship size / power output is always higher than the biggest available weapons then you have a more traditional battleship type with multiple large guns – I’ll call this the Battleship model.


In either case if physical space is available the ship could also have redundant smaller weapons and allocate energy according to need.

For example a 64 ship could have 1 x 64 weapon or 4 x 16s and in both cases firing those weapons would take all the ship’s energy. The ship could also have 16 x 4s in reserve for close range but they can only be fired if energy is taken away from the big guns.


If ship size tech and weapon size tech advance separately then which model is dominant may cycle over time e.g. every time max ship size increases it might take a while for weapon size to catch up so

  • mature size and weapon tech -> dreadnought with single spinal weapon using all power e.g. a 64 with 1×64
  • new ship size and old weapon tech -> battleship with multiple weapons e.g. a 64 with 4x16s
Fleet Escorts (Destroyers)

Fleet escorts are ships designed to protect capital ships from specific threats their long ranged weapons can’t deal with or can be overwhelmed by, like

  • submarine – stealth close range attack
  • torpedo boats – swarmed
  • planes – swarmed
  • missiles – swarmed

If there is no stealth in space (assumption) and if sensors can see and hit small targets at extreme ranges (assumption) then in theory even a few long range weapons might be able to take out a swarm especially if a capital ship has secondary short range weapons that can take over when the swarm gets closer.

For example a 64 with

  • 1 x 64
  • 4 x 16s
  • 16 x 4s
  • 64 x 1s

getting swarmed from range 8 then even if the targets can cover one range band per turn (not necessarily likely given these range bands are measured in thousands of Km) that’s

  • 1 shot each at ranges 5-8, total 4
  • 4 shots each at range 3 and 4, total 8
  • 16 shots at range 2
  • 64 shots at range 1

for a total of 92 shots – so you’d need a pretty big swarm.

However it’s possible so let’s say the second ship type would be fleet escorts called destroyers carrying lots of short range weapons who move with the capital ships as insurance against swarming.

They can range greatly in size, their main distinction being their focus on multiple short to medium range weapons to deal with swarms.

For example

  • size 16 with 4x4s and 16x1s
  • size 8 with 2x4s and 8x1s


Fleet Scouts (Escort Carriers or Shipborne Fighters)

The capital ships main advantage is range so to avoid ambushes – enemy fleets hidden behind planets, moons or inside gas giants etc – a battle fleet needs scouts.

These scouts travel with the fleets and provide recon within the current system as opposed to ships who recon systems away from the fleet.

The main distinction of a fleet scout is to be expendable.

Scouts could be small ships but fighters seem like they would be more cost effective.

They could be carried in an Escort Carrier or alternatively each battleship could have a squadron of scout fighters.



The ships of the line offload some of their cargo, medical, repair capabilities onto auxiliaries to free up space for more weapons and armor.

In particular imtu fuel purification takes a lot of space so fleets generally include specialist fleet tankers as auxiliaries. They are generally not tankers in the usual sense but carry fuel shuttles and fuel purification equipment to refuel a fleet as fast as possible.


Detached Duty Ships
  • cruisers: patrolling, recon, raiding, anti-piracy etc
  • merchant escorts



In modern parlance cruiser generally has a size connotation but originally it was functional – ships that performed cruising duties separate from the the fleet. I am using the functional description.

Cruising duties would include things like

  • couriers
  • anti-piracy
  • patrolling
  • courtesy visits
  • raiding
  • deep raiding

Cruisers can be vary greatly in size depending on task and often labeled by size

  • corvette
  • frigate
  • cruiser

The general pattern will be

  • smallest: corvette to frigate
  • medium: frigate to light cruiser
  • largest: cruiser to heavy cruiser to strike cruiser etc

There are a lot of different varieties for different tasks but the common thread is their primary trait is movement so when necessary cruiser design tends to sacrifice weapons or armor for speed and self-sufficiency.


Merchant Escorts

These might be Corvettes (cluster patrols) up to Frigates (silk road patrols) or sometimes obsolete Destroyers i.e. a fleet escort design from an earlier era that continued in production for merchant escort duties.


Main Battle Fleet Composition

The main battle fleets are generally restricted to the alpha clusters and the trade lines between them partly because defending the alpha clusters is their primary purpose and partly because refueling is such a major undertaking.

