Tubular Nylon and Climbing Cord
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There are many types of webbing and small rope that can be used to connect your rocket's recovery system; each one has its benefits and drawbacks. This article covers the types of rope, cord, and webbing that are used in climbing and applies them to high power rocketry.
Tubular Nylon
Tubular nylon (Fig. 1) is a tube of woven nylon that normally lays flat.
It is more than twice as strong as the same width of non-tubular nylon
webbing, yet it has almost the same thickness. Contrary to popular belief,
tubular webbing only stretches a tiny amount, if at all. It is not
intended to take a shock load, which, in this case, happens to be two
rocket parts almost coming to a stop from a high velocity. While the
tubular nylon will handle the force until it reaches it's breaking point,
the object it is connected to might not. This is not to say tubular nylon
should not be used. On the contrary, it is used quite often due to its
immense strength and reliability. Below is a table showing four of the
most common webbing and each type's specifications, including approximate
price.
| Size (inch) | Strength (kN) | Wt./ft (g) | Approx. Price/ft* |
| 9/16 Military Spec. | 6.8 | 5.7 | $0.22 |
| 9/16 Climb Spec. | 10.2 | 7.9 | $0.22 |
| 1 | 18.1 | 11 | $0.30 |
| 2 | 30.0 | 25 | $0.55 |
Accessory Cord
Accessory cord (Fig. 2) varies in size from 2mm through 8mm in diameter
with the 4mm and up sizes rated for strength. While not as strong as
webbing, accessory cord tends to stretch a little more, probably because it
takes less force to reach a higher percentage of it's maximum strength.
One of the two advantages of using accessory cord is its round shape, which
make it easier to tie knots in. The second advantage is the natural fit
into standard oval quick-links compared to webbing. Below is a table
showing various types of accessory cord and each type's specifications,
including approximate price.
| Brand | Size (mm) | Strength (kN) | Wt./ft (g) | Approx. Price/ft* |
| Mammut | 2 | N/A | N/A | $0.15 |
| 3 | N/A | N/A | $0.15 | |
| 4 | 3.7 | 3.0 | $0.20 | |
| 5 | 5.6 | 5.5 | $0.25 | |
| 6 | 8.0 | 7.0 | $0.30 | |
| 7 | 10.8 | 9.1 | $0.40 | |
| 8 | 15.5 | 12.1 | $0.50 | |
| PMI | 3 | N/A | N/A | $0.15 |
| 4 | 3.3 | 3.4 | $0.20 | |
| 5 | 5.5 | 6.3 | $0.25 | |
| 6 | 6.8 | 7.4 | $0.30 | |
| 7 | 9.3 | 8.9 | $0.40 |
Spectra Cord
Spectra cord comes in 2.0mm and 5.5mm diameters and is much like Kevlar in
its properties. Spectra will not melt or stretch and is extremely strong
for it's size and weight. It is so strong, climbing manufacturers
routinely say it is 10 times stronger than steel. What makes this cord
different is that the spectra fibers are on the inside with a nylon sheath
woven over them (Fig. 3) to protect them from abrasion. Note: The spectra
cord is relatively stiff and slick, because of this, only a triple
fisherman's knot is recommended to tie it with. Below is a table showing
three different types of spectra cord and each type's specifications,
including approximate price.
| Brand | Size (mm) | Strength (kN) | Wt./ft (g) | Approx. Price/ft |
| Kelty* | 2 | 0.8 | N/A | $15/50ft |
| Blue Water | 5.5 | 17.8 | 4.8 | $1.05 |
| Maxim | 5.5 | 17.8 | 5.9 | $1.05 |
* The price is approximate and listed for comparison only.
Dynamic Rope
This type of rope is specifically engineered to absorb shock loads. It is
not rated for strength; instead it is rated based on how many simulated
climber falls it can handle before stretching beyond a certain limit.
Essentially, this rope will not break; it stretches. However, it is not
bungie because it only stretches roughly 5% of it's total length for a huge
impact force (~ 3000lbf). While not sounding like much, this 5% stretch
significantly reduces the impact force that would be felt by a rocket.
Dynamic Ropes' major drawbacks for rocketry are that it's only available in
larger diameters (7.8-11mm) and a minimum length of 164 feet, which result
in a high price.
All the information for the tables came from the 1999 REI Climb Information Guide. To purchase any of the above materials visit your local climbing/outdoor store or visit REI online at www.rei.com.
Submitted by Bryan Flynt
