697: Tensile vs. Shear Strength

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Tensile vs. Shear Strength
Although really, the damage was done when the party planners took the hole punch to the elevator ribbon to hang up the sign.
Title text: Although really, the damage was done when the party planners took the hole punch to the elevator ribbon to hang up the sign.


This comic is set in the future, where engineers have successfully constructed a space elevator, until Black Hat decides to cut the cable linking the ground and space using pruning shears. This demonstrates how tensile and shear strength are not the same. Tensile strength represents how hard you can pull on something without it breaking. Shear strength represents how hard you can try to cut it without it breaking. Many materials have great tensile strength but low shear strength (such as dental floss — try to break it by just pulling on two ends), including whatever this space elevator is made of. The material clearly has extremely high tensile strength because it can hold the elevator in place, with one end on the ground and one in space, but it can be cut with a simple pair of pruning shears. This also highlights the fact that "shear strength" and "shears" are etymologically related.

A space elevator is a proposed construction that would make space travel easier. It consists of a long string attached to the Earth (near equator) on one end and a counterweight (beyond the geostationary orbit) on the other end, kept taut and in one place by the gravity and centrifugal forces. This would make it possible to carry spacecraft into the orbit by simple mechanical means, as opposed to requiring the use of rockets as is the case nowadays, saving a lot of energy and resources.

The phrase "a modern Babel" refers to the biblical story of the Tower of Babel (later referenced in 2421: Tower of Babel), in which humans endeavor to build a tower reaching heaven. Their arrogance angers God and prompts him to sabotage the project. A space elevator can be seen as a modern equivalent of a tower to heaven. Additionally, the expression "a modern Babel" may be used figuratively to describe huge projects (especially buildings or human-made structures) that fail because they are too ambitious.

The title text makes the point that even before Black Hat cut the space elevator's ribbon-like cable in two, it was ruined by the holes in it for the banner. The holes would reduce the surface area of the cross section of the ribbon, reducing its ability to keep the elevator attached to the ground. The flag and holes would also potentially make it impossible for the elevator to travel up the ribbon, making the entire elevator useless.


[A space elevator occupies the height of the frame, consisting of a base, a cable extending out into space, and an elevator unit with standard elevator features such as sliding doors and up/down buttons. A banner flutters in the breeze attached to the cable going up above the elevator. There is text on the banner. Text appear in four lines split across the elevator cable itself, the rhyming portions of the text is on the right side of the cable. Five individuals stand at the base of the elevator. To the left are Megan, a Cueball-like guy with his arms raised, and Ponytail, who is holding a bottle of champagne/sparkling wine which is bubbling out down the neck of the bottle. To the right is Black Hat, who cuts the cable with a pruning shear like it was part of the ceremony as a ribbon cutting. Finally further right is Cueball who sees what Black Hat is doing. He is very alarmed holding a hand to his mouth while holding the other out towards Black Hat.]
Banner: Space Elevator
Banner: Grand opening
After countless engineers
spend trillions over fifty years,
a modern babel disappears
because some fuck brought pruning shears.
Pruning shears: Snip
Cueball: !!

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Does anyone know the tensile vs. shear strength of nanotubes? Djbrasier ([[User talk:Djbrasier|talk]]) 20:33, 3 September 2013 (UTC)

"but it can be cut with a simple pair of pruning shears (also a pun on shear strength)" - I don't know the precise etymology of the word shears, but I would assume they are called such precisely because they operate on a shearing principle. While it is fair to assume that the use of the words 'pruning shears' is intentional (rather than 'scissors' or similar) , I wouldn't call it a pun (exploiting multiple meanings of words). --Pudder (talk) 16:07, 11 September 2014 (UTC) (Retroactively signed)

I agree, it's not a pun. I'm going to change that. Spiral phi (talk) 19:32, 27 August 2014 (UTC)

I can't immediately think of any other rhyming comics. Are there any others? --Pudder (talk) 16:07, 11 September 2014 (UTC)

At least 206: Reno Rhymes, 491: Twitter and 805: Paradise City, and probably others. Pelosujamo (talk) 17:32, 23 September 2014 (UTC)

