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Unmanned Spaceflight.com _ Venus _ Water-cooled lander

Posted by: tanjent Aug 22 2007, 05:22 PM

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.

Posted by: helvick Aug 22 2007, 06:32 PM

QUOTE
..The concept is based around a 60-centimeter-wide pressure vessel, that's thermally isolated from a larger vessel using a vacuum. Inside the vessel are all the electronics you need, insulated from the otuside using 100 kilograms of water..

It appears that what is being suggested is a very small electronics package surrounded by the water within the 60cm internal vessel. A 60cm diameter sphere would hold ~110kg of water so you probably could get 100kg of water + a few kg of smarts into such a package.

The assumption is that you can effectively thermally isolate this internal block from the outer shell where you have the ultra high temperature power components and the sensors. I assume the objection came from someone who had some idea about how hard it is to build a thermal isolation system that keep the heat transfer across a 400deg+ temperature gradient into a 60cm core at <<1 watt. to be fair it does seem a bit fanciful but I'm no thermal expert.

Apart from that the numbers don't seem to add up (to me at any rate).

100kg of water. Specific heat capacity of 4.2 kJ.kg^-1.K^-1. Say we are heating that from an initial (arbitrary) temperature of 300K (25C) to 600K. That requires 4.2 * 100 * 300 kiloJoules. That's 126000 kiloJoules.
A Joule is a watt per second, or a kiloJoule is 0.2777 watt hours so 126000 is 35000 watt hours. Divide by 24 and you get 1458 days. Maybe he's talking about Venusian days?

The good news though is that the error is all in the right direction. This means that you could support an electronics package that produced 30 watts of power instead and still last 50 days before you had to start venting coolant. The other number they talked about then actually makes sense too. The latent heat of vapourisation of water is 2260kJ.kg^-1. Conveniently enough 30watts for one day is close to that at 2592kJ so you lose about 1.1kg of coolant per day once boiloff has started.

So maybe the system is viable for an electronics package that generates 1 watt of heat and a thermal isolation system that leaks 29 watts? That may still be impossible but it's certainly a lot easier to build than a perfect thermal isolation system.

Posted by: tty Aug 22 2007, 08:41 PM

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.

Posted by: djellison Aug 22 2007, 09:03 PM

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

Posted by: RJG Aug 22 2007, 09:59 PM

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

Posted by: tasp Aug 22 2007, 11:08 PM

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 ??

Posted by: Greg Hullender Aug 23 2007, 06:16 AM

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

Posted by: djellison Aug 23 2007, 12:03 PM

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

Posted by: marsbug Aug 23 2007, 01:36 PM

QUOTE (Greg Hullender @ Aug 23 2007, 07:16 AM) *
" 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.
--Greg

I usually try to avoid numbers but this intruiged me so heres a quick and dirty attempt:
Assume the thermal isolation system consists of a double walled shell with a good vacuum in between (say 10^-4 mbar) so that convection and conduction between the walls is close to zero, and that the shell is constructed of a low vapour pressure material such as tungsten, or is being actively pumped to keep it at that level. Assume also that connections between the walls are minimised (could fairly low tech heat tolerant machinery outside the shell be operated entirely by remote by an electronics package inside?) to ten square centimetres of cross sectional area, made of material with a low thermal conduction , say equal to aerogel at 0.03 w/m/k. As a random figure lets say the separation between walls is 10 cm, and the water starts at a temperature of 273 degrees Kelvin. The amount of heat transmitted through the connections would then be:


0.03= q/t * (L/(A*ΛT)
wher q/t is joules per secound, L is the length of the conducting connection, A is the total csa of the connections and ΛT is the temperature difference in kelvin.

So:
q/t= 0.03/ (0.1/(0.01*400)= 1.2 watts. Which looks ok.
BUT:
The major source of heat into the inner vessel will be radiation from the outer wall. Assuming that the outer wall reaches the same temperature as the venusian atmosphere fairly quickly, and that it can be approximated as a black body it will be radiating heat onto the inner wall at
W= σ*A*T^4
Where W is heat transfer in watts, σ is the Stefan-Boltzmann constant, A is the internal surface area of a sphere with an internal radius of 40 centimeters and T is the temperature of the outer wall in Kelvin (673 deg):
A= 4πr^2 = 4*π*0.4*0.4 = 2.01 meters square (ish)
W= σ*2.01*673^4= 23797.24 watts falling onto the inner shell? Or roughly 24 Kj per secound.
So water has specific heat capacity of 4.2 kJ.kg^-1.K^-1, so for 100 kg to reach 373 Kelvin from just above freezing requires 4.2kj*100*100= 42000 Kj. Assuming it heats linearly (which I know it doesn’t but can’t remember how to work it out properly) 42000/24 = 1750 seconds, or just under half an hour.
That doesn’t look very promising! Where did I go wrong anybody (bet its something very simple!)?. But it does assume one atmosphere of pressure, not ninety! Any chance I could look at how you worked it out for that figure doug?

