Alternative Jump Fuel Storage

[Celti]

During some discussions for the upcoming ISW game I'm playing in/Co-GMing, we ran into the topic of alternate jump fuels for longer range. What's to keep someone from having one jump worth of fuel space for liquid hydrogen for the immediate working space of a jump, and storing the rest in some denser format - say, compressed methane, which you can get to at least twice the density, molecularly speaking, of hydrogen than you can of liquid hydrogen. Then you use the fuel processors when you need to jump, converting the methane to the less-dense liquid hydrogen, and jump - the methane could even be stored in collapsible tanks in the cargo bay, for trade-offs of distance versus cargo. This is completely useless for single-jump ships, but for military vessels and couriers, it could be a much more efficient solution.

[Agemegos]

Originally Posted by Celti

What's to keep someone from having one jump worth of fuel space for liquid hydrogen for the immediate working space of a jump, and storing the rest in some denser format - say, compressed methane, which you can get to at least twice the density, molecularly speaking, of hydrogen than you can of liquid hydrogen. Then you use the fuel processors when you need to jump, converting the methane to the less-dense liquid hydrogen, and jump

Well, when you process methane (CH4) you get 75% carbon by mass and only 25% hydrogen by mass.

[thrash]

Originally Posted by Celti

What's to keep someone from having one jump worth of fuel space for liquid hydrogen for the immediate working space of a jump, and storing the rest in some denser format

Nothing at all, really, especially if fuel skimming works as advertised (scoop raw gas giant atmosphere into tanks, then cycle through fuel purifiers until only liquid hydrogen is left).

[Celti]

Originally Posted by Agemegos

Well, when you process methane (CH4) you get 75% carbon by mass and only 25% hydrogen by mass.

However, methane compresses much more than hydrogen does. Even liquid hydrogen is incredibly lightweight, and it doesn't compress very well.

[Agemegos]

Originally Posted by Celti

However, methane compresses much more than hydrogen does. Even liquid hydrogen is incredibly lightweight, and it doesn't compress very well.

Good point. As compressed gases, methane has twice as much hydrogen per unit volume as molecular hydrogen under the same temperature and pressure. Liquid hydrogen has a mass of about 70 kg/m^3, while liquefied methane is about 450 kg/m^3, of which about 113 kg/m^3 is hydrogen. You can get about 60% more hydrogen into a given volume as LNG than as liquid hydrogen. Also, the cryogenics requirement is lower: liquid hydrogen boils at 20 K, LNG at about 110 K.

While we're at it, let's consider water. 1/9 hydrogen by mass at 1000 kg/m^3: 111 kg of hydrogen per cubic metre with no cryogenics needed at all. Liquid ammonia is pretty attractive: 124 kg of hydrogen per cubic metre with a modest cooling requirement of 239 K. Lithium hydride is a bit disappointing: it's solid at room temperature but only 99 kg/m^3 hydrogen.

The best of all might by hydrogen clathrate hydrate, which is basically hydrogen gas dissolved in water ice. I can't find a figure for its density, but it holds 50 kg/m^3 of hydrogen in addition to what makes up the ice. It is stable at 77 K, meaning that liquid nitrogen would be adequate as a cryogenic coolant.

[Anaraxes]

Clever idea.

[Rraurgrimm]

My answer what keeps one from using these: game mechanics...

Otherwise one would have starships with far more interior space available. And I did not really like the L-hyd droptanks of the Gazelle-class escort already. It seems so gamistic and like something player characters would to to gain an advantage over the game universe admittedly. ^_^

[Pomphis]

It´s only useful in specialized situations. Higher density means more mass, so sAccel goes down, which is a problem for warships and commercial ships.

There may be a use for spy and scout ships and raiders who need to be able to jump home again without refueling. There may be a use for ships crossing rifts which have to do 2 jumps in a row without refueling. There may be a use for express couriers who can do 2 jumps along the most direct route possible.

[Voidstar]

An old CT/MT method of hydrogen fuel storage was gravitic compression to create a form of "metallic" hydrogen... I'm no chemist.. But if I remember it right it's a form used in nature where the gravity in a gas giant compresses the hydrogen to the point of the creation of an (I Assume) semi metallic state. Nature usually being the best example...

[thtraveller]

Originally Posted by Voidstar

An old CT/MT method of hydrogen fuel storage was gravitic compression to create a form of "metallic" hydrogen.

This is perhaps the best method with Metastable Metallic Hydrogen density around 9 times that of normal hydrogen (according to wiki) and 3 times the cost (according to G:Spaceships) OR 12 times the density and M$1 per DT according to G:DB p153. The main practical problem being the release of significant amounts of energy when decompressing it.

Hmm, potentially this would give a J2 ship with 60% fuel tanks a 38+ parsec range (46+ at J1, 30+ at J3, 22+ at J4) - assuming they had to decompress it before use. You could jump clean across a sector without ever coming close to a planet. Great for scout ships and x-boats though you wouldn't want to put it in anything that might get shot at.