Hot Air Balloon
Hot Air Balloon
A hot air balloon is a nonporous envelope of thin material filled with a lifting gas that is capable of lifting a suspended payload into the atmosphere. Balloons rise because of the displacement of air, applying the principle that the total upward buoyant force is equal to the weight of the air displaced.
Over the course of history, balloon envelopes have been made of paper, rubber, fabric, and various plastics. The shapes of balloons have also varied over time, but today the most common ones are spheres, oblate spheroids, and aerodynamic configurations. Lifting gases have also varied. Today, the most commonly used gases are helium, hydrogen, and heated air.
In the late eighteenth century, Joseph and Jacques Montgolfier pioneered hot air ballooning in France. In 1782, they discovered that heated air in a lightweight bag caused it to rise. In 1783, they demonstrated their discovery publicly in Annonay, France. A few months later they repeated the experiment at Versailles, this time sending up a sheep, a rooster, and a duck as passengers.
The first manned balloon flight took place in Paris, France, on November 21, 1783. Coordinated by the Mongolfier brothers, Pilatre de Rozier and the Marquis Francois Laurent d'Arlandes were launched into the air in a balloon made of paper and linen. Smoke and heated air were used for the lifting gas.
Early balloons were used in war and sport. In the nineteenth century, the balloon was honed for use in war to spy behind enemy lines. Peacetime uses included taking the earliest aerial photographs. These balloons were useful but they were not steerable. Serious scientific balloon experiments only began late in the twentieth century.
Between 1934 and 1961, crewed balloon flights conducted research in the stratosphere [the atmosphere 6-15 mi (9.7-24.14 km) above Earth's surface]. Pressurized capsules allowed crews to go as high as 100,000 ft (30 km). In 1961, the advent of space flight made many of these experiments obsolete.
Since the early 1960s, the hot air balloon has been used as a free balloon (i.e. released into the atmosphere) to carry people aloft as a sport. In the 1970s, consumer hot air balloon sales soared, and in 1973 the first World championships were held in the United States. Today, there are various ballooning events around the world, but the main objective of most serious ballooners is to make and break records.
Other types of balloons currently used include the meteorological balloon, the zero-pressure balloon, the superpressure balloon (a constant level balloon), the military tethered balloon, and the powered balloon.
Modern hot air ballooning
Today's hot air balloons have two main parts, the envelope (or gas bag) and the basket. The gas bag is usually spherical and constructed of a nonextensible material. The heated air that lifts the balloon comes from a hydrocarbon gas burner attached above the basket.
The basket (also called the gondola) carries the passengers. Lift is controlled by adjusting the burning rate of the gas. A valve at the top of the balloon has a rope attached so that passengers can control descent. A rip cord and rip panel allow the rapid release of gas on landing to prevent the dragging of the load on impact.
Balloons can only go so far up into the atmosphere. The current limit for practical ballooning is 34 miles (55 km).
One of the most interesting aspects of hot air ballooning is the inflation of the hot air balloon. Under normal conditions, a four-man balloon can be inflated and launched with a crew of four to five people. To inflate such a large object, a large space is needed. The basket is laid out on its side. The envelope is connected to the basket and spread out over the ground. A few crew members hold open the mouth of the balloon and while a fan partially inflates a balloon with cold air.
The balloon's pilot enters the envelope at this point to make preliminary pre-flight checks on operational lines, rigging, pulleys, Velcro™ tab, flying wires, parachute, and the fabric of the envelope.
One fuel tank is used when the burner is turned on. A rush of trapped air surges into the envelope. The mouth tends to close behind it because of the rush. It takes about 60 seconds to fill a 20,000 cu ft (6,096 cu m) balloon. As the envelope fills, it rises above the basket.
The crew dealing with the crown of the balloon hold it steady downwind and prevent it from rolling side to side. As the lift increases, the crew walks the crown line to the basket.
After the inflation is completed, the pilot and passengers come aboard to make final checks. The balloon continues heating until the balloon becomes "light" (ready for take off).
Envelopes balance their load with load tapes or cords. Americans prefer a heavy fabric to share the load. Their European counterparts prefer lighter fabric and balance it with more load tape structures.
The fabric is woven from two kinds of yarn, nylon and Dacron (polyester). There are advantages and disadvantages to both. Nylon is lighter and stronger, but Dacron can withstand higher temperatures. The woven fabric is actually a mesh structure that allows air to pass through it. Most fabrics have a tensile strength of 40-100 lb (18.16-45.4 kg) per inch-wide strip.