Edit: Even if it works at ninety atmospheres I'd be impressed by a container that would hold a good vacuum for fifty days under those conditions without an active pump. I'd also be impressed by an active pump that would run for fifty days under venus surface conditions. Not saying it can't be done, just that I'd be impressed! biggrin.gif wink.gif

Posted by: djellison Aug 23 2007, 01:52 PM

QUOTE (marsbug @ Aug 23 2007, 01:36 PM) *
Any chance I could look at how you worked it out for that figure doug?


I didn't work out anything - it's all the presenters work. I'll drag up the abstract PDF later smile.gif

Doug

 EPSC2007_A_00387.pdf ( 42.88K ) : 340
 

Posted by: AndyG Aug 23 2007, 02:22 PM

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

Posted by: marsbug Aug 23 2007, 02:45 PM

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?

Posted by: AndyG Aug 23 2007, 03:23 PM

[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

Posted by: helvick Aug 23 2007, 04:13 PM

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.

Posted by: algorimancer Aug 23 2007, 05:36 PM

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.

Posted by: tty Aug 23 2007, 06:25 PM

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,

Posted by: hendric Aug 23 2007, 07:42 PM

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.

Posted by: djellison Aug 23 2007, 08:52 PM

QUOTE (hendric @ Aug 23 2007, 07:42 PM) *
One idea I've always had for a Venusian balloon is one using water as the lift gas...


BUSTED - for not reading TPS Blog smile.gif

http://planetary.org/blog/article/00001097/

One interesting technique is to use a phase change liquid in the balloon to control its altitude. It turns out that water is perfect for this. The balloon would drop to an altitude of 42 kilometers at which point the water would vaporize inside the balloon, inflate it - and the balloon would rise. Four or so hours later, it would reach 60 kilometers, the water would re-condense, the balloon would drop back to 42 kilometers again - and the process would repeat. You could explore that entire range of atmosphere with a period of around 6 to 8 hours, up and down, using water to autonomously control the altitude.

Posted by: hendric Aug 24 2007, 03:04 AM

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.

Posted by: tasp Aug 24 2007, 03:10 AM

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)

Posted by: rlorenz Aug 24 2007, 07:39 AM

QUOTE (algorimancer @ Aug 23 2007, 01:36 PM) *
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.


Jeff Hall from JPL discusses a 38cm concentric titanium sphere enclosure with aerogel in between -
limits the heat leak to 100W - in a Journal of Spacecraft and Rockets paper vol.37 no.1 (142-144)
2000

First page is at
http://pdf.aiaa.org/jaPreview/JSR/2000/PVJAIMP3539.pdf

My own efforts on the related problem of very small dropzondes released from a balloon
are discussed at http://www.lpl.arizona.edu/~rlorenz/venusmicroprobes.pdf

Posted by: AndyG Aug 24 2007, 08:45 AM

QUOTE (rlorenz @ Aug 24 2007, 08:39 AM) *
Jeff Hall from JPL discusses a 38cm concentric titanium sphere enclosure with aerogel in between -
limits the heat leak to 100W


That's not a bad match to the calculated ~500W in a 60cm sphere. Presumably they're able to improve on the Google-found 2% emissivity I used.

Andy

One quick edit later - http://www.iop.org/Jet/fulltext/JETP98057.pdf mentions practical levels of emissivity down to ~0.5% for aged, thin silver coatings...which would help bump up the insulation factor.

Posted by: Greg Hullender Aug 24 2007, 09:51 PM

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

Posted by: djellison Aug 25 2007, 02:16 PM

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

Posted by: tty Aug 25 2007, 05:51 PM

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.

Posted by: algorimancer Aug 26 2007, 12:21 AM

Of course, after the MER experience, anything less than a mobile rover isn't worth bothering with rolleyes.gif

Posted by: nprev Aug 26 2007, 03:05 AM

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!)

Posted by: JRehling Aug 26 2007, 04:39 AM

QUOTE (nprev @ Aug 25 2007, 08:05 PM) *
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!)


Venus surface science can largely be broken into two categories: Seismic studies, and everything else. Seismic studies need long durations on the surface, and I don't think the water-cooled approach will cut it. On the bright side, a seismometer is a lot easier than most sensitive instruments to engineer for 500C. I think at some point, we have to have 2 to 4 seismometers on the surface working for at least a year. Those landers can have additional instrument packages and it might make sense to engineer only part of the package to survive: Do your landing panorama in the first half hour and who cares (much) if the camera dies after that -- not much is going to change. The trick is that *some* electronics even on a seismography-only package have to keep working at Venus temperatures. Maybe there's a way to refrigerate a tiny electronics "brain" that controls and runs telemetry for a cruder (vacuum tube?! just kidding) seismometer. Just about everything else could play by Venera rules: 90 minutes is plenty. All of the atmospheric studies could actually be done during the end of descent.

