From: zenadsl6186@zen.co.uk   
      
   Reading the thread about cryogenic plastics, I thought it might be an idea   
   to use (reinforced? with what?) polythene for the tanks of my LH2/LOX second   
   stage* goods version.   
      
   Then I thought, it might be possible to use a pressure-fed engine, as the   
   outside pressure is vaccuum and there is no real constraint on the length of   
   the engine nozzle. This might allow a reasonable Isp with a tank pressure of   
   perhaps 100 psi, perhaps 85 psi chamber pressure. Ablative engine chamber   
   and nozzle, very cheap-n'-cheerful.   
      
   My sim says that you lose less than 1% of Isp (and use less than 1% of extra   
   nozzle weight) at around 60 psi chamber pressure, but I don't entirely trust   
   it. I don't know if that would be better overall than eg a RL-10 or three   
   tho', but it would be ... a lot ... cheaper ...   
      
   I don't know the strength of polythene at cryo temperatures either.   
      
      
      
      
      
      
   {- weird idea (math doesn't work very well): you could have a rope centrally   
   vertical inside the tank with a winch at the bottom that collapses the tank   
   and provides the pressurisation - the polythene tank is flexible? - no need   
   for seperate pressurant tanks -}   
      
      
      
      
   *Manned Booster flyback[1] first stage is a kero/LOX/kero skinny   
   re-fuselaged re-winged 747/A380 aircraft with 2 uprated jets and a   
   highly-reuseablised RD-180 LOX/kero rocket engine or similar. There are two   
   types of LH2/LOX second stage, one is a winged reuseable re-entering people   
   carrier, and one is a disposable goods stage. Seperation is at 80 km high   
   and 2 kps vertical velocity; the 60 ton second stages are entirely enclosed   
   until seperation, so there are no aerodynamic forces on them ever.   
      
   Booster flies several times a day with different second stages, some   
   reuseable for people, some disposable for goods.   
      
   People carrier is re-entering, reuseable, high-tech, high safety, but with   
   much lower useful payload.   
      
   Goods second stages do not re-enter, they are for uplift only of bulk mass   
   cargo like fuel, hadrware, heatshields, water, solar cells, food and so on;   
   as long as they don't blow up in the launcher stage (which is manned) it   
   doesn't matter if they fail occasionally.   
      
   They dock with an orbital tug. They have a small reuseable electronics and   
   control package which the tug collects.  Down lift for these and other goods   
   is provided by ablative shields previously uplifted as cargo.   
      
      
      
      
   Can anyone think of a use for lots of 100psi 16 foot diameter reinforced   
   polythene tube in orbit? :)   
      
      
      
      
      
   [1] it climbs and flies the 200-odd westward miles before it does it's   
   eastbound rocket first stage thing, not after, so maybe flyback isn't quite   
   the right word - perhaps prefly is better? Though Booster has enough fuel   
   left after reentry to loiter, go-around or touch-and-go, if eg the runway is   
   busy.   
      
      
      
      
   The sole essential with reuseable tourist spacecraft is not getting them to   
   the right place, or even re-entry; it the final making of reliably soft   
   contact with the planet.   
      
   For that, parachutes are out. Rogallo wings are out. Any unpowered landing   
   is out. Rockets are out. Horizontal powered landing on water with wings is   
   just about admissable, but not for regular use. Any non-helicopter-type   
   vertical landing is out.   
      
   Powered-wings-and-wheels on a runway is about the only method presently   
   approved for scheduled public passenger transportation.   
      
      
      
      
   -- Peter   
      
   "Sweet dreams are made of   
   Anything that gets you in the seam   
      
   And I feel like I'm Seventeen again."                  Eurythmics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)