IPB

Welcome Guest ( Log In | Register )

4 Pages V  < 1 2 3 4 >  
Reply to this topicStart new topic
Water-cooled lander
tty
post Aug 23 2007, 06:25 PM
Post #16


Member
***

Group: Members
Posts: 683
Joined: 20-April 05
From: Sweden
Member No.: 273



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,
Go to the top of the page
 
+Quote Post
hendric
post Aug 23 2007, 07:42 PM
Post #17


Director of Galilean Photography
***

Group: Members
Posts: 713
Joined: 15-July 04
From: Austin, TX
Member No.: 93



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.


--------------------
Space Enthusiast Richard Hendricks
--
"The engineers, as usual, made a tremendous fuss. Again as usual, they did the job in half the time they had dismissed as being absolutely impossible." --Rescue Party, Arthur C Clarke
Mother Nature is the final inspector of all quality.
Go to the top of the page
 
+Quote Post
djellison
post Aug 23 2007, 08:52 PM
Post #18


Administrator
****

Group: Chairman
Posts: 13806
Joined: 8-February 04
Member No.: 1



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.
Go to the top of the page
 
+Quote Post
hendric
post Aug 24 2007, 03:04 AM
Post #19


Director of Galilean Photography
***

Group: Members
Posts: 713
Joined: 15-July 04
From: Austin, TX
Member No.: 93



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.


--------------------
Space Enthusiast Richard Hendricks
--
"The engineers, as usual, made a tremendous fuss. Again as usual, they did the job in half the time they had dismissed as being absolutely impossible." --Rescue Party, Arthur C Clarke
Mother Nature is the final inspector of all quality.
Go to the top of the page
 
+Quote Post
tasp
post Aug 24 2007, 03:10 AM
Post #20


Member
***

Group: Members
Posts: 885
Joined: 30-January 05
Member No.: 162



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)
Go to the top of the page
 
+Quote Post
rlorenz
post Aug 24 2007, 07:39 AM
Post #21


Member
***

Group: Members
Posts: 417
Joined: 23-February 07
From: Occasionally in Columbia, MD
Member No.: 1764



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
Go to the top of the page
 
+Quote Post
AndyG
post Aug 24 2007, 08:45 AM
Post #22


Member
***

Group: Members
Posts: 573
Joined: 20-April 05
Member No.: 279



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 - Work with an eye towards ITER mentions practical levels of emissivity down to ~0.5% for aged, thin silver coatings...which would help bump up the insulation factor.
Go to the top of the page
 
+Quote Post
Greg Hullender
post Aug 24 2007, 09:51 PM
Post #23


Senior Member
****

Group: Members
Posts: 1010
Joined: 29-November 05
From: Seattle, WA, USA
Member No.: 590



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
Go to the top of the page
 
+Quote Post
djellison
post Aug 25 2007, 02:16 PM
Post #24


Administrator
****

Group: Chairman
Posts: 13806
Joined: 8-February 04
Member No.: 1



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
Go to the top of the page
 
+Quote Post
tty
post Aug 25 2007, 05:51 PM
Post #25


Member
***

Group: Members
Posts: 683
Joined: 20-April 05
From: Sweden
Member No.: 273



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.
Go to the top of the page
 
+Quote Post
algorimancer
post Aug 26 2007, 12:21 AM
Post #26


Member
***

Group: Members
Posts: 587
Joined: 20-April 05
From: League City, Texas
Member No.: 285



Of course, after the MER experience, anything less than a mobile rover isn't worth bothering with rolleyes.gif
Go to the top of the page
 
+Quote Post
nprev
post Aug 26 2007, 03:05 AM
Post #27


Senior Member
****

Group: Moderator
Posts: 7092
Joined: 8-December 05
From: Los Angeles
Member No.: 602



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!)


--------------------
A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
Go to the top of the page
 
+Quote Post
JRehling
post Aug 26 2007, 04:39 AM
Post #28


Senior Member
****

Group: Members
Posts: 1572
Joined: 20-April 05
Member No.: 321



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.
Go to the top of the page
 
+Quote Post
tasp
post Aug 26 2007, 02:39 PM
Post #29


Member
***

Group: Members
Posts: 885
Joined: 30-January 05
Member No.: 162



{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}
Go to the top of the page
 
+Quote Post
ugordan
post Aug 26 2007, 02:52 PM
Post #30


Senior Member
****

Group: Members
Posts: 3569
Joined: 1-October 05
From: Croatia
Member No.: 523



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?


--------------------
Go to the top of the page
 
+Quote Post

4 Pages V  < 1 2 3 4 >
Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 26th October 2014 - 09:30 AM
RULES AND GUIDELINES
Please read the Forum Rules and Guidelines before posting.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted. Do not reproduce without permission. Read here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators.
SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society and is funded by donations from visitors and members. Help keep this forum up and running by contributing here.