Engineering Mars commercial rocket propellant production for the Big Falcon Rocket (part 1)
by Steve Hoeser
|The saying goes that ideas are what could happen, while engineering is about making things happen.|
Elon Musk has indicated that SpaceX will exploit this approach to produce the propellants needed to refuel his reusable Big Falcon Rocket (BFR) during Mars transport missions. The BFR propellants include densified liquid methane and oxygen. This series of articles will take a first order engineering look at some of the leading ideas for establishing production plants for the propellants to fuel a BFR on Mars. Also reviewed are associated, potentially marketable, by-products, and especially the energy that may be required for the production processes in the context of emplacement considerations of such a commercial enterprise.
The saying goes that ideas are what could happen, while engineering is about making things happen. So when considering selection and integration of system technologies for the reactant/product recirculation and separation of such a Martian propellant processing plant, many real-life factors, including physics and chemistry, must be the basis for consideration. In this case, this includes addressing separation of ISRU products under relevant operating conditions, regeneration approaches for filters, membranes, and/or sorption beds, the integration of thermal control systems for operation in the colder Mars environment, pressure differences caused by separation processes, and the effects of lower Mars gravity. Specific examples of first order propellant processing plant separation and recirculation engineering design considerations that have already been identified are:
It should be pointed out that NASA’s NextSTEP-2 program seeks to investigate ISRU space resource utilization technology.1 Their vision is to progress from our current “Earth-reliant” approach to exploration and eventually become “Earth independent” in space. NextSTEP uses a public-private partnership model that seeks commercial development of deep space exploration capabilities to support more extensive human spaceflight missions in and beyond cislunar space. NASA anticipates that the results of these efforts over the ensuing years will provide the needed engineering details for full-up ISRU system developments.
1. Sabatier Processing Subsystem: The Sabatier methanation reaction has been recommended by many of the top researchers as the most likely basis for an ISRU propellant production processing plant on Mars. This is driven primarily because this process can exploit the extremely high availability of carbon dioxide in the Martian atmosphere to create both the methane fuel and water, which can be broken down into useful hydrogen and the oxygen oxidizer needed to run the BFR engines.
CO2 (g) + 4H2 (g) ⇋ CH4 (g) + 2H2O (g)
ΔH = –165.0 kJ
The Sabatier reaction or Sabatier process was discovered by the French chemist Paul Sabatier in the 1910s. It involves the reaction of hydrogen with carbon dioxide at elevated temperatures (optimally 300–400°C) and pressures in the presence of a catalyst to produce methane and water. Nickel or, more optimally, ruthenium on alumina (aluminum oxide) acts as the catalyst.2 The methane gas is the end propellant product. The water produced acts as the feedstock for later propellant plant processing and other process products.
|The low temperatures on Mars also mean that careful design attention to proper insulation of the reactor for thermal management is required.|
Since the Sabatier reaction occurs rapidly enough to be useful at about 400°C, a source of heat is needed to get the reactor up to its operating temperature. This will likely be provided by some form of an electric heater. Fortunately, once started, the reaction produces its own thermal energy. This means that, except for reactor conductive losses and process thermal management makeup, once the initial temperature is achieved, the reaction provides the majority of the heat to continue the reaction. So this part of the process is not continuously demanding high energy input.
For this first order analysis it is assumed that the energy for this initial reactor start-up has already been expended and loss make-up is low so that it is not a driving requirement to the conceptual design. Instead it will be booked for more detailed consideration during detailed design engineering.
The low temperatures on Mars also mean that careful design attention to proper insulation of the reactor for thermal management is required. Also, as with any chemical reaction, continuous process control monitoring will be needed in the subsystem design to maintain reaction temperatures within optimal bands to obtain the highest production yields.
Once produced, the Sabatier reactor methane gas product outputs will need to go through a separator to remove process-generated water vapor. The collected water vapor is assumed to be collected, rather than expelled as manufacturing waste. The collected water would be thermally conditioned by taking advantage of the cold Martian atmosphere using exterior radiator to condense the water vapor into liquid water, and then be directed to water storage tank(s) for later processing or product sale.
Design and management for liquid water storage tanks needs to carefully consider. This is driven by the average recorded temperatures on Mars of –63°C. These Martian temperatures are well below the freezing temperature of water. Further, this temperature varies from a maximum temperature of 20°C and a minimum of –140°C at the polar regions depending on time of year and distance from the equator. Consideration of the local conditions in which the plant will operate are important input design factors in engineering of the heat exchanges, pipe insulation requirements, and thermal management subsystem design.
