Last time, I mused over the lethality of Traveller’s low berths. This time, I’m grumbling about starship fuel tanks.
Jump drives
Back in 1981, when I first read Traveller’s little black books, I didn’t fully understand how jump drives worked. I imagined they were like the hyperdrives in Star Wars – and were used throughout to keep the ship in jump space.
Except they don’t work like that.
I realised something was amiss with my concept when I got my hands on Supplement 7: Traders and Gunboats and, in particular, the Gazelle class Close Escort.
The Gazelle class Close Escort
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| The Gazelle then: From Traders and Gunboats |
The Gazelle included a new feature: drop tanks.
“The L-Hyd Tanks: The close escort carries a large fraction of its fuel in droppable tanks mounted longitudinally. With the tanks attached, the ship is capable of jump 4 and 4-G acceleration. When the tanks are dropped, the ship burns the fuel and can achieve jump. Once the tanks are dropped, they must be replaced at a starport or naval base; until they are replaced, the ship is capable of 5-G acceleration, but only jump-2 (due to lack of sufficient fuel capacity).”
This crashed up against my assumptions, and it took me a while to work out what this actually meant: jump drives spent all their fuel before entering jump. And then they could drop their tanks, which is important because, in Traveller, a ship’s volume (measured in, er, tonnage – more on that below) determines how far it can jump.
This was the first time I had seen this. It wasn’t in my little black books, and it wasn’t in The Traveller Book.
It’s not even particularly clear in Mongoose’s latest rules.
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| The Gazelle now: From High Guard (2022 update) |
Core Book 2022 Update
While providing a much clearer explanation of what’s going on during jump, Mongoose’s Core Rulebook Update 2022 doesn’t clearly say that jump drive fuel is consumed to create the jump bubble before entering jumpspace.
And the amount of fuel spent creating the bubble is 10% of the ship’s tonnage x the distance jumped, in parsecs. (While this is implied, I don’t think I’ve seen it in the core rules, although I may have missed it. It’s covered in the Traveller wiki.)
Why is this important?
Because in a world where tonnage is money, why would you ever have internal fuel tanks for your jump drive? Once you enter the jump bubble, those tanks are empty…
In fact, for most ships, the J-drive fuel tanks are empty for most of its life. You normally only need to fill them when you’re about to make a jump – otherwise, it is best to keep them empty.
The cost of an empty fuel tank
So, if we look at a humble 200-ton (or rather, 2,800 m3 volume) Free Trader, its jump-1 drive requires 20 tons of fuel. If a Free Trader had droppable fuel tanks, those 20 tons could be turned into cargo space which would be potentially an extra Cr 20,000 every trip (assuming it was filled with standard cargo).
Why wouldn’t you do that?
Now, drop tanks may only work in civilized places – perhaps only at class A and B starports or where there is a steady trade. I can imagine renting standard drop tanks much in the way you’d rent a skip today. (So that will eat into that Cr 20,000.)
And you might want internal fuel tanks if you didn’t know whether you would be able to refuel at the other end. However, most Traveller ships don’t have the fuel capacity for more than one jump, and the assumption is that fuel is pretty easy to get.
Are there other options?
Alternative fuel tanks
High Guard gives us more options for fuel tanks:
Collapsible fuel tanks: A huge bladder that sits in the cargo space and can be filled (and emptied) with fuel as needed. It's a good idea, but there’s no time to use the cargo space productively between forming the jump bubble and entering jumpspace.
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| Collapsible fuel tank from High Guard |
Drop tanks: As fitted on the Gazelle Close Escort, above. However, they bring a penalty to the jump drive roll, and the tanks themselves have a less than 50% chance of surviving the experience. (I find it odd that they aren’t more reliable – I imagine Toyota kaizen-ing the hell out of these and making them safe and reliable and cornering the market as a result.)
Fuel/Cargo container: Allows fuel to be stored in unused parts of a ship.
Mountable tanks: Fixed tanks that convert cargo space into fuel tanks, and as far as I can see, have no benefit over a collapsible fuel tank. (Well, they take up less space than a fuel bladder, but you’ve lost the use of your cargo space.)
None of these are ideal – they either use up precious cargo space or aren’t particularly reliable.
My solution: collapsible external fuel tanks
Collapsible external fuel tanks
Why not fit those fuel bladders on the outside of a ship? That way, when the fuel is used up, the bladder collapses and preserves the ship’s precious volume. And it can be reused time and time again.
