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Apollo Discussions => The Hoax Theory => Topic started by: Chew on May 09, 2012, 04:13:01 PM

Title: 15 miles of wire: guess the volume
Post by: Chew on May 09, 2012, 04:13:01 PM
What is your gut intuitive guess for the volume occupied by 15 miles of wire? Assume 26 AWG (.405 mm diameter) and the insulation is as thick as the wire itself; e.g. the total diameter is 1.215 mm.

I guessed before I calculated it and I was way off.
Title: Re: 15 miles of wire: guess the volume
Post by: DataCable on May 09, 2012, 04:39:29 PM
210 cubic feet.
Title: Re: 15 miles of wire: guess the volume
Post by: Valis on May 09, 2012, 05:26:28 PM
I didn't get any intuitive grasp of the volume, so I did an order of magnitude calculation in my head. The process is something like this:

15 miles is roughly 24 km (10*1.6=16, add half of that and it's 24).
The area of a cross-section for the wire is about 3*0.6^2, or 3*0.36, or about 1 mm^2, which is 10^-6 m^2.
Finally, the volume is 24*10^3*10^-6 = 0.024 (m^3), or 24 liters. That's the lower estimate, so my guesstimate would be less than 30 l, which would be a bit over one cubic feet.
Title: Re: 15 miles of wire: guess the volume
Post by: Echnaton on May 09, 2012, 05:40:03 PM
My top of the head estimations came out this way 15 miles X 1.6 gives about 24 KM of wire.  If wire was say 1 mm in diameter then 1000 strands could be laid side to side within 1 meter. If it were in 1 m long cuts, then 1 km of wire would be 1000 pieces or 1/1000 of a m3.  So the whole thing would be 24/1000 of a m3.   


So with wire at 1.2 mm my guess is it would be about 1.2^2 * 24 or about 36/1000 cubic meters  A web site tells me that is about 1.25 cubic feet. 



Title: Re: 15 miles of wire: guess the volume
Post by: Ranb on May 09, 2012, 05:48:12 PM
I got .99 cubic feet using the volume of a cylinder 0.0478 inches wide by 15 miles high.  I wasn't going to guess this one; I used a calulator.

Ranb
Title: Re: 15 miles of wire: guess the volume
Post by: Echnaton on May 09, 2012, 06:05:11 PM
I guessed before I calculated it and I was way off.

I didn't guess but was surprised at the results of my volume estimation was so small.  I could never have done this in imperial units, it is so much easier to work in metric
Title: Re: 15 miles of wire: guess the volume
Post by: Bob B. on May 09, 2012, 07:09:01 PM
I didn't get a chance to post my guess, but I was going to say about two cubic [feet] at the most.

Edited to add the word "feet".
Title: Re: 15 miles of wire: guess the volume
Post by: gillianren on May 09, 2012, 07:37:38 PM
I so have no guess for that.
Title: Re: 15 miles of wire: guess the volume
Post by: ka9q on May 09, 2012, 07:55:53 PM
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. Apollo's primary power supply was 28V DC, a low voltage that requires fairly heavy conductors to carry a lot of power. Some of it was also distributed as 3-phase 115V 400 Hz AC, but it was mainly for devices that need AC (like induction motors) rather than to lighten the wiring. In those days, AC inverters were one of the heavier and less reliable subsystems on a spacecraft.

Copper is a dense element, almost 9x as dense as water. A liter of the stuff would have a mass of almost 9 kg; one cubic foot (28.3 L) would be 253 kg! Of course, part of each wire is the much less dense insulation, probably Teflon.

Modern electronics allows spacecraft wiring to be vastly reduced in weight, bulk and complexity, also simplifying connectors. That's important because they're still among the most unreliable of all electrical components.

One advantage of modern electronics is lower power consumption, allowing lighter power wiring. Power semiconductors now make DC-DC conversion easy and efficient, so it's easier to use higher voltages for more efficient transmission over greater distances. The ISS, for example, operates its solar arrays at about 160V DC, and utility power is 124.5V DC. (Of course, the ISS is much larger than Apollo.)

But the real breakthrough in wiring has come from digital electronics. Every spacecraft has zillions of sensors and control devices all over the place, and in Apollo each required its own dedicated wiring and connector pins.

The wiring harnesses were massive, hard to make and easy to damage (remember Apollo 1). Redundancy was hard to provide. Analog signals often had to go significant distances, creating noise problems. Quite a bit of wiring passed between the CM and SM, or between the two stages of the LM, requiring large explosively-driven "guillotine" cutters for separation. That in turn required "deadfacing" relays and switches to disconnect each line and prevent a short circuit when it was cut.

