There is a recent posting on Emily's Planetary Society blog, which must be Doug's because she's not there herself, although her name is the only name on it. The subject is using water to cool a long-lived surface probe on Venus. It sounds far more practical than any of the other proposals for landing giant atomic-powered refrigerators, or developing a whole new family of high-temperature semiconductors, etc.
But I didn't understand the whispered criticism to the effect that the Ekonomov paper assumed that the water would absorb heat only from the one watt of power driving the instrument package itself. I simply can't believe that he went to the podium and presented his model without taking into account the fact that the surface of Venus is a pretty hot place, and that the proposed probe would be absorbing the ambient heat. This is an interesting proposal and I would like to understand both the original calculation of 50 days to bring the water to a boil, and the cited flaw in the calculation. I too find it hard to believe that it would take 50 days to bring water to a boil on the Venusian surface, but where exactly is the error, and what remains after we correct it?
Doug is busy of course, but I hope he will find the time to address this when he returns, if someone else hasn't done so by then.
Why start at 300 K rather than 270 K? By shielding the package from the Sun during passage it should be quite possible to get the water to freeze. Note that melting a lump of ice takes about the same amount of heat as warming the resulting water from freezing point to 80 degrees centigrade. The only drawback is that the water container must be built so it is not damaged by the water freezing (ice is of course less dense than water). However you would probably have to do that in any case for safety.
Unfortunately - the guy's english wasn't very good, and his details were fairly thin on the ground. He talked about the power consumption of the electronics being only 1 watt to minimize the energy put into the water - and perhaps through langauge barrier rather than anything else - it seemd that he was infering that only 1W of energy would be heating the water. (and it was Earth days he was talking about - he wanted to reach a significant part of a Venus year - 100-200 Earth days)
Obviously - the cable from the batteries itself will be sinking more than that, ditto any other connectors to the 'outside' world in terms of instrumentation, comms, the vent for steam etc etc. I don't think the language barrier between the speaker and the audience helped when people were asking about the 1w etc. Think about what the rovers do when they're on 240 whrs - 10 watts average. Now think what a venus lander would do with 1 watt average.
There was another point made to me - a seismic instrument on 1 bps? Forget it. Seismic measurements from inside a boiling kettle? Crazy.
Like I said in the blog - most agreed that the principle would obviously work - but just not how he was describing it.
Doug
These calculations are based on the use of water. I remember using test loads for broadcast TV transmitters that employed the latent heat of evaporation of a Glauber's Salt (sodium sulphate) solution to get rid of the dissipated power. I believe that sodium sulphate has a significantly higher latent heat figure -but Wikipedia etc have let me down so I can't find the number right now.
If the specific heat + latent heat approach is a good one then obviously choosing the best compound will give maximum duration on the surface.
Rob
None of my steam traction engine information sheets goes up to 900F, but I suspect the pressure required to maintain the water as liquid will be large.
In fact, larger than the external atmospheric pressure, so the water will hit an equilibrium with venting into the high pressure atmosphere at a temperature less than the external temp. Might as well design the electronics to tolerater this temp/pressure, for maximum life.
Question for the chemistry majors here, is there a nice stable compound, that would not harm the electronics, and minimize that equlibrium temperature ??
Tasp: the idea was that the water is INSIDE a glorified thermos bottle, so it isn't exposed to 900F at all. Anyway, that's way over the critical temperature of water, so no amount of pressure could keep it liquid.
I'd say the first question to answer would be "given the best thermos we can make, with the atmosphere of Venus on the outside and holding 100 Kg of water initially at 4C and 1 atm on the inside, how will the water temperature vary as a function of time?" If the answer is, "it'll boil in hours," then there's nothing more to discuss. But if the answer is "it won't reach 100C for months, and even then it'll take two years for it to all boil away," then maybe there is something to it.
Anyone know enough about modern thermos technology to estimate this?
--Greg
The principle would be to have the pressure vessel at 90+atmospheres so the water would boil away at that critical point at 300 deg C (numbers are 'roughly')
Doug
Marsbug - you're maths is somewhat awry.
Think of the vessel as a Dewar/vacuum flask. It boils down (!) to the difference of the inner and outer shell temperatures ( = disturbingly high) and the emmissivity of the surface of the vessels, given that it's a radiation source of heat across the vacuum.
