Conclusions Several candidate processes have been presented and discussed for ISMU on Mars. It is not yet possible to choose between them. It is also not clear to what extent Martian water would improve ISMU economics. It does appear that zirconia cell effluent would not be further processed in a Sabatier plant, and a zirconia cell would only be used in a Sabatier plant process if hydrogen were imported from Earth. Methane production using a ceramic membrane process merits further study. Storing propellants just below their critical temperatures and pressures seems to have substantial benefits. Operating plants at a higher pressure (near the storage pressure of the product propellants) appears to have benefits in simplicity of design by eliminating pumps, and reduces the volume of the plant. Substantial complexity is incurred in trying to produce propellants at ratios other than those stoichiometrically determined such as those for maximum I . Sabatier based processes are more energy efficient, but ceramic membrane systems are simpler. No single process can yet be identified as superior. ACKNOWLEDGEMENTS The authors are grateful to the University of Arizona Space Engineering Research Center, a NASA sponsored program under contract number NAGW-1332, for financial support that allowed this study. REFERENCES AND NOTES [1] Ash, R. L., Dowler, W. L., and Varsi, G. (1978) “Feasibility of Rocket Propellant Production on Mars,” Acta Astronautica, Vol. 5, pp. 705-724. [2] Ramohalli, K., Dowler, W., French, J., and Ash, R. (1987) “Novel Extraterrestrial Processing for Space Propulsion,” Acta Astronautica, Vol. 15, No. 5, pp. 259-273. [3] Ash, R. L., Cuda, V. Jr., Stancati, M. L., andNiEHOFF, J. C. (1981) “Outer Planet Satellite Return Missions Using In-Situ Propellant Production,” Acta Astronautica, Vol. 8, No. 5-6, pp. 511-526. [4] Ash, R. L., Huang, J-K., and Johnson, P. B. (1986) “Elements of Oxygen Production Systems Using Martian Atmosphere,” AIAA/ASME/S AE/ASEE 22nd Joint Propulsion Conference, Paper No. AIAA-86-1586, Huntsville, AL. [5] Baker, D.A. and Zubrin, R.M. (1989) “Mars Direct,” Journal of the British Interplanetary Society, Vol. 42, No. 4, pp. 147-160.
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