Author Topic: 15 miles of wire: guess the volume  (Read 15602 times)

Offline LunarOrbit

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Re: 15 miles of wire: guess the volume
« Reply #15 on: May 12, 2012, 11:10:06 PM »
I'm not going to try to guess a number, but am I right that you can get the volume of the wire just by considering it a really long and narrow cylinder?
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Offline Not Myself

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Re: 15 miles of wire: guess the volume
« Reply #16 on: May 13, 2012, 12:06:35 AM »
I'm not going to try to guess a number, but am I right that you can get the volume of the wire just by considering it a really long and narrow cylinder?

That's what I (and some others) did.  But even though it is just under one cubic foot, if you tried to pack it in a box 1 foot to an edge, you probably couldn't do it, because round wire wouldn't stack with perfect efficiency.
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Offline Valis

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Re: 15 miles of wire: guess the volume
« Reply #17 on: May 13, 2012, 02:17:59 AM »
I'm not going to try to guess a number, but am I right that you can get the volume of the wire just by considering it a really long and narrow cylinder?

That's what I (and some others) did.  But even though it is just under one cubic foot, if you tried to pack it in a box 1 foot to an edge, you probably couldn't do it, because round wire wouldn't stack with perfect efficiency.
Yep. For a 2D cross-section of the wound wires, a hexagonal packing is the most efficient, and it has about 10 % empty space between the wires.

Offline Count Zero

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Re: 15 miles of wire: guess the volume
« Reply #18 on: May 13, 2012, 10:28:46 AM »
Right, but since we're talking about how much volume within the capsule was taken up by 15 miles of wire, we can assume that it is unpacked.
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Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #19 on: May 13, 2012, 03:22:06 PM »
Somewhere in my library I have a 200-page book on the physics of wire, written for engineers designing wire harnesses and cable runs.  I can't find it now, but the total volume of the wire is generally not the issue.  Yes, I realize the question being asked, so total volume of the wiring versus total volume of the spacecraft seems like a reasonable question.  But for actual design values we want the cross section of the harness and the minimum bending radius.  Those tell you the most about the geometry of the actual wiring harnesses.

We use 4-24 AWG wiring in spacecraft.  The sheath is typically TKT and varies greatly in thickness depending on the application.  We choose wire based on mechanical properties, thermal properties, and electrical properties, all of which tend to provide conflicting constraints and therefore a careful design choice.
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Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #20 on: May 14, 2012, 02:22:51 PM »
I just got around to digesting this at the level it deserved.

There are of course many different wire sizes. AWG 18 wire has a diameter of about 1 mm. I think most of the signal wiring was smaller than that precisely to save weight, but the power wiring would undoubtedly have been much heavier.
True the latter, you have some high-current wiring in the CSM and it takes thicker conductors to pass it.  That said, thinner wire saves weight, but also heats up more.  In wiring for vacuum, you have to take heat rejection into account.  Thinner conductors also break more easily in a high-vibration environment.  Most spacecraft are a high-vibration environment, at least for the first 6 minutes of their flight (i.e., atmospheric boost).  The LM was notorious for wire breakage in early development shake tests, requiring thicker and thicker conductors to handle the mechanical requirements of the wiring harnesses.  Harnesses made from thicker conductors don't bend as easily, requiring redesigning the harnesses and the cable runs that accommodate them.  I have some samples of high-bandwidth cabling we can use for digital communications.  It has a data bandwidth of 12 GB/s (that's big B for "bytes") but a foot of it is so inflexible that you can hold it at one end and it cantilevers out straight with a cell phone hung from the other end.  Designing runs for this marvelous data channel is... challenging.

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Of course, part of each wire is the much less dense insulation, probably Teflon.

Variously Teflon, Kapton, or TKT.  Kapton is great because it has the best electrical insulation value per unit mass, requiring a thinner sheath and less cross section and mass for the harness.  However it has the ugly property of failing to self-extinguish in a high oxygen environment and having poor abrasion properties.  Teflon has reciprocal advantages.  TKT is a composite sheathing involving an inner layer of Teflon, the bulk of the sheath being Kapton, and finally an outer layer of Teflon again.  It's a very expensive best-of-both-worlds product.

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That's important because they're still among the most unreliable of all electrical components.

Indeed.  Every manner of connector manages to shake loose eventually under takeoff acoustical loads.  This is why we still prefer to hardwire everything and then guillotine/deadface it where separation is a requirement.

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The reduction in wiring complexity is so great that it becomes easy to add another bus for full redundancy.

It also compensates for the problem that the wiring in a digital bus becomes a critical item itself.  With home-run analog wiring, damage to wiring harness gracefully degrades the affected systems.  With a digital bus, damage to the harness that implements the bus can fail the bus entirely, failing large portions of the spacecraft.  Hence the additional bus (preferably routed somewhat differently) is comforting.

