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#11
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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. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#12
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"muldar" skrev i en meddelelse
... it still is one HUGE success. Monumental, really. Just too bad the major news networks didnt even mention it or show a photo tonight. This sorry state of socially irresponsible affairs in the USA must end soon. You think this sorry state of affairs is confined to the USA? Since I don't have cable or sat dish, I can take only the two TV channels that are carried on airwave here in Denmark. One is the old national TV channel that was the monopoly station in the old days. On this channel there was heavy coverage on the new opera house that was inaugurated in Copenhagen on the same day as the Huygens landing, both in the regular news and as a special, hours-long program. There was nothing about the Huygens landing. The other channel did have something, but not much. And also heavy coverage of the new opera house. Thank heavens for the Web. :-) (I'll check it later when there isn't so much run on the relevant sites.) Jon Lennart Beck. |
#13
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Probably both, but mostly engineering.
RTGs generate a LOT of heat, witch would be difficult to deal with inside the closed reentry shell of that small probe. For instance, an RTG generating 200W eletric power generates 2000W heat power. It's also heavy, generates radiation, witch is bad for the instruments... etc. Tim Killian wrote: Was the lack of an RTG on Huygens a political decision, or a true engineering limitation? Scott M. Kozel wrote: The Huygens spacecraft could not utilize an RTG or solar power, so it was limited to un-rechargable batteries, and given the number of instruments on board, battery capacity was limited to a matter of hours. |
#14
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A lifetime of weeks would have required nuclear heaters at least, and
perhaps a nuclear thermal electric source as well. That means more mass, probably more then anyone wanted to pay to send. not to forget that try landing heavier probe somewhere quite unknown would probably lead to assured failure it s good to get back something ! |
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#17
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In article ,
Marc 182 wrote: 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. Exactly. Unfortunately, there was also a requirement that the RHUs not roast the inside of the probe before launch and while in space, including quite a while spent relatively near the Sun. So in practice, they couldn't do the whole job, and a noticeable fraction of Huygens's power consumption was for heaters. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#18
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#19
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In article ,
says... On Mon, 17 Jan 2005 21:20:32 -0700, Marc 182 wrote: In article , 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! I don't disagree with you, and I don't want to second guess ESA and what they did, which was pull off a wonderful success. What I was thinking about is, what can you do in the initial phases of your planning to provide maximum benefit should your probe exceed nominal mission life? Lots and lots of probes live longer than expected, and maybe you should plan to deal with that event. It seems like space probes die in three ways, abruptly when something goes badly wrong, in a long fade as power dwindles or distance increases, or with a smack as someone directs it into a gas giant. No one wants to turn off the switch. So for the Huygens probe, which lasted for hours after the 180 second nominal post touchdown mission, I'm wondering what could have been done early to take advantage of that extra time should it come to happen. Maybe a small post nominal mission team, with greatly relaxed testing requirements, could have had something ready to go should the probe live long enough to execute it. Planned for early, requiring no significant hardware changes, and executing only after nominal mission, such a team might have returned quite a bit of bonus data. Marc |
#20
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Marc 182 wrote:
I don't disagree with you, and I don't want to second guess ESA and what they did, which was pull off a wonderful success. What I was thinking about is, what can you do in the initial phases of your planning to provide maximum benefit should your probe exceed nominal mission life? In case they had done so and huygens had failed everybody would have bashed them for bothering about "post mission" stuff instead of ensuring that the mission itself gets done properly. Beagle2 was such an afterthought. It was tacked onto mars express because someone figured out there could be a few more kilos on the orbiter. Gladly it didn't cause mars express to fail but the pr desaster was big enough nevertheless. I don't know whether ESA or NASA overdesign with your thoughts in the backs of their minds but if they were they sure wouldn't let on. Much better to cash in (pr wise) on a surprisingly long mission than to get bashed for a surprisingly short one. Lots of Greetings! Volker |
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