Many methods are used in rocketry to attach recovery harnesses together. This article will cover four of the most common methods. Each one has benefits and drawbacks and only the reader can decide which method is best for them.
Oval Quick-Links
These commonly used pieces are found on almost every high power rocket that takes flight. They work extremely well for attaching skinny pieces of webbing or round cord to metal anchors (eyebolts or u-bolts) on your rocket. The problem arises when wide (1” or larger) webbing is attached with a quick-link. The quick-link will spin when loaded so that the anchor is making contact at one of the ends. This bunches the webbing at the other end, weakening it. The significance of this weakness must be evaluated by each individual on a case by case basis to determine if it is worth concern.
Delta Shaped Quick-Links
While over twice the price as ovals, these triangle-shaped quick-links tend to shift where the narrow end is touching the metal anchor, which, in turn, puts the webbing in contact with a flat side (Fig. 2). This allows the webbing to stay its normal width, which distributes the load more evenly over the entire surface of the webbing. Another added benefit is the wider design, which makes joining three different pieces easier.
Knots
If tied properly, knots are a strong and reliable method of attachment. There are many different knots, each having it’s own specific use. Knots add versatility in any recovery harness by having the advantage of being able to untie and re-tie numerous times in any configuration. Unfortunately, they also add weight and additional bulkiness to already cramped payload compartments. Another drawback is that they weaken the rope by nature of the sharp bends that form the knot (Table 1). For a rope or webbing to not be weakened by a bend, the radius of the bend must be five times the diameter of the rope. A good rule of thumb is to assume any knot weakens the rope or webbing by 50%
Relative Strengths of Knots for Kernmantle Rope
| Knot | Strength % |
|---|---|
| Without Knot | 100 |
| Double Fisherman’s | 65-70 |
| Bowline | 70-75 |
| Water Knot | 60-70 |
| Figure 8 | 75-80 |
| Clove Hitch | 60-65 |
| Fisherman’s | 60-65 |
| Overhand | 60-65 |
Knots must have a tail longer than three inches.
Source: Mountaineering Freedom of the Hills 5th Ed., The Mountaineers, 1992
Sewing
This is the answer to reducing weight and bulk and improving the look of your recovery harness. It does not really improve the strength of the system, however, because you still have the webbing or rope bending over whatever the loop is attached to (refer to above section). Webbing that is professionally sewn is typically attached to itself with bar-tacks. Two bar-tacks are typically as strong as the material it is sewn into, but most manufacturers sew three to five bar-tacks to assure safety.
Unfortunately, a special machine is needed to sew the bar-tacks. To avoid the added cost of a bar-tack sewing machine, many rocket enthusiasts sew an ‘X’ in a box to join the webbing. However, this method should be evaluated/tested before use.
Conclusion
No single method of attachment is the answer to every situation. A good rule of thumb in designing any rocket recovery system is to assume any knots or sharp bends, including sewn loops, in the rope or webbing weakens it by 50%. Nonetheless, the requirements of each rocket recovery system should be evaluated and the best method for that situation should be used.
Submitted by: Bryan Flynt