Revisiting one of my favourite RPGs of all time I stumbled over the Mechanical Design or Engineering rules provided in the books again. They are complicated and pretty hard to understand (at least for me), so I tried to build a spreadsheet out of the formulas and options given, to double-check vehicle data sheets found in the books as well as create new vehicles correctly and without effort.

This attempt proved to be pretty difficult, though. For one, there are several versions of construction rules in existence, as they were changed with the releases of new books. Also the data sheets of vehicles in different rulebooks seem to vary drastically (even for the very same vehicle), besides typos, it seems that the original designers also had to struggle with constant changes of the engineering rules. In addition, the rules are quite complex and inconsistent. Sometimes a required stat is suddenly mentioned under a different name a few steps down in the process and you don’t know for sure if you are still using the correct one. In the end I never get the results that are shown in the data sheets I tried to check and I can’t truely say whether I made a mistake or maybe the authors got it wrong. Yet, the System Designer, as I named it, is ready and available online here.

I will try to make sense out of the easiest part of the construction rules: the Offensive & Defensive system design. This is used to generate weapons or defensive systems like shields for vehicles.

On rules versions

The first set of rules was released in 1997 in the book Companion to Jovian Chronicles – Advanced Rules & Background (DP9-302). The Chapter is called Engineering and starts on page 98. The rules for Offensive and Defensive System Design (ODSD) run from page 124 to 135.

These rules were reworked and re-released in 2003 in the book Silhouette Core Version 003 (DP9-900) and can be found in Chapter Four: Mechanical Design starting on page 62. The ODSD rules run from page 78 to 81 and have been cut down a bit as you will see.

I will call these versions ODSD-1 (the original Companion rules) and ODSD-2 (the redesigned rules from Silhouette Core) from now on. I guess no one is still using ODSD1 from 1997, probably people have turned to OGL rules provided in Silhouette Core rules, too. I will start with the ODSD1 rules, though, as I am more familiar with them and will probably do the same with ODSD2.

Please note: I changed some terms for clarification. In the rulebooks the terms Base Weapon Rating, Weapon Rating, Base Cost, and others were used alternately and confused me a lot. I hope I used my terms in a more precise way,

The project: Re-building a Kill Kinetic Cannon applying the rules from ODSD-1

Data source: Datasheet Alexander-Class Destroyer from Ships of the Fleet: Volume 1 -> Section KKC Turret

On page 38 of the mentioned book we find the game stats for a KKC Turret section. At the end of the section table we find the Offensive and Defensive Systems that are installed in that section. I will use these stats to double-check the results from my System Designer:

Qty	Name	Arc	DM	BR	ACC	ROF	Ammo	Special	MS	WC	AC
3	KK Cannon	L/R	x30	7	-2	3	300ea	AP	12	3800	12

STEP 1: The Basics

Damage Multiplier (DM)
First of all we define the desired damage multiplier (DM) – in this case this is x30 giving us a rather heavy naval cannon with a solid impact.
The costs formula is DM² which gives us 900 for this position.
Costs: 900

Base Range (BR)
The base range is set to give us a decent firing range. The KKC get’s 7 hex, that’s a cost rating of 343 (formula: BR³) for range. A hex represents a specific distance in km.
Costs: 343

Accuracy Rating (AR)
The accuracy rating has been set to -2 as the weapon is not very accurate. Checking the accuracy modifier table we get the modifier 0.5 – which we will keep for later and apply to the final weapon costs.
Multiplier: 0.5

Rate of Fire (ROF)
Set a rate of fire – this KKC gets 3 and is therefore capable of rapid fire. This produces a costs rating for this feature of 272.25 based on the formula (0.5 x (DM+ROF))².
Costs: 272.25

STEP 3: Choose characteristics

Choose characteristics (perks/flaws) now. We only select Armor Piercing (AP). This is a general multiplier, which means costs for weapon and ammo will be multiplied by this multiplier (later). Perks or flaws can change weapon cost or ammo cost or both.
Multiplier: 5

Base Cost (BC)
We can now sum up the costs so far and continue by applying our multipliers. But, as several of the next stats require a different sum, we will calculate two different Base Weapon Costs here to make life easier later on:

Base Cost 1 (BC1): DM Costs + BR Costs + ROF Costs x AR Multiplier = 758
Base Cost 2 (BC2): DM Costs + BR Costs = 1243

Weapon Cost (WC)
We now multiply BC1 by the Characteristics Multiplier (x5) giving us 3790, which was obviously rounded to 3800 in the books. Please note, that this is still an abstract cost rating, not a buying price in the in game currency!

Ammo Cost (AC)
For ammo costs ROF costs are not considered and the AR Multiplier will be applied differently from Weapon Cost. The formula is (AR Modifier x (DM Costs + BR Costs))/250. We can use BC2 here and make this (0,5 x 1243)/250 = 2.846 or 2.5 The AP characteristic (x5) is now applied to this Ammo Cost value resulting in Ammo Cost of 12/shot (rounding down from 12.43).

The amount of ammo is taken from a table in the Companion book that compares the section size to Weapon Minimum Size (we will calculate Minimum Size in the next step).

Weapon Minimum Size (MS) tells us how large a vehicle should be to be able to mount our weapon. Weapons without the HEAT characteristic use the formula Cube root (Total Weapon Costs) to get MS. In this case this would be 758^0.3333333 which gives us 9 (rounded down). So the KKC cannon can be mounted on any vehicle or vehicle section size 9 or larger. Size 9 corresponds to about 24021 kg and a volume of 172 m³.

Ammo Mass (per Shot) is Weapon Mass (kg)/1000, which is 24 kg per shot (quite a large slug producing way more kinetic energy than required).

Conclusion

Our calculations result in the following game stats for that KKC weapon.

Name	Arc	DM	BR	ACC	ROF	Ammo	Special	MS	WC	AC
KK Cannon	L/R	x30	7	-2	3	300ea	AP	9	3790	12

Please note, that the Minimum Size calculation result is different from what is given in the source book. Stats for the same KKC vary throughout the books, as different versions of the mechanical design rules were applied and some editing errors have crept in, too. Applying the original formulas will give you a MS of 9 not 12, though.

Several formulas are odd in my oppinion. Taking ROF out of the Ammo Cost calculation does not make a big difference. Also, Accuracy Multiplier and ROF Multiplier seem to be too low for major aspects of a ranged weapon system. Range Costs should be higher, also. Ammo Mass is way too high, unless you have rockets or missiles. Weapon Cost and Ammo Cost have to be multiplied by a specific multipier to get a value in a valid ingame currency (credits).

In the System Designer sheet I have experimented with adjusted modifiers and apply AP multiplier only to ammo cost, not weapon cost. I think that range and ROF costs are way too low in the original rules. You will see a second result at the end of the sheet that uses my “mod” values.

Let me know what you think and if you find any errors in my tinkering.