A rigorous design process will use heat exchanges and water circulators to tap the incoming warm water vapors from the Sabatier process to help maintain the water in liquid form. For purposes of this study, the energy necessary for circulating pumps is anticipated to be minimal, compared to other primary demands in the processing. The circulating pumps should thus account for a minimal amount of overhead energy estimated in the processing plant budget.
The Sabatier process needs a source of free hydrogen to react with the carbon dioxide to produce methane. A partial source that can be used to obtain the needed hydrogen is through the electrolysis of the water by-product that results from the Sabatier reactor. Unfortunately, for every 44 kilograms of carbon dioxide that is converted into 16 kilograms of methane in the Sabatier process, you need four kilograms of hydrogen. The Sabatier process only produces 36 kilograms of water in the balanced reaction. This means an external source of additional “make-up” hydrogen is required for continuous reactor processing. Unfortunately, free hydrogen gas on Mars is essentially nonexistent.
Instead, an additional source of water is needed to obtain the make-up hydrogen feed stock for the Sabatier reactor. Potential sources of this water could come either from Mars ISRU or could come as an imported commodity from Earth or other celestial bodies, such as the Moon. Water sourcing and further propellant processing to obtain the needed hydrogen and to produce propellant oxygen oxidizer is covered in section 3 below. (Authors clarification note: For purposes of this first order analysis, the extremely small amounts of water obtained from the Martian atmosphere are an infinitesimal contribution to water supplies needed and are not included in the results).
A 2011 prototype test operation was conducted that harvested carbon dioxide from a simulated Martian atmosphere and reacted it with hydrogen. This test produced methane gas at a rate of one kilogram per day, operated autonomously for five consecutive days maintaining a nearly 100 percent conversion rate. An optimized system of this design massing 50 kilograms “has been projected to produce 1 kg/day of O2:CH4 propellant… with a methane purity of 98+% while consuming 700 Watts of electrical power.” Overall unit conversion rate expected from the optimized system is one metric ton of propellant per 17 megawatt-hours energy input.3 So assuming all feedstock is available to feed the processing the power to produce a full load of methane (in gaseous form) for a BFR (240 tons) is estimated to be in the neighborhood of 4.1 gigawatt-hours.
2. Raw Material Sources for Sabatier Process: Before Sabatier reactor production can occur, the raw materials for processing must be obtained. Surveys and spectral findings to date indicate that the carbon dioxide required for the Sabatier reaction is an abundant resource in the Martian atmosphere. Even though the atmospheric pressure on Mars is very low—less than one hundredth of Earth’s—the Martian atmosphere consists of almost 95 percent carbon dioxide. Despite its thin atmosphere, this Martian concentration is almost 20 times as much carbon dioxide as is in an equivalent amount of the Earth’s atmosphere.
|The other primary issue of the raw Martian atmosphere is the presence of moderate to significant amounts of dust.|
However, like on Earth, the Mars atmosphere contains other gasses. In addition to carbon dioxide, the Martian atmosphere also has about three percent nitrogen (N2) and trace amounts of oxygen and water. (For completeness, it should be noted that there are also miniscule amounts of other gases. These miniscule gases should not impact propellant processing. These include argon, neon, krypton, and xenon.) This means a gas separation system is needed prior to feeding the Sabatier processing to remove nitrogen gases and trace amounts of oxygen and water to obtain processing-quality carbon dioxide purity.
Carbon molecular sieves (CMS) are the method commonly used on Earth in the separation processing of air. CMS’s belong to the activated carbons family and can be obtained by various procedures to create pores narrowing to smaller sizes than 10 angstroms. This allows CMS’s to be used to separate the carbon dioxide from the nitrogen. A desiccant, oxidizing reactive bed or adjusted chill/pressure cycle to freeze out the oxygen and remove water should also be considered as an engineering solution method to separate these trace constituents from the atmospheric carbon dioxide in Martian propellant processing plant designs.
However, with high costs of transport for the various CMS and desiccant materials from Earth, it may be more practical to simply expend the additional power instead using a multi-stage chiller pressure cycle subsystem to separate the Martian atmospheric constituents. An example of this processing is the fractional distillation process. On Earth, this process uses cryogenic air separation units (ASUs) to separate nitrogen, oxygen, and often to co-produce argon. The process separates gases by cooling the gas at specified pressures until the individual gas components condense at a given temperature into a liquid which is then extracted. The downside to using this process is that it is a very energy-intensive operation due to the operation of multiple turbo-compression cycling and associated condensation.
