In-Space Manufacture of Storable Propellants

Our exploration of the feasibility of in-space production of storable propellants from resources available on asteroids, and also on Mars and the Moon, has considered the process of sample acquisition; the energy requirements for heating and volatile release; the thermodynamic behavior of gas release from minerals and organic polymers containing hydrogen, oxygen, carbon, sulfur, and nitrogen; purification of the released water and carbon dioxide; the storage and transportation of these materials in the condensed state; synthesis of fuels including methanol and dimethyl ether (DME); and the co-production, concentration, stability, and storage of the complementary oxidizer high-test hydrogen peroxide (HTP). We call attention to the ability of the storable propellant/oxidizer combination of DME and HTP to carry out deep-space missions and to perform retrieval and relocation of any and all space-derived resources, such as retrieving asteroidal metal to high Earth orbit. This study's analyses are based on returning resources to a storage/processing/dispensing facility in a Highly Eccentric Earth Orbit (HEEO) with a perigee above geosynchronous orbit and an apogee approach or beyond the Moon's orbit. The methane/LOX option, notable for good engine performance, has not been included in this study because our scope includes only fully storable propellants. This work in Phase I has led to a number of conclusions. These are: 1. Thermodynamic theory shows that extraction of water and carbon dioxide from carbonaceous (C-type) near-Earth asteroids by means of direct solar heating is feasible and efficient. This is in agreement with experiments carried out by Joel Sercel in an independent NIAC Phase I project, using both CM type meteorite material and a C-type asteroid simulant that we have developed at DSI and the University of Central Florida under an SBIR grant running concurrently with this (and his) NIAC grant. 2. The same theory also predicts that attempts at full extraction of volatiles from a C asteroid will require calciner temperatures of at least 700 K, at which temperature not only does the native organic polymer in the C asteroid material react with magnetite (Fe3O4) to generate carbon dioxide and water vapor, but also these gases react with coexisting sulfide and sulfate minerals to release copious amounts of sulfur dioxide. This prediction has also been qualitatively verified by Joel Sercel's work on meteorite and DSI simulant materials. At these temperatures, release of H, C, O, N, and S produces gases amounting to about 40% of the total mass of the asteroidal material. 3. The principal sulfur gas released, sulfur dioxide, is a source of some concern for several reasons. It is a toxic and offensively odorous gas that must be removed from water intended for life-support or hydroponic use. It also spontaneously generates elemental sulfur and sulfuric acid, highly undesirable materials that can corrode or clog water-handling systems such as Solar Thermal Propulsion engines. Further, some of these sulfur compounds can poison the catalyst beds used in several critical processing steps in manufacture of propellants and metal products. 4. Sulfur impurities may be removed on the asteroid from freshly generated impure water to make it safe and suitable for these uses. The application of Reverse Osmosis (RO) or other relevant technology provides the requisite purification in a simple and safe manner, using equipment of high TRL. The purified water is then suitable for use in everything from STP thrusters to chemical reagents to chemical propellant manufacture to life-support fluids. (4alt.) Alternatively, SO2 release may be minimized by using lower calcining temperatures, which also severely limits CO2 release. This option suggests retrieval of only water to HEO on the first mission, with production of only H/O propellants. Such a mission would also provide an opportunity for retrieval of unprocessed asteroid material to HEEO for use in process-development experiments there, relegating CO2 and SO2 production to HEEO. 5. The rejected sulfur compounds from water purification are a valuable feedstock for manufacture of industrial grade sulfuric acid, a critically important and useful chemical reagent for mineral and metallurgical use, and do not represent waste. We have not yet studied the handling and processing of this sulfur-rich material since it is not the main focus of this project; for present purposes we envision sequestration and storage of sulfuric acid against future demands. In scenario 4, the sulfuric acid would be made and stored on the asteroid; in scenario 4alt this would take place on the HEEO facility. 6. Manufacture of HTP and DME (or methanol) requires only CO2 and H2O as raw materials, in the proportion of 2 CO2 molecules per 9 molecules of H2O for making DME/HTP (1:5 for methanol/HTP). Several candidate processes are available for manufacture of each product, all requiring the presence of hydrogen and oxygen gas. Thus electrolysis of water is an essential step in storable propellant manufacture. This process is carried out routinely as part of the life support (oxygen recovery) system of the International Space Station, and has a very high TRL, with years of flight experience. 7. CO2 and H2O can be co-condensed as a methane clathrate hydrate, CO2.6.75H2O, close in composition to the desired ratio for coproduction of HTP and DME, with some excess water. Hydrate formation appears to be a very useful way of storing and transporting its component gases for in-space industrial use, and may be a patentable process. If clathrate formation proves too challenging in our Phase II laboratory tests, transport as solid CO2 and H2O is feasible. 