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Balloon Envelope Design

Copy Existing Balloon Design
Many homebuilders have found a good first project in replicating the envelope of a commercially produced balloon and STCing (Supplemental Type Certifying) it to an old balloon basket.

Cone and Sphere
Used by Brian Boland and others this design technique is very simple and widely used for amateur construction. The envelope shape consists of a half-sphere on top of a truncated cone.

Smalley Factors
These formulas were developed by Mr. Smalley to build mylar balloons for atmospheric research. The formulas produce so-called "natural shape" envelopes with no circumferential stress. Smalley's equation does however not account for internal pressure differentials and fabric weight. Bob LeDoux, editor of the Balloon Builders' Journal, designed an original gore calculation spreadsheet. Tom Deering significantly enhanced the spreadsheet in 2001, incorporating materials, weight and lift calculations. You can download the spreadsheet from Tom Deering's website.

Software by Arnaud Deramecourt
French balloon builder Arnaud Deramecourt has developed a sophisticated software package that can design envelopes using three different profiles. The software also produces .DXF files that can be fed directly to a computer cutting table. You can download the Deramecourt software at the website for Montgolfiere France Records.

Software by Bob Nungester
From a post by Bob Nungester to rec.aviation.balloon: "I've been interested for several years in how to calculate the shape of a "natural" (zero circumferential stress) balloon. Justin Smalley developed tables of factors for this problem in the 1960's but his solution, using differential equations, could not take into account the fact that hot air balloons have the bottom portion of the envelope cut off (the mouth), and it required a series of tables to account for various fabric weights. Another approximation was the assumption that the gas density is uniform, but it actually varies due to temperature variations in the envelope.
I worked on this problem for some time just to see if I could solve the problem, and I finally developed a finite element analysis method to calculate the shape of a balloon, including the effect of mouth location and fabric weight. The method can also incorporate the effect of variable temperature, but I've been unable to find a study that shows the actual variation of temperature in hot air balloons.
I wrote a Windows-based program using Visual Basic to implement this method so anyone can design any balloon. The program works for anything from 10-foot toy balloons to statosperic balloons of 30 million cubic feet or larger. The output is a text file containing the 1/2 gore pattern layout for the balloon, and a second text file containing the detailed values of R, Z, area, volume, etc. for the balloon. I'll be writing a more detailed article in the next edition of the Balloon Builder's Journal, but the program is available now.
This program, as well as another program used to set up clearances for 3-rope tethers, is available from the following web site:
Each program is a ZIP file that you put in an empty subdirectory, unzip it using PKUNZIP, and then run the SETUP.EXE program to install it. Anyone interested in balloon design or construction is welcome to try the program. It's shareware, so you're welcome to try it for free, but if you plan to use it I'll send you the version with a few more "bells and whistles" for the registration fee of $25.
If anyone knows of a study showing the variation of temperature versus height in a hot air balloon, please let me know and I'll include this in the next version of the program."

CAD/CAM Systems
Most modern Balloon manufacturers use sophisticated CAD/CAM systems that include finite element analysis of the structure. Unfortunately, I haven't heard of private homebuilders using any such systems.

Warning! Materials and ideas listed here are in part unproven and may be hazardous to use in the construction of experimental hot air balloons and airships. Building and flying experimental aircraft involves significant risk and may lead to serious injury or death. Always obtain professional advice when building or flying human-carrying balloons and airships!

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