Enough people commented/emailed/IMed me saying "Jesus Christ do you know what you're saying? Ammonia is one of the worst fucking substances in the universe!" that I was pretty much forced to reconsider my stance on the subject. I am now embarrassed to admit that I didn't realize just how hazardous ammonia was.
This leaves me in a bit of quandary. The obvious solution (pun intended) is to say that the deuterium is stored as Heavy Water. This actually makes the most sense from an OSHA point of voice, since it's not flammable, naturally liquid, and barely reactive or hazardous.
On the other hand, this makes fuel leaks extremely boring from a storytelling perspective. The only way to make it exciting it to have the hole become large enough that drowning is an issue. While that is kind of cool, any damage which will make a hole that large is also likely to crack the structural integrity of the hull, in which case all the water is going to vent outwards, along with the atmosphere, via explosive decompression.
In other words, it's not sufficiently fuel-like for dramatic purposes.
On the other hand, some folks suggested methane as a storage medium: it's less nasty than ammonia, is stable, and is an actual fuel -- but then we're back to "wrapping yourself with explosives" again. Yes, I understand that it burns only when mixed with air and that "liquid methane does not burn unless subjected to high pressure (normally 4–5 atmospheres)", which probably means it's a good choice. But I don't know. There's something about it that bugs me, but nothing I can vocalize.
Still, it works. It's a stabilized hydrogen compound, isn't as insidious as hydrogen, and is sufficiently fuel-like for storytelling purposes. Therefore, I am now revising my opinion and stating that In My Traveller Universe, starships store fuel as Tetradeuteromethane, which means it's your farts that make the ship go instead of your pee.
As a point of interest, all this talk about fuel and refinement and whatever made me realize something that's been bugging me for a while now. In Mongoose Traveller, if not in earlier editions, it's possible to scoop up a shipful of unrefined fuel and refine it on your way out to the jump point. This bothered me for some vague reason, and it was made worse by the fact that 99% of the time, starships have their fuel processors next to or behind their fuel scoops.
Um -- if you're refining fuel already in the tank, why must the processor be at the entry point?
And then as I considered the ramifications of using deuterium instead of hydrogen, it really hit me: Deuterium refinement is basically separating all the D from the H and venting out all the rest. If D is less than .02% of all H in the universe, then if you load up your tanks with unrefined fuel and then start the refinement process... you're going to end up with empty tanks, since 99.98% of their volume will be unusable.
So that's another sacred cow slaughtered right there: clearly, the refinement must take place as the ship is taking on fuel to begin with. Mechanically, this isn't game-breaking: we're just shifting the time-sink of fuel refinement to the front of the line. Dramatically, however...
Frontier refueling takes a hell of a lot longer now. It's not 1-6 hours to fill your tanks, it's however long it takes your processor (at 1 dton/hour*) to refine a full tank. This has the delightful effect of making a typical murderhobo crew either have to interact with people or part with their hard-earned money, because:
- 20+ hours on the surface of a planet refining fuel results in more adventure. Either their ship is stuck at a spaceport (in which case, you can hit them with adventure hooks and charge them additional docking fees if it takes more than a day for them to fill a tank), or they're in the wilderness for hours and hours, where you can hit them with planned or random encounters.
- 20+ hours skimming gas giant atmosphere is just asking for trouble. Not in the sense of "This is a stupid and dangerous thing to do"; more like "Gas giant skimming is not a routine procedure and there is always the possibility that something can go interestingly wrong with a failed roll or two." This was reinforced last game session when the group's pilot decided he would rather spend two days in transit to a mainworld, where they could safely refine fuel from seawater, instead of spending 20 hours of refueling operations in the gas giant that they were practically orbiting.
- If the PCs are running a merchant ship and time is worth money, buying refined fuel for 500 Cr/ton suddenly looks a lot more appealing than spending days refining the cheap stuff. A 400 ton Fat Trader needs 52 tons of fuel and only has one processor aboard -- if you're on a deadline, the math does itself.
For the Game Master, this is a win-win decision.
* Okay, technically it's 20 tons/day. I like mine better, thanks.