8. Production of methanol can be effected by passing a mixture of hydrogen and carbon oxides (either CO or CO2) through any of several types of industrially validated catalyst beds. DME can be produced efficiently either by dehydration of methanol (one common practice is dehydration of methanol by reaction with sulfuric acid) or using a two-layer catalyst bed with the methanol product being immediately passed at higher pressure through a gamma-alumina catalyst layer to manufacture DME. Either methanol or DME is a credible propellant. 9. Most terrestrial manufacturing schemes for HTP use organic catalysts of questionable utility in space; however, one proposed (and partially patented) scheme is simply an adaptation of conventional H/O fuel cell technology optimized for HTP production rather than electrical energy generation. Further development of this scheme and validation of various candidate electrode materials looks very promising for efficiency, robustness, and simplicity. 10. The essential on-asteroid processing steps and flow diagrams (heating, condensation, reverse osmosis, and freezing for transport) seem readily manageable, whereas the ensuing processing steps for co-production of HTP and DME are a mixture of high-TRL and low-TRL steps of a higher degree of complexity. It would seem prudent to carry out these steps in near-Earth space where human-tended or tele-operated equipment would carry out the synthesis. 11. HTP production in this scheme is remarkably free of sources of contaminants that could catalytically destroy HTP during storage. We have developed a quantitative model of the kinetics of HTP decomposition that shows extremely low decomposition rates (order of 10-4 to 10-5 %/year) when stored in appropriate containers near or below the freezing point with ppm concentrations of sodium or potassium stannate as stabilizer. 12. The insensitivity of propellant performance (for example, the Isp for DME/HTP fueling) on the water content of HTP makes extreme purification of the HTP to <5% H2O unnecessary. 13. It appears that the best system architecture would involve water production and purification at the asteroid, use of STP with water as the working fluid for the retrieval of large masses of asteroidal material to Earth orbit, and processing of retrieved water and carbon dioxide in Earth orbit to manufacture storable (or cryogenic) propellants for use in outbound missions to asteroids or any other destination. 14. C asteroid processing schemes (but not sample acquisition schemes) would be applicable to any source of CO2 and H2O, including lunar polar volatiles and Martian volatiles. 15. Whether future missions will employ deep cryogens (LH2/LOX), mild cryogens (methane/LOX), storables, or simply water for STP or NTP use, retrieval and purification of asteroidal volatiles (water and carbon dioxide) is a key technology. 16. Water-based STP, with its low power (governed by the solar flux), has sufficient Isp (at least 200 s) to serve well for launching a water payload from an asteroid and directing it to Earth intercept, but lacks the acceleration to be effective in capturing into Earth orbit at approach speeds too high for lunar capture. Chemical propulsion, with its high thrust level, is better suited for both Earth capture (via inverse Oberth effect at low altitude) and Earth departure to deep-space targets. Initially, carrying some of the propellant derived from earlier missions to assist in Earth capture may be desirable. 16alt. In the same way, Earth capture via aerobraking with or without a lunar swingby is an extremely promising way to improve overall performance (mass payback ratio) of the system. Aerobrake manufacture and use are not within the scope of this study. 17. Looking forward, manufacture of propellants at the asteroid target will require significant experience in Earth orbit to validate and fully automate the processes, suggesting that it will be significantly farther downstream and that current research should concentrate on improving the low-TRL parts of the processing scheme in Earth orbit. 18. Development of the technologies for in-space propellant manufacture can easily be approached as a stepwise, evolutionary sequence. The logical sequence appears at present to be: a) Extraction of water from an NEA and return to Earth orbit via Solar Thermal Propulsion using part of the water as the STP working fluid. All water processing into marketable products would occur in HEEO. Products available in Earth orbit would include water, hydrogen, oxygen, and hydrogen peroxide, addressing markets such as radiation shielding (water), life support (water and O2), cryogenic propellants (LOX/LH2), and monopropellants for station-keeping and microsatellite propulsion (H2O2). Critical technologies include water extraction and purification, development of a water-based STP system, adaptation of ISS water-electrolysis technology, and production of high-test peroxide (>95% purity H2O2; HTP). If deep-cryogen propellants are desired, then liquefaction of oxygen and hydrogen are required. b) Extraction of carbon dioxide from the same NEA and return of water and CO2 via water-based STP to HEEO, quite possibly transporting these materials as a solid clathrate hydrate. With water and carbon dioxide available as feedstocks, carbon-based storable propellants such as dimethyl ether (DME) or methanol (CH3OH) can be co-produced with hydrogen peroxide. Results from Phase I indicate that the first missions actually may be able start with this phase \u2013 combined water and CO2 harvesting \u2013 and a critical part of the Phase II research is verifying that this leap is feasible. c) Using the same starting materials, methane/LOX production in HEEO becomes possible. This is an attractive intermediate between deep cryogens (LH2) and fully storable propellants (HTP/DME) in terms of both technical difficulty and performance. 19. In the longer run, once experience in storable propellant manufacture is in hand, it would be desirable to transfer that process to the surface of the asteroid where mining and extraction are taking place. This would open up a variety of new mission opportunities to, for example, Beltasteroids and the outer planets.