[ Freelance Traveller Home Page | Search Freelance Traveller | Site Index ]

*Freelance Traveller

The Electronic Fan-Supported Traveller® Resource

Getting Around in Traveller

This article originally appeared in the September/October 2019 issue.

Engines and power plants are the life blood of every spacecraft. The most common systems used are M-Drives for in-system travel, fusion-based power plants to provide the vast energies needed to travel through space, and Jump engines, the magnificent engines which inflate a private pocket universe and make interstellar travel possible. In my preferred versions (Cepheus Engine and Mongoose first edition), however, there are several issues with engines and power plants. Some were corrected in second edition Mongoose Traveller (although introducing other problems that keep me from switching), and one that seems to be common to all Traveller versions. Here I offer a few minor changes to improve game balance and make several theoretically possible ships more practical.

Jump Engine Fuel

Jump engines provide the only means available to travel from one system to another in most standard forms of Traveller. Typically a ship needs 10% of its volume in refined hydrogen fuel per parsec Jumped. As technology progresses, longer and longer jumps become technically possible, but since the amount of hydrogen required stays constant there is a practical upper limit to the distance a ship can Jump. External drop tanks can extend that limit, and in well-established worlds that works reasonably well. For unexplored or undeveloped systems, however, drop tanks simply are not practical.

A second problem with the large fuel tanks required on high-jump ships is the reduced cargo capacity available aboard those ships. For Jumps using the full capabilities of the engines this is not a problem and those cargoes move fairly efficiently. For shorter jumps, though, the extra fuel storage is basically wasted space and the same tonnage ship carries less cargo and makes less profit per trip. The only way to carry cargo efficiently is to always travel at maximum Jump.

To help reduce this penalty, I allow larger, higher tech jump engines to be more efficient than smaller engines. In particular, ships capable of Jump-3 and Jump-4 need only 9% of their volume in refined hydrogen per parsec while those capable of Jump-5 and Jump-6 need only 8% of their volume per parsec jumped. This allows high-tech, high-Jump ships to have smaller fuel tanks than they would otherwise require. The ships become more efficient, increasing their profitability while maintaining the flexibility of their high jump capacity.

So how would this work out in practice? A standard 400-ton merchant freighter at Jump-3 would require 120 tons of fuel under the standard rules. The modified rules would only require 108 tons, a savings of 12 tons that could be dedicated to cargo or any other use desired. If the ship only needed to make a Jump-2 to reach its destination, only 72 tons of fuel would be needed.

For a 400-ton Jump-5 express freighter, a massive 200 tons of fuel would be required for a single jump, or 50% of the total tonnage of the ship. Using the modified rules, however, it would only need 58% or 40% of the ship’s volume. While the ship still requires 160 tons of fuel, those extra 40 tons of available space make a significant difference. Furthermore, when the Jump-5 vessel only has to make a Jump-2, it only uses 64 tons of fuel. Saving 24 tons of refined hydrogen would cost Cr12,000 less (at Cr500/ton) and allow more marginal trips to still be profitable.

While useful, this fuel efficiency change will never allow a high-jump ship to be as efficient as a lower-jump freighter for short jumps. It does, however, open up far more design possibilities for high-jump ships that simply aren’t available to smaller vessels under the standard rules. They can carry more cargo per trip, and refueling even on the shorter trips is less expensive because of the more efficient engines.

For example, trying to construct a 100 ton Jump-6 express courier using standard Cepheus Engine rules is an exercise in futility. 66 tons of fuel, 20 tons of Type C Jump engine, 10 tons Type C power plant, 10 ton bridge… oops, over size already. Granted, 12 tons isn’t a lot of fuel space saved using the modified rules (48 tons needed for the Jump-6 + 6 for the power plant for 2 weeks), but a stateroom and a Type A maneuver drive does barely fit. Add a hefty computer and standard sensors… sounds pretty much like an X-boat to me. There would even be 2 tons of cargo room if they are willing to sacrifice independent acceleration in-system. It may not be a ship that any player would want, but at least it makes the small Jump-6 courier possible.

Finally, a high jump transport for priority cargo is something many militaries might really appreciate. In Mongoose Traveller a Heavy Freighter carries 500 tons of cargo, 216 tons of fuel, has a Type H Jump drive and a Type H Power plant (70 tons), and a total range of Jump-2. Bumping that up to a Jump-5 requires a Type S Jump drive and power plant (142 tons total) plus 534 tons (!!) of fuel. Without any other changes, the extra fuel and power systems take up 400 tons of space, leaving 100 tons for cargo. Note that Jump-6 (Type X Jump engine and power plant and increased fuel consumption of the power plant) would require removing all cargo and 50 tons other systems to even fit within a 1,000-ton hull. With the modified Jump rule, the power systems still have to be Type S but the fuel requirement drops to 434 tons (8105, +34 for 2 weeks of powering a Type S power plant). Cargo capacity doubles to 200 tons compared with the standard rules. Granted, this is still a modest cargo capacity compared to the 500 tons carried by the normal heavy freighter, but it might very well be a worthwhile tradeoff for priority military cargo.

