From: rupertg@charlieindiaxraydotcharlieoscar.uk.retro.com   
      
   On Mon, 17 Jan 2005 21:20:32 -0700, Marc 182   
    wrote:   
      
   >In article , henry@spsystems.net says...   
   >> In article ,   
   >> Tim Killian   wrote:   
   >> >Was the lack of an RTG on Huygens a political decision, or a true   
   >> >engineering limitation?   
   >>   
   >> Yes. :-)  Within the priorities, mass limits, and cost constraints of the   
   >> project, there was definitely no engineering possibility of an RTG.  (The   
   >> "priorities" part is that Huygens was mainly an atmosphere mission with   
   >> only a secondary role as a lander, as witness its primary mission being   
   >> 153 minutes -- 150 minutes of descent, 3 minutes on the surface.)   
   >>   
   >> The priorities, mass limits, and cost constraints were ultimately mostly   
   >> political decisions at one level or another.  I don't believe there was an   
   >> explicit political "no RTG" decision -- Huygens did have a whole bunch of   
   >> RHUs (plutonium heater capsules) -- but the mission as defined couldn't   
   >> really afford one (in dollars, mass, or engineering complications) and   
   >> didn't really need one.   
   >   
   >So an RTG wasn't practical for several reasons on this mission; however,   
   >it's been reported that the batteries were rated for 7 hours.  If that's   
   >true, what if they had included a programmable timer in the probe?  Just   
   >before release the timer could have been programmed with the expected   
   >times when Cassini would be in a position to again receive data.   
   >Between data windows the probe would mostly power down, just running low   
   >current instruments like temperature and air pressure, recording data,   
   >conserving batteries, and keeping warm with it's RHUs. If need be,   
   >Huygens could have powered down completely between passes, but only   
   >getting current data during a pass would be much less interesting. When   
   >Cassini was again in the sky Huygens would again wake up and transmit   
   >the collected data and take a new snapshot.  They might have gotten 4 or   
   >5 more passes out of the thing.   
   >   
   >Of course for any of that to work several other things would have to be   
   >true, not the least of which is that the RHUs would be enough to keep   
   >the inside of the probe warm during weeks of near or complete power down   
   >on that cold cold moon.   
   >   
      
   The RTUs are fine - they've got a half-life of around a hundred years,   
   so aren't going to run down any time soon.   
      
   Huygens was limited by three things - in order of importance, they   
   were cost, mass and the unknown. The biggest one was cost, which has a   
   direct correlation on complexity. Even adding something entirely in   
   software costs a lot, because of the testing (Logica, who wrote the   
   Huygens software, spent eight times as long and wrote eight times as   
   much code on the testing side of it as they did on the actual flight   
   code). The same goes for Cassini: every manoeuvre has a price, and   
   it's quite complex getting stuff back from Huygens.   
      
   And nobody knew what to expect. What would people be saying now if   
   Huygens came down into a snowdrift during a thunderstorm, and returned   
   nothing? It's easy in hindsight to say what would have been a   
   worthwhile tradeoff, now the thing's sitting in an interesting   
   landscape, but it could have been very different.   
      
   We're just going to have to go back!   
      
   R   
      
      
      
   >Marc   
      
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