Freelance Traveller

The Lyman Drive in Mongoose Traveller 2nd Edition

by Alex Goodwin

This article originally appeared in the November/December 2020 issue.

Introduction

Page 14 of MGT2’s High Guard (hereinafter, HG) provides Jump options ranging from the humble jump-1, all the way up to jump-9 (at TL19). HG also provides options for building kit at different TLs, not merely its baseline, on p.48.

Obvious Question: how do these interact with Jeff Zeitlin’s alternative Lyman Jump drive?

To quote the original article:

“The Lyman Drive offers a possible alternative that makes it easier for a small ship to be economically sustainable, while not completely eliminating the need to scrimp and save between payments. It also offers the possibility of designing small ships with extended jump range – ideal for exploratory ships.”

How is that premise modified in MGT2?

Unless otherwise contradicted in this article, Lyman drives function the same in MGT2 as the original article indicates.

Fuel usage

For ease of reference (and to give credit where due), I’ll present the original data with a yellow background. Figures I’ve calculated for this article will be rounded to two decimal places.

Inveterate physicists and chemists may well recognise the minimum-TL fuel burn from the reduced Rydberg constant for hydrogen. Higher-TL fuel burns are, for jump N, by analogy with Lyman transitions from the 1st level to the (N+1)th level.

Gain In Available Tonnage

The gain in available tonnage gets more non-trivial with increasing Jump capability:

Increased Cost

A ship mounting a Lyman drive pays for its reduced fuel bill in the following ways:

• Higher Jump drive capital cost;
• Higher Jump drive operating/repair cost;
• Refined fuel is mandatory – using unrefined fuel in a Lyman drive should result, if the GM is feeling charitable, in catastrophic misjump at best.

Using the baseline HG rules, ships of jump-8 and higher are, at best, impractical. A J-8 ship devotes 20% of its displacement to its jump drive, 80% of its volume to fuel tankage, and then has to wedge in luxuries like power, M-drive, crew, armour, etc. Likewise, a J-9 ship devotes 22.5% of its volume to jump drive, 90% of its volume to fuel tankage, etc.

High Guard, High Technology

p.48 of HG quoth:

“When selecting prototype (lower TL) or advanced (higher TL) components, use the Prototype/Advanced table to alter its TL, tonnage, and cost, and to determine how many Advantages or Disadvantages it will acquire.

“Advantages and Disadvantages are then selected from a suitable category in this chapter, or the referee and Travellers can create their own.”

Relevant extract from the Prototype/Advanced table:

Although multiple advantages for Jump drives are described on p.48 of HG, I’ll only focus on one, and apply it once, twice or thrice:

“Decreased Fuel: The drive uses 5% less fuel then normal.”

I will assume cost premia compound, instead of adding.

For example, building an Advanced Lyman Jump-4 drive at TTL15 would attract two cost premia:

• +40% for J-4/TTL15 Lyman drive;
• +10% for Advanced.

Overall cost would not be 100% + 40% + 10% = 150% of base, but 140% * 110% = 154% of base.

Lyman J-1 remains dominated by Standard J-1, so I won’t bother looking at J-1 further.

Per HG p.216’s writeup of the Tigress-class dreadnought, it would seem that fuel-burn reductions add together.

A 500k dton J-4 ship with conventional Jump drives would burn 200k dton of fuel per jump.

The Tigress’ jump drives are listed as “Decreased fuel×2”.

With 444 GW of power generation (occupying 22k dton) aboard and 5 months of fuel burn, powerplant fuel tankage works out to 50% of powerplant volume, or 11k dton.

Total fuel tankage carried by a Tigress is 191k dton. 11k dton goes to power generation, leaving 180k dton for Jump – 90% of what would be needed for conventional Jump (with the twice-over fuel-burn reduction of 5% apiece). Thus, fuel-burn reductions from Advanced construction must add, instead of compound.

