Tethered model hot-air balloon
Twelve tissue-paper gores make a 1.5 m balloon that lifts about 250 g over a ground fire and flies on a tether: the Montgolfiers' principle, measured, with no flame in the air.
Home: In the air, Level 3 · Mill, level 2 of 6 in its track. Serves 1 step.
Every build on this site has hazards that injure or kill — quicklime, carbon monoxide, molten metal, collapsing ground. Read the safety notice before starting, and read this build's own failure modes below.
What it is and why this shape
This is the Montgolfier balloon of 1783 at a size where nothing can go wrong that a bucket of water will not fix. It teaches the one number that decides every balloon — lift per cubic metre at a temperature — by measuring it, and it shows how an envelope is cut from gores, the way full-size balloons still are. It is never released: a free-flying fire balloon starts fires wherever it lands.
One of 12 gores, for a 1.5 m model balloon
Fig 1
Loose fill / grain
Filling and tethering the model balloon
Fig 2
Earth / subsoilTimberLoose fill / grainFire / hot zone
Keyed parts
| 1 | Envelope | Twelve identical gores of tissue, glued edge to edge into a sphere 1.5 m across. |
| 2 | Mouth ring | 0.5 m cane or wire ring glued into the open bottom. |
| 3 | Chimney | 1.2 m of metal pipe over a small fire, so only hot air, never flame, reaches the mouth. |
| 4 | Fire | Small, on bare ground, under the chimney. |
| 5 | Tether | A two-point bridle from the ring to a line on a reel. |
Specifications
| Size | 1.5 m sphere, 12 gores | About 1.76 m³; mouth cut at 70° south, about 0.5 m across. |
| Gore | 2.09 m long, 39.3 cm at its widest | Widths in the drawing; a 1.5 cm glue flap on one edge. |
| Envelope weight | about 150 g | About 6.9 m² of tissue at about 17 g/m², glue and the mouth ring. |
| Lift | about 0.4 kg gross, 0.25 kg net | Air inside at about 80 °C against 15 °C outside. |
| Heat | a fire under a 1.2 m chimney | On bare ground; the flame never reaches the balloon. |
| Tether | two-point bridle to a line on a reel | Never released. |
Bill of materials
| Material | Quantity | Notes and substitutes |
|---|---|---|
| Tissue paper | about 8 m² | The thinnest, strongest you can get; several colours help show the gores. |
| Glue | a small pot | Paste or glue stick; a thin, even line. |
| Mouth ring | 1.6 m | Thin cane, split bamboo or light wire, bent into a 0.5 m ring. |
| Top disc | 1, 15 cm across | Tissue, to cover the point where the gores meet. |
| Chimney | 1, 1.2 m × about 15 cm | Stovepipe or rolled sheet metal on three stones, over a small fire. |
| Tether | 30 m of thread or light cord, and a reel | Tied to the ring at two opposite points. |
| Bucket of water, and gloves | — | At hand the whole time. |
| Spring balance and thermometer | — | To measure the lift and the air temperature. |
Tools: Long ruler or measuring cord · Scissors · Pencil · A long table or clean floor
Construction
Make the gore template
On a long strip of paper, draw a 2.09 m centre line. From the top, mark every 19.6 cm (each 15° of latitude) and at each mark draw the full width across the line: 0, 10.2, 19.6, 27.8, 34.0, 37.9, 39.3 cm to the equator, then 37.9, 34.0, 27.8, 19.6 and 13.4 cm at the mouth. Join the ends in a smooth curve and cut it out. Each width is 2π × 75 cm × cos(latitude) ÷ 12.
Cut twelve gores
Join tissue sheets end to end where one is too short. Trace the template twelve times, adding a 1.5 cm glue flap on one edge only, and cut them out.
Glue the gores together
Glue gore to gore along the flaps, flat on a table, one seam at a time, and let each dry. Close the last seam to make the sphere. Glue the 15 cm top disc over the crown where the points meet: a hole there loses the hottest air.
Fit the mouth ring
Bend the cane or wire into a 0.5 m ring, glue the bottom edges of the gores round it, and tie the tether bridle to two opposite points of the ring.
Check it
Hang it by the top and blow cool air in gently: look for holes and open seams, and patch them with tissue. Weigh it: about 150 g.
Set up the fire and chimney
On bare ground or wet grass, in calm air, with nothing within 30 m that can burn and no power line in sight: light a small fire and stand the 1.2 m chimney over it on three stones. A thermometer held at the chimney's top should read about 80–100 °C: hot air, no flame, no sparks.
Fill it
Two people hold the balloon by the ring over the chimney top, never lower, while it fills and stands up. When it pulls upward, lift it off the chimney and let it rise on its tether, paying out the reel. It flies for as long as the air inside stays warm.
Measure the lift
Before letting it go up, hook a spring balance to the tether and read the pull: about 250 g with air at 80 °C inside. Compare with 1.76 m³ × 0.23 kg/m³ − 0.15 kg. Record the air temperature inside and out.
Tolerances that matter
| Dimension | Target | Why it matters |
|---|---|---|
| Gore widths | within 3 mm of the template | Unequal gores make a lopsided balloon that spills its hot air. |
| Air at the mouth | 80–100 °C, never flame | Tissue scorches and burns long before the lift is worth it. |
| Weight | under 0.25 kg | Every gram of glue is a gram less of lift. |
How this fails
Flame or sparks reaching the mouth: the balloon burns. Keep the chimney 1.2 m tall and the fire small.
Released free: it lands burning. Always tethered.
Wind: a gust tears tissue and blows the balloon into the fire. Calm air only.
A hole at the crown: the hottest air escapes first. Top disc glued on.
Too much glue: the balloon is too heavy to lift.
How you know it worked
VerificationThe balloon weighs under 0.25 kg, its measured lift matches the sum within about a quarter, it has flown on its tether in calm air, and no flame came near it.
Scaling and variants
Double the diameter and the volume, and the lift, rise eight times while the tissue weighs four times as much: bigger balloons lift far more for their weight. Full-size balloons are built the same way, from gores tapering to a crown, in nylon with heat-resistant fabric at the mouth (Cameron Balloons). Anyone who will fly in one learns from an instructor first.