Anatomy of a Hot Air Balloon
The hot air balloon consists of three parts: envelope, basket, and burner system. It's a fairly simple piece of equipment, and for what it delivers, relatively inexpensive when compared to other aircraft.
There are many differences among balloons made by various manufacturers, so for discussion purposes, we’ll focus on the Raven S-55A, one of the most popular types and sizes of balloon in current use.
Balloons are categorized by the Federation Aeronautique Internationale (FAI), according to how much air their envelopes can contain.
The part of the aircraft that looks like a balloon is actually called an envelope. Some pilots also affectionately refer to it as "the bag." It is connected to the gondola, or basket, in which pilot and passengers ride, by means of stainless steel suspension cables.
The fabric in the envelope is rip stop, fire-resistant nylon, similar to material used in the backpacks or lightweight tents carried by hikers and mountaineers.
It is woven in panels, two panels making up what is a gore. There are 24 of these gores, or large vertical sections, in the entire S-55A envelope. The gores are held together by stitching and by heavy duty load tapes, webbing similar to the material used in seat belts in automobiles, which help support the weight of the balloon and minimize strain on the fabric, thus prolonging the useful life of the vessel.
The envelope is usually treated with a polyurethane coating to reduce porosity, and in the coating is an ultraviolet inhibitor to help the fabric withstand the rays of the sun.
The top cap or crown of the balloon may be designed in one of two ways. The standard top features a circular deflation port that is closed off by a circular panel, which is held sealed during flight by a flexible hook and-loop closure.
A deflation port line, usually called the rip line and colored red, extends from the top to the basket and is pulled by the pilot upon landing to effect an instantaneous deflation of the envelope. Pull on the line, and the hook-and-loop closure comes open, and the hot air rushes out.
The parachute top also features an opening in the crown, along with a similar circular panel to fill the opening. In this case, the panel is rigged up with lines, as the name suggests, exactly like a parachute. It is used both for effecting a complete deflation of the envelope after landing, and also during flight to vent a limited amount of hot air as a means for controlling vertical ascent or descent.
After landing, the pilot pulls on the top line of the parachute top and holds it open for as long as he can until the envelope has been deflated.
Balloons built with the standard deflation port also feature an opening in the side known as the maneuvering vent, also initially sealed, like the deflation port by hook-and-loop closure. This vent is operated by a rope called the maneuvering vent line, which extends from the vent to the basket. The pilot pulls the line, causing the side vent to open and allowing the hot air to escape to stop a rate of ascent or initiate a rate of descent. Once the line is released, the vent automatically recloses.
A skirt is connected to the bottom section of the envelope with peelable tabs and helps create a stovepipe effect when the burner system is running, channeling the warm air into the balloon and improving fuel efficiency, especially on windy days, by protecting the burner from the wind.
Attached to the inside of the balloon envelope, near the top or crown, are temperature indicators known as telltales. These telltales are white when installed but turn black if the air in the envelope exceeds a certain temperature. This permits the pilot to tell at a glance when his balloon may need repair.
We mentioned earlier that the cubic capacity of an envelope dictates how many people can be carried aloft safely in a balloon. However, this capacity may be adjusted upward or downward depending on other factors, principally air temperature, and altitude.
The key is to determine the maximum safe gross weight that can be carried by the balloon on a given day. All balloons are provided with FAA-certified curves for estimating temperature-limited gross weight. These curves show gross weight in pounds versus air temperature in degrees Fahrenheit for operation at various pressure altitudes, usually sea level, 5,000, 10,000, and 15,000 feet MSL.
Currently, open-air passenger vessels for balloons are made either of wicker or aluminum and fiberglass. A rigid tubular framework extends from the basket upward where it attaches to the 24 stainless steel cables suspended from the envelope.
This rigid framework actually serves three purposes:
- It helps protect passengers;
- It supports the burner system, which is installed overhead at the top of the basket;
- Thanks to the mechanical-fulcrum effect, it provides the basket with stability and less of a tendency to tip over upon landing.
Baskets normally come in square or rectangular shapes. Triangular shapes also are available, and on special order one can obtain completely enclosed gondolas, or deluxe baskets with built-in banquette seats and insulated champagne holders. As a minimum, there must, be room on board for securing the necessary fuel tanks and flight instruments.
Wicker is the classic or traditional look in a basket, but it is actually not just nostalgia that makes it a popular material, even though it costs more than aluminum-fiberglass varieties.
The strong and resilient rattan that is woven around a tubular aluminum frame happens to provide an excellent shock absorber during less-than-perfect landings. The aluminum/fiberglass baskets are quite strong and lighter than wicker. Although they do not have as much "give" on impact, these baskets are often used for commercial ventures where more rugged conditions are the rule.
The Burner System
If the envelope and the basket are the most visible parts of the balloon, our third major component, the burner system, is the motive force, for it produces the heated air which the pilot uses to lift off and to control his vessel's up-and-down motion once aloft.
Normally the burner or burners point directly into the center of the balloon envelope. There is a pilot light very much like the kind found on ordinary gas stoves, and this assures rapid access to heat whenever necessary.
There is also a blast valve allowing the pilot to adjust the rate at which fuel flows from the tanks. To draw fuel into the burner, the pilot simply pulls the trigger on his blast valve. The liquid propane gas is carried under pressure from the fuel tank to the burner, where it is set on fire by the pilot light.