The other primary issue of the raw Martian atmosphere is the presence of moderate to significant amounts of dust. In some cases, dust storms have covered virtually all of Mars. When considering design solutions, the dust removal methods that require minimal maintenance, lowest operating costs, and that will maintain continuous plant intake are most desired. For this reason, remove-and-replace mechanical filters, such as those in home furnaces, are not desirable. Vortex swirl particle separator designs would likely be a better option. Regardless, this dust will need to be removed from the Martian ISRU air intake as well during atmospheric pre-processing.
For purposes of this first order review, the key design selection metric for Mars propellant production is to reduce electrical power needs. Since the CMS units will only require minor ongoing electrical needs, they are used for carbon dioxide atmospheric extraction in this review. The CMS power needs only includes the electricity needed by fans to force the gasses over the CMS materials and dust filtration to remove any dislodged CMS particulates. The CMS electrical needs are accounted for in the propellant plant power overhead budget.
Further, the import costs to transport and the set-up needed for the CMS material in the propellant production facility is assumed to have already been completed. Future engineering detailed design and economic viability investigations should take closer looks at these system engineering factors.
(Author’s additional note: It is interesting to note that with profitable expansion of propellant production, the technology appears to exist for making CMS by using local 3-D printing units and ISRU materials. The additional expenses associated with the electricity for mining, refining, and printing the CMS versus the use of multi-stage chiller pressure cycles will be a design/cost trade for future process expansion selection considerations.)
A key to efficiently designed Martian propellant plant—or, for that matter any off-world processing plant—is that the production system reaction products should not waste potentially marketable or usable manufacturing byproducts. For example, the nitrogen extracted from the atmospheric gas separation processing is a potentially exploitable/marketable product. Nitrogen is often used for flushing of tanks and lines through which other gases pass because it is reactive neutral. Then too, the oxygen and water atmospheric separated byproducts are both potentially valuable feedstock for making propellant oxidizer or for life support. For this reason, designing engineers will need to consider options, means, and costs in any facility design with business analysts to determine the costs to market value of any manufacturing byproducts.
3. Water ISRU Sourcing on Mars: Most space scientists currently agree that vast deposits of water appear to be trapped within the ice caps at the north and south poles of Mars. Frozen water also now appears to lie beneath the Martian surface at lower latitudes. Scientists have discovered a slab of ice they estimate is as large as California and Texas combined in the region between the equator and north pole of the Red Planet. Other regions of the planet may contain undiscovered frozen water as well. Specifically, some of the high-latitude regions seem to boast patterned ground shapes that may have formed as permafrost in the soil that froze and thawed over Martian geologic history.
|A key to efficiently designed Martian propellant plant—or, for that matter any off-world processing plant—is that the production system reaction products should not waste potentially marketable or usable manufacturing byproducts.|
Water mining on Mars will require the development of its own extraction and processing facility. Shipped in from places like the Moon, water may also be an early economically viable purchase alternative. This imported water would require development of lunar water mining, production, and Martian logistics transport architecture. If such commercial capabilities are in place at the establishment of a Mars BFR propellant plant, a certain amount of imported water might prove cost-effective (or simply unavoidable) during the build-up transition to full ISRU water mining and processing production. The selection and cost-effective balance of water sourcing will require additional future detailed system engineering and business trades plus operational business management monitoring over the production plant’s operating life.
For this first order review, it is assumed that an affordable means of extracting and processing of pure quality water will be identified from Martian ISRU water sources. In a 2016 planning study for NASA to examine Martian ISRU methane/oxygen processing, the use of Martian water ore was investigated.4 That study examined four cases of water ore processing. Robotic vehicles, such as the NASA KSC Regolith Advanced Surface Systems Operations Robot (RASSOR) prototype or the OffWorld Inc.5 smart robots, are likely candidates for mining the raw “water ore”.
For this investigation’s ISRU review, the NASA study four-kilowatt-hour figure was used for the power estimates used by robotic water extraction/mining unit operations. Including the power needs to then load the water ore and transport this water ore feedstock adds another 25 percent to this number, for a total of five kilowatt-hours to mine 4,150 kilograms of water ore. The 7.5-kilowatt-hour examples from the NASA study were then used for power estimates of the separation and vitalization processing to produce 33 kilograms of refined pure quality water from the mined Martian water-bearing regolith ore. This gives an energy cost of extracting and processing (with an overhead factor) to produce purified water from Martian raw ore materials at an estimate of about 230 watts per kilogram. Using this ISRU water source to make up roughly half of the needed water to fuel the Sabatier processor will require roughly two gigawatt-hours of power to mine, produce, and deliver the makeup hydrogen for a single BFR propellant load.
The next article in this series will cover the remaining propellant production processes, while the third installment will investigate options for producing the power necessary for BFR propellant ISRU-based production.