Parachute Design & Fabrication

Team Design Projects

Hermes Disk Gap Band 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 and Poynte, Section 4):

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 parachute

A common type of nylon for parachutes is MIL-C-7020.

Fabrics are specified in $//<![CDATA[ \begin{array}{l}\frac{oz}{yd^2}\end{array} //]]>$ and have permabilities between 80-120 cubic feet per minute (cfm).

• Teflon
• Rayon

This outdated material burns easily and has poor performance when wet.

Tapes

Specs superseding MIL-T-5038

Treated Fabrics

A note on treated fabrics from Poynter (4.021): coating/treating a fabric may decrease tear strength by preventing the fibers from adjusting around a tear.

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

Relevant MIL Specs

Procedures

Gores

Block vs. Bias Construction relates to at what angle you cut the gores out the fabric. Advantages of block are: simpler fabrication. Advantages of bias are: better canopy strength, less line loading and less required slack. (Poynter 8.1.5 and 8.1.9)

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."

From Poynter (8.1.5): "Because of the uncertainty of canopy loading due to non-uniformity, suspension line systems should have a designed margin of safety of 2.0. Lines lose about 35% of their strength due to sewing and the attachment of fittings."

Vent

Using a vent helps stabilize a parachute and eases construction.

Sewing Standards

Full details on sewing standards can be found here.

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

Dan Poynter, The Parachute Manual: A Technical Treatise on Aerodynamic Decelerators, Third Ed.