From: henry@spsystems.net   
      
   In article <402183C9.3B9F5CB1@electrosphere.org>,   
     wrote:   
   >Can someone think off the top of their head of suitable materials for   
   >use as reentry shielding/ external shell material that are compatible   
   >with either a fresh water or salt water environment after reentry?   
      
   Salt water plus lightweight structure is a nasty corrosion/maintenance   
   problem no matter how you slice it.  The US Navy experimented with jet   
   seaplanes in the 50s, and eventually gave up on them, at least partly   
   because they were so maintenance-intensive.   
      
   Fresh water is a better situation, although still not terrific.  I'd say   
   that you probably want to operate in fresh water as much as possible, and   
   treat salt water as an "emergencies only" case.   
      
   The most promising reentry protection approach for either of these is   
   active cooling, either transpiration or circulation, with heat-sink as a   
   runner-up.  (Ablators are fine too, but I assume you want reusability.)   
   Heat sinks would be copper, beryllium, or beryllium-based composites.   
   Active cooling would quite likely be either copper or aluminum, with a   
   small possibility of steel.  Copper and beryllium don't get along with   
   seawater, steel is better, aluminum is fine.  Transpiration-cooling   
   systems would have to worry about clogging of orifices by waterborne   
   debris, and if they used a porous skin, about it taking up water.   
      
   Shuttle tiles and the like are a disaster, they'll take up water and it's   
   hard to get it out.  Most of the metallic hot-skin alternatives aren't   
   really any better -- the outer skin can't be waterproof because of   
   expansion joints, and generally there's porous insulation inside   
   somewhere.   
      
   >The obvious problems of corrosion and sealing aside, how many TPS   
   >materials can stand up to the shock of being immersed into the sea or a lake?   
      
   The outer skin generally will be cool by landing time anyway, because it's   
   got cool air flowing over it for several minutes before that.  Some of the   
   concepts will still have quite a bit of heat inside, mind you.  (Even the   
   shuttle tiles do.)  I'd say it's not likely to be a big issue, given the   
   use of materials that are suitable for water anyhow.   
      
   >If a suitable material might exist, how will it affect overall design of   
   >a TSTO/SSTO?   
      
   Depends greatly on details, and on the type of TSTO/SSTO.  Heat sinks are   
   probably too heavy for SSTOs, but might work for TSTOs.  Active cooling   
   should work for either.  The exclusion of hot-skin approaches will push   
   strongly away from winged designs and toward semiballistic low-L/D   
   concepts, which make short sharp reentries that work well with active   
   cooling or heat sinks.   
      
   >I just keep thinking of some SSTO that lands and takes off in water,   
   >bypassing the need for landing gear completely, as well as some safety   
   >issues that get resolved by launching over water. Does this seem like a   
   >crackpot idea or is there some merit to this?   
      
   It has both merit and problems.  There's a lot of water around, and that's   
   useful, but it's a somewhat difficult environment, especially salt water.   
   --   
   MOST launched 30 June; science observations running     |   Henry Spencer   
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