The main component of the battle fleets are battleship squadrons where each squadron is a unit comprising one battleship and its various support ships and auxiliaries.

Battleship Squadron
  • 1 x battleship
  • 1 x escort carrier
  • 3 x escorts
  • 1 x auxiliary (medical, workshop, supply)
  • 1 x tanker
  • 1 x frigate (odd jobs)

A battle fleet is then made up n battleship squadrons and a flagship squadron.

Flagship Squadron
  • 1 x largest battleship
  • 1 x escort or maybe a strike carrier
  • 3 x escorts
  • 3 x auxiliary (large medical, large workshop, large supply)
  • 1 x large tanker
  • 1 x frigate (odd jobs)

and one cruiser squadron (frigates) for recon in systems around the main fleet.


Cruiser Squadrons

Cruisers are organized in squadrons of eight by function but generally operate in smaller groups.

Raiders and deep raiders may operate in squadrons or half squadrons.

Sizes vary greatly – from smallest to largest

  • colonial cruisers – sub-sector patrol
  • frigates – silk road patrol or battle fleet recon
  • heavy cruiser – raider
  • battle cruiser – deep strike raider

A standard colonial patrol squadron might be

  • 2 x colonial cruisers
  • 6 x colonial frigates

“Colonial” in front of the name generally means one size lower than the fleet equivalent so

  • a fleet corvette of c. 1200-1600 dtons -> a colonial corvette of c. 400-600
  • a fleet light cruiser (frigate) of c. 3k to 5k dtons -> a colonial cruiser of c. 1200-1600 dtons


Relative Sizes

just for example assuming max size was 500k dtons

  • colonial corvette/frigate 400-600
  • colonial cruiser 1200-1600
  • fleet corvette 1200-1600
  • patrol cruiser (frigate) 3k-5k
  • strike cruisers (raider) ??
  • battle cruiser (deep raider) ??
  • fleet escorts (destroyers: ??
  • capital ships 500K


Player Encounters

So in my 3I the type of naval ships players might encounter would be:

(1) in alpha systems and occasionally other important space, some or all of a batron or multiple batrons of

  • battleship
  • carrier
  • fighters
  • escorts
  • tenders
  • corvette/frigate

which could be IN or colonial navy, plus any of the below.

(2) silk road truck stop systems

  • patrol frigates from alpha to alpha usually in pairs
  • fighters from orbital station
  • SDBs

(3) hinterland systems and truck stop systems in between

  • colonial cruisers from colonial navy – solo or in pairs
  • gazelles – local planetary navy

(3) backwater

  • colonial cruisers from colonial navy – solo or in pairs

(4) anywhere but generally only in wartime

  • patrol frigates – solo or in pairs
  • strike cruisers – squadrons or half squadrons
  • battle cruisers – squadrons or half squadrons



Trillion Credit Squadron uses a rough rule of thumb of 500Cr per person as the yearly navy tax so that’s 500 billion Cr per billion population (modified by TL etc).

TCS then says multiply that by ten to get total allowable fleet tonnage, say

  • 20% of it goes on static defenses and sundry other admin functions
  • 40% goes on colonial navy
  • 40% goes to the sector

Total populations are given in the Spinward Marches supplement.

So for example Glisten is listed as c. 4.5 billion so that’s c. 2,250 billion x 10 in total, or x 4 for just the colonial fleet so that’s 9000 billion Cr. This sum can be used to roughly figure out the size of Glisten’s colonial navy.

A Tigress is c. 340 billion, so say that is ballpark for a capital ship generally and with the other ships a standard Batron comes to c. 500 billion in total.

So we can guesstimate Glisten’s colonial fleet at 8 Batrons plus sundries.



So a Glisten sized Colonial Fleet might be:

  • main battle fleet of 8 x Batrons and 1 x cruiser squadron (frigates)(recon)
  • cruiser squadron (frigates) (silk road patrol)
  • colonial cruiser squadron (hinterland and backwater systems)
  • various SDBs and orbital stations in important systems

As a defensive force a colonial fleet generally won’t have raider or deep raider cruiser squadrons but might do in some situations.