Author Topic: 15 miles of wire: guess the volume  (Read 15601 times)

Offline BazBear

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Re: 15 miles of wire: guess the volume
« Reply #30 on: May 15, 2012, 06:51:44 PM »
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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>
Ditto! Good find DAK.
"It's true you know. In space, no one can hear you scream like a little girl." - Mark Watney, protagonist of The Martian by Andy Weir

My Youtube Apollo playlist http://www.youtube.com/watch?v=8SfyE9qsG8k&list=PL2aEC7cUMrGCNrtGMMWRXYob-kqCz2zz8

Offline ka9q

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Re: 15 miles of wire: guess the volume
« Reply #31 on: May 16, 2012, 07:03:58 PM »
it's helpful when modeling an n-body gravitational problem, where n is the identifiable objects in the Kuiper Belt.
So what's 'n'? Must be pretty large if you have that much interprocessor communication.

Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #32 on: May 16, 2012, 07:49:19 PM »
N ~= 800 for model validation runs using known objects, N ~= 5,000-70,000 for investigative runs involving speculative KBO populations.

The biggest N I've had to crunch was N=1017, on a full-hull CSD model for an oceangoing vessel.  N=109 is not uncommon for aerodynamics CFD models, full fuselage and airfoil.
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Offline randombloke

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Re: 15 miles of wire: guess the volume
« Reply #33 on: May 24, 2012, 05:36:36 PM »
Personally I'm not that surprised (intuitively or otherwise) by the small volume of the wire in the OP, but that might be because I'm a biologist for the most part; anyone familiar with the packing arrangements of DNA ought not to be surprised by great lengths taking up small volumes (off the top of my head: the average human genome is ~2m long, per cell. There are on the order of 2e13 cells in an adult. And you still have a lot of room left over for the bits of the cell that do all the work too).

Offline Chew

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Re: 15 miles of wire: guess the volume
« Reply #34 on: May 24, 2012, 06:56:02 PM »
One gram of activated charcoal has a surface area of 500 m².

Offline slang

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Re: 15 miles of wire: guess the volume
« Reply #35 on: May 24, 2012, 07:55:09 PM »
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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.

I can wrap my head around guillotining, but what is "to deadface"?

Offline ka9q

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Re: 15 miles of wire: guess the volume
« Reply #36 on: May 24, 2012, 09:24:21 PM »
I can wrap my head around guillotining, but what is "to deadface"?
Deadfacing refers to deactivating the electrical cables between the stages prior to severing them with the explosively driven guillotine blade. This prevented momentary short circuits that in some cases (battery feeds) could cause problems.

I believe the electrical interruptions were usually done with relays. As mechanical devices, relays have a poor reputation for reliability. But I guess they're still better than connectors with lots of pins.

A re-engineering of Apollo with today's technology could vastly reduce the number of discrete wires passing between the stages. I'd probably use optical communications links wherever possible, as it is essentially immune to electrical interference. You could sever a fiber cable with a guillotine or just pull a connector apart. If the links don't have to be too fast, you might even avoid a direct mechanical connection between the stages by having a LED or laser on one side illuminate a photodetector on the other.

I always wondered what would happen if one of the guillotine cable or bolt cutters failed to fire at ascent stage ignition. It would probably be fatal for the crew. Fortunately, the ordnance seemed to work flawlessly.

Edited to add: As you might expect, NASA has detailed handbooks for just about everything, including electrical deadfacing. Look for CR-2002-211839 at ntrs.nasa.gov.




« Last Edit: May 24, 2012, 09:38:11 PM by ka9q »

Offline Chew

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Re: 15 miles of wire: guess the volume
« Reply #37 on: May 24, 2012, 10:07:10 PM »
I always wondered what would happen if one of the guillotine cable or bolt cutters failed to fire at ascent stage ignition.

Yeah, I've often wondered about that myself. I see the ascent module pitching or rolling around, dragging the descent stage with it by the unsevered connections, and plowing head first into the regolith.

I can't shake the mental image of the origin of Murphy's Law whenever I watch a LM liftoff.

Offline slang

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Re: 15 miles of wire: guess the volume
« Reply #38 on: May 25, 2012, 02:53:48 AM »
Deadfacing refers to deactivating the electrical cables between the stages prior to severing them with the explosively driven guillotine blade. This prevented momentary short circuits that in some cases (battery feeds) could cause problems.

Thanks, that makes sense.

As you might expect, NASA has detailed handbooks for just about everything, including electrical deadfacing. Look for CR-2002-211839 at ntrs.nasa.gov.

Wow... and that's just to aid analysing whether deadfacing is required or not.. how to actually do it is in other documents. A quick go over the introduction part shows that a major reason for deadfacing is to prevent arcing. Interesting read, thanks again!

Offline Al Johnston

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Re: 15 miles of wire: guess the volume
« Reply #39 on: May 25, 2012, 06:15:15 AM »
I always wondered what would happen if one of the guillotine cable or bolt cutters failed to fire at ascent stage ignition. It would probably be fatal for the crew. Fortunately, the ordnance seemed to work flawlessly.

