From: mikkelhaaheim@gmail.com   
      
   Le mardi 20 septembre 2016 12:00:08 UTC+2, elie....@gmail.com a écrit :   
   > Why use water? Liquid hydrogen is bulkie, but it will be a dramatically   
   better heat sink.   
      
   Simple answer: I knew the approximate data for water at the top of my head.   
      
   That said, there are advantages and disadvantages of using either.   
      
   The bulk of hydrogen is a serious problem.    
   First, for a given H2 vs H20 mass, you need much more structural mass to   
   contain LH (much more than the 1400% required to simply coontain the volume).   
   This is because ice can act as its own structural support, while LH can not   
   (it is even possible to use    
   ice without ANY additional structure... until the moment it melts). This added   
   mass means more thrust is required to attain the same acceleration. You might   
   want to argue that you will not need as much H2 mass because of its better   
   performance. I will    
   explain shortly why this is wrong.   
   Second, more bulk means more surface area. By using LH, you are increasing   
   your chances of being detected by 1400%. I will point out that skin temp is a   
   function of insulation efficiency... and that there is no reason you can't use   
   14°K (the melting    
   point of H2) as the starting ice temperature. I will also point out that   
   reflectivity is a function of outer coat material.   
   Third, at a very low boiling point, H2 will expand explosively to over 55   
   times its volume, converting from liquid to gas. Attempting to contain this   
   (which was not suggested), would require either much greater volume or much   
   heavier containment mass (in    
   any case, you have increased mass).   
      
   For a given temperature range, H2 greatly outperforms H20 as a heat sink.   
   However, in the given proposal, a mass of H2 can only perform as a heat sink   
   up to 19°K. At best, you MIGHT be able to extract 600 J/g, even assuming you   
   start with "solid" H2 at    
   4°K. Otherwise, ou can only extract 569 J/g assuming that you start with   
   "solid" H2 at its melting point of 14°K (taking advantage of the heat of   
   fusion), or 511 J/g if you start with LH at its melting point. After that, you   
   have no choice but to expel    
   the gas at the moment it boils. H2O is effective as a heat sink up to a much   
   higher boiling point. I presented a functional load of 733 J/g from 73°K to   
   293°K. This increases to 793 J/g if you allow room temperature to rise to   
   just below skin temp (310   
   °K). If you also begin with a common starting temp of 14°K, the useful load   
   increases to 911 J/g. If you take it a bit further, and use heat pumps, the   
   useful load increases to 3420 J/g at its boiling point, where it, too, must   
   finally be ejected. Thus,    
   the useful load of H20 is anywhere from 150% to 500% the useful load of H2   
   (this is why I say that H2O has a better heat sink performance).   
   If you combine useful load with volumetric efficiency, H2O offers 70 times   
   greater load per volume performance.   
      
   H2 is very dfficult to contain, as Mark states. Not only does H2 leak through   
   smaller seams, there is also loss through a quantum effect called "tunneling"   
   (I think this is why Mark puts "leak" in quotes). It is also much more   
   susceptible to containment    
   rupturing. Furthermore, the extreme low temperatures required tend to render   
   most materials extremely brittle, so tankage does not survive as long before   
   it needs to be completely replaced.    
   Containment aside, H2 is extremely volatile. Overheating can take place at   
   very low temperatures (below 20°K), and can result in explosive expansion. It   
   is also quite reactive, creating a risk of combustive explosion.   
      
   H2O is somewhat rare in space... although there ARE places with considerable   
   high quantities (the martian poles, Europa, etc.). Additionally, H2 is quite   
   abundant... s is oxygen. Both can be readily extracted from just about   
   anywhere, and very easily    
   combined into water. I would say availability is not much of an isue.   
      
   It is somewhat debatable which is a better propellant. H2 is, of course,   
   extremely superior as a combustive propellant. Also, given its low boiling   
   point, it has a considerable advantage in thermal expansion rate, driving its   
   Isp. However, it is very low    
   mass, which is a marked disadvantage in thrust, in principle.   
   My understanding is that thermal expansion of all gases is 1/T per °K. This   
   means that the thermal expansion of H2 at 20°K (1°K above boiling point) is   
   1/20, as it is heated to 21°K. The expansion of H2O at 1°K above its   
   respective boiling point (   
   374°K) would then be 1/374, as it is heated 1°K to 375°K. This should give   
   H2 an Isp performance benefit of 1870% (18.7x)over H20  Taking into account   
   the atomic mass, you would need to expand 18 molecules of H2 to achieve the   
   same thrust as a    
   molecule of H2O. This would reduce thrust performance benefit to about 104%   
   (1.04x) over H2O. So far, H2 is still superior... but at this point, heat   
   capacity becomes a disadvantage, as it takes 7x the amount of energy to heat   
   gaseous H2 as it does to    
   heat water vapour.   
   This is all very rough calculation, so I might have made some mistakes in the   
   previous paragraph... notably in the particulars of the analysis. Take it with   
   a grain of salt, and feel free to correct me on any of these points.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)