This kind of mission wouldn't cover all of the needs we have at Venus, but nothing is going to get around the need for a seismic network sooner or later. Some of the other surface science can ride along.

Posted by: tasp Aug 26 2007, 02:39 PM

{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}

Posted by: ugordan Aug 26 2007, 02:52 PM

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?

Posted by: JRehling Aug 26 2007, 03:47 PM

QUOTE (tasp @ Aug 26 2007, 07:39 AM) *
{Going out on a limb here}
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.


It would be quite easy to do this if the location we were scanning happened to be the place where a sizable quake were happening at that moment. Earthquakes of magnitude 7 to 8 can produce ground velocities (horizontally) of 50 cm/sec or so. Probably easy to detect.

I think trying to look at a seismic station at one location on Venus and detect (via doppler) the vibrations of a quake on the other side of the planet would probably be impossible. The ground velocity would be tiny indeed.

So an alternate approach would be to bet on picking the right location of a quake and to scan while one is happening. That seems like we'd have to get awfully lucky to get one detection, and wouldn't learn all that much from it.

I wonder if the solution isn't simply to take a "vacuum tube" approach to electronics that function on the surface of Venus (forget modern microchips) and plunk down a lander that survives 500C. Power source, seismometer, transmitter and nothing else.

Posted by: Greg Hullender Aug 26 2007, 06:20 PM

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

Posted by: Greg Hullender Aug 26 2007, 07:12 PM

ugordan: As I calculate it, the Sun-Venus L1 point is 1,002,000 km from the surface of Venus.

--Greg

Posted by: tty Aug 26 2007, 07:20 PM

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).

Posted by: Gsnorgathon Aug 26 2007, 09:21 PM

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

Posted by: tasp Aug 27 2007, 02:25 AM

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.

Posted by: Greg Hullender Sep 13 2007, 12:42 AM

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

Posted by: JRehling Sep 14 2007, 06:20 PM

QUOTE (Greg Hullender @ Sep 12 2007, 05:42 PM) *
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


That's great news. I wonder how much of the workload that chip can handle, or if it's of toylike simplicity and is more of a proof of concept. And if it adheres to any other standards so that programming it doesn't require a boatload of software technology development.

I think this could go one of two routes: Either the new chip technology ends up producing a crude but servicable brain for Venus missions, which end up resembling 1970s computers, or they put still more effort in to interface the CPU and memory chips into prevailing standards, so that the programming work is similar to what it would be for using off the shelf technology on a typical space mission.

But this is a huge step. The steps that remain are something we know can be done.

Three Venus landers with camera and seismometers -- the Venusian Viking, after so many decades -- let's see it happen!

Posted by: nnyspace Nov 7 2007, 05:58 PM

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?

Posted by: djellison Nov 7 2007, 07:05 PM

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

Posted by: dvandorn Nov 7 2007, 07:12 PM

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

Posted by: AndyG Nov 7 2007, 08:31 PM

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

Posted by: nnyspace Nov 7 2007, 10:01 PM

QUOTE (djellison @ Nov 7 2007, 07:05 PM) *
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


Well yes water does have a very high heat of fusion and specific heat capacity. But others come close for example it would take water 34.8 hr to go from -20 to 60C with 500 watts of input heat, for formic acid it would take 24.5hr, and if formic acid has a much small density difference between it phases (can't find density of solid phase formic acid) you could get a weights savings by using formic acid without the need for another containment layer/vessel.

--nny

Posted by: JRehling Nov 8 2007, 05:21 AM

QUOTE (AndyG @ Nov 7 2007, 12:31 PM) *
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


Now, yes, but Apollo got its start 40+ years ago, before Benjamin Braddock was told that the future was plastics.

I've seen a Mercury training capsule up close (and at length), and was quasi-surprised to see leather used inside. But, hey, it works, so why not?

Posted by: tedstryk Nov 8 2007, 11:32 AM

Also, natural materials had been used for a long time. Plastics were relatively new.

Posted by: ugordan Nov 8 2007, 11:45 AM

QUOTE (JRehling @ Nov 8 2007, 06:21 AM) *
Now, yes, but Apollo got its start 40+ years ago,

OT, but I just realized tomorrow's the 40th anniversary of Apollo 4 launch, the first Saturn V flight. Has it been that long?

Posted by: nnyspace Nov 8 2007, 06:15 PM

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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