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## Netted Balloon Systems 

The netted design [Figure 11-2]  dates from the 18$^{th}$ century. A spherical balloon envelope is encased in a diamond mesh net of light weight cord that runs around the balloon from the apex valve at the top of the envelope down to support points on the gondola. 

Inflation is accomplished by first bringing the inflation hose to the center of a ground tarp. The envelope is next laid over the hose with the hose inserted into the appendix and with the apex of the balloon on top and centered. The net is deployed over and attached to the envelope and as the lifting gas is flowed, sandbags are hung on the diamonds of the net at ground level. As the balloon fills and forms a rising hemisphere, the crew moves the sandbags down the net to keep them at ground level. Enough bags are added to keep the balloon anchored to the ground. 

After the envelope is completely filled, it is raised and the basket is brought underneath and attached to the net. The hemispherical shape is very stable in moderate to high ground winds as the flow is over the top of the shape. One drawback of netted balloons is that more crew is required to manage the sandbags during inflation. 

## Quick Fill Balloon Systems 

The quick fill design dates from the mid 20th century. A natural shape envelope is surrounded by vertical support tapes that run from the apex of the balloon down to the gondola. [Figure 11-3] Several horizontal tapes extend circumferentially around and are attached to the vertical tapes to stabilize the structure. 

Inflation is accomplished by laying out the envelope on the ground along its full length with the apex at one end and the appendix (base) at the other. The gondola is attached to the load lines and is bagged down. As the lifting gas is flowed in through the appendix, several crew members hold the apex of the balloon down. When enough gas has entered the envelope to generate approximately 100 to 150 pounds of net lift, the apex is released, the envelope rises, and the inflation is completed. 

## Lifting Gases 

Modern gas balloons normally use helium or hydrogen as the lifting gas. Anhydrous ammonia and methane are two other less common options. Helium is a monotomic, inert gas and must be refined to reach a purity of greater than 99 percent from raw well gas at a purity of a few percent. Hydrogen typically exists in the molecular form H$_{2}$. It is flammable when combined in a mixture of 25 percent H$_{2}$ to 75 percent air. This means that this mixture supports an existing flame. 

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