Sohl Cluster Starship Design Rules




This document outlines rules for designing starships for a science fiction role-playing game. It is heavily influenced by the Traveller High Guard ship design system, and features a roughly similar level of detail. It tends to be slightly more math-intensive, but with an appropriate spreadsheet, a ship can be designed, tweaked, and finalized within only a few minutes.

The starship operations and combat rules for the system have not yet been developed, though certain features of the design system imply details of the combat system.

Many of the traditional types of Traveller ships (Scout/Couriers, Free Traders, Close Escorts) can be closely approximated in this system, though some important details will differ. A good way to start using the Sohl Cluster design system is to convert an existing High Guard design.



Five primary metrics are tracked in the design system: tonnage (mass), cost, power, computing, and crew.

Tonnage, unlike in High Guard, refers only to mass, not volume; translating tons to deck plan squares is thus less straightforward. The overall tonnage of the ship is selected, and each component (including the hull framework) contributes toward that tonnage. Modular designs, dismountable fuel tanks, and external cargo should be handled by constructing the ship in its most massive possible configuration; only maneuver drive performance will change in other configurations, and is easily recalculated. Tonnage is measured in metric tons (or tonnes, if thatís your preference).

Cost for all components is specified per ton of component. Costs as given in the tables are on roughly the same order of magnitude as those in High Guard. Obviously the cost values can be easily altered to suit the campaign. Cost is measured in megacredits (MCr).

Power is the equivalent of High Guard energy points; itís what is produced by the power plant and consumed by other components. By convention, the power requirements of all of a shipís systems are totaled, then rounded up to the next whole number to determine the expected capacity of the shipís power plant. It is naturally legal to build a power plant which delivers a non-integral number of power points, or a power plant which does not provide full power to all systems simultaneously, but it is required that such deviations are well-documented. Power is measured in abstract power points. An assumption that a power point is equivalent to a High Guard energy point and thus equal to about 250 megawatts is not unreasonable.

Computing is a resource consumed by most ship systems (rather than only a few, as in High Guard) and provided by shipsí computers. As with power points, it is conventional to total the computing requirements of all of a shipís systems, then round up to the next whole number to determine the expected capacity of the shipís computer. Computing is measured in abstract computing points which imply a mixture of processing, storage, networking, and I/O capability; a single computing point is far more power than a typical home computer.

Crew is similarly a resource demanded by most ship systems; the small fractions of a crew member are added up and (naturally) rounded up to a whole number of people. Actual assignments of individual crew to required roles are left up to refereeís discretion; in practice, despite what the spreadsheet may imply, a single crew member generally doesnít spend his time on several different jobs. As an example, the standard scout ship crew requirements include 1.24 crew for the warp drive, 1.0 crew for the power plant, and 0.84 crew for the maneuver drive; the likelihood is that each station gets exactly one crew member. Additionally, crew positions can be automated, which requires a great deal of computing power, but reduces life support needs. Ordinarily, no more than half the crew may be eliminated by automation, rounding down, but for things like unmanned probes with very limited decision-making capability, or remote-controlled vessels, the entire crew complement may be eliminated.



The hull frame component masses a flat percentage of the final ship tonnage; this includes a pressurized interior, at least one airlock per 100 tons or fraction of hull, and the framework to attach all other components to, and attitude controls via gyroscope and/or reaction jet. Typically, almost all components are in the pressurized section of the ship, though this is not required. Incidentally, in these rules, unlike Traveller, there are no rules changes at the 100-ton mark, yet High Guard ships and small craft tend to convert fairly straightforwardly at their former tonnage, mainly by the method of reducing small craftís endurance. Hulls do not require power or crew, though stealthy and deceptive hulls require computing. 

By default, the hull is unstreamlined; it cannot enter an atmosphere or skim gas giants for fuel. Partial, full, or airframe streamlining may be purchased at additional cost and tonnage. Partial streamlining allows gas giant skimming and entry into a very thin atmosphere. Full streamlining allows gas giant skimming and entry into any atmosphere. Airframe streamlining not only allows safe entry into any atmosphere, but provides airfoil and control surfaces which effectively double the power of the maneuver drive in dense or standard atmosphere. In other words, a fully streamlined ship with a 1-g motor cannot land safely on a 1.8-g world with a standard atmosphere, but an airframe ship can.  

