Hi Leland,
The tale of the porous plug is very interesting. I can't wait to mention "sintering" in my next conversation about superfluidity. Thanks!
Mike
>Hi,
>
>Here is the Gravity Probe B Mission update for July 29,2005
>
>#---------------------------------------------
>
>
>=============================================
>
>GRAVITY PROBE B MISSION UPDATE FOR 29 JULY 2005
>
>==============================================
>
>
>
>GP-B STATUS AT A GLANCE
>
>=============================
>
>Mission Elapsed Time: 465 days (66 weeks/ 15.29 months)
>
>Science Data Collection: 336 days (48 weeks/11.05 months)
>
>Current Orbit #: 6,860 as of 1:00PM PST
>
>Spacecraft General Health: Good
>
>Roll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)
>
>Gyro Suspension System (GSS): All 4 gyros digitally suspended in science mode
>
>Dewar Temperature: 1.82 kelvin, holding steady
>
>Global Positioning System (GPS) lock: Greater than 98.7%
>
>Attitude & Translation Control (ATC): X-axis attitude error: 165.8 marcs rms
>
>Y-axis attitude error: 215.4 marcs rms
>
>Command & Data Handling (CDH): B-side (backup) computer in control
>
>Multi-bit errors (MBE): 2
>
>Single-bit errors (SBE): 8 (daily avg.)
>
>Telescope Readout (TRE): Nominal
>
>SQUID Readouts (SRE): Nominal
>
>Gyro #1 rotor potential: -1.4 mV
>
>Gyro #2 rotor potential: +4.1 mV
>
>Gyro #3 rotor potential: -3.3 mV
>
>Gyro #4 rotor potential: -1.7 mV
>
>Gyro #1 Drag-free Status: Backup Drag-free mode (normal)
>
>
>MISSION DIRECTOR'S SUMMARY
>
>=======================
>
>As of Mission Day 465, the Gravity Probe B vehicle and payload are in good health. All four gyros are digitally suspended in science mode. The spacecraft is flying drag-free around Gyro #1
>Preliminary calibration testing of gyros #2, #3, and #4 continued this week and will be completed by Monday. These calibrations do not impact science data collection, but they do help us learn more about the characteristics of these gyros. These tests included a modulation of the Gyroscope Suspension System (GSS) preloads at roll rate, a SQUID configuration test, and a SQUID off test.
>
>On Thursday, we performed the final heat pulse meter to determine the amount of liquid helium remaining in the Dewar. Preliminary analysis supports previous estimates for the helium lifetime. Final results of this test are pending analysis.
>
>
>MISSION NEWS—LIQUID IN, VAPOR OUT: THE POROUS PLUG
>
>===============================================
>
>As we mentioned in last week’s highlights, the GP-B mission is in the final weeks of science data collection before the helium in the Dewar is exhausted. Our tests and calculations indicate that the liquid helium will completely evaporate in early September. All told, this will give us more than eleven months of relativity data to test Einstein’s theory about curved spacetime. It is therefore appropriate to look back at one of the critical pieces of equipment that has allowed the mission to collect so much data for so long -- the “porous plug”.
>
>The porous plug is a small device located near the top of the 650-gallon Dewar. Its primary purpose is to provide a controlled vent for helium vapor while retaining the liquid helium inside for as long as possible. It does this by taking advantage of the superfluid nature of the liquid helium.
>
>“Superfluidity” is a characteristic of liquid helium that only appears when it is 2.18 kelvin or colder (our liquid helium is holding steady at 1.8K). When liquid helium is superfluid, it has no viscosity. This allows it to move about on a surface or through a porous substance without friction. A remarkable effect of this is that if superfluid helium was left in an short open beaker in a supercooled environment, the liquid would crawl up the inside of the beaker and down the outside until the beaker was empty!
>
>(For more details, check out GP-B’s Scientific Papers under “The Engineering Story” or NASA’s GSFC Introduction to Liquid Helium or CERN’s Teaching Materials .)
>
>The role of the porous plug is to control this flow of superfluid helium just enough to let some of it evaporate without letting the liquid leak out. A simple vent would not work because in a zero-g environment, the superfluid helium would simply migrate out of any opening in the Dewar, no matter how small we made it or how quickly we opened and closed it. Instead, we needed to make a porous plug.
>
>The plug itself is made of stainless steel, but it has gone through a “sintering” process that makes it slightly porous (like a sponge). Sintering is a process of powdering a material into tiny granules and then heating the granules until they coalesce into a porous mass, without heating them so high that they melt together. The pores in the resulting mass are extremely small (in fact, invisible to the naked eye), but they are large enough for the superfluid helium to find its way through the plug.
>
>Before the oozing superfluid helium leaks completely through the plug, it reaches a point where it begins to evaporate (called the liquid-vapor interface). When it evaporates, the helium vapor takes heat energy out of the liquid helium. Now the liquid helium near the outside of the plug is colder and it sinks back into the Dewar. At least, this is how it is supposed to work.
