Redefining Hull Characteristics for Traveller: The New Era
This article originally appeared in the December 2013 issue.
The assumptions made in the descriptive text do not tally with the figures shown on the Hull Size table in Fire, Fusion, and Steel p.11. The relationship between Rate (tons of displaced liquid hydrogen) and Volume (cubic metres, or kilolitres) is obviously correct, for it assumes a simple equation (Rate × the volume of 1 ton of liquid hydrogen). But beyond this, the table fails.
For example, taking a Rate of 1, where 1 ton of liquid hydrogen occupies 14m3, gives a total internal volume of, obviously, 14m3. All calculations for material use and so forth assume a shell 1cm thick.
Thus the length (diameter) of a spherical hull should be equal to the diameter required to produce 14m3 in volume, plus 2cm. The following formulae are required:
Vol is the volume of the sphere
r is the radius of the sphere
π is a constant equal to 3.142 (rounded)
Thus the length L of a sphere required to enclose 14m3 is 2.99m. Adding on the 1cm shell gives a grand total for the length of a spherical hull of 3.01m (rounded). Therein lies the first problem: the value of L listed for a hull rate of 1 is 3.2m, which would require a hull thickness of 10cm to achieve.
To calculate the material volume MV, the diameter of the hull (inclusive of the shell) is fed back into the spherical volume formula, and the enclosed volume is subtracted from the result. Using a diameter of 3.01, MV is thus calculated to be 0.28m3. If the initial calculation for the diameter of the enclosed hull is rounded from 2.99m to 3m (thus giving an overall value of L of 3.02m), then the value of MV would be 0.4m3 (rounding to one decimal place)—providing the value indicated on the table.
If the given value of L is used, MV comes to 3.16m—a far cry from that listed for MV.
The MegaTraveller design sequences assume that one ton of liquid hydrogen occupies not 14m3, but 13.5m3 (a vessel with a UCP hull rating of 100 has an internal volume of 1,350m3—see the Referee’s Manual). For those looking for better design compatibility between Traveller: The New Era and MegaTraveller (particularly useful when re-engineering MegaTraveller equipment under Traveller: The New Era rules), I suggest using this value of 13.5m3 in place of 14m3 when dealing with Fire, Fusion, and Steel volumes. For simplicity’s sake, multiply all volume calculations expressed in terms of liquid hydrogen (including the volume of hydrogen fuel etc.) by 0.96 before applying it to your design.
The Formulaic Approach
Two approaches to the design of hulls can be used: the formulaic approach, providing more accurate results, or the table approach (coupled with interpolation) for a more easily referenced angle.
For the formulaic approach, start with the desired hull rate, then apply the following formulae:
Vol = 14×Rate (or Vol = 13.5×Rate for MegaTraveller compatibility)
VehicleMV = MV×1.7
The Table Approach
This approach follows the same manner as the published Fire, Fusion, and Steel rules, but replace the standard table with the ones below.
Two tables are provided. One uses 14m3 Traveller: The New Era compatibility, the second 13.5m3 MegaTraveller compatibility. VehicleMV is provided as an alternative to the Chassis Size table, Fire, Fusion, and Steel p.17.
In both preceding tables:
Rate is the displacement of the enclosed volume of the hull, in tons of liquid hydrogen
Vol is the volume of enclosed hull, useable for installation of equipment, in m3
MV is the material volume of the hull’s shell, assuming a shell thickness of 1cm, in m3
L is the length of the hull (diameter for spheres), in m
VehicleMV is the material volume of a vehicle’s chassis, assuming a material volume multiplier MVM of 1.7
Material Volume and Hull Volume Usage
Because a hull thickness of 1cm has already been taken into consideration, through the formulae and the values on the tables, do not subtract total hull volume from the useable volume of the vessel if the hull has a thickness of 1cm.
Because additional hull plating does not affect a vessel’s length, it is assumed to be added inside the useable volume of the vessel. Calculate the volume occupied by additional plating by working out the total volume of the hull as per the standard Fire, Fusion, and Steel rules, then subtract from this the volume of material required to plate the hull in a 1cm shell (basically, MV × hull configuration MVM).