Hot-air ballooning

Envelope design

Hot-air balloons vary considerably in design and materials. Lightweight coated-nylon and polyester fabrics are the most common materials for envelopes. Cotton is very serviceable but has a comparatively poor weight-to-strength ratio. Specially shaped balloons, which are literally pneumatic sculptures, are popular at public events. They utilize special tailoring and many internal baffles and cords to attain the desired designs.

Sport balloons typically have a silhouette similar to the natural shape of fully inflated gas balloons. They can be assembled with many vertical gores (fabric sections, or panels) or fewer horizontal gores. The gore material can be cut straight (with the fabric’s natural grain) or on the bias (diagonal to the fabric’s natural grain). If straight gores are used, excess material can be gathered to create a fluted pattern that provides some flexibility. Gores made of bias-cut materials have greater stretch, which provides a natural flexibility. With horizontal gores, the individual panels can be gathered to provide a bulbous gore, which gives even greater flexibility. Because of the greatly reduced effective radius of curvature, a bulbous gore balloon experiences much less stress on the envelope fabric.

With bias gores, load tapes known as tensors are generally sewn loosely into ducts formed in the vertical seams, much like the shroud lines sewn into a parachute’s radial seams. With other balloons, the load is carried in tapes sewn or heat-sealed directly to the vertical seams. One load tape can and should have more than enough strength to carry the whole load without an excessive weight penalty. The stress in the skin of the balloon is so low that normal handling will cause visible damage if the fabric is weakened by wear or exposure long before it would fail in normal flight. The importance of load tapes and adequate excess strength in them in balloon construction cannot be overemphasized. Catastrophic failure in a properly designed balloon is extremely rare.

Deflation systems

Landing a bag with some four tons of air in it at 30 km (20 miles) per hour without wheels, steering gear, or brakes presents some problems. Prior to John Wise’s discovery of the rip panel deflation system in the mid-19th century, a gas balloon could be dragged along for several kilometres before coming to a stop. In particular, balloons on extended flights seemed to be drawn into low pressure systems, which often resulted in stormy landings.

The basic hot-air balloon rip panel is a simple large sleeve at the apex that is drawn into a bunch and tied off with a cord. The cord is typically cut remotely by an electrically actuated explosive squib cannon or released with a pull cord moments before touchdown in order to deflate the envelope rapidly. This configuration is known as a pop top. Other balloon models use a circular panel held in place with a hook-and-loop (e.g., Velcro) closure that can be opened and closed progressively for finer adjustments in buoyancy. That system is more often replaced with Tracy Barnes’s “parachute top,” which is a combination venting and deflation panel. The parachute top consists of a simple hole at the apex of the balloon, usually about one-quarter of the balloon’s diameter, plugged with a parachute of slightly greater diameter. The parachute is positioned by cords radiating out a few metres from the parachute’s edge to anchors located on the puffy gore centres. A venting cord leads from the juncture of the parachute’s shroud lines to the basket. Sometimes a mechanical advantage is gained by a pulley system. Pulling the cord draws the parachute down into the balloon, thereby letting hot air escape. Releasing the cord snaps the parachute back into the closed position with the force of the hot air. If the parachute is pulled far enough into the balloon, it will collapse, letting the balloon completely deflate rapidly. This is a great advantage in high-wind landings, where it might be difficult to maintain tension on the cord. Some versions use separate retracting cords to force the opening to close, redundant cinch cords to bunch the parachute for hands-off deflation, and elastic centring cords to provide automatic rapid setting and resetting of the parachute.

Burners

Hot-air balloons today use liquid propane burners exclusively. The burner’s vaporizing coils are generally made of steel, but copper tubing has been used with good results. A secondary system without vaporization and slower air mixing can provide much quieter operation and give redundancy for safety. Many designs have used multiple duplicate burners and fuel supplies because there has been a history of leaking seals on control valves and occasionally contaminated fuel.

Baskets

Passenger baskets, or gondolas, vary as much in design as envelopes. For easy transportation, collapsible tubular frames with stout fabric covers can meet minimum requirements. Originally, wicker baskets had manila rope woven into the basket for suspension, but because of the potential for rot, these were replaced with synthetic or steel cables. Heavy wickerwork frames, using rattan up to a few centimetres in diameter, typically have also been replaced with metal or plastic framing.

Wicker construction has an advantage over metal skeletons and hard fibreglass shells in its absorption of the kinetic energy of impacts, however. Wicker is also favoured for its nostalgic artistic appearance. Any basket can have closed-cell foam padding on the inside for passenger safety and comfort.

Metallic components present hazards in the event of contact with electrical power lines. However, wickerwork by itself provides little protection to the passengers or fuel tanks from such contact. Vacuum-formed monocoque plastic baskets are slowly coming into use. They provide advanced impact and electrical contact resistance.