Knots form the link between the recovery system and the rocket. It doesn't matter if you are using bungee cord, tubular nylon or kernmantle climbing rope, the ability to tie a variety of knots is fundamental to recovering your rocket safely. Any knot used should be as strong as possible yet be small and neat, it should not come undone during normal use and it should be appropriate to the situation.

Knots are only temporary connections and have a worrying ability to work loose on their own and become untied, thus they must be checked before each flight to make sure they are still secure. The stiffer the tape or rope used the more likely the knot is to come undone.

All knots weaken the rope by 25-45 per cent, depending on the knot itself and the size of the rope used. This weakening is due to the bends put into the rope by the knot; the tighter the turn, the weaker the knot.

Knots that are used to form a permanent join, such as when tying rope or tape to form a sling, should be tied as tightly as possible. Knots for all other situations are worked tight by pulling the individual strands forming them. These knots should be firm but not over tight, so that if they are loaded they can tighten up further and thus absorb some of the energy of deployment.

Not all knots are useful in rocket recovery systems, they are either not suitable for the application or weaken the rope too much. You should never use a Clove Hitch or an Overhand knot if your recovery system is using rope or bungee, they are far too weak compared to other knots. Ideally you should use a Figure-of-eight on a bight knot as this weakens the rope the least, the table below shows how different knots affect the strength.

As you can see it isn't really worth considering the Overhand or the Clove hitch when constructing recovery systems. Similarly if you have to join two ropes together a simple Fisherman's knot is inferior to a Double fisherman's knot.

The figure-of-eight is a very versatile knot, it's used predominantly by climbers for tying into the rope for safety. It has a distinctive shape, is easy to tie, strong and fairly easy to untie after it's been loaded. Also if it's been tied wrongly with either one turn too many or too few, it will still produce a usable knot, an overhand loop or a figure-of-nine. It is advisable to tie this knot with the main rope lying along the outside of the first bend.

A figure-of-eight can be tied anywhere in the rope, but if tied in the rope end the tail should be secured with a stopper knot which is then worked up close to the main knot. Although this stopper knot does not make the main knot any stronger, it does ensure that a sufficiently long tail has been left. Too short a tail could work itself loose.

The bowline is a strong knot which is easy to adjust and easy to untie after it's been loaded. It is a popular choice of securing a recovery system. However, if it is tied wrongly it can be unsafe, it can work loose fairly easily and it doesn't function will if subject to a three way loading, when it can slip or even invert. It should be secured with a stopper knot tied around the rope in the loop.

A bowline can be tied anywhere in the rope by doubling it and tying a bowline in this bight of rope. However, this isn't recommend for the parachute attachment point due to the three way loading this leads to. You can reinforce the bowline by lying a second loop on top of the initial turn and then completing the standard bowline. This double-knotted bowline is stronger than the basic bowline and less prone to inverting.

The tape knot is the most suitable for joining tapes. An overhand knot is tied in one end and the other end then follows it through. This knot can work loose very easily and long tail ends must be left (approximately five times the tape width). Load with at least body weight to tighten, check every time you fly and re-tie if the ends become too short.

A useful mid-rope knot, the Alpine butterfly's main advantage is the ease with which it can be untied after it has been loaded. It can be loaded along either of the main strands or by the loop, this makes it an ideal parachute attachment knot. In this method of tying the knot a bight is twisted to give three loops, then the top loop is taken behind and up through the bottom loop and the knot is finished by firming it up.

As you can see that are only a few knots that are useful to rocket recovery systems. The overhand knot and the clove hitch are too weak to use and in my opinion the bowline is too complicated and the risk of it being tied wrongly is too high. This just leaves the figure-of-eight, which is a really easy knot to tie and still safe if tied with one loop too many or too few. The figure-of-eight also just happens to be the strongest of the bunch and if climbers trust their lives to it then we can safely trust it in our rockets.

The parachute attachment point is a little trickier, the bowline should definitely not be used due to its weakness under three way loading. The figure-of-eight can be used but it's not ideal in this situation, however it is really easy to tie. The ideal knot for this application is the Alpine butterfly as it can be loaded quite happily in all three directions and still be very easy to untie if the attachment point needs moved.

If you are using tape of any kind then you should use the tape knot. This is the safest knot for use with tape and again climbers trust it to tie slings to protect their climbs. It's easy to tie and when loaded can be quite difficult to untie.

Finally always check your knots before each flight. There is no point tying the perfect knots if they are loose and lead to a recovery failure. Check and double check and if in doubt retie the knot.

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Last update: Sunday, 04-Mar-2007 10:46:11 EST