The Case for Mars (26 page)

The only raw materials thus required from Mars are carbon and oxygen, which are the two most plentiful elements in the 95 percent carbon dioxide Martian atmosphere and can be acquired “free as air” anywhere on the planet. The atmospheric pressure measured at the two
Viking
sites varied over a Martian year between 7 and 10 millibars (1 bar is Earth sea-level atmospheric pressure, or 14.7 psi; 10 millibar, or mbar, is 1 percent Earth sea-level atmospheric pressure), with a year-round average of about 8 mbar observed at the higher altitude
Viking 1
landing site on Chryse Planitia. Pumps that can acquire gas at this pressure and compress it to a workable pressure of 1 bar or more were first demonstrated by the English physicist Francis Hawksbee in 1709. Even better pumps are available today. However, you don’t even need a pump to compress carbon dioxide. Instead, you can use a sorbant bed that will act like a sponge, soaking up carbon dioxide. All you need to do is take a jar and fill it with either activated carbon or zeolite, and then expose it to the Martian atmosphere a
t night. Given the chill (-90°C) nighttime temperatures, the material bed will soak up to 20 percent of its weight in carbon dioxide. Then when day comes, you warm the bed up to 10°C or so, and the carbon dioxide will outgas. You can generate very high pressure carbon dioxide gas this way, with essentially no moving parts and very little power expenditure. In fact, you can even use the waste heat generated by other components on your propellant maker to drive the outgassing process. At my lab at Martin Marietta we built such a system and it worked quite well.

Now, to ensure quality control in the propellant production process, no substances of unknown composition, to wit, Martian dust, should enter the chemical reactors. This can be accomplished by first placing a dust filter on the bed inlet or pump intake to remove the vast majority of the dust, and then compressing the Martian air to about 7 bar pressure. When carbon dioxide gas is brought to this pressure and then allowed to equilibrate to ambient Martian temperatures, it will condense into a liquid state. (We don’t see liquid carbon dioxide on Earth because the pressure is too low.) Any dust that managed to evade the pump filters will go into solution, or precipitate to the bottom of the CO
₂
tank, while the nitrogen and argon constituents of the air will remain gaseous and thus can be removed, to be either discarded, or, better yet, kept for use as life-support system buffer gas. If carbon dioxide is then vaporized off the holding tank, it will be distilled 100 percent pure, as all dust will be left behind in solution. Distillation purification processes working on this principle have been widely used on Earth since the mid-1700s, when Benjamin Franklin introduced a desalination device for use by the British Navy.

Once pure carbon dioxide is obtained, the entire process becomes thoroughly controllable and predictable, as no unknown variables can be introduced by Mars. With the design of adequate quality control in the carbon dioxide acquisition process, the rest of the chemical production process can be duplicated on Earth under precisely the same conditions that will be present on Mars, and reliability guaranteed by an intensive program of ground testing. Very few of the other key elements of a piloted Mars mission (engines, aerobrakes, parachutes, life support systems, orbital rendezvous or assembly techniques, etc.) can be subjected to an equivalent degree of advance testin
g. This means that, far from being one of the weak links in the chain of a Mars mission, the in-situ propellant process can be made one of the strongest.

Once carbon dioxide is acquired, it can be rapidly reacted with the hydrogen brought from Earth in the methanation reaction, which is also called the Sabatier reaction after the chemist of that name who studied it extensively during the latter part of the nineteenth century.

The Sabatier reaction produces methane and water from carbon dioxide and hydrogen and is written as:

This reaction is exothermic, that is, it releases heat, and will occur spontaneously in the presence of a nickel or ruthenium catalyst (nickel is cheaper, ruthenium is better). The equilibrium constant which determines the completeness of the reaction is extremely strong in driving the reaction to the right, and production yields of greater than 99 percent utilization with just one pass through a reactor are routinely achieved. In addition to having been in wide-scale industrial use for about a hundred years, the Sabatier reaction has been researched by NASA, the U.S. Air Force, and their contractors for possible use in Space Station and Manned Orbiting Laboratory life-support systems. The Hamilton Standard company, for example, has developed a Sabatier unit for use on the Space Station, and has subjected it to about 4,200 hours of qualification testing.

The fact that the Sabatier reaction is exothermic means that no energy is required to drive it. Furthermore, the reactors used are simple steel pipes, rugged and compact, that contain a catalyst bed. In fact, on the basis of results obtained from a lab program at Martin Marietta, I believe that the Sabatier reactor setup required to perform all the methane production needed for the Mars Direct mission could consist of no more than a set of three reactors, each one meter long and 12 centimeters in diameter.

