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Author Topic: A brief introduction to ISRU for Mars  (Read 5005 times)
aftercolumbia
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« on: October 14, 2006, 11:38:01 PM »

 Shocked

During the development of Mars Challenger's design, I learned a lot about the challenges of In-Situ Resource Utilization, especially the partial ISRU plans that dominate piloted Mars mission designs, such as Mars For Less, Mars Direct, or the particularly infamous NASA Design Reference Mission (the second most expensive way to get to Mars after the Space Exploration Initiative.)  Basic Sabatier/Electrolysis ISRU uses the following two chemical reactions:

(1) Sabatier Reaction (Also known as Reverse Water-Gas Shift)
4H2 + CO2 -> CH4 + 2H2O

(2) Hydrolysis (a.k.a. Water Electrolysis)
2H20 -> 2H2 + O2

The first one is super simple, slightly exothermic catalyst bed type reaction.  The reactor resembles a car's catalytic converter and the first one used for ISRU related testing was designed and assembled by Dr. Robert Zubrin in a matter of hours in 1989 (with a couple months of waiting for ordered parts in between.)

Hydrolysis, primarily used for making high purity hydrogen, is much more "expensive" and difficult.  There are a lot more design choices, including type of electrolyte, cell voltages, etc.  The reaction is highly endothermic, but as we shall soon see, the electrical requirements of hydrolysis for Mars ISRU are not such a big deal after all...

Physical Processes:

It wasn't a surprise when I found out that the ISRU production concept is dominated by its physical processes.  In Mars Challenger, I'm trying, very literally, to pull over two tonnes of finished cryogenic propellant out of thin air.  Rabbits and top hats notwithstanding, the dominant process having to do with thin air is compression.

On Mars:

You actually need two compressors, one for the Sabatier and one for the fridge.  The best fit of Earthly equipment to the task of liquifying the propellant is Cryomech (www.cryomech.com) Gifford-McMahon helium cryocoolers (i.e. the AP300/CP970 system.)  As far as compressing Mars air...well...you need a compression ratio of about 1200.  The equivalent on Earth would generate some 18000psi...typical of handgun chambers.  There is no such compressor, so it must be developed from scratch.

The compressed Martian air is given an appropriate dose of hydrogen and then injected into the Sabatier.  What comes out is a mixture of methane, water vapor, surplus carbon dioxide, and a tiny bit of hydrogen, along with other constituent gasses on Mars.  The first task is to get the water out, which happens in a distiller.  The distiller needs no refrigeration, as the record high on Mars is about 11deg.  In fact, it may even need heating because of water's tendency to freeze.  Once the water is out, what's left goes to the first stage of the liquifier, which will condense out everything but the methane and hydrogen.  The second stage of the liquifier liquifies the methane.  The hydrogen is extracted from the vapour and run back into the Sabatier either in the fridge, or in the storage tank, where it will occupy the bubble in the top.

A typical cryocooler operates by running a helium compressor of the Copeland Scroll type.  Helium is an excellent refrigerant with a really high specific heat ratio (aka. gamma) and low heat capacity (meaning it can change temperature quickly.)  The compressor is in the vertical position and is oil flooded.  80% of the adiabatic compression heat is removed by the oil, while the remaining 20% stays with the helium, which is now inconveniently entrained with little oil droplets.  After going through a heat exchanger (water or air cooled) it goes into an oil separator, which uses a big silica cotton puff to gather the oil droplets up and drain them to the bottom.  A charcoal adsorber deals with the vapours.  This is important because impurities can freeze in the cold head and cause it to go on strike.  The cold head uses a displacer piston to expand the helium, leading to efficient adiabatic cooling.

Um...perhaps an oil flooded compressor is a bad idea.  The oil can freeze on Mars, and it means having yet another fluid to bother with.  TAI (http://www.techapps.com/images/03CECPaper.pdf) has a better idea: use a two stage scroll with intercooling so that 100% of the heat is removed by the helium, and the compressor can stay dry.  Copeland Scroll parts do not rub against each other, they press, forming a good enough seal between the compression parts.  This makes the compressor a tad more complex, but everything else having to do with it as complex or simpler.  The radiator, instead of having an oil loop and a helium loop, has two helium loops at different pressures.  The oil separator is replaced by a tiny filter to eliminate wear particles, and the adsorber, if it is needed at all, can be made much smaller.

During the trip to Mars, looking after the hydrogen, especially if it is in your boosters ascent tanks (like it is on Mars Challenger) is also rather difficult.  Fortunately news from the Hershel Planck mission (http://www.rssd.esa.int/herschel/Publ/1998/cryo98-fp-bc.pdf) shows that this is relatively easy and flight proven.  Because the ISRU application is not vibration sensitive, we don't have to get fancy with J-T sorption stuff and 3He diffusers...whatever all that is...  Also liquid hydrogen may be cold, but not _that_ cold.  Cryomech's pulse tubes are good enough.  They are also compatible with the same compressors as the liquifier cold heads.

So, during cruise, we run the same compressor as we will on the surface, but with a different type of cold head better suited to LH2 tempuratures.  With that, we don't need to get as fancy as Herschel with the insulation, and our tanks are built more for the requirements of a sample return booster rather than a cryogenic space telescope.

With basic ISRU, one of the difficulties is caused by the fact that you are likely going to be bringing along more hydrogen than you will have final propellants...by volume, that is (bulk oxymethane propellants are over eleven times as dense as liquid hydrogen.)  This leads to juggling tanks and figuring out how to store propellants without mixing oxygen and something that could explode with it in the same room.  I recommend storing water, the intermediate product from the Sabatier reactor.  It's the easiest thing to store, Mars Challenger uses its hover propulsion system tanks.  In fact, the hydraulic system water tanks on the Shuttle used to be satellite hydrogen tanks...although they're not using exactly the same tanks, like mine.  If you're trying to solve a piloted missions ISRU problem, do be aware that storing water in an ex-hydrazine tank is not a good idea for potable supplies.  Hydrazine is well... Yuck!...actually, it's Jolly Rogers type stuff (typical WHMIS, HazMat, DG symbol...you might notice it on your cleaning chemicals at home.)

That's about it...see easy as  Shocked, enjoy!
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aftercolumbia
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« Reply #1 on: October 15, 2006, 10:17:32 AM »

In my second look at this article, I realize that I made it sound like storing hydrogen in space was easy.  Herschel uses a helium tank similar in size to Judith's entire supply.  It is too heavy to be used in the ascent application, and the real problem with long term liquid hydrogen storage is not the cold temperature, but material compatibility.  A lot of metals absorb hydrogen and become brittle, especially titanium.  This is the problem that demands an all new tank.
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