nasa-celebrates-successful-mars-landing

[Jon Kirwan]

Oh no, I could not possibly have a suggestion. My spectrometers
don't seem to have an overlapping wavelength range with yours,
mine are xray/gamma. I am practically clueless when it comes to
those in the UV and below range, mine generally count single
photons.

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.

Efficiency calibration is costly on those for gamma, too,
though - the calibration source costs thousands. The rest, hm,
has become much less expensive since the release of the
netmca-3 (one can still spend tens of thousands on it but
does not have to any longer).

Now THAT I totally believe. I probably couldn't afford it.
But I still could learn.

I would be curious what gamma spectrometry the thing is
carrying on board, too (if any, but likely so). Unlikely
HPGe, but then who knows, Mars is a cold place, they may
have figured some practical way to cool things down to
about liquid nitrogen.

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

 

[George Neuner]

The blockbuster-movie-preview-preview-style-over-hyped-bullshit just
detracted from what in reality was an utterly brilliant job. Even
though nothing _did_ go wrong (AFAICT), and they hit center of the
bullseye, I'm confident that there was both redundancy and margin for
error designed/built into almost everything.

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

 

[Tim Williams]

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.

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

 

[Adrian Jansen]

Good point.

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.

 

[Jon Kirwan]

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.

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

 

[Jon Kirwan]

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.

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

 

[Jon Kirwan]

Thermal conduction is independent of pressure until the mean free path
becomes comparable to the dimension of the object, or to the gap between
objects.
<snip>

Yes, I just remembered the Knudsen number.

Thanks!!
Jon

 

[Tim Williams]

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

 

[josephkk]

Good point.

What triggered my comment is remembering that heat buildup
was most certainly a problem during broadcasts with the high
gain antenna system towards Earth. They can't operate it
continuously, if I recall correctly. Made me wonder about the
rest. I had also remembered the case of satellites and "knew"
that the Martian atmosphere could be as little as a few tens
of Pascals. So I projected that heat accumulation may be a
problem. Apparently, it isn't.

I gather your point. To understand it more fully, I need to
think more closely about mean-free-path, I guess. Thanks.


The problem for students is that they cannot afford the $800
it takes to get a $8 tungsten bulb calibrated. (It can cost a
lot more than that, too.) Then they need to set up a careful
station for making the calibration. It's just plain too
expensive to do. And I don't know a way around it. Wavelength
calibration is CHEAP!! $12 buys a merc-argon calibration
lamp, which fires up with the Argon first (for about a minute
or two where you get some very nice Argon lines in the longer
wavelengths) and then the Mercury kicks in and dominates with
it's lines. A couple of well-timed snapshots and you've got
enough (with software) to completely calibrate your pixel
positions in a cheap ($10) megapixel camera.

I'd LOVE to figure out a cheap intensity calibration setup,
though. So would a lot of other people, I suppose.


But that's the problem. I can get a cheap tungsten lamp. They
are VERY CHEAP. In fact, the EXACT SAME ONE I would buy,
calibrated, is dirt cheap uncalibrated. It is the calibration
data that comes with a newly calibrated lamp that costs so
darned much. It needs to be traceable to standards. And you
pay for that. Plus, you need an expensive power supply to
operate it (0.1% current control is standard.) And it ages,
too. Within 100 hours or so, it's drifted enough that it
needs recalibration.

Thanks,
Jon

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.

?-)

 

[Jon Kirwan]

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.

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

 

[Walter Banks]

On Earth once the temp difference is above a few degrees I think
convection is the biggest source of heat transfer. But I have no idea
of how that changes with pressure. Maybe some day I can build a
convectron pressure gauge. The patent on that must be expired.

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..

 

[Uwe Hercksen]

Of all those single points of failure, the ones that worried
me the most were the pyros. You can test a lot of things --
like the 65,000 pounds of force the parachute had to bear.
But you can't test the ACTUAL pyros you will use. No matter
what you do, the ones you place in there can't have been
tested. We know how to make explosives of great uniformity,
of course. But all of that has to go into a system and it
must fire exactly correctly, under buffeting circumstances,
without a single point of failure in a single pyro. Just one
and that is it. Not that the rest wasn't also difficult. But
some of the things, since as the novel use of an imbalance in
weight distribution in order to permit direction control
during entry, can have errors in one part of the software be
compensated by the outer control loop in another part. So
even there, there is a backup hope. But the pyros either
work, or don't. That's what I was watching mostly for, though
the rest was also sincerely knuckle-whitening as well.

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

 

[Uwe Hercksen]

There was certainly lots of redundancy. One number that is still
impressive was that with all of the redundancy NASA identified about
250 remaining single point possible failures. The math on that many
series terms makes the comment understandable.

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

 

[Jon Kirwan]

Sure, I'm always putting my foot in my mouth!
(I must enjoy the taste of toe jam :^)

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.

On Earth once the temp difference is above a few degrees I think
convection is the biggest source of heat transfer. But I have no idea
of how that changes with pressure. Maybe some day I can build a
convectron pressure gauge. The patent on that must be expired.

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

 

[Jon Kirwan]

OK first a disclaimer, I know squat about the details of spectrometer
calibration.

It would seem to break down into two pieces. Absolute sensitivity (at
one wavelength) and then relative wavelength sensitivity.

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.

For the first part you could maybe do something with a laser diode
into photodiode and then the same laser (though a bunch of
attenuators) into the spectrometer.

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.

For the second you have the students use the black body curve and the
tungsten lamp. Operate the lamp at two different temperatures.
(Maybe build a nice stable DC supply for the lamp.) It seems like you
should be able to extract an approximate sensitvity curve from two
black body curves, even if you don't know the exact temperatures.
(But I'll leave that as an excersise for the students :^)

How good a calibration do you need?

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

 

[Tim Williams]

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

 

[George Neuner]

I don't know that I've ever met a customer who would consider that a
success.

Well, it got 100% of the way there

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

George

 

[josephkk]

I know you were laughing. Don't.

I wasn't. I thought it was an idea worth investigating, not necessarily
the right approach to a solution.

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....

That is why it is just a suggestion for a maybe workable approach.

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.

Agreed. I have hit the drift wall a few times myself. Not fun.

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.

Yep i was worried that it could be this bad.

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

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).

?-)

 

[josephkk]

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

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).

?-)

 

[Mark Borgerson]

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.

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