As the reaction (1) is run, the methane so produced is liquefied either by thermal contact with the super-cold input hydrogen stream or (later on after the liquid hydrogen is exhausted) by the use of a mechanical refrigerator. (Methane is liquid at about the same “soft cryogenic” temperatures as liquid oxygen.) The water produced is condensed and then transferred to a holding tank, aft
er which it is pumped into an electrolysis cell and subjected to the familiar electrolysis reaction, which splits water into its components, hydrogen and oxygen:

The oxygen so produced is refrigerated and stored, while the hydrogen can be recycled back to the Sabatier reaction (1).

Electrolysis is familiar to many people from high school chemistry, where it is a favorite demonstration experiment. However, this universal experience with the electrolysis reaction has created a somewhat misleading mental image of an electrolysis cell as something composed of Pyrex beakers and glassware strung out across a desk top. In reality, modem electrolysis units are extremely compact and robust objects, composed of sandwiched layers of electrolyte-impregnated plastic separated by metal meshes, with the assembly compressed at each end by substantial metal end caps bolted down to metal rods running the length of the stack. Such solid polymer electrolyte (SPE) electrolyzers have been brought to an extremely advanced stateIn fact, velopment for use in nuclear submarines, with over seven million cell-hours of experience to date. Testing has included subjecting cells to depth charging and loads of up to 200 gs. Both the Hamilton Standard and the Life Sciences companies have developed lightweight electrolysis units for use on the Space Station. These units have the capacity to perform the propellant production operation for the Mars Sample Return ISSP mission. The SPE units that Hamilton Standard has supplied for use by Britain’s Royal Navy have the correct output level to support the propellant production requirements of the manned Mars Direct mission. These units have operated for periods of up to 28,000 hours without maintenance, about four times the utilization required for Mars Direct. The submarine SPE electrolysis units are very heavy, as they are designed to be so for ballasting purposes. SPE electrolysis units designed for space missions would be much lighter.

If all the hydrogen is expended cycling the propellant production process through reactions (1) and (2), then each kilogram of hydrogen brought to Mars will have been transformed into 12 kilograms of methane/oxygen bipropellant, with an oxygen-to-methane mixture ratio of 2:1. Burning the bipropellant at such a ratio wou
ld provide a specific impulse of about 340 seconds. This would be okay, but the optimum oxygen-to-methane combustion mixture ratio is about 3.5:1, as this provides for a specific impulse of 380 seconds and a hydrogen-to-bipropellant mass leveraging of 18:1. This is the level of performance we need to reach for the optimal design of the manned Mars Direct mission.

To obtain this optimal level of performance, an additional source of oxygen must be procured beyond that made available by the combination of reactions (1) and (2). One possible answer is the direct reduction of carbon dioxide.

This reaction can be accomplished by heating carbon dioxide to about 1,100°C, which will cause the gas to partially dissociate, after which the free oxygen so produced can be electrochemically pumped across a zirconia ceramic membrane by applying a voltage, thereby separating the oxygen product from the rest of the gas. The use of this reaction to produce oxygen on Mars was first proposed by Dr. Robert Ash at JPL in the 1970s, and since then has been the subject of ongoing research by both Ash (now at Old Dominion University), and Kumar Ramohalli and K. R. Sridhar (at the University of Arizona). The advantage of this process is that it is completely decoupled from any other chemical process, and an infinite amount of oxygen can be produced without any additional feedstock. The disadvantages are that the zirconia tubes are brittle, and have small rates of output, so that very large numbers would be required for the manned Mars Direct application. It also requires about five times as much power per unit oxygen produced as does water electrolysis. Improved yields have recently been reported at the University of Arizona, so the process may be regarded as promising, but still experimental.

An alternative that would keep the set of processes employed firmly within the world of Gaslight Era industrial chemistry would be to run the well-known (to chemical engineers) “water-gas shift” reaction in reverse. That is, recycle some of the hydrogen produced in the electrolysis unit into a third chamber where it is reacted with carbon dioxide in the presence of an iron-chrome catalyst
to produce carbon monoxide and water as follows:

This reaction is mildly endothermic, but will occur at 400°C, wich is well within the temperature range of the Sabatier reaction. If reaction (4) is cycled with reactions (1) and (2), the desired mixture ratio of methane and oxygen can be produced with all the energy required to drive reaction (4) provided by thermal heat output from the Sabatier reactor. Reaction (4) can be carried out in a simple steel pipe, making the construction of such a reactor quite robust. The disadvantage of reaction (4) is that in the temperature range of interest it has an equilibrium constant of only about 0.1, which means that in order to make it go it is necessary to run a condenser to remove water from the reactor on an ongoing basis. (Water is one of the products on the right-hand side of equation (4); so long as it is continuously removed, chemical principles dictate that the reaction will keep moving to the right, producing water so as to try to maintain the appropriate equilibrium concentration in the reactor.) This is certainly feasible, and actually constitutes a fairly modest chemical engineering design problem. However, a number of alternatives that are at least equally promising have been advanced. One of the most elegant of these would be simply to combine reactions (1) and (4) in a single reactor as follows: