ISRU and Synthetic Fossil Fuels

[Laie]

This is cropping up in the suggestions forum and got me curious (http://forum.kerbalspaceprogram.com/threads/97174-Squad-s-accounced-there-will-be-Resources-in-Beta-how-should-they-go-about-it/page6). Rather than derail that thread (though I'm afraid it has already happened), I'd like to post the questions here:

Synthesizing hydrocarbons from elements, or a host of other materials (straw, wood, what have you) is not only possible, but Germany has done it on an industrial scale during WWII. So, no question about it being possible at all. Building a space-worthy reactor could be difficult, but certainly not impossible.

My main question: What suitable raw materials could you expect to mine from an asteroid? What kind of refining would be necessary to make either usable in a reactor? In the case of water, that seems to be easy. But how much hassle before you have usable carbon(mono|di)oxide)?

How does the mining even work? I expect that it would be necessary to scoop/dig up the material, break and heat it, then collect the vapors. Doing that in near-zero gravity sounds like some real fun. Could that even be done without a permanent crew? I guess that all parts close to the heating/collection will quickly be covered with gunk...

[lajoswinkler]

If asteroids were made of typical Earth rocks like limestone, then you could heat them over 1000 °C to get quicklime and CO₂. However, that's not the case with asteroids. Carbon in asteroids is not likely to be fixed with oxygen into carbonates. Asteroids are mainly silicates contaminated with iron and nickel alloys. There's also carbon in those alloys.

[dansmithers]

Why? Why on earth would you want hydrocarbons in space? Before you say plastics, remember that it takes more than oil to make useful plastic, it takes a factory.

[Bill Phil]

Why? Why on earth would you want hydrocarbons in space? Before you say plastics, remember that it takes more than oil to make useful plastic, it takes a factory.

The point is that you aren't on earth. Hydrocarbons like methane are very efficient, especially compared to RP-1. So, yeah.

[Kryten]

The point is that you aren't on earth. Hydrocarbons like methane are very efficient, especially compared to RP-1. So, yeah.

RP-1 is a hydrocarbon. Why produce any hydrocarbon if you have the option of producing hydrogen? The higher density of hydrocarbons and higher thrust of hydrocarbon engines are a lot less useful when you're not starting in a deep gravity well.

[andrewas]

Hydrogen is difficult to store, though, so synthesizing methane may be more useful.

[Laie]

Why? Why on earth would you want hydrocarbons in space? Before you say plastics, remember that it takes more than oil to make useful plastic, it takes a factory.

Because everything around KSP engines points towards them burning Kerosene, and people in the thread about ISRU in KSP wanted to create the "right" kind of fuel. Whether that's a good idea, gameplay-wise, belongs in that other thread (see OP). I only wondered how much more difficult this would be if one tried to do it for real. And, coming to think about it, even mining for water isn't all that easy.

Hydrogen is difficult to store, though, so synthesizing methane may be more useful.

Well, that seems like a good reason. And if I understood it right, the reaction to produce hydrocarbons strongly favors methane anyway.

[Bill Phil]

RP-1 is a hydrocarbon. Why produce any hydrocarbon if you have the option of producing hydrogen? The higher density of hydrocarbons and higher thrust of hydrocarbon engines are a lot less useful when you're not starting in a deep gravity well.

RP-1 is a terrible hydrocarbon though. Methane and lower 'anes are much more efficient. Propane is of special interest, because it has a decent ISP and has a similar density to RP-1.

Hydrogen is difficult to store, and difficult to freeze, especially in space where there is often a lot of light and heat on one side and the opposite on the other side.

[Streetwind]

Robert Zubrin has been advocating using combustion engines for Mars ground vehicles in his Mars Direct proposals, fueled by methane (or higher hydrocarbons) and oxygen produced locally through ISRU. The argument is that the ISRU unit, being stationary, can be easily supplied with large amounts of electricity from solar farms and nuclear reactors, while the mobile exploration vehicles are constrained in the amount of energy they can carry. And combustion engines running hydrocarbon fuels have a higher energy density than current electric drivetrains, allowing these Mars exploration vehicles to cover larger distances between refuelings.