So your W= σ*A*T^4 wants to be W= e*σ*A*T^4
e can be as low as 0.02 for polished gold or silver. Plugging that in, the radiation input onto the inner sphere will be around 500W. Toastie!
This'll make for a vessel that'll last hours at most. Certainly not days and weeks.
Whilst looking at this (but you beat me to the post) I did spot some work on Si IC's conducted (!) at 600+C. That might be a better way to go (albeit more expensive).
Andy
Thanks Doug and AndyG! This makes it seem a bit more plausable, and use of gettering answeres the objection of how to maintain a vacuum (shoulda thought of that!). Fifty days still seems like a long time to me, but a long term venus lander; what an idea! Going a bit off topic here but what science objectives could you fulfill with one?
[Excessive quote removed, hopefully before Doug caught it!]
Other than seismological studies - and multiple landers would be needed for that - I think the dipping balloon has more going for it. With the right selection of heat-absorber, using the suggested technology, you could cool off above the altitude of your choice (Sodium freezes at 45km, Water freezes at the cloud tops around 58km) allowing for short-stay trips to the surface coupled with long duration study of Venus's meteorology and surface.
Andy
All the above makes sense but my understanding is that you can further reduce the heat flux by adding additional intermediate isolation layers between the inner and outer flask walls (see page 16 of this set of lecture notes for the detail). The net effect is an additional 1/(n+1) reduction in heat flux where n is the number of additonal layers so (in theory) you could reduce the flux rate down to ~50 watts with 9 additional inner walls. This approach is similar to that adopted by the JWST for its main shield.
The materials issues remain very tough though - the components all have to be very stable at up to 600K.
I wonder whether a silica aerogel might be a better solution as an insulator. Light-weight, great insulation, in contrast with the problem of maintaining a vacuum on the surface of Venus. Melting point of 1200 degrees celsius seems appropriate. Apparently the variant with carbon added to the mix makes an even better insulator.
If we assume that 500 W is a realistic heat input what will happen?
This is very much a back-of-the-envelope calculation, mind you
I assume that we start with 100 kilos of ice at 273 K. To melt 1 gram of ice takes 333 J so 500 Watts will melt 1.5 grams of ice per second. It will take about 67000 seconds to melt 100 kilograms, so temperature will stay stable at 273 K for about 18 hours.
Temperature will then start rising more or less linearly as the water warms up. It takes 4.19 J to raise the temperature of one gram of water one degree. 500 Watts will therefore raise the temperature of about 120 grams of water one degree. So it will take about 830 seconds (slightly less than 14 minutes) for the water to heat up one degree. For the temperature to rise 100 degrees will thus take about 23 hours. However the water won't boil at 100 degrees since the steam can't vent into the atmosphere until the pressure is higher than atmospheric pressure. The phase diagram I have is logarithmic so it's a bit difficult to be sure but it seems that water boils at about 500 K on the surface of Venus. That temperature should be reached after about 50 hours. The water then starts boiling and vents to the atmosphere. It takes 2260 J to boil one gram of water so 500 Watts will boil about 0.22 grams of water per second. To boil away 100 kilos will then take about 450,000 seconds i e about 125 hours. The temperature will then quickly go to ambient.
So temperature should stay at 273 K for about 20 hours then rise to about 500 K over about 50 hours and then stay at 500 K for about 120 hours, altogether a little more than a week.
And yes, I'm aware that the constants I've used vary with temperature and that the heat flux is also dependent on the thermal gradient, and yes, the venting steam will actually cool the surroundings a little (and could probably be routed to maximize this effect).
P.S. It just struck me that by adding a pump to evacuate the steam you could have those 120 hours at 373 K rather than 500 K. It would have to be a very good pump though,
One idea I've always had for a Venusian balloon is one using water as the lift gas. As it rises, above a certain altitude the water would start condensing, causing the balloon to start drifting down again until the water boils off enough to cause it to lift again.
On the temp. side, the ice could actually start well below freezing, modern industrial temp Si is down to -40 C.
Aw dang, all my best ideas get stolen! First the ultrasonic backup detector for car bumpers, now this! What's next, a front door answering machine?!
Anyways, I was thinking that the frequency could be modified by changing the size and contents of the balloon water. More water takes longer to heat&cool down, plus with the right mixture you can extend the freezing/boiling points to make the range larger. The balloon could catch and contain the water flowing down the inside of the balloon within a container to let it sink even further. Hell, combine this with Ekonomov's idea of using water as the phase-change material, and you get a balloon that can lift-off when its instruments get too hot, and come back down once the water jacket has refrozen. Keep a little filler gas (neon) in the balloon to keep it from completely collapsing on the way down, and you get yourself a free parachute too.
