Water-cooled lander Options

[tanjent]

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.

[Greg Hullender]

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