Team Design Projects
Materials
Porosity
There are two types of porosity related to parachutes: fabric and geometric porosity. Fabric porosity relates to the inherent permeability of the parachute material. Geometric porosity relates to what percent of the parachute's canopy is "cut out." The equivalent metric between fabric and geometric porosity measures is 27.4 ft3/ft2/min, at 1/2-inch water pressure ≈ 1% geometric porosity (Knacke, 5-71).
Both forms of porosity influence opening forces, drag, and stability by means of the following trends (Knacke, 5-71):
Porosity \uparrow: C_{D} \downarrow, F_{opening} \downarrow, Oscillation \downarrow
Fabrics
The following fabrics are used in parachute fabrication (information gathered from Knake, Section 6):
Natural Fibers
- silk
- cotton
Man-made fibers
- Kevlar
- Dacron
- Nylon
This is the most-commonly used fabric for parachute gores. All of the MIT Rocket Team's Parachutes to date (January, 2018) have been fabricated out of the ripstop variety. It is important to note that nylon retains most of its strength up to 250 degrees Fahrenheit. Nylon has the following, other general property ranges:
- elongation 29 \rightarrow 40\%
- tenacity: 2.5 \rightarrow 9.5 \frac{g}{denier}, usually 6.6 for parachute
A common type of nylon for parachutes is MIL-C-7020.
- Teflon
- Rayon
This outdated material burns easily and has poor performance when wet.
Terminology
tenacity: a strength metric for yarn or fiber; \frac{fiber \ breaking \ force}{denier}
denier: fiber mass per 9000 meters
Procedures
Shroud Lines
From Knacke (6-84), "all lines and tapes are [traditionally] measured under a preload. A preload of about 1% of the ultimate material strength is required to align all fibers in the woven or braided material before the individual fibers are stretched." Furthermore, "[it] is practical to let all fabrics relax for a short period after rewinding from the storage spool before measuring the material."
Vent
Using a vent helps stabilize a parachute and eases construction.
Terminology
take-up: the shortening of two pieces of fabric around the seam. Ideal parachute design takes this into account.
Resources
T.W. Knacke, Parachute Recovery Systems: Design Manual
Greathouse and Schwing, Study of Geometric Porosity on Static Stability and Drag using Computational Fluid Dynamics for Rigid Parachute Shapes
Heinrich and Haak, Stability and Drag of Parachutes With Varying Effective Porosity