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Discussion in 'Hobby Electronics' started by Don McKenzie, Aug 6, 2012.

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  1. Jon Kirwan

    Jon Kirwan Guest

    For all those reasons, that is why I would LOVE to see the
    "optics" (and the schematics, as well.) Totally new area for
    me to learn about. In my region, it's all about electron
    transitions -- and no molecular dissassociation.

    I'd expect you'd count single photons at those energies --
    just one probably creates quite a shower to deal with. I'd
    like to know, in practice, how one measures the energy.

    Also, in space there is a serious problem with "brown crud",
    which is smashed, charged particles that come from the
    satellite's own fabrication scattered into space, which
    because it is charged comes back to the spacecraft at some
    later time, but sticks elsewhere (not where you want it.) I
    don't know if there is a problem with this on Mars -- there
    is an atmosphere of sorts. But I'm curious just the same if
    there is an accumulation problem and how it is dealt with.
    Now THAT I totally believe. I probably couldn't afford it.
    But I still could learn.
    Well, they certainly don't have much atmospheric pressure to
    worry about.

    Which reminds me of another thing. How do they convect the
    internally generated heat away. I recall hearing about
    multiple power systems, operating around 32V at near 2A. More
    than one. This power must be convected/conducted away -- no
    way they want to radiate it. I wonder if they push it
    (costing more heat) towards a vane or arm and drag it around
    on the surface to get rid of it? The atmosphere won't help
    much.

    Jon
     
  2. Watching the control room feed, it seemed to me that there was a
    communication bobble surrounding the parachute deployment. The
    conversation wasn't clear, but from what little I could make out I had
    the impression that there was an unexpected telemetry gap that spooked
    them for a few moments.

    George
     
  3. Tim Williams

    Tim Williams Guest

    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.

    Tim
     
  4. In earth atmosphere, I understand most heat above around 300K ( 'room'
    temp ) is carried by radiation and convection. Conduction is a minor part.

    No idea what convection is like at 5 torr in a CO2 atmosphere, but I
    suspect its a lot lower.
     
  5. Jon Kirwan

    Jon Kirwan Guest

    The shell is now gone. I was curious about Curiosity, itself.

    You can only emit so many watts into space via radiation
    given a surface area: A*e*s*T^4. There is no convection or
    conduction, of course. But as George pointed out, the
    existing atmosphere likely provides some relief here.

    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.)

    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.

    All this poses some interesting questions, at least.

    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.

    The ISS, for example, used hundreds of kilograms of silicon
    purely for thermal protection.

    I'm curious how these issues were examined, explored, and
    either discounted or solved. Anyone know the details?

    Thanks,
    Jon
     
  6. Jon Kirwan

    Jon Kirwan Guest

    Then that begs the question, again. Perhaps George spoke too
    soon.

    I'll go dig. I'm ignorant and don't like the feeling.

    Jon
     
  7. Jon Kirwan

    Jon Kirwan Guest

    Yes, I just remembered the Knudsen number.

    Thanks!!
    Jon
     
  8. Tim Williams

    Tim Williams Guest

    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.
    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.

    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).
    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
     
  9. josephkk

    josephkk Guest


    It may be possible to start with a selection of calibrated LED sources
    (kind of like the near IR ones used for light source & light meter used
    for fiber optic cable measurements). With a nice selection of say a dozen
    wavelengths in the range of interest, it may become reasonably feasible.
    With a decent calibration cycle as well.

    Internet patent dated 7 August, 2012, all rights reserved.

    ?-)
     
  10. Jon Kirwan

    Jon Kirwan Guest

    I know you were laughing. Don't. I've been there. I've
    already built calibration LEDs. Not for the purpose of
    calibrating visible wavelength spectrophotometers, though.
    They were very special-purpose standard-candle devices
    because dispersion isn't anything like a tungsten lamp and
    alignment of the LED die severely limits practical use.

    If you had ANY idea just how HARD it is to calibrate LEDs for
    optical output....

    First, you buy 1000 LEDs and request that they all come from
    the same FAB and batch. They will need to be operated at an
    elevated, controlled temperature when they are finally used,
    so you set each one up to run at that temperature and then
    continuously observe them for 200 hours of operation. In that
    time, most of them will drift all over the place and will NOT
    settle down entirely in that time. You throw those away.
    Others will not drift all over but will drift way too fast to
    be any good. Throw those away, too. Of the 10 or 15 LEDs left
    in the batch that happen to have stabilized well enough in
    200 hours of operation to appear to be useful, you go through
    the trouble of cutting away the epoxy with precision tools
    and then operating them some more while producing a
    calibration table for each in their final module.

    It's not cheap. Their wavelength skirts are wide, too. Now do
    this for a variety of visible areas to produce what you are
    talking about?

    Tungsten gives nice coverage. Here's a calibration table I
    got from Optronix for a 45W tungsten lamp (actually a 50W),
    model 245M, using a standard distance away using a standard
    optical arrangement. Filament must be vertical, current
    controlled to 0.1%, etc. It gives an example of how many
    points are sometimes used, though you can always pay for
    more. Given the black body curve behavior, interpolation at
    different points is manageable.