And it's not like there are environmental concerns on Mars...

[magnemoe]

RP-1 is a terrible hydrocarbon though. Methane and lower 'anes are much more efficient. Propane is of special interest, because it has a decent ISP and has a similar density to RP-1.

Hydrogen is difficult to store, and difficult to freeze, especially in space where there is often a lot of light and heat on one side and the opposite on the other side.

If its so terrible why is it used so much over methane or propane? As I understand methane would have higher ISP but as you say propane has higher density.

None of the stuff is any problems storing or working with unlike hydrogen.

Hydrogen is nice for upper stages where the low density is less of a problem as you need less but ISP is more important.

You might want to make methane in space to avoid problems with storing hydrogen.

yes, you could crack water into H2 and O2 before use but this takes lots of power and is better done over time.

[shynung]

If its so terrible why is it used so much over methane or propane? As I understand methane would have higher ISP but as you say propane has higher density.

Boiling points consideration, along with cost.

Before RP-1 was developed, US rockets used JP-4, which is designed for jet engines. At the time, JP-4 has an inconsistent composition; John D. Clark once wrote that 'no two barrels are alike'. This hinders its use in rocket engines. When nuclear ICBMs was first developed, a new fuel with a higher standard was developed. It was decided that the new fuel must come from oil, as it was cheap back then. That fuel becomes today's RP-1.

One advantage of using RP-1 instead of propane or methane is that RP-1 is liquid at room temperature and pressure. Propane and methane are gas at those conditions. To keep it liquid, propane must be stored inside a pressurized tank (which is heavy), and methane in a cryogenic one (the insulation of which also has considerable mass). By using RP-1, the mass at launch can be minimized because RP-1 requires neither pressurized or cryogenic tanks.

[magnemoe]

Boiling points consideration, along with cost.

Before RP-1 was developed, US rockets used JP-4, which is designed for jet engines. At the time, JP-4 has an inconsistent composition; John D. Clark once wrote that 'no two barrels are alike'. This hinders its use in rocket engines. When nuclear ICBMs was first developed, a new fuel with a higher standard was developed. It was decided that the new fuel must come from oil, as it was cheap back then. That fuel becomes today's RP-1.

One advantage of using RP-1 instead of propane or methane is that RP-1 is liquid at room temperature and pressure. Propane and methane are gas at those conditions. To keep it liquid, propane must be stored inside a pressurized tank (which is heavy), and methane in a cryogenic one (the insulation of which also has considerable mass). By using RP-1, the mass at launch can be minimized because RP-1 requires neither pressurized or cryogenic tanks.

You are right, however propane could also be stored cryogenic, methane can be stored in a bit higher temperature than oxygen so the cooling system for oxygen would also work for it.

As I understand a future spacex engine will use methane and oxygen.

Problem has probably been that the increased effect has not been worth the extra work, if you want highest possible performance you use hydrogen.

[Bill Phil]

If its so terrible why is it used so much over methane or propane? As I understand methane would have higher ISP but as you say propane has higher density.

None of the stuff is any problems storing or working with unlike hydrogen.

Hydrogen is nice for upper stages where the low density is less of a problem as you need less but ISP is more important.

You might want to make methane in space to avoid problems with storing hydrogen.

yes, you could crack water into H2 and O2 before use but this takes lots of power and is better done over time.

Because it's easily available. If you could choose in space, you should use propane. It's simple compared to kerosene and is much more efficient.

[KSK]

Apparently carbonaceous asteroids contain significant amounts of water, which may be bound up in hydrates or other minerals (http://www.galleries.com/rocks/asteroids.htm) and also significant quantities of organic molecules (http://www.esa.int/Our_Activities/Space_Science/Asteroids_Structure_and_composition_of_asteroids).

If that's correct then heating up raw asteroid should drive off the water. Electrolysing that to produce oxygen and then, literally, burning whatever is left after heating, should produce CO2. Once you have water and CO2, you're good for lots of useful chemistry. Potentially anyway.