A stealthy hull configuration uses special materials and hull shapes to make a ship harder to detect. In addition, the shipís computer and passive sensors conspire to keep the ship oriented in such a way as to minimize its visibility to active sensors (i.e. turning the minimal cross section towards the emitter).  

A deceptive hull configuration makes the ship appear to be something it is not. It can make a warship look like a liner or vice versa, look like higher or lower technology than it actually is, or make a ship look like a planetoid. It can even make a ship built by one company, culture, or species look like it came from another. Deceptive configuration can be bought multiple times Ė a high-tech alien warship can look like a low-tech terran liner for the cost of 3 deceptive configurations. Apparent tonnage canít be altered more than about 15%, and actual performance isnít altered. This deception is fixed at design time, but for one extra "level of deception", the ship can morph between two configurations. 

Armored hulls include both internal and external structural improvements. The cost is per "armor pip", and the as yet undeveloped combat system will specify how armored hulls affect battle; the effect will probably be comparable to High Guard armor.


Maneuver Drives


Sohl Cluster offers three fundamentally different types of maneuver drive: reaction, gravitic, and reactionless. Combinations of the three may be purchased and can operate together, adding their effects. All three of them have a base cost plus a per-gee, per-ton cost. Maneuver drives are usually bought in whole gee numbers, but may be bought in tenths of gees. All three require power, computing, and crew. Only reaction motors consume fuel directly.

Reaction motors are cheap and consume large amounts of fuel. They also create a large, easily detected trail in space. They are hazardous; many worlds may have restrictions on in-atmosphere use of reaction drives. It is not uncommon for a large ship with a weak gravitic or reactionless drive to have a small "booster" reaction motor with a small fuel supply to assist in takeoffs and landings from high-gravity worlds; a half-day fuel tank is usually sufficient for one takeoff and one landing.

Gravitic drives are fairly efficient, but their gee potential is multiplied by the gee distortion of local space. Thus, a 1-G gravitic drive canít take off straight up without a streamlined hull, but can easily enter and leave orbit from space, and far away from any planets, will have almost no maneuvering ability. Piloting a gravitic ship is harder than using a reaction or reactionless drive; it is akin to sailing a wind-powered vessel as opposed to a motorboat. They do not emit any detectable exhaust. A "2-G" (or any other value) gravitic drive is a slight misnomer; itís a "times 2" drive, which provides 2G only when located in a 1G gravitational potential region.

Reactionless drives are expensive and complex, but they donít require a fuel supply, they arenít dependent on the local gravity well, and they donít emit harmful or detectable exhaust.

"Compact" reactionless and gravitic drives, but not reaction drives, can be purchased. These have 1/10 the usual base tonnage, but the usual per-gee/per-ton costs. They cannot be repaired except at a well-equipped base, or aboard a ship with a maintenance hangar equal to 100 times the tonnage of the compact drive to be repaired. These are intended primarily for very small vehicles.

Power Plants


Sohl Cluster provides two types of power plants, which require fuel to generate power, and batteries. Fusion and antimatter reactors have a base cost, per-power-point cost, and fuel tankage cost. Batteries only have a per-point-per-day cost. A ship with both reactors and batteries can divert small amounts of power to recharging its batteries; a ship with batteries only must be recharged by a starport or another ship.

Fusion reactors are fairly cheap and consume moderate amounts of fuel. In operation, they produce a very small amount of emissions which are detectable by nearby ships.

Antimatter reactors are expensive and bulky, but are far more fuel-efficient. They emit more detectable radiation than fusion reactors.

Batteries are more efficient than reactors for short durations, and so are frequently found in small craft. They emit no detectable radiation, and as such, they are well-suited for installing in stealthy ships.

Fuel is bought per point per day. It is therefore very straightforward to trade off tonnage for endurance when tuning a design. The Traveller convention is to allocate fuel for 30 days of power.

If antimatter power plants are abhorrent to the nature of the campaign, they can be called "highly advanced fusion plants".