A pressure gauge on the burner tells what this pressure is. The propane is transferred from tanks in the basket through a flexible hose to the burner system. The fuel goes through coils on the burner which vaporize the liquid gas.
Then the pilot light ignites the vapor, sending a six-to eight-foot flame into the envelope, making a loud whooshing sound and adding heat at the rate of 12 million BTU's (British Thermal Units) per hour.
The BTU is defined as the quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
To give a better idea of the tremendous output of a burner system in a hot air balloon, one burner produces at a rate per hour that would be enough to heat 120 three bedroom homes comfortably.
Propane, the fuel used almost exclusively in hot air ballooning, has the additional advantage of being readily available and quite inexpensive in today's energy-scarce world.
Also called liquefied petroleum gas, or LPG, it is a by-product of the petroleum-manufacturing process. Propane is odorless, incidentally, so as a safety measure refineries introduced an ingredient called mercaptan into the gas.
Mercaptan has a strong, sweetish scent, which makes gas leaks readily detectable.
The propane is stored in fuel tanks, either ten-gallon aluminum or twenty-gallon stainless-steel tanks. The advantage of the more expensive stainless-steel tank is that it is lighter, so you can carry more fuel with less tank weight.
There are two ways of heating air inside the envelope. One is by triggering the blast valve, as mentioned, which when wide open can add heat at the rate of 12 million BTU's per hour. Intermittent blasts during flight enable the pilot to maintain a standard temperature in the balloon, and a standard temperature generally provides a standard altitude for that flight.
The second method of adding heat to the envelope is by using the "cruise" or metering valve. By monitoring the exact flow of propane to the burner, you can then maintain a steady output from the burner, which will provide a standard temperature. This would enable you to maintain a straight and level flight without having to touch the blast valve that often. It's as close as you can come to flying on automatic pilot in a hot air balloon.
Other Ballooning Gear
We've covered the three basic components of the hot air balloon-envelope, basket, and burner system-but there are other items of equipment that pilots need and sometimes use frequently. Some of these items are required (by the FAA), some recommended, and some optional in the same sense that mud flaps and FM radios are optional on new cars.
Altimeter: This tells you how high you are above the ground, normally giving readings in feet above sea level, and can be adjusted to the local ground elevation where the ascent is taking place.
Variometer: This is basically a rate of-climb meter and lets you know first of all if you are moving up or down. That may sound ridiculous, but at altitudes of 1,000 feet or more, where visual references are few and far between, it is often quite difficult to judge such basic facts of life as whether you are on the way up, or on the way down. The variometer also measures the vertical speed of the balloon in feet per minute or second.
Pyrometer: An electrical sensor installed near the top or crown of the envelope sends information to this meter in the basket. It tells the temperature of the air in that area. As with RPM's in an automobile, there is a "red line" not to exceed in terms of envelope temperature, beyond which you risk damaging fabric strength. The pyrometer helps you monitor this important factor.
Fuel gauge: This shows how much propane is left in the tanks on board, and because fuel supply is important to a safe and sane balloon flight, such gauges should be as reliable and visible as possible.
Sparker: This gadget comes in handy for quickly reigniting a pilot light that has inadvertently gone out. Most pilots carry two sparkers-plus a cigarette lighter and a pack of dry matches, to be on the safe side Sparkers are also used by welders and by campers for starting up Coleman lanterns, so they are readily available.
Headgear: Protective hats or helmets are vital safeguards in the event of a fast or bouncy landing. As with lifeboats on a ship, there should be enough on board for all occupants.
Protective Clothing: There is no point in totally ignoring the remote but real chances for injury involved in ballooning. In that spirit, let's note a few simple precautions to incorporate in your standard garb for the sport.
The pilot should wear some eye protecting device-goggles or simply a pair of sunglasses-during the inflation stage. This will shield his eyes from any backlash of heat from an unexpected gust of wind blowing flames from the burner toward him instead of toward the envelope, and conceivably save a nice set of eyebrows.
Pilot and crew should wear heavy-duty gloves as protection from the heat of the burner and its flames, and also from possible rope burns.
Pilot and crew should wear long sleeved shirts and long pants, again to protect themselves from the heat and flame of the burner system during inflation, cotton or fire-retardant materials are preferred, not nylon and polyester.
Pilot, crew, and passengers should wear sturdy boots or high shoes so everyone is more comfortable in the rough or slippery terrain one might conceivably land in.
Fire Extinguisher: It should be of the type useful in combating propane fueled fires, and carried not only in the chase vehicle but on board balloon. The 2A10BC is the type often recommended.
First Aid Kit: It should include silver compound medication for burns and, like the fire extinguisher, be carried both on board and in the chase vehicle.
Balloon Log Book: The pilot records the date, place, and other pertinent details of every one of his flights in the given balloon.
Electric Blast Valve: This is a push-button valve control that can be operated from anywhere in the basket.
Electronic Ignition: Similar to above, this operates off a battery and keeps the pilot light going.
Compass: Until recently, FAA regulations required balloonists to carry a magnetic compass on board, but this device plays no practical role, at least not on short flights. On long flights in unfamiliar country, it does give you an idea of the exact direction in which you are gradually getting lost.
Communication System: On short flights there is no particular need to be in touch with Houston Control. Aviation radios and transponders are useful on board for long flights at high altitudes, and radios are required to be carried if you are flying through any airport traffic control zone.