Thanx for clarifying what I was trying to convey.
Appreciate seeing the math done, to.
(not one of my strengths to be able to cipher the digits, but I do get an intuitive order of magnitude estimate often enough to be useful)
Taking tty's figures and using 100W instead of 500W, I think we get 40 days, which, considering the roughness of our estimates, seems quite close to the claimed 50 days.
I note that the abstract in the paper suggested that they had electronics that would work at 500K, but not at 700K. Maybe this idea isn't so crazy after all.
Oh one other point; the heat dissipated by the electronics inside the shell will obviously shorten the time somewhat, but if they can really get that into single-digit watts, then it's inside the error bars.
--Greg
That 50 days is to get up to boiling point - and then you have the latent heat of evaporation until you use up enough of the water to expose the electronics I guess.
Doug
Unfortunately to use the heat of evaporation requires that the water boils and it won't do that until about 500 K unless you have a pump efficient enough to evacuate the steam despite the atmospheric pressure on Venus.
Of course, after the MER experience, anything less than a mobile rover isn't worth bothering with
True enough, actually; there's only so much data you can get from a stationary location with a fixed instrument set, However, I think that Venus is still ripe for such initial forays...the Veneras provided very limited data (but simultaneously acknowledging the fact that they were MAJOR engineering achievements!)
{Going out on a limb here}
Could we put a satellite on the (IIRC) L1 position between Venus and the sun, and put some retroreflectors on the surface of Venus. The satellite would continuously illuminate whatever retroreflector was visible with an appropriate microwave frequency and monitor the reflected signal for frequewncy shifts ??
Retroreflectors near the limb of Venus would be sensitive to vibrations parallel to the surface, and the retroreflector directly below would be sensitive to up and down motions.
We would be looking for rapid (but tiny!) frequency variations caused by seismic vibrations in the return signal, and we could ignore the slow shifts caused by the (slow) Venusian rotation.
{I remain innocent of the frequency stability requirements for the satellite, but the retroreflectors seem to be feasible from a materials science standpoint}
I'm no expert on radio waves by any means, but your idea leaves me with two questions:
1) L1 and L2 points are unstable so you would need either station-keeping or a quasi-orbit around those points. This would affect ranging and tracking.
2) What effect would changes in the overhead atmosphere have on microwave beams passing through. Temperature/density gradients slightly affecting index of refraction and apparent ranging.
Both of these effects would probably be only apparent on longer timescales than your typical seizmic signal so they could plausibly be filtered out.
Additionally, while retroreflectors would pickup up-down motions, lateral motion would be undetectable - isn't that the major component of seizmic waves anyway?
EDIT: an additional point, what is the L1 distance anyway? It might be a really long way off and your return signal strength would inversely vary with 4th power (!) of distance. Why the need for a reflector anyway if you're working with microwave?
Summary:
According to a 2002 NASA publication, it looks like Silicon Carbide-based semiconductors will eventually enable electronics that will work up to 600C, with enough Earth-based applications to spur their development.
Details:
Wondering whether we could do better than vacuum tubes, I poked around and found this NASA link depicting a diode operating at 600C.
http://www.grc.nasa.gov/WWW/SiC/SiC.html
On the same site, under publications/review papers, I found this 2002 IEEE paper on very high-temperature semiconductors:
"High-Temperature Electronics—A Role for Wide Bandgap Semiconductors?" PHILIP G. NEUDECK, SENIOR MEMBER, IEEE, ROBERT S. OKOJIE, MEMBER, IEEE, AND LIANG-YU CHEN, PROCEEDINGS OF THE IEEE, VOL. 90, NO. 6, JUNE 2002
http://www.grc.nasa.gov/WWW/SiC/SiC.html
It's not a hard read, but here are some highlights:
The two materials of most interest are SiC (Silicon Carbide aka Carborundum) and GaN (Gallium Nitride). SiC is the more developed of the two -- "mass produced single-crytstal wafers are commercially available." High imperfection rates in these crystals are one big obstacle at the moment. Another issue is the need to develop "high-temperature passive components, such as inductors, capicitors, and transformers" (although those don't sound nearly as challenging).
