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from the next-capsule-arriving-at-platform-one dept.
Jeff Williams, Alexey Ovchinin, Oleg Skripochka have returned to Earth from the International Space Station:
Less than a week after winding up a successful spacewalk, outgoing space station commander Jeff Williams, America's most experienced astronaut, joined two Russian cosmonauts for a fiery return to Earth Tuesday, closing out a 172-day mission with an on-target landing in Kazakhstan.
[...] The four-minute 41-second burn slowed the ship by 286 mph, just enough to drop the far side of the orbit deep into Earth's atmosphere. After a 25-minute free fall to just above the top of the discernible atmosphere, the three modules making up the Soyuz TMA-20M spacecraft separated. A few moments after that, the central crew cabin, the only module protected by a heat shield, slammed into the atmosphere at an altitude of 62 miles and a velocity of some 17,000 mph. Using atmospheric friction to slow down, the descent module's main parachute deployed at an altitude of a little less than seven miles and the spacecraft settled to the steppe of Kazakhstan.
[...] For Williams, the end of the mission marked a personal milestone. On Aug. 20, he surpassed the U.S. record for most cumulative time in space -- 520 days -- that was set earlier this year by astronaut Scott Kelly at the end of his nearly yearlong mission. With landing Tuesday, Williams' mark will stand at 534 days aloft over four missions, moving him up to 14th on the list of most experienced spacemen. The overall record is held by Gennady Padalka, who has logged 878 days in space over five missions.
(Score: 3, Informative) by LoRdTAW on Thursday September 08 2016, @03:07PM
Vacuum is something I know a thing or two about after dealing with electron beam welders for the past few years. A shaft passing from atmosphere to a vacuum is called a mechanical feed-through. It can rotate, move linearly or both. There are also electrical feed-throughs as well. Air is a fluid composed of various gasses. A vacuum is the absence of most of that fluid creating a pressure differential. It's the same thing as sealing an air compressor system which has a much higher pressure differential. The seal is nothing more than an Oring or quadseal (looks like a cross shaped o ring with four sealing surfaces instead of two). The material is usually a synthetic rubber such as the very common buna (nitrile) rubber or more exotic fluroelastomers can be used. A bled of the two is also found and one blend commonly used is called viton. It has excellent vacuum characteristics such as low out-gassing and good heat resistance on top of good wear resistance. Out-gassing is a problem as it can degrade the material by "sucking out" the binders which hold it together. They become brittle and either fail mechanically by falling apart or crack causing leaks in the case of a static seal.
How long do they last? Depends. A rotating shaft that sees constant use might last a few years. A much less used shaft might last many years, or decades. We have similar rotating shafts in our machines. One which drives a moving table sees daily use running at a few hundred RPM. It can last 1-3 years using buna wetted with a vacuum compatible grease which both lubricates and keeps it from drying out. A very similar shutter to the one you described which protects the operator view port from heavy weld spatter and vapor have seals that I know on some machine haven't been touched in over 20 years. The less use, the longer they last.
Another method of sealing a linear loving shaft is to use metal bellows which usually cover the shaft on the vacuum side. They are welded or bolted with a metal seal to the housing and the shaft which provide a very long lasting vacuum seal. They are commonly found in poppet and gate type vacuum valves. They also are found on micrometer type linear feed-throughs to precisely adjust the innards of sensitive vacuum processes such as the specimen in a scanning electron microscope. that can't rotate so are linear only. Quad seals are the less costly method of sealing a linear shaft and also permit rotation as well. There is also the cupseal which is a sort of U shaped rubber doughnut that allows for axial misalignment and play. They each have their place and our machines have a combination of all of these types of vacuum seals throughout its various parts.
And finally, the best static seals are made with a soft metal gasket. The standard CF, or conflat flange used for high vacuum hardware uses a flat copper gasket that is sandwiched between two stainless steel flanges. The flanges have a knife edge that bite into the gasket and form a really good vacuum seal. As far as I know, they last almost indefinitely but are one time use. Fun fact: they use aluminum in a lot of vacuum chambers for large high vacuum experiments because it's easy to form and machine vs stainless as well as being cheaper. But it's softer than copper. So they use explosive welding to bond a stainless steel flange to an aluminum flange using a thin layer of tantalum between to help the bond. The stainless is harder than the copper for sealing and the aluminum side is welded to the apparatus.
(Score: 0) by Anonymous Coward on Thursday September 08 2016, @04:02PM
Very informative answer, especially on the seals, many thanks.
I wish they had some schematic out and some details on the hydraulics of the ISS, as it is always mandatory to service those systems against vacuum since the ISS never lowers its altitude. I can only assume that when an outer component fails, they will have to create a live bypass and weld/replace on spot while 'live', otherwise all fluid will be sucked outside of the system, and even the best serviceman would never manage 0% leaks (especially in a clumsy astronaut suite)
I am still very much in doubt as to what is the material used in seals, valves and (even worse) pumps, as the wear-and-tear has to be enormous tolerating that temperature range, vacuum which evaporates and petrifies lubricants, all while maintaining its geometry and acting as an effective seal. They did show two o-rings on that 'Smarter Every Day' video so I can only guess that this might be the 'viton' you mentioned. I still don't get the 'cupola' shutter contraption, though, as demonstrated live by the astronaut: it has no actuators at all!
(Score: 2) by LoRdTAW on Friday September 09 2016, @05:03PM
They use a lot of connectors and valves between modules and components to avoid needing to weld. Connectors I see are similar to VCR/VCO and JIC flared fittings for tubing. They probably design everything to be removable and easily replaced without cutting or joining. So its just a few basic tools like wrenches, sockets and screwdrivers. Same with electronics, they use circular pin and sleeve connectors as well as D-sub, coaxial and so forth.
No such thing as zero leaks. We deal with them all the time. You just have to perform basic PM and have a procedure for leak checking. If you think about it, the pressure differential between the inside and outside is atmosphere or 14.7PSI/1013kpa. So it's not very hard to seal.
The trick is dealing with the radiation, temperature, and vacuum. Most metals are fine in a vacuum as with radiation. But common plastics will out gas in a vacuum becoming brittle and also don't like ionizing radiation for the same reason. Nylon wire ties in our vacuum chambers used to support cables running to our CNC fixtures will become brittle and crumble because of vacuum and x-ray bombardment. So we use kapton tape and metal supports or replace nylon ties when necessary. They may last a few months or more depending on quality.
As for the temperature, more exotic alloys and metals are used such as inconel, chromoly, molybdeum Invar, Kovar, titanium and more. Invar and Kovar are the two commonly used alloys for high temp differential stuff. We weld a lot of that stuff going into satellites.