Large Engine Sizes

A second change I have implemented has to do with the limited size of engines in standard Cepheus Engine or first edition Mongoose Traveller rules. (This was corrected in Mongoose High Guard and Second edition.) I like the size limitation of 5,000 tons on Jump-capable ships, but main ship engines only go up to size Z. For a 5,000-ton battleship, this only allows a low Jump-2 capability. Even a small 2,000-ton cruiser can only reach Jump-4 or 4G acceleration. A second limitation is that hull sizes under the standard rules only go up by 1,000 tons at a time. For a 2,000-ton to a 3,000-ton ship, that is a 50% size increase. Finer gradations would be useful.

To correct this, I developed tables 1 and 2, which includes ship hulls that are 500 tons apart as well as larger engines and power plants. Size increases for both engines and power plants are larger in these heavy ranges because ship size increases by 500 tons per column rather than by 200 tons in the medium size ranges. Fuel use is calculated based on power plants size using ⅓ of volume in fuel per week, rounded down as per p.124 of Cepheus Engine SRD.

Table 1: Oversize Power Systems
Rating J-Drive M-Drive P-Plant  
  Vol. MCr Vol. MCr Vol. MCr Fuel
AA 135 260 51 104 79 206 52
BB 145 280 55 112 85 222 57
CC 155 300 59 120 91 238 61
DD 165 320 63 128 97 254 65
EE 175 340 67 136 103 280 69
FF 185 360 71 144 109 296 73
GG 200 390 77 156 118 320 79
HH 215 420 83 168 127 344 85
JJ 230 450 89 180 136 368 91
KK 245 480 95 192 145 392 97

Vol. = Volume in displacement tons; Fuel=displacement tons per two weeks

Table 2: Drive Performance by Hull Volume
Drive Type Hull Volume
100 200 300 400 500 600 700 800 900 1,000 1,200 1,400 1,600 1,800 2,000 2,500 3,000 3,500 4,000 4,500 5,000
A 2 1
B 4 2 1 1
C 6 3 2 1 1 1
D   4 2 2 1 1 1 1
E   5 3 2 2 1 1 1 1 1
F   6 4 3 2 2 1 1 1 1 1
G     4 3 2 2 2 2 1 1 1 1
H     5 4 3 2 2 2 2 2 1 1 1
J     6 4 3 3 2 2 2 2 2 1 1 1
K       5 4 3 3 3 2 2 2 2 1 1 1
L       5 4 3 3 3 3 3 2 2 2 1 1
M       6 4 4 3 3 3 3 2 2 2 2 1 1
N         5 4 4 4 3 3 3 3 2 2 2 1
P         5 4 4 4 4 4 3 3 3 2 2 1
Q         6 5 4 4 4 4 4 3 3 3 2 2 1
R           5 5 5 4 4 4 4 3 3 3 2 1
S           5 5 5 5 5 4 4 4 3 3 2 1
T           6 5 5 5 5 5 4 4 4 3 3 2 1
U             6 5 5 5 5 4 4 4 4 3 2 1
V               6 5 5 5 5 4 4 4 3 2 1 1
W                 6 5 5 5 4 4 4 4 3 2 1 1
X                   6 5 5 5 4 4 4 3 2 1 1 1
Y                     6 5 5 4 4 4 3 3 2 1 1
Z                       6 5 5 4 4 4 3 2 2 1
AA                         5 5 5 4 4 3 3 2 2
BB                         6 5 5 5 4 4 3 3 2
CC                           6 5 5 5 4 4 3 3
DD                             6 5 5 5 4 3 3
EE                               6 5 5 4 4 3
FF                                 6 5 5 4 4
GG                                   6 5 5 4
HH                                     6 5 5
JJ                                       6 5
KK                                         6

— = Drive insufficient for use in this hull
Blank Cell = Drive offers no improvement in performance over smaller drives

These tables provide expanded high-acceleration options for larger ships such as warships and priority transports. In addition, jump tugs designed to ferry non-jump spaceships can now be built to move more tonnage while still staying within the 5,000-ton limit in Cepheus Engine. Combined with changes to the fuel efficiency of high-jump starships, more ships can actually approach maximum jump range where the limit becomes the technology level of the ship rather than the maximum size of the engine.