>
>It was a tricky challenge to choose the right size for the porous plug. If it was too small, the liquid-vapor interface would be below the plug, which would choke the plug and prevent the release of helium vapor. If the plug was too large, the interface would be above the plug and the liquid helium would ooze out of the plug before it had evaporated and would be lost. The proper size turned out to be 6.9 cm disk, 0.635 cm thick with a permeability of 3.8 x 10^-10 cm squared. The material for the plug is a low-carbon version of stainless steel provided by the Mott Corporation in Connecticut (which also produced our sintered titanium filters to purify the helium gas and the sintered titanium blades for the cryopump – See June 24 Highlight).
>
>Now that we have gone to the trouble of making a sophisticated porous metal plug, why are we doing this? Why is the porous plug so critical to the life of the mission? Because without finding some way of continuously releasing the helium vapor that builds up inside the Dewar, the pressure and temperature of the science instrument would rise to unacceptable levels.
>
>The challenge begins with the fact that the Dewar and satellite, while orbiting the Earth, are warmed up by radiation from the Sun and Earth. The Dewar has thermal barriers to insulate the helium from this radiation, but some residual heat does enter the Dewar and heats up the liquid helium. If left alone, the liquid helium would gradually transition into helium vapor, thereby increasing the pressure and temperature inside the Dewar. So we need to regularly vent the Dewar.
>
>By releasing the vapor, we do two things. First, we prevent the pressure from rising and maintain it at a steady level. Second, when the helium evaporates out of the Dewar, it takes heat energy with it. Therefore, any heat energy that has leaked into the liquid helium through the Dewar is released out of the liquid helium. This is the same effect that happens when your sweat evaporates; your skin is releasing heat energy and it cools your body off.
>
>By using the porous plug, we have been able to maintain a steady pressure and temperature inside the Dewar. The liquid helium has stayed within a few millikelvin of 1.8K ever since we launched sixteen months ago. This has allowed the science instrument to operate properly at superconducting temperatures and has given us more than eleven months of relativity data to test Einstein. Without this sophisticated plug, the mission would have been over before it had even started.
>
>
>===================
>
>PREVIOUS GP-B UPDATES
>
>===================
>
>If you wish to read any of our previous updates, our GP-B Web site includes a chronological archive of all the updates/highlights (with photos and drawings) that we have posted over the past 8 years:
http://einstein.stanford.edu/highlights/hlindexmain.html
>
>
>
>=============================
>
>OTHER LINKS THAT MAY INTEREST YOU
>
>=============================
>
>Our GP-B Web site, http://einstein.stanford.edu contains lots of information about the Gravity Probe B experiment, general relativity, and the amazing technologies that were developed to carry out this experiment.
>
>Visual tour of the GP-B spacecraft and payload from our GP-B Web site: http://einstein.stanford.edu/content/vehicle_tour/index.html
>
>PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that you can download print out, and assemble: http://einstein.stanford.edu/content/paper_model.
>
>NASA's Marshall Space Flight Center also has a series of Web pages devoted to GP-B: (http://www.gravityprobeb.com)
>
>Photo, taken through a telescope by Swiss physics teacher and amateur astronomer Stefano Sposetti, of GP-B spacecraft in orbit, passing near IM Pegasi: http://aida.astronomie.info/sposetti.
>
>The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto), with contributions from the Observatoire de Paris, have been studying the motions of the guide star, IM Pegasi for over a decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/
>
>In addition, you'll find information in the Guide Star FAQ on our Web site: http://einstein.stanford.edu/content/faqs/faqs.html#guidestar and on pages 18-20 of the Gravity Probe B Launch Companion: http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf.
>
>Track the GP-B satellite on the Web using NASA's Java-based J-Pass satellite tracking application at: http://science.nasa.gov/realtime/JPass/ Also, you can track the GP-B satellite on Personal Digital Assistants (PDAs) using either the Palm OS or Pocket PC operating systems with software from Big Fat Tail Productions: http://www.bigfattail.com.
>
>The Einstein Exhibition at the Skirball Cultural Center in Los Angeles has closed. However, you can visit the American Museum of Natural History's virtual Einstein exhibit on the Web at: http://www.skirball.org/exhibit/amnh_frame.html.
>
>==========================
>
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>
>==========================
>
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>
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>
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>
>
>
>--
>
>**********************************
>
>NASA - Stanford - Lockheed Martin
>
> Gravity Probe B Program
>
>"Testing Einstein's Universe"
>
> http://einstein.stanford.edu
>
>
>
>Bob Kahn
>
>Public Affairs Coordinator
>
>Phone: 650-723-2540
>
>Fax: 650-723-3494
>
>Email: kahn@relgyro.stanford.edu
>**********************************
>
>
>--------------------------------------------
>Mr. Shannon K'doah Range
>Educational Outreach Director
>NASA's Gravity Probe B: Testing Einstein's Universe at Stanford University
>http://einstein.stanford.edu/
>range@relgyro.stanford.edu
>415-867-4689
>
>[Curved spacetime] cannot be held responsible for people falling in love.
>-- Albert Einstein
>-------------------------------------------------------------------
>
>#-------------------------------------
>
>Regards,
>
>LelandJ
