From: henry@spsystems.net   
      
   In article ,   
   Carey Sublette  wrote:   
   >> ...The presence of some water was partly to improve the fuel's   
   >> coolant properties, and partly to reduce flame temperature...   
   >   
   >Quoting Willy Ley in "Rockets, Missiles, and Space Travel" (1959), p. 451:   
   >"The fuel of the V-2 was ordinary ethyl alcohol - in this case made from   
   >potatoes - to which enough water had been added to bring its strength down   
   >to 75 per cent by volume. ... The combustion productions   
   >of burning ethyl alcohol are CO2 and H2O, and of course the CO2 molecule is   
   >by far the heavier. By adding water to the alcohol the proportion of water   
   >molecules in the exhaust is increased and the average molecular weight   
   >depressed..."   
      
   Unfortunately, adding water to alcohol *reduces* performance, although not   
   quite as much as you might think.  Molecular weight is actually a complete   
   red herring, the result of misunderstanding a performance equation(*).   
      
   (* People see an equation with "temperature/molecularweight" and fail to   
   realize that those two values are not independent in a chemical rocket.   
   With minor simplifying assumptions, temperature/molecularweight is easily   
   shown to be simply combustion energy release per unit mass.  For example,   
   Hill&Peterson's "Mechanics and Thermodynamics of Propulsion", 2nd ed,   
   shows this.  But energy/mass *cannot* be increased by adding non-reacting   
   mass.  The more subtle issue, which for example makes it better to run   
   most fuel combinations fuel-rich, is nozzle efficiency as affected by gas   
   properties.  People *think* molecular weight matters because both the   
   energy/mass and the gas properties are generally better for lighter and   
   simpler molecules, which also tend to have lower molecular weight.  See   
   the "Performance" chapter of Clark's "Ignition!".)   
      
   Earlier in the same book, Ley's account of how the VfR first came up with   
   the alcohol-water mix mentions a vague (and fallacious) notion of higher   
   performance due to reduced LOX requirements (Ley's idea), but also the   
   ability to add cooling water to the fuel (Riedel's idea).  The VfR engines   
   tended to burn out except when they had running water for cooling, and a   
   fuel that could have water mixed into it was a lot simpler than a separate   
   water supply.  Riedel ran some experiments and determined that about 60%   
   alcohol seemed best.  ("Rockets, Missiles, and Men in Space" -- R,M&ST was   
   an earlier edition of the same book -- pages 181-2.)   
      
   The Army project that followed on from the VfR's efforts used the same   
   general fuel (with a bit less water) simply because it had proven   
   workable, at a time when making a liquid rocket engine work at all was   
   quite a challenge.  And the V-2 inherited that decision.   
      
   Clark's "Ignition!" comments (pages 8-9):   
      
   "Late in 1931 Klaus Riedel of the VfR designed a motor for a new   
   combination, and it was fired early in 1932.  It used liquid oxygen, as   
   usual, but the fuel, conceived by Riedel and Willy Ley, was a 60-40   
   mixture of ethyl alcohol and water.  The performance was somewhat below   
   that of gasoline, but the flame temperature was much lower, cooling was   
   simpler, and the hardware lasted longer.  This was the VfR's major   
   contribution to propellant technology, leading in a straight line to the   
   A-4 (or V-2)..."   
      
   (Interestingly, Clark notes that Reaction Motors in the US later got   
   successful operation using LOX/gasoline/water, with the water mixed into   
   the gasoline during injection.)   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
   since Oct; first surprises seen; papers pending.        | henry@spsystems.net   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)