Consider the case of a 4,000 ton system monitor whose job is to protect assets within a single star system. Using a maximum maneuver engine of size Z, it could only accelerate at 2G which would severely limit how quickly this vessel could reach a pirate incursion. The new modification would give an acceleration of 3G with Type AA engines (costing 18 additional tons for increased engine, power plant, and 4 weeks of fuel compared to Type Z), 4G with Type CC engines (56 additional tons), 5G with Type FF engines (110 additional tons), and a whopping 6G with Type HH engines (164 additional tons). Because Jump engines are not present, these monitors can accelerate much more quickly for only a modest increase in power plant size and fuel usage. It only seems to make sense that shipyards would want to provide the maximum flexibility for their clients travel needs whether moving quickly within a system or having extended jump ranges for larger ships.

Small Craft Engines and Power Plants

Small craft are an important part of most space fantasy settings. Who can’t imagine a small fighter whizzing around a giant space station while big guns fire uselessly around them? More practically, any ship (or space station) not designed to enter an atmosphere needs a reasonable way to get cargo and people from the surface into orbit and vice versa. Small craft are perfectly suited for this role.

In Cepheus Engine and the Mongoose Traveller SRD, though, there are a number of situations where using the engines and power plants of the small ship design rules simply make no sense. For example, take a 50-ton small craft with 4G acceleration. This is very reasonable performance; it gives decent acceleration, doesn’t take forever to get off the surface, and isn’t pushing any rules limits. According to both Cepheus Engine and Mongoose Traveller, this would require an sK engine (5 tons, 11 MCr) and an sK power plant (3.9 tons, 7.5 MCr), and the power plant would use 2.6 tons of fuel per 2 weeks in Cepheus Engine, 2 tons in Mongoose Traveller. It would also be able to power only a single energy weapon.

That same 50-ton craft, however, could achieve the same 4G acceleration using a ship type A Maneuver Drive (2 tons, 4 MCr) and type A power plant (4 tons, 8 MCr) and 2 tons of fuel per 2 weeks. So, for exactly the same size ship and same acceleration, the ship type A power setup is 1) smaller by nearly 50%, 2) cheaper by better than 50%, 3) uses the same fuel at most (depending on your rules), and 4) could fully power a triple laser turret. The overall cost of the ship would drop by about 25%. There are no disadvantages. None. As small craft can use the same components as larger craft there is no reason to use the small systems.

The same logic applies in many situations. 40-ton craft with 5G acceleration (such as the Cepheus Engine Pinnace) have identical numbers as the situation described above. 90-ton craft with 2G acceleration need type sH engines and power plants at 7.3 tons and 15.5 MCr under SRD rules, or type sJ systems under Cepheus Engine at 8.1 tons and 17 MCr. Compared to type A ship engines and power plants (treating 90 tons as 100 tons) this is again heavier and more expensive. Table 3 breaks down the trade off points for the combined engine and power plant ratings compared to their small craft equivalents, where the ship system is the combined power plant and maneuver drive.

Table 3: Ship vs. Small Craft System Break-Even Points
Ship System Small Craft Systems to Break Even
Type Cost Volume Cost Volume
type A 12 MCr  6 tons sF - sG sF-sG
type B 24 MCr 10 tons sQ sK-sL
type C 36 MCr 15 tons sU sP

A second issue regarding small craft engines and power plant systems is their relative sizes. For regular ships, power plants are larger than M-drives and continually get larger as their size increases. The ratio of M-drive to P-plant gets closer together as sizes increases. The engine:power plant ratio for type A engines is 0.5, the ratio for type E is 0.563, for type K is 0.613, or for type P is 0.628. For small craft, though, power plants increase in size much much faster than M-drive engines do. For example, the ratio of M-drive to P-plant for sA engines is 0.417, for type D the ratio is 0.952, for type K the ratio is 1.282, type P is 1.429, and type T is 1.494. The huge change in ratio makes it hard to justify ever using a mid-size or larger small craft engine because of the relative size increase.

It is quite easy to use normal ship engines and power plants in mid-size or larger small craft. Performance and power are improved, while costs are reduced. Rounding acceleration down is somewhat inefficient, but those losses are more than acceptable given the dramatic difference in size and cost ratios for everything but the smallest small craft. Table 4 lists the ship performance in various size small craft, keeping in mind that under standard Cepheus Engine and First edition Mongoose Traveller rules the maximum acceleration is 6G. Note that type C engines in any small craft are usually overkill and provide little if any advantage over Type B engines.