To contain the air, the woven fabric is coated with a sealant. The most common is polyurethane, plus additives like neoprene (synthetic rubber) or silicone, and an ultraviolet inhibitor to protect the coating from breaking down because of the sun. The number of coats is determined by air tightness balanced with material fragility.
Two other important parts of the envelope are the parachute and the rip panel, both of which help control the balloon, especially when descending. The parachute is fail-safe, so it has become the dominant control. It is made of fabric similar to the envelope. The rip panel is sealed by Velcro™, and has a secondary opening called a vent that is also made with fabric similar to the envelope.
A net supports the basket and distributes the weight of the basket evenly over the balloon.
The basket's body is usually made of rattan and willow woven together. The floor can be made of plywood. The edges of the basket are commonly bound in leather, suede, or rawhide. Stainless steel wires and/or upright rigid supports attach the basket to the burner frame. Some manufacturers suspend the basket from a load ring that is itself suspended from the envelope. This load ring can also work as the frame for the burner.
The burner is a single-unit propane burner powered by two or more fuel tanks. The fuel tanks are joined to the burner with a permanent hose coupling. All the burners are constant-burning pilot flames.
There are a couple of components to the burner. The liquid valve regulates the amount of fuel used by the burner. The pressure gauge shows the amount of gas pressure as it enters the burner coil indicating the amount of power available. The pilot light burns the vapor off the top of the tank. The vaporized propane emerges through jets that produce a flame with a fat base and a long tip. The stainless steel vaporizing coil passes the propane to produce heat.
Envelopes are designed to reduce fabric stress by producing a lightly curved gore (the sectional panels that are sewn together to make the envelope). Curved gores are longer in the center than they are in their ends. There are three main types of gores: vertical, horizontal and diagonal. Diagonal panels are the most economic because they waste the least amount of fabric. Many gores are computer designed.
There are two prevalent balloon shapes: teardrop and round. The teardrop can climb faster using less energy, but the round shaped balloon uses less fabric, thereby having less surface area to heat. An envelope can expect to last 400 flying hours.
Baskets have remained essentially the same since the 1700s. They are made of wicker and are square. They flex on impact. In the early 1970s, one company manufactured a gondola that was made of aluminum and fiberglass, but it tended to shatter on hard landings. The other big change is a triangle-shaped basket which allowed for some innovations with instrument panel placement. Baskets last about 800 flying hours.
- 1 Envelope construction basically involves the sewing of the gores together. Whether done by hand or industrial sewing machine, there are three stitches. The double lap seam features two rows of parallel stitching along the folded over fabric seam. Preferred by manufacturers for its strength and lightness, the seam features about eight stitches in every inch (3 per cm). A few manufactures use a flat seam (straight parallel stitching holds two pieces of fabric together) and the zigzag (zigzag parallel stitching with a double lap of fabric). The load tapes and cords are also sewn in.
- 2 After the envelope is stitched, it is coated. The coating is applied mechanically and under pressure.
- 3 Finally, if the envelope is to be used for advertising purposes, an applique is attached with a slogan or name. It can be applied with acrylic spray paint, or ready-made adhesive letters or banners can be attached. If the artwork is large, it can be sewn into the envelope proper by being cut directly into the gores. This can be a complex, demanding process.
- 4 Baskets are manufactured base-first. On top of the plywood base with runners, a frame of cane is built up to 1 in (2.5 cm) in diameter. At the corner, the frame surrounds stainless steel wires and load frames. Around the frame, the rattan or willow is woven. Holes are left in the body of the basket for cylinder straps. The finished basket is coated with varnish to help maintain its shape and set the cane together. Finally, the edges are protected with sewn in rawhide, leather, or suede. The instrument/dashboard is built in as are the propane tanks for the burner unit.
- 5 Many balloon manufacturers outsource these components and assemble them between the basket and the envelope after the other parts are put together.
Innovations that will allow hot air balloons to go higher, for a longer period of time, and under more control will continue to happen. Many of the innovations center on improving the burner and the deflation system.
Where to Learn More
Gibbs-Smith, C.H. Ballooning. Penguin Books, 1948.
Hildebrant, A. Airships Past and Present. Archibald Constable & Co., Ltd., 1908.
"A Brief Look at the History of Ballooning." http://www.geocities.com/TheTropics/4744/hist.htm (April 30, 1997).
Jervis, Mark. "A Short History of Ballooning." http://www.bris.ac.uk/Depts/Union/BUHABS/first.html (April 30, 1997).