🚀 go-pugleaf

"Jon Kirwan" <jonk@infinitefactors.org> wrote in message 
news:eb8328ddvil626apeui0pq7jnprtcgbbp5@4ax.com...
>>In addition to what the others have said, keep in mind the thing floated
>>through space for six months -- you can't cool Pu238 down, so *all* that
>>heat had to be dissipated, as radiation, from the backshell and heat 
>>shield,
>>into space.  The vehicle itself may've had cooling to the shell, but the
>>shell still had to dissipate it.
>
>The shell is now gone. I was curious about Curiosity, itself.

Point being, it probably had a harder time dissipating in flight than on the 
surface...  Real question is, is the ratio of surface areas reasonable for 
the respective conditions?  The shell was flat and surrounded by vacuum, but 
large; the rover chassis is smaller, but has more surface area, and has a 
little air around it.

I've got a vacuum tube that's rated for flight up to 80,000 feet, derated by 
half at that altitude (only 15W, it's 6L6GC sized).  Mars is roughly in that 
range, or up to 100kft, I forget where exactly.  That's a 50% derating for 
similar pressure, but the tube's operating temperature is way higher (peak 
envelope temperature 300C!), so radiation is dominant.  At normaler 
temperatures (40-60C), I'd guess 80-90% derating would be reasonable, 
relative to STP figures of dissipation.

>But speaking only about radiation outward, even with e at 0.9
>(it usually isn't -- but I've measured silicon carbide, for
>example, at around .9), this works out to about 490 watts/m^2
>of radiating surface with e=0.9 and T=40C. (Anodized aluminum
>is about 0.75.)

So again, it comes down to surface area, and how they distribute the heat.

Wouldn't be surprised if they have an inner loop around the RTG, to burn off 
most of the power at a high temperature, and secondary loop(s) to keep the 
rest of the thing warm.  Extra temp on the RTG casing or whatever would do a 
fine job of bleeding off any extra heat the frame doesn't need; the casing's 
temperature will swing with demand, but that's fine because it's already 
designed to run warm or hot.

>The problem is compounded by the fact that the same (or
>nearby) surfaces will absorb insolation. On Mars, this is
>about 1/2 the amount Earth gets, per m^2. But the Gale Crater
>is at 4.5 degrees south latitude (basically on the equator)
>and so it would experience, I think, on the order of 1362/2
>or 680 Watts/m^2 of insolation, ignoring the albedo of the
>atmosphere (which probably [but I don't really know] doesn't
>reflect or thermalize a lot away before it reaches ground
>level.) There would also be differential heating (sides where
>sun is striking vs other sides), too.


If Curiosity had a bubble and a seat to sit in, I bet it would feel a lot 
like sitting in a parked car, on a cold winter's day, in bright daylight. 
When heat's trapped, it's warm, but not sweltering or anything; open the 
windows and you'll freeze your ass off (but that'd be convection, of 
course).

>Cyclic heating and cooling can seriously cut down on the
>lifetime of instrumentation, structures, solar panels, and so
>on. UV radiation, also, is energetic enough to break polymer
>bonds such as C-C and C-O, as well as many functional groups.
>Obviously, still higher radiation from the sun causes
>ionization, photon excitation, and atomic displacement. I
>remember also that tests on a CCD not so long ago showed that
>100 Rads generated hot spots on 10% of the CCD exceeding the
>limits of an 8 bit ADC.

On the upside, there's no oxygen to move in and latch onto said free 
radicals.

Even without oxygen, I wouldn't be surprised if weaker plastics end up 
shredded in a few years, much as they do here.

I'd expect they use an awful lot of epoxy composites, kapton film and kevlar 
and carbon fiber.  These are a whole lot more robust than the average nylon 
or rubber, and endure well even in harsh conditions.

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms