Parachute Design

Team Design Projects

Hermes Disk-Band Gap Drogue

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):

$//<![CDATA[ \begin{array}{l}Porosity \uparrow: C_{D} \downarrow, F_{opening} \downarrow, Oscillation \downarrow\end{array} //]]>$

Fabrics

The following fabrics are used in parachute fabrication (information gathered from Knake, Section 6.6.1):

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  $//<![CDATA[ \begin{array}{l}29 \rightarrow 40\%\end{array} //]]>$
  • tenacity: $//<![CDATA[ \begin{array}{l}2.5 \rightarrow 9.5 \frac{g}{denier}\end{array} //]]>$, usually 6.6 for parachutes
    
    
• Teflon
• Rayon

This outdated material burns easily and has poor performance when wet.

Terminology

tenacity: a strength metric for yarn or fiber;  $//<![CDATA[ \begin{array}{l}\frac{fiber \ breaking \ force}{denier}\end{array} //]]>$

denier: fiber mass per 9000 meters

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