Water, Water Everywhere!

 It's made of the most and third most common elements in the Universe. 

It is essential for life-as-we-know-it (TM). 

It can be a propellant, radiation shielding or heat sink. 

Water! The original beverage of choice. I'm going a little more hard science on this than usual (hah!) which means I looked up some numbers on the web and scanned the Atomic rockets site.

Many people bitch comment about Traveller ships using hydrogen for fuel. Hydrogen weighs 70 kilos per cubic meter or one ton per 13.5 cubic meters. It's a form of measurement. A ship displacing 100 tonnes (volume) will displace 100 tons (mass) of liquid hydrogen. We will specify displacement (volume) with dtonsGo ahead, dip your Scout ship in an ocean of liquid hydrogen. I will wait.

Anyway some people feel schlepping your H around as liquid H in inefficient. Why not use water instead and electrolyze it as you need hydrogen? Seems legit. A couple problems though.

Water is heavy compared to liquid H. A Scout ship's Jump fuel masses 40 tons. Some sources use 10 tons per ton to figure she mass. The total weight of the Scout would be 1000 tons. The fuel masses about 4% of the total weight. Assuming you can just fill the fuel tanks with water the fuel will now mass 560 tons -more than the weight of the rest of the ship. 

Your ship's performance may suffer is the takeaway. 

Another takeaway: fuel tanks have mass and cost. This is assumed in the 100 grand per ton you pay for a hull. Generally fuel tanks mass about one sixth the mass of the fuel. This is 7 tons of the Scout ship normally. A pittance. It's 93 tons for water filling the same volume. Non-trivial. The numbers may vary with bonded superdense and such materials but it will be more than for hydrogen. 

The tanks need to be constructed differently. You want the hydrogen to stay liquid, which requires cryogenics to chill it. You want your water for stay liquid. This is because a ton of ice takes up 10% moe volume than a ton of water. Yeah, 40 tons of fuel taking up 44 dtons of tankage that hold 40 tons is not a good thing. Yes, ships generally try to get rid of heat so it is unlikely but shit stuff happens. So assume we only fill 12 dtons with water. People are very safety conscious on spacecraft, when anything done wrong can kill you.

Ask yourself for additional cost (which I leave to you to rule) and impaired performance, how much more hydrogen can I store as water? It turns out that a ton of water (one cubic meter) holds 100 kilos of hydrogen. So our 12 cubic metes will net us 1.32 dtons of hydrogen. That hydrogen needs separate tankage and the same cryo gear as before. You want separation of the two liquids otherwise the ice expansion might come into play. So there is about one third more fuel you could carry using water. In addition, most setting indicate you need that fuel for the jump five damned fast, so you will need fuel tankage for the hydrogen you electrolyze and chill.

No I don't know how the hydrogen is used in a jump drive. If I knew how jump drives worked I'd be a billionaire and buy Amazon, and allow the employees pee breaks. 

But refueling from an ocean is way easier than flying through a gas giant! Yeah, but easier is a relative term. If your 100 dton Scout lands in an ocean about three fourths of it will be submerged. In possibly salty water. The naval men among my readers are cringing at the thought right now. Of course you can run maneuver drives in third gear to float more of your ship or even hover. If you hover you need hoses and suction pumps. If you lighten the ship, you're more vulnerable to waves. Having your space legs s one thing, having your sea legs is quite another. How the m-drives react near the ocean is up to you. I know some people say something about them generating static electricity. MT said TL 9-10 drives put out a lot of heat and were almost as hot as chemical rockets on a take off.

There is a bright side to water, it makes fine reaction mass. The section drives can use it for propellant. They don't even have to sweat that refined or unrefined question. Likewise, the ship reactors in Cepheus Deluxe may carry their hydrogen fuel in the form of heavy water. 

Liquid natural gas (LNG) is another way to carry hydrogen. Natural gas is also known as methane. Methane is a greenhouse gas (regardless of your views on climate change, it does hold in heat) and a vital component of cow farts. Before you start chasing ruminants to refill, read further. It boils at 112 Kelvin as opposed to 20 Kelvin for hydrogen. It still Neds cryo gear but it's a sauna compared to H2. Four hydrogen and a lone carbon atom form methane. So there s a higher percentage than in water, about 25% by weight. A displacement ton of LNG masses six tons and 1.5 tons of that is hydrogen. In effect you hav 50% more hydrogen per displacement ton. Our scout would have enough fuel with methane for 3 2-pc jumps. It would also weight in at 200 tons more than usual. As with water, breaking down the methane to hydrogen is a concern. Unlike water, the waste is carbon, which will gunk up your ship. Oxygen will rust stuff but you get rid of that fast by opening a window. Not the fuel we were hoping for. 

TL;DR stick to hydrogen, lest you have all manner of fiddling with math to figure out your ship's performance.

However, there are other ways to carry hydrogen...




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