There's a very impressive list of prospective applications for these devices (Table I), ranging from Automotive (components in the cylinders), Turbine Engines, Industrial, Deep-Well drilling, and (yes) Spacecraft (Venus and Mercury Exploration). Based on that, even though "formidible developmental challenges remain," I'd expect there's a good chance that electronics suitable for use at Venusian surface temperature and pressure will end up getting developed.
Sort of that, existing SOI (Silicon on Insulator) work up to 300C (commerical devices rated to 225 exist), and GaAs (Gallium Arsenide) adds "perhaps an additional 100C". In fact, they cite three papers demonstrating short-term GaAs operation at 500C, but note that "long-term operation of these electronics appreciably beyond the capability of SOI remains undemonstrated." Still, that puts GaAs within the range claimed by the authors of the water-cooled-lander presentation.
On the whole, this looks very encouraging to me. That Venus rover we've been dreaming of may not be so ridiculous after all.
--Greg
ugordan: As I calculate it, the Sun-Venus L1 point is 1,002,000 km from the surface of Venus.
--Greg
Completely mechanical seismometers were used for a long time and worked quite well. Most types were quite heavy, but at least one type (Galitzin's vertical seismograph) was quite small and handy. So what you need in the way of electronics is some type of electronic or electromechanical device to pick up the data and a simple transmitter to transmit it (perhaps to an orbiter) plus a power source. It doesn't sound impossible to build that to work at 700 K. It may be more difficult to build a lander that will guarantee a good coupling between seismometer and the ground (this problem in itself may well preclude elaborate insulation around the seismometer).
Since the topic's already wandered somewhat from the OP, I'll just post this bit that I've been reminded of:
Mike Malin's http://www.msss.com/venus/vgnp/vgnp.txt.html
I think we would want a retroreflector (and to use a microwave frequency absorbed by the surface materials) so we would be observing a point source on the surface. If we were monitoring an appreciable area the signals from the perimeter of the expanding shock would cause the beam reflections to interfere.
I think the drift of the spacecraft at the Venusian Lagrange spot would be held slow enough that we could distinguish the (relatively) faster surface jolts.
We can also simultaneously illuminate the retroreflectors with 2 different microwave frequencies and correct for atmospheric scintillations.
As Venus rotates, the reflectors directly below will affect the return signal via verticle oscillations, retroreflectors illumed near the Venusian limb will reveal motions parallel to the surface.
If the technique would work from the 60 degree leading and trailing Venusian Lagrange positions also, we might be able to simultaneously study specific retroreflectors from 2 sats, and be able to characterize ground motions more precisely.
For some reason, NASA issued a press release yesterday saying they've built an integrated circuit that runs at 600C.
http://www.nasa.gov/home/hqnews/2007/sep/HQ_07189_Silicon_Chip.html
From the article, "This chip exceeded 1,700 hours of continuous operation at 500 degrees Celsius - a breakthrough that represents a 100-fold increase in what has previously been achieved. The new silicon carbide differential amplifier integrated circuit chip may provide benefits to anything requiring long-lasting electronic circuits in very hot environments."
Can't find a relevant paper about it yet though.
--Greg
Could a liquid/solid that has a lower density change (volume change) between solid/liquid phase be found, So then it could simple be place in the space between the electronics and would not need its own containment vessel?
The problem is that the latent heat of evaporation of water is so huge -I don't know of anything else that can match it.
Doug
Apollo's Lunar Rover used bee's wax to cool its Lunar Communications Relay Unit (LCRU), the self-contained comm system that allowed good comm (and TV) from wherever the Rover was parked. It cooled at the phase change between solid and liquid, and was pretty effective up to about 150 degrees C. (It *may* also have used the phase change from liquid to vapor for cooling, I just don't recall right now. But I know it used bee's wax.)
-the other Doug
So we have wood used in Apollo hatches (is that right, or am I misremembering details?) and cork in some ablative coverings...it seems strange in such a high tech frontier of plastics and alloys that any natural products could find a role in such harsh environments.
Andy
Also, natural materials had been used for a long time. Plastics were relatively new.
Speaking of biomaterials, fats, fatty acids and glycerine could be used as a heat absorber for a venus lander, the density difference between these liquids and their solids is minimal and they have high melting points (around room temp), also they would not be to corrosive or conductive, so bathing the electronics and instruments in them would not be that bad.
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