Collapsible external tanks will affect a ship’s streamlining – although I can imagine blisters on the hull that conceal the tank (but at a cost – maybe 2% of the fuel tank tonnage) and protect the ship’s streamlining.
Implications
The real problem with this is that it invalidates almost all the Traveller ship design assumptions since 1977. Ships should have more cargo space and less fuel space. You might still want internal fuel tanks for some situations, particularly if you need an armoured hull, but most ships would have external fuel bladders.
Footnote: Traveller tonnage
Traveller tonnage is a bit weird. Ships are measured in displacement tonnage, which is actually a measure of volume (equivalent to the volume of liquid hydrogen displaced, measured in tons – so one displacement ton is 14 m2).
I can see how this works for jump drives – the J-drive needs to create a jump bubble big enough to take the ship. So volume is important.
But for manoeuvre drives? I would have thought that a ship’s mass was more important than its volume for an M-drive. So, is a ship more agile with its fuel tanks empty? Apparently not – but maybe that was one calculation too many. (Interestingly, M-drives use power from the power plant but no actual fuel themselves.)
Next
Next time I look at the most populated world in the Regina subsector: Rethe.
Real-world "wet" ships are registered for size in Gross Tonnage, GT, which is an indirect measure of volume (it replaced Gross Register Tonnage, GRT, which was a direct measure of volume).
ReplyDeleteGiven that the fuel used in Traveller is liquid hydrogen (temperature = -253C; 20C above absolute zero), I can only assume that whoever came up with the idea of collapsible fuel bladders had no knowledge of materials science. To hold liquid hydrogen (and to keep it liquid) the bladders would need to have a very high insulation factor and also be strong enough to withstand the pressure required to keep it liquid or if any vaporises the pressure caused by the gas. At the same time, it would need to flexible enough to collapse down when empty. Whilst it's possible that at a high enough TL such a material could exist, I seriously doubt that they'd be available at TL9 as implied by the rules.
If you allow them to be mounted externally, any blister to protect them and improve the streamlining would be permanent and increase the volume of the ship, even when they are empty.
For the (de)mountable fuel tanks, there are two ways they can be used (canonically, Tukera lines uses them both ways for different ships):
1. To allow a J1 ship to cover two parsecs in two jumps (eg. the 5000 dTon Hercules-class freighter).
2. On high-jump capable ships, to reduce the fuel tankage allowing more cargo to be carried on shorter jumps (eg the J4 capable 1000 dTon Type RT Long Liner - it has permanent tankage for power plant and 1 parsec plus three 100 dTon mountable tanks).
Tukera facilities have highly trained ground crews able to fit/remove the mountable tanks in a couple of days.
The problems with L-Hyd drop tanks are the cost, availability and time to fit them.
I agree that M-drives should take mass into account; in some versions, eg MegaTraveller and New Era, it did. I suspect that they went with volume in most versions for simplicity's sake. To a large extent the mass/volume ratio will be in a fairly narrow range across all ships.
Thanks for your comments. I didn't know about the Tukera lines stuff - there's so much Traveller background out there!
DeleteI've just had another thought about L-Hyd drop tanks - the dropped tanks would need to be recovered by some other vessel in the system for re-use and to prevent them becoming a hazard to shipping. That means an in-system craft large enough to carry multiple tanks, from multiple potential jump points in the system and return them to the starport for re-use.
DeleteYes, drop tanks would need to be recovered and reused, so probably only at Class A or B starports with the right infrastructure. I imagine it would be a bit like hiring a skip (although maybe not that basic...)
DeleteIf you look up Tukera Lines on the Wiki there are links to some of their ships (apart from the 3000 dTon Prosperity-class freighter where the link is broken - just search for "Prosperity freighter" and you'll find the design).
ReplyDeleteI think originally it was just the Hercules that had the mountable tanks - that would have been in The Traveller Adventure; I don't recall the Long Liner having them in that campaign.
Thanks. I do have The Traveller Adventure (a relic from 1983) but I haven't opened it in forty years...
DeleteIn a lot of ways, it makes sense for the Type RT to have (de)mountable fuel tanks - one ship design capable of J4 can be used for J1, J2, J3 or J4 routes rather than needing separate designs for each route. They can also be quickly switched if markets change, to rotate ships (and crews) between services or to cover for ships that are being repaired or having their annual maintenance.
DeleteAt Junidy, for example, Tukera would have a few J1 ships for the Junidy-Towers route and a lot more J2 ships for the Junidy-Marz and Junidy-Nasemin routes.
