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.







