> To be fair, Elie has not contested this. Rather, the argument has been that   
   the extra bulk would be insignificant in the context of the mission. My   
   argument is that extra bulk is ALWAYS significant.   
      
   The problems with bulk are:   
   - A bigger ship to build: this shouldn't be a problem at this point of   
   military space industrialization/budget.   
   - A bigger surface to see: It is as close to 'a hole in the Universe' black as   
   you can get, even with today's materials. The only way to see it is when it is   
   occulting something, and apparently this doesn't work at large distances as it   
   suffers from the    
   same diffraction problems than lasers.   
   - A longer, thinner ship: it won't do any hard burn, so structural strength   
   can be extremely low. The exception is during a catapult launch, but this   
   would be constant acceleration on the long axis, so extra length should not be   
   much more of a problem.   
   - A bigger surface to collect radiation: most radiation comes from the Sun,   
   which won't lit the black surface, so even a much bigger surface won't make   
   much of a difference in collected heat.   
   - A bigger surface to cool down: this would raise complexity and mass.   
   Complexity is not a problem at this point (it will simply make it more   
   expensive, but again, military budget). As for mass...   
   - More mass: we would need to run the numbers, but I suspect we still come   
   ahead in dV. I would have been concerned with increased risks of gravimetric   
   detection, but it seems that ship gravimetric detection is not a concern   
   anyway.   
   - There is a maximum volume: filling the entire shadow cone. However, the   
   shadow cone is half a degree at Earth distance, so this is not particularly   
   limiting even near the inner planets.   
      
   None of those seem particularly bad apart from the last one if too close to   
   the Sun, compared to what is gained.   
   Of course, I may have missed some issues.   
      
   > This point has been conceded as well. Elie has accepted that a closed Helium   
   loop would be required to achieve the 3°K objective; with the added   
   requirement of the heat pump, I believe.   
      
   The heat pump would not necessarily be required if the heat sink is initially   
   below 3K, but it would probably help dumping more heat from the 3K surface and   
   thus increase autonomy.   
      
   Also, if this stealth design is not good enough to stay undetected until   
   impact, then it can be used as a first stage for a terminal vehicle with, for   
   example, chemical rockets for high-thrusts defensive manoeuvres. Much of the   
   craft may be discarded if    
   it is cheaper than making it all thrust-resistant, plus the added   
   manoeuvrability, but if it can't be heavy enough for KKV duties, then the   
   terminal vehicle(s) should carry nukes instead.   
      
   > It appears that we both missed something. If you read carefully,you will   
   notice that your equation applies to heating... that is, if the final   
   objective is to heat a space. The article goes on to directly state that this   
   is the inverse of efficiency    
   for a heat engine (the equation I was using was the efficiency of a machine   
   intended for using heat to do work). However, it apears that we both missed   
   the equation for the condition we actually desired, which is to cool a volume   
   of space. For    
   refridgeration purposes, the equation for COP(cold) is Tcold/(Thot-Tcold).   
      
   Oh.   
   Right.   
   So, this gives us an efficiency of 0.28, which is one minus the previous   
   number. Which is the normal relationship between those two numbers, according   
   to the article.   
   So yeah, with 1/4 best possible theoretical optimal efficiency, liquid   
   hydrogen is definitely out.   
   Solid heatsink and helium coolant loop it is, then!   
      
   > H2O has better overall energy absorbtion, considering the range of   
   temperature through which it can continue to absorb energy before having to be   
   ejected, as well as its phenomenal latent heat of evapouration. Once you have   
   extracted all its value as a    
   heat sink, it becomes a better suited propellant.   
      
   Leaving aside bulk (working on per-mass basis) I still don't follow how H2O is   
   better as a propellant.   
      
   Let's arbitrarily say the exhaust temperature is 2501°K.   
   The theoretical max temperature (without heat pump) is the surface temperature   
   of the Sun, which 5777°K according to our friend Google, but I'll assume we   
   can't so efficiently heat it up at the moment.   
   In fact, we could theoretically raise the exhaust temperature beyond 5777°K,   
   for example using photoelectric cells to drive a heat pump, but I'll ignore   
   this here.   
      
   At equal exhaust temperature, H2 (being a lighter molecule) has a higher   
   exhaust velocity, and thus higher dV per mass.   
      
   We can increase the aperture to heat hydrogen more, though this would decrease   
   autonomy.   
   Instead, we can decrease mass flow. Acceleration will be lower, but it will   
   keep its superior dV.   
   This is what we see with nuclear rockets, in fact (can you tell I am playing   
   Children of a Dead Earth at the moment?): heavier molecules give higher thrust   
   but lower exhaust velocity, and thus lower dV, for a given energy consumption.   
   The logical    
   extremes being photon drive on one end and things like  mass drivers on the   
   other.   
      
      
   (Please take all the following calculations with a lump of salt)   
      
   To heat 1kg of mass up from 1°K to 2501°K, it requires (heat capacity)* 2500   
   + heat of fusion + heat of vaporisation.   
   Heat capacity is varying with temperature, which is going to be a problem for   
   precise calculations. Let's see what ballpack estimates will give us.   
   Assuming an average 14 J/g/m for H2, 4.2 for liquid water and 2.1 for ice and   
   steam; heat of fusion + vaporisation, for H2 and H2O respectively, of 0(I   
   couldn't find coherent numbers, but it seems quite low anyway)+460 and   
   334+2257 J/g:   
   H2 => 14*2500 + 0 + 460*1000 = 495000   
   H2O=> 4.2*100 + 2.1*2400 + 334*1000+2257*1000 = 2596460   
   Per mass, H2O is able to absorb five times more energy than H2. Even with all   
   the approximations used all around, this clearly makes it a much better heat   
   sink, thanks indeed to its great heat of fusion. I had not realized it was by   
   that much.   
   Even if we get at higher temperatures, we shouldn't see a significant   
   difference in temperature.   
      
   So water has better autonomy, better acceleration and lower dV. Weird.   
   Unless I missed something, it means that water won't give thrust immediately,   
   as it will first need to melt and vaporise.   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)