A free toolkit for hot-air balloon designers & builders — lift, gore patterns, and flying-wire geometry, plus the classic legacy programs.
These calculators reproduce the math behind the well-known builder spreadsheets and DOS-era design programs (Bob Nungester's BALLDSGN/TETHER and Steve Griffin's design sheets). Every result is computed live in your browser. Use them for planning and estimation — always cross-check against your manufacturer data and an experienced builder before cutting fabric.
Gross lift from envelope volume, air & envelope temperature, and field elevation.
Gross lift = total weight the envelope can hold up. Subtract envelope, basket, burner, fuel & passengers to get free lift.
Individual flying-wire lengths from the burner load-frame corners up to each gore attachment point around the mouth — for a 4-point load frame.
The full natural-shape cutting pattern — half-gore width at every station from base to crown, scaled from envelope volume. Reproduces Steve Griffin's design sheet.
B(s) is the captured zero-stress natural-shape profile (51 stations, crown→mouth). Print at 1:1 and add seam beyond the cut line.
Suspension-cable length from the basket attachment ring up to the envelope mouth — the mouth forms a straight cone, so each cable runs along its slant.
| Vertical drop (mouth → ring) | — |
Method from BBJ Issue 5; the 40° angle (cot = 1.1918) is Phil MacNutt's measured correction in Issue 24. A square basket of side S gives ring diameter S·1.414.
A balloon can fly under FAR Part 103 with no airworthiness certificate and no pilot certificate — if it stays within every limit below.
Over 155 lb, or carrying anyone else? You're into amateur-built experimental territory — airworthiness + pilot certificate (FAA paperwork covered in BBJ Issues 9 & 11). Verify current 14 CFR §103 before flying.
The theory and the original programs the calculators above are built from.
The classic BALLDSGN program computes a zero-circumferential-stress ("natural shape") envelope — the tear-drop where the fabric carries only vertical load. It integrates the gore profile up from the mouth:
dr/ds = sinθ · dz/ds = cosθ · dθ/ds = κ, with the curvature
set so hoop stress stays zero as buoyancy pressure builds with height.
Inputs: payload, mouth & parachute-hole diameter, fabric weight (oz/yd²), number of gores, seam allowance, specific lift. Outputs: full gore table, volume, surface area, mouth angle, envelope weight.
The TETHER program lays out a 3-rope tether anchored to the burner frame. Enter the three anchor-to-anchor distances, the inflated balloon diameter and deflated length, and the rope lengths.
It solves the 3-D high point by trilateration and reports the rope slack to add at each anchor when the basket sits on the ground beneath the high point — so you can step out the spot, inflate, and dial in the slack. It also maps the basket, envelope-edge and top clearance limits.
Steve Griffin's spreadsheets open in Excel, LibreOffice or Numbers. The two .EXE programs
are 16-bit Windows (Bob Nungester) and need DOSBox or a Windows 3.x/XP environment to run.
Credits: BALLDSGN & TETHER © Bob Nungester; design spreadsheets © Steve Griffin. Shared for the balloon-building community.
The complete archive of Bob LeDoux's Balloon Builders Journal — 28 issues published 1993–1998 plus the unpublished 29th, nearly 400 pages of amateur balloon-building know-how. Click any issue to open the PDF.
Start here: index & introduction Download entire archive (.zip)