From: mikkelhaaheim@gmail.com   
      
   Le vendredi 23 septembre 2016 14:35:48 UTC+2, elie....@gmail.com a écrit :   
      
   > At this temperature, heat pumps are too inefficient to be useful.   
   Theoretical max efficiency is Thot/(Thot-Tcold), which for liquid hydrogen is   
   14/(14-3) = 1.28, meaning you produce several more watts of waste heat per   
   watt of heat removed, and that's    
   at the most perfect theoretical performance.   
      
   Is there a reason you are inverting your expression of efficiency? With   
   maximum efficiency expressed as "1", the formula for theoretical maximupm   
   efficiency is (Thot-Tcold)/Thot. This is multiplied by 100 if you want to   
   express efficiency in percentage,    
   with max efficiency being 100%.   
   You are making my point regarding the preference of water/ice over H2.   
   However, I find it rather silly to limit your "hot" temperature to 14°K. Your   
   H2 is suitable to absorb heat up to 20°K, so this should be your "hot" temp.   
   Still, your heat pump    
   efficiency will be limited to 0.85 (or 85%). However, using H2O as the heat   
   sink, you can bring the hot operating temperature of the pump to 373°K,   
   yielding a maximum theoretical efficiency of 0.99 (99.2%).    
   Actually, it occurs to me that the efficiency of both would be slightly   
   better, as the He coolant loop would have to be chilled BELOW 3°K in order   
   for heat to flow from the shell to the coolant. Given that you don't want the   
   coolant to get above 3°K    
   outside the insulation layer, and that the shell would not be able to heat it   
   further anyway, you would then want the coolant to circulate through hotter   
   sections of the craft to bring up its "hot" temp. The good news about this is   
   you may not actually    
   need to pump it (add heat).   
      
   >    
   > helium will be neither good enough at absorbing heat per kg, per m^3 nor   
   even that good as a propellant, so it won't cut it by itself.   
      
   No. I did not intend that helium should be used as either heat sink or   
   propellant, but rather as a closed-loop primary coolant transfering heat to   
   the heat sink.   
      
   > So you will need solid hydrogen (as cold as possible, let's arbitrarily say   
   1K), and a liquid helium loop to keep the surface of the ship at 3K. This   
   complicates the design.   
      
   Correct. Except that H2O would be a better choice.   
      
   > On the other hand, wouldn't solid hydrogen be less prone to escape and   
   embrittle everything?   
      
   Actually, it is much more prone to embrittlement, but I think you are probably   
   correct that it would be less prone to escape (although I don't know how this   
   would affect it's propensity toward quantum tunneling loss). It might even   
   provide some    
   structural reinforcement. However, it will still underperform H2O.   
   A little bit of intellectual honesty: most of the problem with embrittlement   
   is due to temperature, so H2O would produce some of the same problems here.   
   OTOH, much of the problem with H2 is it's volatility. H2O is far less   
   volatile... unless you go in    
   the opposite direction and superheat it at high pressure.   
      
   >    
   > So as long as we have a (initially) solid heat sink and a liquid helium   
   loop, it is probably not that much more complicated to have one type or   
   another, or even several at the same time.   
      
   Correct.   
      
   > Still, I would stick with hydrogen, at least for the most part, for its   
   capacity to absorb more heat per kg at low temperature, and for its   
   performance as a propellant.   
      
   Except this is more heat per kg PER °K. The fact that H2O has a much greater   
   useful temperature range means that it has far superior performance per kg   
   itself.   
      
   >    
   > Hydrogen absorbing more energy per kg is a feature, not a liability: energy   
   is free at as great a quantity as needed with the Sun, thanks to solar-thermal   
   propulsion. If you need more energy for a given dV, simply increase the mirror   
   aperture.   
      
   Energy absorption is a feature, not a liability, FOR A HEAT SINK. It becomes a   
   liability when you are trying to use it for propulsion. This is why Argon is   
   actually a prefered propellant for thermal propulsion. It has considerable   
   mass for thrust, and    
   requires 1/4 the energy to heat than water. It is not so good as a heat sink,   
   though, which is why H2O would be a better candidate for such a hybrid   
   function.   
      
   > But in the inner Solar System, pretty much the only important factor is dV   
   per kg, regardless of how much energy it requires. (As long as the plume   
   itself is not visible, so light sail or photon drive is out)   
      
   H2O converting at BP will produce much greater thrust per mass than H2, at a   
   ower enrgy cost. This means a higher dV per kg, whether you are looking at it   
   as a function of energy per unit of thrust or a function of thrust per unit of   
   mass.   
      
   >    
   > Bulk is a drawback, but not such a big one I suspect. By far the most energy   
   received is from the Sun, and this is taken care of with the solar-thermal   
   engine. For the rest, you end up with a long, thin cone, but this craft   
   doesn't have to manoeuvre    
   anyway.   
      
   Maybe, maybe not. You would probably be correct if the crat is used strictly   
   as a ferry... but you have to take into account ALL of the internal systems.   
   Life support, etc, probably would not add considerable waste heat... at least   
   that has also been MY    
   argument. However, other military systems, or even recreational systems (or...   
   lights, anyone?) can add up.   
   A little extra bulk might, or might not, be a significant drawback. 1400%   
   extra bulk poses a drawback for many reasons. It absorbs 14x as much energy...   
   which is an advantage if you can use that energy, a disadvantage if it goes   
   into waste heat. It    
   reflects 14x as much energy. It is easier to see because it reflects more   
   energy (14x), but also because it fills up 14x more FOV. It provides a 14x   
   bigger target to hit. It is also 14x more likely to suffer accidental damage   
   from stray objects. The    
   extra bulk also means extra structural support and shielding, which means   
   extra mass, which means less dV. It also means a larger coolant loop, which   
   means a greater length of piping to suffer damage. Etc.   
      
   > A numerical analysis would be necessary to see how this plays out, but with   
   the better dV of hydrogen, I suspect it still holds the advantage.   
   >    
      
      
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