Though the thing would have buckled before all this could even have a chance to happen 16:22, 11 October 2014 (UTC)

Buckling is specifically a failure under compressive load. I'm assuming we are looking at a cable under tensile load caused by centrifugal force. (The old swinging a bucket on a rope trick). Wikipedia has an interesting Space Elevator article. --Pudder (talk) 12:23, 12 December 2014 (UTC)

Aargh! The first line lacks a syllable... E.g. THEN after countless engineers...Mumiemonstret (talk) 10:31, 17 October 2014 (UTC)

I'm pretty sure the 3rd line's been changed; when I first read it, it ended with either "turns to tears" or "ends in tears". Can anyone confirm? Promethean (talk) 19:03, 31 May 2015 (UTC)

More accurate to refer to punched holes in terms of being stress concentrators than in terms of cross-section reduction. Sharp corners can reduce stress to failure massively while only marginally reducing cross-sectional area 16:09, 24 December 2015 (UTC)

In reality, I believe that any space elevator would be nearly perfectly balanced, to the point that it would not even need to necessarily be connected to the ground. That is of course why a space elevator in and of itself is possible and why a material with great tensile strength is needed, as it would be perfectly balanced. (talk) (please sign your comments with ~~~~)

Attaching to the ground provides stability to some designs. It does not need to be perfectly balanced as the moving cargo will be changing the center of mass anyway. SDSpivey (talk) 00:21, 29 October 2021 (UTC)

I'm pretty sure you guys missed the reference to Burma Shave. The setup of the text is identical to the "burma shave" campaign (4 rhyming lines + "Burma Shave"). In this case Pruning Shears is used instead of Burma Shave which sounds quite similar (this is why he choose that over "scissors" or similar). See comic 491 for another reference. 15:11, 24 August 2016 (UTC) msx80

The Burma Shave connection is weak. The signs used only three or four syllables per line and just one rhyme (e.g., "Ben met Anna / Made a hit / Neglected beard / Ben Anna split"), and the "Burma Shave" was on a fifth, separate sign, not part of the rhyme. Gmcgath (talk) 20:23, 25 October 2016 (UTC)

I find it cool there is a melody "Cerebral Plumbing" by Rom di Prisco, which fits the pattern of the poem. 07:21, 28 July 2019 (UTC)

It’s written in Common Meter (mostly) which means that it fits to quite a number of tunes (especially if one is prepared to force the syllables a bit). The second line is the main one to break a little from this but with a bit of effort it can be sang to O Little Town or Bethlehem or even the Original Pokemon Theme.

I can't see how these holes were supposed to ruin the whole project. Apparently, the elevator was still in one piece (although probably couldn't be used). All they had to do is to raise the base a couple of meters (the banner doesn't seem that high) so that the damaged region of the ribbon is not relevant (possibly they could even cut the ribbon right above it). Surely, it will cost a lot, but not quite as much as a new elevator. Bebidek (talk) 17:59, 13 March 2024 (UTC)