Now you make a high speed digital bus with just a few wires, route it all over the spacecraft and attach it through bus interface electronics to each sensor or control. Being digital and usually differential (a balanced twisted pair), the bus is inherently much less susceptible to noise. Analog sensor signals are converted locally to digital before going on the bus. The reduction in wiring complexity is so great that it becomes easy to add another bus for full redundancy.
Title: Re: 15 miles of wire: guess the volume
Post by: Not Myself on May 12, 2012, 01:36:02 PM
I didn't get a chance to post my guess, but I was going to say about two cubic at the most.

Hard to be wrong, if you don't specify the units :)

My guess was much too high.
Title: Re: 15 miles of wire: guess the volume
Post by: Bob B. on May 12, 2012, 06:06:39 PM
I didn't get a chance to post my guess, but I was going to say about two cubic at the most.

Hard to be wrong, if you don't specify the units :)

Oops.  That should have been two cubic feet.
Title: Re: 15 miles of wire: guess the volume
Post by: Not Myself on May 12, 2012, 06:13:58 PM
I didn't get a chance to post my guess, but I was going to say about two cubic at the most.

Hard to be wrong, if you don't specify the units :)

Oops.  That should have been two cubic feet.

Then that's actually a pretty good guess.  Substantially better than mine :)
Title: Re: 15 miles of wire: guess the volume
Post by: Chew on May 12, 2012, 06:26:07 PM
Then that's actually a pretty good guess.  Substantially better than mine :)


Was it worse than 10 cubic feet? That was my guess.
Title: Re: 15 miles of wire: guess the volume
Post by: Not Myself on May 12, 2012, 06:36:33 PM
Was it worse than 10 cubic feet? That was my guess.

 :-[

I went with a cubic metre :(
Title: Re: 15 miles of wire: guess the volume
Post by: Chew on May 12, 2012, 09:13:09 PM
Well, now I don't feel so bad.
Title: Re: 15 miles of wire: guess the volume
Post by: LunarOrbit 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?
Title: Re: 15 miles of wire: guess the volume
Post by: Not Myself 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.
Title: Re: 15 miles of wire: guess the volume
Post by: Valis 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.
Title: Re: 15 miles of wire: guess the volume
Post by: Count Zero 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.
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.

Quote
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.

Quote
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.

Quote
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.
Title: Re: 15 miles of wire: guess the volume
Post by: Valis 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.
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.
Title: Re: 15 miles of wire: guess the volume
Post by: ka9q 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.

Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.
Title: Re: 15 miles of wire: guess the volume
Post by: DAKDAK 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
Title: Re: 15 miles of wire: guess the volume
Post by: Count Zero on May 15, 2012, 03:24:28 AM
Quote
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>
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.
Title: Re: 15 miles of wire: guess the volume
Post by: cjameshuff 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.
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.
Title: Re: 15 miles of wire: guess the volume
Post by: BazBear on May 15, 2012, 06:51:44 PM
Quote
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.
Title: Re: 15 miles of wire: guess the volume
Post by: ka9q 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.
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.
Title: Re: 15 miles of wire: guess the volume
Post by: randombloke 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).
Title: Re: 15 miles of wire: guess the volume
Post by: Chew on May 24, 2012, 06:56:02 PM
One gram of activated charcoal has a surface area of 500 m².
Title: Re: 15 miles of wire: guess the volume
Post by: slang on May 24, 2012, 07:55:09 PM
Quote
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"?
Title: Re: 15 miles of wire: guess the volume
Post by: ka9q 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.




Title: Re: 15 miles of wire: guess the volume
Post by: Chew 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.
Title: Re: 15 miles of wire: guess the volume
Post by: slang 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!
Title: Re: 15 miles of wire: guess the volume
Post by: Al Johnston 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?
Title: Re: 15 miles of wire: guess the volume
Post by: Chew 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.
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah 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.
Title: Re: 15 miles of wire: guess the volume
Post by: ka9q 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.
Title: Re: 15 miles of wire: guess the volume
Post by: ka9q 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.

Quote
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.

Title: Re: 15 miles of wire: guess the volume
Post by: Chew on May 26, 2012, 12:11:57 PM
Quote
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

Quote
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 (http://history.nasa.gov/afj/loressay.htm)

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.
Title: Re: 15 miles of wire: guess the volume
Post by: JayUtah on May 26, 2012, 02:41:34 PM
Trivia: the connections were severed 5 seconds before liftoff.

By "connections" do you mean the electrical and piping connections severed by the guillotine?  I think there's some confusion over the supply connections versus the mechanical connections holding the stages together.