Hunting for a knot

[roo]

Sure, a little more info is to point out that my knot search is for reasons of developing improved designs of PCB-based deployable solar panels for nanosatellites (Satellites in the region of 1-3Kg, usually educational or technology demonstrators that hitch a ride with larger ones). The Dyneema monofil is used to hold the deployable solar panel shut against the body mounted panel, and is then subsequently melted by a thermal knife circuit to release the panels when in orbit (It's a fairly standard practice in concept, but in practice there's few fixed methods).

Some key problems inherent with it are the panels are relatively heavy for a fine filament thread and very expensive, the threads MUST release with redundancy (hence the inability to multi-wrap) and any leftover parts must be bonded to the craft so they don't float in front of lenses etc. The whole system undergoes vibration testing and significant vibration at launch, where the panels and cord can see loads tens of times higher than normal static loading in a large range of frequencies. If the tie down slips, the solar panels deploy within the launch structure, damaging the panels and jamming in the launch vehicle. Also, the total height envelope for both PCBs, cells, hinges and tie down/release mechanisms is 6.5mm.

So far a few knot types have been tested in full vibration testing and we're confident with their performance but I'm looking for a ratcheting knot to make assembly easier to pre-load the dyneema (and hence the panels) without sacrificing locking potential.

I wonder if there might be some suitable material that would shrink upon mild heating to give you tension after a knot is tied.

Shrinking Thread

[DonaldMcewen]

Sure, a little more info is to point out that my knot search is for reasons of developing improved designs of PCB-based deployable solar panels for nanosatellites (Satellites in the region of 1-3Kg, usually educational or technology demonstrators that hitch a ride with larger ones). The Dyneema monofil is used to hold the deployable solar panel shut against the body mounted panel, and is then subsequently melted by a thermal knife circuit to release the panels when in orbit (It's a fairly standard practice in concept, but in practice there's few fixed methods).

Some key problems inherent with it are the panels are relatively heavy for a fine filament thread and very expensive, the threads MUST release with redundancy (hence the inability to multi-wrap) and any leftover parts must be bonded to the craft so they don't float in front of lenses etc. The whole system undergoes vibration testing and significant vibration at launch, where the panels and cord can see loads tens of times higher than normal static loading in a large range of frequencies. If the tie down slips, the solar panels deploy within the launch structure, damaging the panels and jamming in the launch vehicle. Also, the total height envelope for both PCBs, cells, hinges and tie down/release mechanisms is 6.5mm.

So far a few knot types have been tested in full vibration testing and we're confident with their performance but I'm looking for a ratcheting knot to make assembly easier to pre-load the dyneema (and hence the panels) without sacrificing locking potential.

I wonder if there might be some suitable material that would shrink upon mild heating to give you tension after a knot is tied.
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Shrinking Thread

Thanks for sharing wonderful information..It will really help me out..

[DerekSmith]

Hi coyoteboy,

The solution which sprang immediately to mind is the ever so simple Packers Knot.  In one end (a) you tie a figure 8 around the other end (b).  As you pull on 'b' the fig 8 tightens around 'b' and grips it vigorously, although in mono-filament and with the pressure of the springs, it may be necessary to relieve a little of the spring tension by applying a little hand assist while you tie off the Packers Knot.

Having tightened up the loop sufficiently, simply tie off by passing a half hitch using 'b' over the end of 'a'.

Pressure on the loop simply further tightens the fig 8 and clamps what little force survives the nip of the fig 8 against the end of 'b'.

Easy to tie even in fine stuff and in tight places, grips like a cable tie and locks off securely.

This little knot is surprisingly strong, still, it would not have survived as a parcel / packing knot if it broke at the first time someone hoisted the package up by its string.

Dyneema however, is hellish for its lack of grip.  Given the nature of the application, you may need to consider a little security in the form of taping off the ends with high tack tape or fusing the ends to blobs, or even a drop of cyanoacrylate to keep your baby tight in flight.

Here is a link to a Youtube video of how to make the knot - you will have to imagine that the two cords actually pass around an object and that they were drawn up tight before the HH was applied. http://www.youtube.com/watch?v=1KCRjLF8Rgg

You have a really interesting project.  Does it have a web site?  Would you mind keeping us up to date on how it is progressing?

We could do with a whole new section -- Knots In Space --

Derek