I imagine the Prosperity-class freighters would also have (de)mountable tanks for the same reason, although the design on the Wiki doesn't have them (nor does the one in The Traveller Adventure).
The benefit of demountable fuel tanks over collapsible tanks in MGT2e is that the collapsible ones for whatever reason can't have their fuel pumped from them fast enough to be used directly by the jump drive, so its only useful if you are trying to do multiple short jumps in a row. If a ship with a high rated jump drive wants to be able to both use space for cargo when doing short jumps and fuel when doing long jumps, it needs either fuel/cargo tanks which take %5 of the tonnage, or dismountable tanks that take no extra tonnage but 4 weeks to swap. Personally I think the 4 weeks is a bit punishing and would put nearly all captains off from using them, one or 2 weeks seems like a trade off more people would be tempted by.
ReplyDeleteYes, you'd think for a fairly standard ship, fitting demountable tanks would be almost routine.
DeleteAs for why no external collapsible tanks, as written collapsible tanks can't directly feed jump drives, but I'm not really sure why that limit exists in fiction, I think its mostly game balance. I could imagine you could install better pumps. One thing I don't think you could get around easily though is getting the tanks stowed away in time. The description of drop tanks implies the amount of time you have to safely get rid of the tanks after emptying them is pretty brief, requiring they be flung away with explosive collars. I could see the flaw with collapsible external tanks being you can't really fold them away quickly enough.
ReplyDeleteMy guess is that whoever invented internal collapsible tanks didn't think about putting them outside the ship... A couple of people have mentioned game balance (on forums), but I'm not sure what they're getting at - I'm not sure what becomes unbalanced if ships carry more freight? Are the freight rules that finely balanced? (Maybe they are, I've not used them.) And if ships can carry more freight, that would (in the real world if not the Traveller rules) depress shipping prices - evening out the balance. Anyway, thanks for commenting!
DeleteI think the balance people are referring to is the balance between the different jump ratings. If a jump 6 ship doesn't have to devote more than half its volume to fuel then it would dramatically change how practical it would be. X boats could be jump 6 instead of jump 4, fast cargo ships in the jump 3 and above range would be way more common, it would have some pretty big knock on effects on the setting. Suddenly the trade off between large cargo and flexible routes mostly vanishes. Ships with Jump1 are now pretty much just strictly worse, rather than there being an interesting trade off.
DeleteThanks - I think I see what you mean. Yes, it does change things a bit - I've got no idea whether the changes would be significant or not. Perhaps someone should have thought about that before adding drop tanks in Supplement 7?
DeleteDrop tanks have similar consequences, but its a bit more limited do to them requiring advanced starports at each end and the extra expense of the service. So their use is more situational. Whereas collapsible tanks would pay for themselves pretty quickly and would have greater flexibility cause they could be used at any starport and even be used with wilderness refueling, so long as you can find a comet or have a shuttle to go gas giant skimming for you.
DeleteAnd also in LBB High Guard: "These L-Hyd Tanks are fitted to the outside of the ship, and drop away before jump. The result is more interior space available for cargo and passengers. Such tanks must be replaced each time they are used, so they are practical only on runs to civilized areas, or to increase fuel capacity to allow several jumps."
DeleteTo go back to a previous point for a moment though, I do still think a reasonable case can be made that collapsible tanks are not a thing for in universe reasons as well, not just a pure game balance handwave. Its pretty hard to imagine getting a tank to collapse and fold up into a blister in the same handful of seconds it takes to explosively fling a drop tank away. And in the default settings even drop tanks are cutting it pretty close in terms of being feasible. Even at the speeds drop tanks are jettisoned they still regularly get mulched by the jump field. I know you're not a big fan of drop tanks being unreliable, but I think it's meant to show how tricky and fast paced an operation forming a jump bubble is, which provides a pretty good reason why a more complex and time consuming option like collapsible external tanks isn't a thing in the default setting. They’re just not fast enough to avoid getting shredded reliably.
DeleteYes, you make good points.
DeleteExternal collapsible would be a possibility. However, considering the preference for mobility, a ship with internal tankage and streamlining is much better in the kind of environments most Traveller players go to. Limited orbital services, and places they want to leave fast. Internal tankage fits in with streamlining.
ReplyDeleteI agree for most Travellers that drop tanks aren't really feasible, given the types of perilous places they like to visit.
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