Faster than Light Drives


Sohl Cluster assumes a "warp drive" style FTL drive rather than Travellerís jump drive; this can be altered according to the requirements of the campaign. The assumption is that "warp class one" can travel about one parsec every 20 days (~60 c), and higher warp classes show a linear progression in speed. Warp drives may be installed in ships under 100 tons; useful probes and courier ships can be built in the 30-50 ton range, or even smaller with advanced technology. Warp drives do not require fuel other than that required for the power plant, but they do require power, computing, and crew.

A ship in warp drive cannot be interacted with, but it emits a distinctive spectrum of electromagnetic radiation which can be easily detected. Of course, the ship is moving at least 60 times faster than that emission! A ship can start or stop warp drive on about 15 minutesí notice at any time, and thus can drop into real space at the edge of a system, use its sensors, then proceed deeper into the system on warp drive.

A warp drive requires at least 30 minutes of "preheating" before use, which requires its full energy point budget. This should discourage building ships without enough power capacity to simultaneously use warp and maneuver drives; such ships are unable to dodge enemy fire while preparing to enter hyperspace.

Warp speeds beyond warp class 5 require double the per-class expense. Thus, buying warp class 7 on the spreadsheet gains warp class 6 performance, 9 yields 7, and 11 yields 8.


Bridge, Controls and Sensors


All crewed ships, including small craft, require a bridge. A bridge has a base and a per-ton-of-ship cost. Unmanned vessels with completely automatic programs (probes, etc.) can omit the bridge. Any bridge can function as a flag bridge (i.e. allow the ship to operate as a combat group leader), but the number of other ships in the group cannot exceed twice the base-10 exponent of the shipís tonnage (i.e. a 100- to 900-ton ship can lead up to 4 other ships; a 10000-ton ship can lead up to 8 other ships). Larger group control can be achieved by buying the bridge multiple times; each additional bridge adds the base group size again; a 10000-ton ship with a triple-size bridge can lead 24 other ships. The additional space largely consists of tactical displays showing the situation with respect to other ships in the group, and communication stations to better coordinate with the captains of the other ships. Multiple purchases of bridge must be designated as "flag bridge" which adds to the leader capability, or as "emergency bridge" to back up the main bridge. Emergency bridge may also include flag capability. In other words, if you buy 5 times the basic bridge requirement, you must specify if itís 5 separate standard bridges, a single bridge with 5x lead capability, or a 3x bridge with a 2x emergency bridge.

Vehicles with no advanced navigational requirements can opt for purchasing Basic Controls instead of the bridge base cost. Basic controls cost 1/10th of the base tonnage of a starship bridge, but have the same per-ton-of-ship cost. Vehicles with basic controls cannot use sensors for anything except basic detection of other objects; they cannot have any fire control stations or weaponry installed; they have no group leadership ability. Thus, basic controls are only suitable for atmospheric vehicles such as aircars or Gcarriers, or possibly spacegoing lifeboats.

Every weapon battery requires a fire control station, with a flat per-station cost. Weapon batteries must consist of a whole number of identical weapons. Weapon batteries are defined at ship construction time.

Active and passive sensors have per-class costs. Higher sensor classes are more sensitive and more likely to detect objects in space. The exact effect of sensors on combat must await the development of combat rules.

Active sensors work by emitting electromagnetic radiation and detecting the reflected radiation. Any ship using active sensors can be detected by any ship with passive sensors within a very long range. A typical warship will have a class 5 or better active sensors; civilian and auxiliary ships will usually have class 4 and under. Active sensors greatly improve target tracking in combat, but expose a shipís position when in use.

Passive sensors detect radiation emitted and reflected from other sources. A typical warship will have a class 5 or better passive sensors; civilian and auxiliary ships will usually have class 4 and under. Stealthy ships, and ships designed to hunt them, tend to have very good passive sensors. Passive sensors can be used for target tracking, but are less accurate than active sensors.


Computers and Automation


Many components require computing power, which is provided by installation of a computer. Note that computers require power and crew in turn. Itís worth noting that power plants have no per-point computing requirement, in order to avoid a feedback loop between computer and power plant allocation which makes Excel complain.

Crew may be replaced with automation. The cost is per replaced crew member. For full function of a ship, no more than half the crew, rounded down, may be replaced. This doesnít become tonnage-efficient until very long missions are involved. If human judgement isnít required for a mission (i.e. if a single written page suffices to describe its behavior), the entire crew can be replaced; this provides for unmanned courier drones and the like. Adding automation increases computing requirements, which can increase crew size, which can allow more crew positions to be automated, so a couple of iterations through the spreadsheet may be necessary to optimally automate a crew.