I can see that a bolt cutter failure would be fatal, but wouldn't the designers arrange it so that a single wire bundle didn't have the mechanical strength to lift the descent stage?
"Cheer up!" they said. "It could be worse!" they said.
So I did.
And it was.

Offline Chew

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Re: 15 miles of wire: guess the volume
« Reply #40 on: May 25, 2012, 08:56:35 AM »
There was also some piping between the two modules.

Trivia: the connections were severed 5 seconds before liftoff.

Offline JayUtah

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Re: 15 miles of wire: guess the volume
« Reply #41 on: May 25, 2012, 12:48:32 PM »
The cutter blades are ceramic and (allegedly) non-conducting.  There are two pyrotechnic pistons, basically the same principle by which automotive airbags are deployed.  Both are activated along redundant firing circuits and only one has to fire in order to activate the blade.

For many aircraft and spacecraft applications in the 1950s up through the 1970s, you'd use terminal blocks.  But wiring those up correctly is error-prone (e.g., Apollo 6 second stage) and requires skill.  Then you have the Bendix connector, which is far less error-prone since it's just a keyed connector, but its mechanical strength is lengendary.  Any other kind of connector that pulls loose simply upon extraction force is bound to shake loose upon launch.  And with any pin-in-socket type connector, bent pins during assembly is a chronic problem requiring reworking the whole harness.

Yes, mechanical relays were used in electrical deadfacing.  And no, they're not especially robust items.  However there are special deadfacing relays that are useful only for that purpose.  With most relays, there's a spring-loaded armature that is held in one position by the spring and moved to the other position by a control force, typically an electromagnet.  They fail in space environment for any number of reasons, typically by contamination that prevents the armature from operating or by failure of the spring.  In a deadfacing relay there is no "must continue to act" force holding the circuit closed, so it operates in failsafe mode until the control force acts.  The relay mechanism is simpler and less prone to contamination, and assembled under clean-room conditions then sealed.  They achieve higher-than-usual reliability.
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Offline ka9q

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Re: 15 miles of wire: guess the volume
« Reply #42 on: May 26, 2012, 05:52:56 AM »
Wires can short to each other even while they're being cut with nonconductive cutter blades.

But many circuits would still require deadfacing even if momentary conductor-to-conductor shorts didn't occur. As slang pointed out, the NASA document I cited refers to the suppression of arcing as a main reason for deadfacing.

LM loads are very high during descent and ascent; just about everything is powered up. So if you didn't deadface the descent batteries before staging, even a 'clean' cut of the power cables to the ascent stage could generate a series arc and a nasty transient on the power buses in the ascent stage that could crash the avionics at one of the most critical moments in the mission. The computer is especially sensitive, as demonstrated during the Apollo 13 emergency. A computer restart due to a momentary droop in DC bus voltages was one of the first things noticed, but by the time someone checked the bus voltages at least one had already recovered to where it could support the computer. So the cause of the restart was not immediately obvious.

Offline ka9q

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Re: 15 miles of wire: guess the volume
« Reply #43 on: May 26, 2012, 05:58:31 AM »
There was also some piping between the two modules.
Yes, such as the oxygen tanks in the descent stage. Looking at the diagrams it is not clear to me that anything keeps the remaining O2 in the descent tanks from immediately venting as soon as the ascent stage lifts off. That seemed a little odd.

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Trivia: the connections were severed 5 seconds before liftoff.
Is that true? I've heard elsewhere that it was nearly simultaneous with ignition. I do wonder about the stability of the ascent stage sitting completely loose on the top of the descent stage for as many as 5 seconds.

In every LM ascent there is a very noticeable "clack" sound in the downlink audio at the moment of staging and ignition. I think we discussed this before but I don't remember if there was any consensus about the cause. So many things happen at that moment that the "clack" could come from anywhere. It might even be an actual cabin sound picked up by a headset microphone.


Offline Chew

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Re: 15 miles of wire: guess the volume
« Reply #44 on: May 26, 2012, 12:11:57 PM »
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Trivia: the connections were severed 5 seconds before liftoff.
Is that true? I've heard elsewhere that it was nearly simultaneous with ignition. I do wonder about the stability of the ascent stage sitting completely loose on the top of the descent stage for as many as 5 seconds.

According to the Flight Journal's article on Lunar Orbit Rendezvous

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At five seconds before ignition, the Commander presses Abort Stage, which detonates explosive bolts holding the two stages together, and fires other charges to propel guillotines that sever wire bundles and plumbing that pass between the stages. At the same time, the Display and Keyboard (DSKY), which has been counting down, blanks and replaces its display with a flashing Verb 99, requesting that the crew authorize ignition. When the LMP presses Pro(ceed) on the keyboard, the normal display returns, and the engine ignites when the count reaches zero.Link

As for the ascent module sliding off the descent module after the connections were severed I believe there were guide pins that held the ascent stage in place. I recall seeing a diagram of the descent module with these pins but I can't find it now. If anyone has more info I would appreciate it.