A digital air- or spacecraft bus greatly simplifies recording and telemetry.  In a modern airliner, where miles of analog wiring used to prevail, the important (and previously difficult) task of flight recording has been greatly simplified.  DFDR aggregators just plug into the bus controller now.
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Offline Valis

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Re: 15 miles of wire: guess the volume
« Reply #21 on: May 14, 2012, 03:29:12 PM »
JayUtah, very interesting stuff. Could you please elaborate on the composition of the high throughput data cable, surely it uses small diameter filaments because of skin effect?

A somewhat parallel anecdote: You probably know Monster cables, where the current or signal direction is marked with arrows (total rubbish, of course). Well, I've seen from a very close distance the testing a startup company wanted to make for their spin-polarized speaker cables, as their hypothesis was that if all the electrons have their spins aligned to a certain direction, the sound is better (there were some other hypotheses in other relevant areas too, but that was the main point). Needless to say, it didn't really go that way, all you do is to degrade your signal (basically higher resistance), and even in the best case, any physical property of the cable is just similar to your normal analog cable. I don't know if the company has folded since, but I'm very sure they didn't have any product available for sale. The moral of the story is that you might want to have physicists in your team for a reality check, or forget the testing, and get a very good marketing team.

Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #22 on: May 14, 2012, 05:01:41 PM »
JayUtah, very interesting stuff. Could you please elaborate on the composition of the high throughput data cable, surely it uses small diameter filaments because of skin effect?

It's very similar to 10GBASE-CX4, if you're familiar with copper Ethernet.  Multiple twinaxial paths.
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Offline ka9q

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Re: 15 miles of wire: guess the volume
« Reply #23 on: May 14, 2012, 06:50:51 PM »
What's so special about that cable that makes it so heavy? I can send 125 MB/s (megabytes) over the four twisted pairs in a Cat-5a cable, and I can do it up to 100m. Beyond that, fiber is the way to go.


Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #24 on: May 15, 2012, 01:14:27 AM »
It's not heavy, per se, just inflexible.  Twinaxial is like coaxial, only with two central conductors.  That's what creates the bulk.  At peak speed it's 12 gigabytes per second.  But you can run it at slower speed under heavy EMI with high reliability.

In spacecraft it's helpful when you need guaranteed throughput to a transponder bank under questionable conditions.  In terrestrial applications, it's helpful when modeling an n-body gravitational problem, where n is the identifiable objects in the Kuiper Belt.  In real time.

It's not a typical-use item.  It's just indicative of the mechanical constraints of some kinds of wire.  You can't bend this stuff easily, so you have to plan your cable runs accordingly.  Fiber has similar constraints on the radius of bends.
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Offline DAKDAK

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Re: 15 miles of wire: guess the volume
« Reply #25 on: May 15, 2012, 02:59:10 AM »
The only pictures of the wire or harnesses I have found are on this website near the bottom of the page

http://www.spaceaholic.com/apollo_artifacts.htm
« Last Edit: June 06, 2012, 08:43:24 PM by LunarOrbit »

Offline Count Zero

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Re: 15 miles of wire: guess the volume
« Reply #26 on: May 15, 2012, 03:24:28 AM »
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The only pictures of the wire or harnesses I have found are on this website near the bottom of the page

http://www.spaceaholic.com/apollo_artifacts.htm

Cool site!  <bookmark>
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Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #27 on: May 15, 2012, 10:27:43 AM »
Cool site!  <bookmark>

Indeed, I love that guy's site.  And I have to say he has the most understanding spouse in the solar system.
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Offline cjameshuff

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Re: 15 miles of wire: guess the volume
« Reply #28 on: May 15, 2012, 10:59:38 AM »
In terrestrial applications, it's helpful when modeling an n-body gravitational problem, where n is the identifiable objects in the Kuiper Belt.  In real time.

A bit faster than that, I hope, considering the orbital periods of those objects.

Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #29 on: May 15, 2012, 12:33:04 PM »
A bit faster than that, I hope, considering the orbital periods of those objects.

True, they do move so very slowly.  The joy comes in comparing the prediction with real-world observations of them.

On the plus side, the same system can raytrace reasonably complex scenes in real time at about 10 frames per second.

Back to wiring, DAKDAK's link shows a "potted" assembly, the master event sequencer or something.  Potting is where you wire it all up, then dump molten plastic all over it and let it solidify.  This provides exceptional protection against acoustic loads.  One of the little-known facts about space engineering is that it's not the raw g-load that wrecks stuff on the ascent; it's the vibration.  Badly designed payloads literally shake to pieces on the ascent.  Also, when you pot the assembly in a form, and then match that form to a chassis, you can have superior heat rejection from the potted component if you arrange for cooling liquid to pass through cooling jackets in the chassis.
"Facts are stubborn things." --John Adams