Lyman/Advanced-construction Interaction

Economics

As said previously, both in this article and in the original article, Lyman J-1 is dominated by Standard J-1 – the latter costs less, burns less fuel and makes more cargo space available.

J-2 still provides only marginal CER increase – not necessarily a slam-dunk guarantee of common-carrier profitability. That may work in a GM’s favour as the Legitimate Business can go places and haul things that a conventionally-powered ship would be forced to skip – more story hooks, more opportunities for shenanigans, etc.

J-3 under MGT2 seems to provide greater CER increases than under other editions – possibly due to jump drives taking up (2.5×N)% of displacement for a Jump-N ship (rather than CT’s (N+1)%).

J-4 and up would seem to be near slam-dunks, due to the massive increase in tonnage that is almost completely available for revenue.

I have no data for J-7+, and wasn’t enthused enough to design any such ships for this article.

By contrast, the jump-drive advantages were fairly marginal across the board – I only bothered looking at a combination where the base jump-drive advantage improved the CER over baseline.

Jump-2 examples

Jump-3 examples

Jump-4 examples

Jump-6 examples

Non-commercial implications

Different incentives are in play here, where although building an Advanced version of a Lyman drive may not strictly be justified on cost grounds alone, the mission space gains (especially on larger ships) may well be considered worth the trade.

Military implications

At J-2, the extra space made available by Advanced construction above and beyond a plain Lyman drive could be put towards ammunition storage, somewhat better armour and/or auxiliary systems, etc. Not only would such a vessel retain a plain-Lyman design’s advantage over a conventionally powered same-displacement vessel, it would have a slight edge over the plain-Lyman parent design.

At J-3, the small space gains from Advanced construction interact well with out-and-back capability to also allow gains in mission space (approx 5% of vessel displacement at the Advanced level). Such a vessel can hit-and-run while bringing somewhat more firepower to bear than conventionally-powered J-3 defenders can manage.

At J-4, 2-jump capability is well-established alongside significant gains in mission space (from approx 14% of displacement at Advanced level up to approx 18% at High Tech level). Another possibility is trading the gains in mission space for a third unrefuelled jump – allowing approach from angles that conventionally-powered forces simply cannot pull off without support. For example, crossing an 8 parsec gap and still arriving with enough fuel to run.

Likewise, J-5 delivers 3-jump capability alongside significant gains in mission space (from approx 10% of displacement at Advanced level up to approx 17% at High Tech level), or (as at J-4), trading the mission space for an extra unrefuelled jump. A 20-parsec unrefuelled range would fundamentally change fleet intruder operations, while a 2-jump battlerider tender would be able to deliver more battleriders than a conventionally-powered J-5 tender, and be able to run without refuelling.

These trends continue at J-6 and J-7, and make J-8/J-9 ships feasible.

Non-military implications

As the original article said, the benefits will be some combination of increased range, mission space, or both.

J-3 vessels will, like their military analogues, gain both mission space and out-and-back capability.

Conclusion

The MGT2 rules on Advantage construction barely interact with Lyman drives, and are not really economically competitive with them.

Large merchant lines, with large fractions of liner (as opposed to tramp) traffic would be natural candidates to use Lyman drives – the operational savings gained on comparatively large ships (and nontrivial increases in mission volume) would tend to more than offset the salary premium required to keep the chance of mishap the same.

Military operators, despite having almost-by-definition large fleets, would not necessarily be automatic candidates. Yes, the gains in operational flexibility, firepower and unrefuelled range (especially at higher jumps) are very tempting – but when a military vessel has to jump on unrefined or partly refined fuel, it tends to have to jump right now. Maybe this could be offset by also mounting a conventional unrefined-fuel-capable J-1, but that’s more expense and another system that can go wrong and savage you in a tight spot.

Large communication-focused operators (such as the Third Imperium’s Xboat network), like large merchant lines, would also be natural candidates, for similar reasons.