    250,.000249
    270,.00818
    290,.00217
    300,.00332
    310,.00489
    320,.00697
    330,.00960
    340,.0130
    350,.0173
    370,.0285
    400,.0531
    450,.118
    500,.212
    555,.337
    600,.446
    654.6,.574
    700,.669
    800,.830
    900,.902
    1050,.892
    1100,.865

    Jon
     
  11. Walter Banks

    Walter Banks Guest

    Convection flow requires gravity. Laptops on various space labs
    starting with the skylab and Shuttle found out about this. For
    convection to work it needs gravity so the lighter heated air
    moves *up* being displaced by cooler heavier air. On Mars
    no problem except the amount of heat the atmosphere can carry.

    w..
     
  12. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    pyros have been used for more than five decades in space missions and
    they were critical for each mission. They were used for stage
    separation, four or more for each stage and all should work perfect.
    But it is possible to test the cabling and the fuse of the pyro by
    measuring the resistance without blowing them. I guess there were more
    than twenty pyros necessary for this mission.
    If hundred pyros were built within one lot, you may test fifty or more
    and use only the rest if all tests were perfect. You may even test some
    just before the start, if one fails all pyros of this type and lot have
    to be replaced. Not cheap, but a failing pyro is much more expensive.

    Bye

    Bye
     
  13. Uwe Hercksen

    Uwe Hercksen Guest

    Hello,

    redundancy is possible for small and critical parts, but there are also
    some large critical parts, the main rocket engines, the tanks, the heat
    shield. You may use three instead of one parachute, but does it work
    with only two of three parachutes?

    Bye
     
  14. Jon Kirwan

    Jon Kirwan Guest

    Nah. You nailed it, George. As I'd guessed earlier, it WAS
    about mean free path. And I was the ignorant one. Not you. I
    just needed an additional clue from Phil (characteristic
    size) to put the pieces together again.

    Makes me feel the fool.

    But then, not for the first time or the last. Been there so
    many times now.
    I got a chance in re-studying to see a beautiful 3D chart
    with pressure on one side, temperature on another, and the
    height being a constant of heat flow. Very interesting (non
    trivial) shape to it.

    Thanks so much for taking a moment to knock me on the head!

    Jon
     
  15. Jon Kirwan

    Jon Kirwan Guest

    The two calibrations I need are:

    (1) Wavelength calibration which assigns a specific center
    wavelength to each pixel in the array (or matrix in the case
    of a camera.) Wavelength span is another issue, but it can be
    finessed a bit with several different alignments of the
    grating, if details are needed.

    (2) Intensity calibration which assigns sensitivities to each
    pixel (no two pixels have the same sensitivities and there
    are other losses elsewhere, as well.) The whole system as a
    unit needs to be calibrated for intensity on a per pixel
    basis so that you know how much you are getting when you get
    it. This is an absolute level calibration, not a relative
    one.

    Between the above two, you get accurate wavelength and
    accurate intensity. Getting sources to help calibrate
    wavelength is cheap. Getting sources (and setups) to help
    calibrate intensity is not cheap.
    I'd like to hear more about this and how I might make that
    work for a megapixel camera over the entire CCD matrix. I'm
    at a loss regarding absolute calibration here. And keep in
    mind that it needs to account for the entire system's losses.
    So I can't go tickering around with the optical path to move
    the laser beam around because it will never be the same
    again. (I don't think so, anyway.) Maybe that would work with
    a monochromator? I don't have experience with those, though
    in any case it's not what I'm caring about right now.
    Good enough to do real work that is traceable to standards.
    Otherwise, it's not repeatable across instruments.

    For now, the students get something useable on wavelength
    alone. And there is a lot of "science" that can get done
    without more than that. Examining fluorescence wavelengths of
    plant materials, for example. Stuff like that. They don't
    have to have intensity to replicate some of the science
    results they see in books.

    But it would be very interesting to come up with a scheme
    that is cheap for intensity calibration. But like temperature
    calibration (freeze points, etc) it's not so cheap to do.

    Jon
     
  16. Tim Williams

    Tim Williams Guest

    Convection is the transfer of heat to and from a fluid. This is true
    whether the fluid flows by change in density or by an external force, and
    includes diffusion, which occurs in space even when the laptop's fan is off.

    The component of heat transfer due entirely to mass flow is called
    advection, and does not include diffusion.

    Tim
     
  17. Well, it got 100% of the way there :cool:

    Problem is - it may have been sitting there completely functional
    except for it's transmitter(s).

    George
     
  18. josephkk

    josephkk Guest

    I wasn't. I thought it was an idea worth investigating, not necessarily
    the right approach to a solution.
    That is why it is just a suggestion for a maybe workable approach.
    Agreed. I have hit the drift wall a few times myself. Not fun.
    Yep i was worried that it could be this bad.
    Yep i have fought those curves as well. Tiresome but doable. You get the
    same kinds of thing is radiation dosing semiconductors (bipolar
    transistors mostly, measurements interest included 1/ H(fe), I(cbo), and a
    couple of others). Also combined radiation (includes annealing effect
    between the radiation types gamma vs neutron).

    ?-)
     
  19. josephkk

    josephkk Guest

    In the instances i have been privileged to look at two of three works as
    some acceptable damage risk, one of three generally includes serious
    damage risk usually considered casualty mode case (barely walking
    wounded).

    ?-)
     
  20. Would good ol' Sol be an adequate calibration source? You can
    probably look up the daily solar intensity somewhere on the net.
    Then you correct for local atmospheric effects. Of course,
    if this project has to work in Oregon, it will probably have
    to be used only during summer school! ;-)

    Oh, and the spectral output is pretty well known and even
    includes a few good marker lines.


    Mark Borgerson
     
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