How Things Work: Hot Air Balloons

Hot air balloons seem ever-ready to take to the skies, standing firm and upright, bobbing in the fresh air.

After reviewing the weather forecast, the pilot marks a wide, open space — free from any trees, poles, or power cables — as the take-off point. Reassured by the clear sky and mild breeze, the pilot slips on flame resistant gloves and climbs into the bowl-shaped wicker basket.

The basket is still attached to the ground crew vehicle, so it does not elope with the wind before the passengers climb aboard.

The average-sized hot air balloon is filled with 105,000 cubic feet of hot air and can carry four passengers. However, if hot air balloons were equipped with a larger wicker basket, they could carry up to 16 passengers.

Besides the scientific fact that hot air rises amid cool air, a passenger’s ballooning journey depends on three structural components: the balloon envelope, burner, and wicker basket.

The balloon envelope is the inflated structure (shaped like an upside-down dome) that stores hot air. The envelope of this balloon is made out of a ripstop nylon fabric with integrated, flame-resistant Nomex technology. Therefore, it is not only strong, lightweight, and waterproof, but it is also fireproof.

Because of the close proximity of the balloon envelope to the burner, it is crucial for the envelope to be fireproof. The burner sets ablaze propane, which is a liquefied, high-energy gas, heating up the air enclosed inside the balloon. In turn, this heating process helps the balloon rise higher up in the sky.

The propane in the hot air balloon comes from one of the central components of the balloon — the wicker basket. The wicker basket not only carries passengers, but also contains some of the most essential equipment required for hot air ballooning, including navigational tools and two narrow cylinders that serve as propane tanks.

These propane tanks contain extremely dense propane and are attached at the top to an intake hose which leads down to the hose of the cylinder in order to extract the propane.

Once the ground crew releases the balloon from the ground, it is the pilot’s job to actually get the balloon off the ground.

The pilot turns a lever that unbolts the propane valve and boosts the flow of gas through the hose. This process starts the burner, causing the propane to flow out of the hose and catch fire on a small, natural gas flame known as the pilot light.

Due to the increased density of the propane (it is reduced in volume by the pressure in the cylinders), it flows smoothly — though rapidly — through the hose and reaches the heating coil. The heating coil, which is a narrow section of steel tubing, is placed in a spiral around the burner and heats up as the pilot light glows.

As the heating coil grows hot, it warms up the propane that flows through it, thus transforming it into a gas before it comes in contact with the pilot light. This process sparks a mightier flame and provides increased efficiency in terms of fuel consumption.

In summary, when the amount of gas that flows through the hose increases, the flame becomes larger, and the air inside the envelope heats up faster. By doing this, the pilot bolsters the vertical speed of the balloon.

As the flame intensifies, the balloon rises at an increasing rate. To slow the ascent of the balloon, the pilot must open the parachute valve at the top of the envelope. The parachute valve is basically an opening that allows hot air to escape. In order to open it, the pilot pulls a cord that extends from the top of the balloon all the way down to the basket.

The opening of the parachute valve allows some of the trapped heat to escape into the atmosphere, causing the air temperature inside the envelope to fall.
Since the temperature outside the envelope and the temperature inside the envelope are now equal, the ascent of the balloon slows. If the pilot leaves the valve open for a long period of time, the balloon begins to descend.

Other than ascending and descending, the pilot does not have much control over the direction that the balloon takes. Nevertheless, while the wind is largely responsible for maneuvering the hot air balloon, the pilot can move the hot air balloon horizontally, to the right or left, at desired speed by adjusting the vertical position of the balloon.

The key here is the altitude of the balloon. Wind blows at different speeds and in different directions at different levels of height. Conventionally, wind speed increases as the balloon drifts higher into the atmosphere.

As the balloon descends, the pilot spots the ground crew in the distance, waiting down below to provide support in the case that the wicker basket tips upon landing. Unlike a plane or helicopter, the wicker basket does not have any wheels or landing skids. Also, the pilot lands in an area enclosed by a number of trees, which blocks much of the wind and makes the landing much smoother.

Hot air ballooning is a vivid demonstration of principles in physics and how nature’s movement allows people to travel.