Science not my thing, could use help here...

[Carlobrand]

If you're using liquid hydrogen, you have to assume you're also using tanks capable of storing liquid hydrogen - in other words very good cryogenic storage tanks. A futuristic society capable of building a ship that will get you repeatedly from orbit to ground and back without frying you along the way knows how to insulate a tank against heat. Short of something happening that damages that tank, I don't think there's anything to worry about on that front, certainly not during the brief life remaining to your poor players if they don't find some way out of their predicament.

A much bigger problem, I think, is what your players are planning to breathe once the air grows stale and their suit tanks run empty. That's likely to be the most immediate of the coming crises if they don't manage to get some sort of emergency power going.

After that its - more or less in order of priority - how they're going to get the CO2 out of the air before it builds up enough to kill them (which will most likely get solved at the same time as the O2 crisis, if their solution involves finding some way to get power to the air recycler ... unless of course the outage damaged the recycler so that it no longer functions quite to specs), how they're going to stay warm (or cool, if this misfortune occurred while they were close enough for the local sun's heat to be an issue), how they're going to stay hydrated, how they're going to stay fed, and how they're going to summon help in whatever time they can manage to buy with their solutions.

There are an assortment of other minor-to-major inconvienences as well (like whether their available light sources, if any, will last long enough to solve all these problems), but the really fun ones are those first few.

 

[Travellerspud]

hydrogen is a gas at room temperature, but if you could capture it (say in a balloon) and compress it, you could turn it to liquid just by squeezing it). Squeeze it harder yet, and it becomes a solid (like ice).

to really answer your question you need to understand how hydrogen changes as pressure and temperature change...and for this you need a phase diagram

http://www.chemguide.co.uk/physical/phaseeqia/phasediags.html

it looks like hydrogen stays liquid above 5-10 atmospheres of pressure (not too hard even today)...but that is at cold temperatures (say -50c) so if it were to heat up, it would expand - unless vented (to space) the tanks would blow up. But the opposite is also true - if allowed to cool (and space is very cold) that gas turns to solid...

I imagine the tanks are normally "balanced" heat-wise using the cold of space to keep the gas in liquid state (much denser this way, so smaller size tanks needed). The side of the tanks facing the crew area AND engines are insulated pretty good (no power needed)...but, if power is lost, the little pumps that control pressure (and the gauges to monitor them) would be lost (maybe they have a battery back-up system)....so after some time (hours? days?) the tank pressure could increased or the liquid could freeze...rotating the ship somehow (using pinnace?) could put different sections of the hull in or out of sunshine (if in the inner or habitable zones, otherwise not) which might be a way to "manually" adjust the tank pressure state.

Or you just say there are fail-safes, and multiple redundant backups...

 

[Enoki]

You can also gel it:

http://www.grc.nasa.gov/WWW/Fuels-And-Space-Propellants/China/sld012.htm

If you have it liquified or near metallic, then you would also have to have either a vent system (inefficent) or, circulation and recovery system (better) to allow for off gassing as the hydrogen slowly heated up. Storage tanks would be dewers of some sort.

 

[Icosahedron]

Most power outages (apart from explorations in deep-space hexes) will take place close enough to a star to deploy emergency solar panel arrays, either automatically or by manually humping them out of the cargo bay and clamping them to the exterior of an airlock. I would imagine that most ships will be able to deploy a solar array sufficient to provide emergency life support operations such as fuel re-compression and atmospheric recirculation.

Having said that, the ideas above regarding insulation and metal matrix storage would be sufficient to reduce fuel loss to negligible quantities for the several weeks that food supplies will last.

As for planet surface conditions, these would not create significantly more complex problems for insulation, but for long term ( years+) forget it, you'd need to refuel your long-abandoned base, and you'd better check the integrity of the materials against corrosion before you do.

 

[Blue Ghost]

So my gaming group just started playing Traveller again (TNE), and a science thing came up, which is very much not my field.

They were on a ship that had lost power. One of players asked what the fuel was. I said it was liquid hydrogen. He asked if the ship lost power, what was keeping the liquid hydrogen cool enough to stay in a liquid state.

What would be doing it?

I'm just curious here, why are you playing Traveller if science isn't your deal?