Netted Balloon Systems Quick Fill Balloon Systems Lifting Gases 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. 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. 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.  $_{Figure 11-3.}$  Quick fill design gas balloon. $_{Figure 11-2.}$  “Spirit of Springfield,” a netted gas ballon. 11-3