MMM Classics Year 7: MMM #s 61-70

The Ice-Water Cycle EngineIn attempting to choose a design for a heat engine forMars, the conditions of electrical power from the sun and lowtemperatures (-75° C, -103° F) were the major ambient factors.The lack of oxygen made internal combustion engines impossible[unless the oxygen is provided from an onboard tank]. Asteam engine is possible, with a large solar concentrator providingthe heat. But on Mars, it is possible to go over to the otherphase transition, water‹fiice, with a weight saving oversteam pressure fittings and only a small loss of efficiency. Aheat engine cycle across the liquidus‹fisolidus line usingH2O as a working fluid, i.e., an Ice-Water Cycle Engine, offersadvantages.The Ice-Water Cycle Engine is a cylinder filled withwater and a piston. When the water freezes, it expands, andwork is done against the piston. The solid is then returned tothe liquid phase by joule resistance heating. Energy is thustransferred from the solar panels to the atmosphere through aphase transition which also produces work. The greatest advantageis the large force on a piston of modest area; the slope ofthe equilibrium curve is so sharp (dP/dT= -130 atm/K) thatenormous forces can be generated by the expanding ice. Thelimit is reached when higher phases of ice with a specificgravity greater than 1 are produced. Operating between -17° Cand 0° C [1.4° to 32° F], 2100 atmospheres (2.1 x 108 pa) canbe generated on the piston. This makes the ice-water cycleengine ideal for situations on Mars where crushing, pulverizingand heavy lifting are desired. It also has a weight savingover electro-inductive/hydraulic systems, especially valuableon automated Mars rovers which must be lifted up from Earth.A small operating ice-water cycle engine wasconstructed and tested at the Allegheny OIC1 technical schoolin McKeesport, PA during the winters of 1978-79. Pistonreturn was facilitated by a simple oblique spring after meltingwas performed by an external coil connected to an automobilebattery. Cycle times were about 90 minutes2 depending on theexternal temperature and the battery was drained rather rapidly.These test were not rigorously scientific but were simplydesigned to see if the concept worked at all.3Calculation of Engine Efficiency: heat into the engine is -79.9 cal/g= 333.1 j/g. The work function is generallyW = V(PT) P dE(P,T)Considering the upper pressure limit of 2100 atm (2.1 x 108 pa) andthe volume change ofThenFor which the thermal efficiency isDV = 0.093 cc/g = 9.3 x 10-8 m3/g PdV = P dV = 19.53 joules/gh = W out = 19.53 = 5.8%Q in 333.1 .This is comparable to the actual efficiency of a steamengine. Due to thermal gradients, the actual efficiency of anoperating ice-water cycle engine will be somewhat lower.Additional controlled experiments are required.In conclusion, solid/liquid phase heat engines maywell become part of a menu of technology useful to applicationsin space economies. Undoubtedly, many other possibilitieson that menu specific to extraterrestrial conditions remainto be discovered. 4 FGFootnotes:1 Opportunities Industrialization Center.2 In a phone conversation 1/15/93, Graham suggested thatthis long cycle time could be brought down at least to a fewminutes by using an internal heat source, perhaps a laser, incombination with a very heat conductive outer cylinder. Theidea of his experiment was just to see if it worked at all, notto optimize the engineering.3 A rather thorough patent search showed no prior work onthis type of device. Graham welcomes hearing from anyoneelse who has thought or tinkered along similar lines. Writehim c/o MMM.4 Graham also reports on solid-liquid Bismuth enginessuitable for use on Mercury. MMM will publish that articleseparately.Acknowledgements: The author wished to thank DaleAmon, Hans Moravec, and Norman Wackenhut for fruitfuldiscussions, and the Allegheny OIC for many kindnesses.References(1) Kennedy, G.C. and LaMori, F., in Gray, D.E., ed.,American Institute of Physics Handbook, McGraw-Hill,NY: 1963(2) Loebel, R. in Weast, R.C. Handbook of Physics andChemistry, CRC, Boca Raton: 1980, p. B-253.MMM #63 - MAR 1993The Industrial Roots of Lunar Settlement Self-SufficiencyA lunar settlement basedsolely on the twin foundationsof Science (geology,mineralogy, astronomyetc.) and Exploration can,like bases in Antarctica,survive as long as the political and military will needed tosecure public funding is high enough. That approach wouldmake it a fragile under-taking, perennially threatened by thetwin axes of back home budget priorities and fickle publicsupport. But let civilians (people with families) take over andstart doing something to pay their own way, and begin turninga profit, and the lunar frontier will soon take on an unthreatenedlife of its own.In this issue weexplore the industrial basisnecessary to secure selfsufficiencyand truepermanence.Moon Miners’ Manifesto Classics - Year 7 - Republished January 2006 - Page 15