Optionally, if Travellerís "Solo Scout" concept is important to the campaign, automation between 50% and 100% of the crew can be allowed, but there will be indirect effects on the efficient running of the ship. It may take longer to perform certain tasks; first-class passengers wonít be impressed with the cabin service; the need to dodge enemy fire might make it impossible to concentrate on plotting a safe hyperspacial trajectory, etc.


Cargo and Vehicles


Cargo space is designated for free; vehicle facilities are designated at 1.1 tons per ton of vehicle regardless of whether the vehicle is carried internally or externally. Vehicle crews should normally be designated as extra crew in the carrying shipís complement.


Crew and Passenger Accommodation


Crew and passenger accommodation has a base cost per person and a cost per person per "comfortable day". Time spent aboard ship beyond the comfort period requires a morale check every time half the original comfort period elapses. For example, a ship with 30-day accommodations will require morale checks at 45 and 60 days. Morale failures may result in penalties to job performance or interpersonal confrontations. Repeated morale failures can result in psychotic episodes ("space rage" or "cabin fever"). After twice the original comfort period, the ship will be on short rations. After three times the original comfort period, all food will be gone (exception: ships with 120 day or more comfort periods are assumed to have recycling systems; no one will starve, but the food will be unappealing).

 Different classes of passenger and crew have different accommodation needs. "High Passage" or First Class passengers require 1.5 comfortable days allocated for each day spent aboard ship; if necessary, they will "rough it" at 1 day allocated per day spent, but will demand a 50% discount. "Middle Passage" or Second Class passengers require 1 comfortable day allocated for each day spent aboard ship; they will "rough it" to 0.75 days allocated at a 50% discount. Starship crew officers require 1 comfortable day allocated; they will rough it to 0.5 days allocated for an additional 15% pay. Non-commissioned starship crew require 0.75 comfortable day allocated per day, and will also go to 0.5 days for 15% extra pay. Beyond those time periods, passengers will not pay at all, and crew will negotiate for as much extra pay as they can get (if they donít fail morale checks) or refuse to work (if they do fail).

 A 5-day crew accommodation may be designated as a "Low Berth" or suspended animation chamber; this will keep one human alive, well, and unconscious for any length of time. This is a very cheap but unpleasant and sometimes risky way to travel.



Weapon specification is pretty abstract, and can be tuned to the needs of the campaign. Since the combat rules are undeveloped, the following items outline the basic facts:




Shields may be purchased. The maximum level of shielding is 20. Shields defend equally against all types of weapons; they are equivalent to a combination of High Guard nuclear dampers and meson screens and then some: they protect against lasers and conventional missiles.


Technology Levels (Optional)


Since technology levels tend to be defined very differently across SFRPG systems, this design system keeps technology levels very simple. Any component can be built with "primitive", "standard", or "advanced" technology, except for the following:


With "primitive" technology, the tonnage of the item is multiplied by 1.5 and the cost per ton is multiplied by 0.5 (yielding an overall cost factor of 0.75).

With "standard" technology, the tonnage and cost of the item is unchanged.

With "advanced" technology, the tonnage is multiplied by 0.75 and the cost per ton is multiplied by 4.0 (yielding an overall cost factor of 3.0).


Component technologies may be mixed and matched, even to the point of putting primitive per-gee-per-ton reaction drives in ships with advanced reaction drive bases.

Converting Traveller TL to Sohl Cluster component availability is left as an exercise for the GM.

Deck Plans


The Traveller convention of "two 1.5m deck squares equals one ton" no longer applies; a ton is mass and not volume in Sohl Cluster. Machinery is far denser than liquid hydrogen, and so probably rates one square or less per ton, plus access space. Crew accommodations probably should still be constructed at two squares per ton. Fudge as required Ė thatís what you do anyway, right?


Example Ship: The Scout/Courier


As an example, consider the translation of the venerable "Type S Scout/Courier" from Traveller terms into Sohl Cluster. Download the spreadsheet to see how I chose to do it.

Author: Russell Bornschlegel, Last revision: 1999/10/12.

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