www.MyAirship.com 
The Reference on Airships 
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
halfsphere 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 socalled "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 Windowsbased program using Visual Basic to implement this method so
anyone can design any balloon. The program works for anything from 10foot 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 3rope
tethers, is available from the following web site:
http://www.jps.net/chuck1/empty_pockets.htm
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 humancarrying
balloons and airships! 

