(NOTE: All photographic images below can be clicked to view at higher resolution.)
In the last few days some of the first high resolution color images of Mars Rover Curiosity's
deck have been taken. These included some of the best images yet showing the knots visible on the exposed wire and cable bundles.
While a few of the folks here are no doubt aware, it might surprise most people to learn that knots tied in cords and thin ribbons have probably traveled on every interplanetary mission ever flown. If human civilization ends tomorrow, interplanetary landers, orbiters, and deep space probes will preserve evidence of both the oldest and newest of human technologies for millions of years..Image Credit: NASA/JPL-Caltech/Malin Space Science Systems(Original)
Knots are still used in this high-tech arena because cable lacing
has long been the preferred cable management technique in aerospace applications. That it remains so to this day is a testament to the effectiveness of properly chosen knots tied by skilled craftspeople. It also no doubt has a bit to do with the conservative nature of aerospace design and engineering practices. Proven technologies are rarely cast aside unless they no longer fulfill requirements or there is something substantially better available.
While the knots used for cable lacing in general can be quite varied -- in some cases even a bit idiosyncratic -- NASA has in-house standards for the knots and methods used on their spacecraft. These are specified in NASA Technical Standard NASA-STD-8739.4 -- Crimping, Interconnecting Cables, Harnesses, and Wiring
. As far as I've been able to identify in the rover images below, all of the lacings shown are one of two of the several patterns specified in the standard.
The above illustration shows the so-called "Spot Tie". It is a clove hitch topped by two half-knots in the form of a reef (square) knot. In addition to its pure binding role, it is also used to affix cable bundles to tie-down points, as can be seen in many of the Curiosity rover images below.
Knot history buffs might find it interesting that a "Spot Tie"-like knot, with opposite Clove Hitch end orientation and topped only with a single half-knot was illustrated in 1917 by A. Hyatt Verrill under the name "Gunner's Knot". This was seemingly due to Verrill copying from J.T. Burgess, who had oversimplified "Bowling's" description of what possibly was the first known textual description of the Constrictor knot. But that's a whole different can of worms!
So why has NASA standardized on this knot instead others which might serve the purpose? The following reasons are merely my own musings. I'd be interested to hear others' comments on this knot's strengths and weaknesses.
The Reef Knot and Clove Hitch are extremely ancient. Both were discussed in detail as surgical and orthopedic knots and slings by Greek physician Heraklas
in the 1st Century AD. The Reef Knot is depicted with varying degrees of realism in ancient Egyptian statuary and hieroglyphics as far back as 4000-5000 years ago. I presume there would be little disagreement here that these two knots must be among the oldest of the purposeful, standardized knots used by humans. You simply cannot get more field-tested than this!
But why combine these two well-known old knots in a somewhat novel way that, at first, might seem a bit "belt-and-suspenders"?
The inner profile of the clove hitch is smooth. Both turns bear on the bound object evenly throughout their contact. The contact area is increased by having two turns. When the reef knot is added, the ends are pulled up and away from the object. There is some extra pressure exerted by the reef knot on the riding turn, but this is distributed onto the two underlying turns. Evenness of pressure is important for the same reasons as the next item.
Overtightening of cable management bindings can cause conductor breakage, insulation damage, excessive chafing, and deformations between the conductive, dielectric, and shield parts of a cable, and no doubt a host of other issues. It is one of the classic problems with ratcheting plastic cable ties (i.e. "zipties") that they only have quantized adjustment steps and cannot be easily loosened. While zipties with a metal tooth insert do allow for smoother tightening, the possibility of this tiny metal part coming loose near electronics generally excludes their use. That zipties cannot easily be loosened or adjusted during tightening makes them more prone to being left in an overtightened state. Difficulty of adjustment might also be considered a possible strike against using the Constrictor Knot (and similar knots) for this application.
The clove hitch is not known as a particularly good binder alone, but that may be an advantage in this application. If the clove is initially made too tight it is easily loosened and readjusted. Once the proper snugness is achieved the addition of the first half-knot produces only a small and predictable amount of additional tightening. One thing I did notice in my tests is that if the first half-knot is made in the opposite orientation than shown in the standard, it tends to produce more tightening and also separates the underlying turns of the clove hitch.
- Resilience to errors in tying
As mentioned above, I did some tests tying the knot incorrectly in different ways. While these forms generally seemed inferior to the specified knot, they were not obviously destined to fail. Using these two basic knots in a compound form seems to be a reasonable way to make errors of tying less detrimental to the resulting knot.Image credit: David J. Fred/Wikimedia Commons (Original)
Above is a high resolution photo taken of these Spot Ties made in Gudebrod Nomex
lacing tape. These types of lacing tapes are often coated or impregnated with materials (e.g. synthetic rubbers) to increase their knot-holding properties. I'm not sure what the tapes visible on the rover are made of, but I'd suspect the material was chosen for its behavior at extremely low temperatures and pressures as well as very good UV resistance.
The keen observer may note that some of the Spot Ties in the rover images show the ends perpendicular to the cable bundle and some parallel. Based on general experience with reef and granny knots, one might be tempted to assume the parallel examples are improperly finished with granny knots. Experimentation with Nomex lacing tape seems to show that it's more a matter of the knot preserving the orientation of the ends as the reef knot was tightened. I found that when the Spot Tie is finished improperly in the granny form but with the ends kept perpendicular to the wire bundle they tend to stay that way. While these experiments are hardly definitive, it doesn't seem to me that one can tell from orientation of the ends whether the knot was properly tied or not.
There is also the issue of the handedness of the first half-knot with respect the ends emerging from the Clove Hitch portion. The relative orientation shown in the NASA spec does appear to be preferable to the alternative.
I won't go into much discussion about these stiches for the moment, but I believe the one on the left (the running clove hitches) appears on the extreme right edge of detail image "1" below.
And now for the pretty pictures...Image Credit: NASA/JPL-Caltech(Original)
Multi-image panorama giving context of rover, deck, and its suroundings. The rim of Gale Crater is visible in the distance.Image Credit: NASA/JPL-Caltech(Original)
Annotated context image showing locations of following five detailed images. Outlines do not quite align to following image borders due to panorama projection.1 Image Credit: NASA/JPL-Caltech/Malin Space Science Systems(Original)
Examples of one of the flat stitching methods (shown above) appear on the extreme right edge of this image.2 Image Credit: NASA/JPL-Caltech/Malin Space Science Systems(Original)3 Image Credit: NASA/JPL-Caltech/Malin Space Science Systems(Original)4 Image Credit: NASA/JPL-Caltech/Malin Space Science Systems(Original)5 Image Credit: NASA/JPL-Caltech/Malin Space Science Systems(Original)
[Edited 2013-09-11 to move images back from AWS S3]