At 20% of rated strength, these will elongate the following amount:
- Kevlar 49: will elongate 0.8%
- Mil-spec 1" tubular nylon: 7.5%
When I throw these numbers into a 14 lb rocket under drougefall of 56 mph, with a 40' total length of recovery harness, we see the following impact forces. Compared are two materials roughly analogous in strength, 3/8" Kevlar @ 3600 lbs, and 1" Mil-spec nylon.
- 3/8" Kevlar - 131 g's w/ 1839 lbs of force on hardware
- 1" tubular - 35 g's w/ 490 lbs of force on hardware
This of course assumes that the parachute opens instantaneously, and there is no "swing-under" as the rocket assembly swings into vertical from a stretched-out horizontal configuration.
At first blush, one would assume that the only advantages for the kevlar would be heat resistance and low pack volume. However, in the example above, the nylon is nearing the limits of allowable elasticity, and higher loads will damage it, thus reducing future strength. Kevlar, however, will remain inside its elastic curve almost to the point of failure. Thus, high load exposures (up to it's yield elasticity of ~2.5% at failure) generally do not deprive kevlar of later strength.
Another subtle advantage of kevlar stems from it's relative inelasticity. When loaded, there is very little energy stored in the kevlar. This results in far less recoil when loaded and released, such as when a loaded rope is cut.
Each material has a place. My next rocket will have a recovery harness that is a blend of the two materials.
All the best, James