Table 4: Small-Craft Performance with Ship Systems
System Small-Craft Hull Displacement (tons)
10 20 30 40 50 60 70 80 90
type A - 10 6 5 4 3 2 2 2
type B - - - 10 8 6 5 5 4
type C - - - - - 10 8 7 6

- = Ship System of this type cannot fit into this hull

It is still better to use small craft engines compared to their ship type equivalents in a few situations, particularly when looking at low performance/price options. For a slow 40-ton pinnace with a thrust of 1G, type sB small craft engines and powerplants only take up 2.5 tons and cost 5 MCr. For a 30-ton launch with 3G thrust, small craft systems sE weigh 4.9 tons and cost 9 MCr. The value equation changes in mid-size small craft, though. For a 60 ton ship with a thrust of 2G, small craft systems sF cost 5.7 tons and 11.5 MCr; type A ship engines give 3G for a measly 0.3 tons and 0.5 MCr more. For a 90-ton shuttle at 2G, a small craft power system H costs 7.3 tons and 15.5 MCr which is about 20% more than ship type A systems for the same performance.

As an alternative, I revised the small craft engine/power plant table to make the smaller engines more similar to the larger ship types. Small craft power plants were left virtually unchanged. Maneuver drives were shrunk substantially and reduced in price to make them more competitive.

Table 5: Revised Small-Craft Engines and Power Plants
Maneuver Drives Power Plants
drive type size(tons) price(MCr) size(tons) price(MCr) tons fuel per 2 weeks
sA 0.5 1.0 1.2 3.0 0.8
sB 0.7 1.5 1.5 3.5 1.0
sC 0.9 2.0 1.8 4.0 1.2
sD 1.2 2.5 2.1 4.5 1.4
sE 1.5 3.0 2.4 5.0 1.6
sF 1.8 3.5 2.7 5.5 1.8
sG 2.1 4.0 3.0 6.0 2.0
sH 2.4 4.5 3.3 6.5 2.2
sJ 2.7 5.0 3.6 7.0 2.4
sK 3.0 5.5 3.9 7.5 2.6
sL 3.3 6.0 4.5 8.0 3.0
sM 3.6 7.0 5.1 9.0 3.4
sN 3.9 8.0 5.7 10.0 3.8
sP 4.2 9.0 6.3 12.0 4.2
sQ 4.5 10.0 6.9 14.0 4.6
sR 4.8 11.0 7.5 16.0 5.0
sS 5.1 12.0 8.1 18.0 5.4
sT 5.5 13.0 8.7 20.0 5.8
sU 5.7 14.0 9.3 22.0 6.2
sV 6.0 15.0 9.9 24.0 6.6

So how does the revised table compare to the standard Cepheus Engine system? For a 50-ton cutter with 4G acceleration, type sK drop from 5 tons to 3 tons and 11 MCr to 5.5 MCr. This would give a cutter 2 extra tons to add cargo, a larger cabin, better sensors, or whatever seems most appropriate. Compared to using ship type A engines and powerplant it would still be slightly larger overall (6.9 tons vs 6), more expensive (13 MCr vs 12) and less fuel efficient (2.6 tons vs 2) but at least it is a lot closer. A 90-ton shuttle aiming for 2G thrust needs either sJ or ship type A equipment. Combined, the sJ engines and power plant are 6.3 tons and 12 MCr, again nearly equal to the size and cost of the type A ship system. Bumping the shuttle up to 4G acceleration requires sP or type B engines. Here the combined values favor type B systems by size (10.5 vs 10 tons) and the small craft systems by cost (21 MCr vs 24). The small craft equipment would also burn slightly more fuel in the same time.

The new table provides the largest advantage for smaller and midsize small craft in the 2G-4G ranges. The extra tonnage available means a lot for this size ship and allows a very different vessel to be constructed. For example, a 10 ton fighter thrusting at 6G needs a Type sC power system. The new table reduces the size of the maneuver drive by 0.6 tons, or 6% of the fighter’s total displacement. That’s equivalent to 4 points (nearly 5) of crystaliron armor.

A slow modular cutter designed with an acceleration of 2G using the new rules requires Type sE maneuver drives and power systems. Together they total 3.9 tons and 8 MCr, which saves 1 ton and 1 MCr compared to Cepheus Engine values. An extra ton for a small craft is a big deal.

Overall, these changes make getting around the systems and subsectors of the Traveller Universe more balanced and opens up several new ship designs. The revised small craft tables basically improve the effectiveness and affordability of the small craft engines without substantially changing the nature of the ships. For my campaigns, I tend to have a fair amount of in-system travel between bases, moons, and habitable worlds. Small craft become the semi trucks of the solar system, ferrying around modular cargo containers and passengers to various outposts and scattered habitats. Similarly, large engines and power plants gives larger ships the opportunity to get to their destination much more quickly than before. High-tech Jump-capable starships also become more affordable and efficient for shorter jumps. High-jump ships still have smaller cargo holds because of the amount of fuel they must be able to carry, but at least they are more efficient at using their fuel which partially compensates for the decreased tonnage available. The change is modest enough that low-jump ships still have many uses. I hope other referees find these alternative rules as balancing as I do.