Well, the covering up of the holes would mean a 'reopening' as it was retrofitted. But BH hasn't helped.
In the (theoretical) construction of a space elevator, the ribbon would be lowered downwards from geostationary orbit whilst a separate ribbon (or the other end of a 'double-wound' one) would be trailed outwards. Perhaps with a handy counterweight to make it unnecessary to have quite so much cable unspooled/generated outwards while keeping the CoG at the geostationary point.
Until it reaches the ground, and gets attached. Then you would rebalance (set a bit more mass outwards) to create a nonzero amount of useful tension at the landward side (over and above its 'natural dangle'. This would stabilise the system. Too much tension, and you're making problems for yourself, but too little and an attached cabin (or several!) would cause issues masses and forces are now incompatible with the 'CoG orbital point'. Enough availabls slack to let you subtly 'skip' the cable around potential collisions that you can predict.
(Typically LEO orbits, less frequently MEO ones... polar/Sun-synchronous, and skewed orbits such as the current trend for GPS-like and/or Starlink-like 'constellations', would be a threat depending upon the precise equatorial intersections generated at the given longitude. Near-GEO would be rare, and geosynchronous-but-not-geostationary ought to be easy to deal with, but ones designed for highly elliptical and inclined orbits with long-loiter over polar regions might be awkward. Anything below LEO is more a matter of controlling the general airspace to high altitudes.)
Severing right above the anchor is probably the least worst place. The closer to GEO, the more 'upwards' force the now grossly unbalanced system succumbs to (sending the GEO waypoint station further out, quickly, maybe into solar orbit), and more ribbon is now below the cut that needs to fall. Above GEO, the GEO station is now 'down-heavy', falling out of orbit, just the counterweight end (probably a 'reaction free, centripetal launching station', where interplanetary craft can be just 'dropped' outwards at the most opportune time). In contrast, severing near the anchor will have the least amount of upwards (and driftwards) movement.
...but not insignificant. I'd be foolish to try to grab the cable before it got out of reach. It might very soon become me that is out of reach (if I don't let go again before I get too high for it to be a new problem). The tension is probably greater than my bodyweight (if the system needs to carry loads no less than myself plus the 'car' I ride in). Perhaps I should also try to hook my foot into the ground-anchor/find another handy bit of ground structure? But only if my own tensile strength is sufficient - i.e. do I compare favourably with a hi-tech carbon nanotube composite?
Unless there's some preanticipated measures, I think the cable is 'lost upwards' too quickly to engineer a re-anchoring. It might help to have emergency thrusters ready to fire, on the GEO station or even elsewhere in the cable (already useful to oscillate the cable slightly laterally when it needs to get out of the way of an orbiting or passing-through space-item). Although there's also a lot to be said for having the system, detecting a sever, automatically making additional severs of the cable just above and below the GEO station (down-cable now falls down across the Earth's surface... might not hit anything important; up-cable and counterweight station now heads outwards... emergency thrusters might be useful there to prevent unintended 'long term space tourism' by anyone manning it), so that you could at least provide the GEO station with a new cable (or material to make it) and it could fairly quickly become its own replacement.
Maybe you can grab the now free cable end. But surely the anchor would be more than just dipping the free end in wet concrete. It might (originally).have features like a properly engineered end with width and/or loops, to mechanically tie into the ground-anchor (which might be quite deep). There's friction on it enough for the 'cars' to ride up and/or down, but that's less weight, even full, than the force of tension required to deal with the presence or absence of one or more such cars.
The GEO would already probably need a still-spooled length of cable anyway, to take-up/let-out a degree of inward (and outwards) ribbon under different gravitational forces (as the Moon is in conjunction/opposition to the elevator system, it would change the idealised 'GEO' height, so maybe there'd be a bimonthly cycle of adjustment to maintain CoG).
The ultimate fallback might be to (depending upon the sever being detected as close enough to ground, by the system that can at least cue up this automatic reaction for a human overseer to confirm) spool out extra 'down' (and a little extra 'up', depending upon relative masses of cable and 'nodes', letting the cable 'fall' down (still under control of GEO), along with judicious emergency thrusters (again, depending upon what is available/how the mass of the system is distributed), giving time for the ground-station to bring the "vertical towtruck" machinery into play (just out of shot from this comic frame) and have the "just touching the ground" effect as per a failing helium party balloon whose string alone is just enough to keep it from flying off (but if too much string is on the ground it rises up, until a given length is held up in equibreum).
...all engineering problems to be very closely looked at. But not necessarily until solving (conceptually, and but sufficient intermediate practical testing) the issue of the actual Space Elevator. Only then can you have an elevator system that you know enough about to exactly work out the necessary contingencies for various (foreseeable) emergency situations. One can certainly extensively postulate various ways to deal with various dangers, but it's hard to be exactly sure if that sheer-snip is something trivial enough, to solve through quickly tying a brick to the end, or potentially no less disasterous than sending a whole group of waiting astronauts of an impromptu grand tour of the solar system when they'd only signed on to staff a simple space-hub facility!
(Either way, it shouldn't be so very dangerous to those standing around that ground-station, for that particular failure, leastwise none whose reactions aren't quicker than a moment of considered common sense! Unless the cable whip-frays, but BH likely wouldn't be so casual a out it, if that was the case.) 20:38, 13 March 2024 (UTC)