Lifting Gases

Several lighter-than-air gases could be employed as the lifting medium—hydrogen (H2), helium (He), methane (CH4), and hot air (Federal Aviation Administration, 2007).

Hydrogen is the lightest of all gases and is used occasionally for SFAP (Keranen, 1980; Gerard et al., 1997; Seang et al., 2008). However, hydrogen and methane are both explosive and highly flammable; they are not considered further for obvious safety reasons, which leaves helium and hot air as the gases of choice for most balloon and blimp applications.

Helium has a lifting capacity of about 1 g/L; whereas, hot air lifts only about 0.2 g/L. Thus, for a given volume, helium lifts approximately five times more weight than does hot air. In addition to a larger and heavier envelope, a hot-air platform also needs to carry a gas tank and burner in order to stay aloft for more than a few minutes. This added weight reduces the potential payload a hot-air system could carry.

Helium is created as a byproduct of radioactive decay within the solid Earth. Continental crust, which is enriched in uranium and other radioactive elements, is a constant source for helium. Because it is inert, helium does not combine with minerals in the crust, but it does readily dissolve into fluids such as ground water and natural gas; the latter typically contains 0.2-1.5% He by volume. Eventually the helium reaches the surface, for example in hot spring water (Persoz et al., 1972), and is released into the atmosphere. Earth's gravity is too weak to retain the helium molecule (single He atom), so it ultimately escapes into space.

Helium was little known prior to the twentieth century. This changed with the discovery that helium is a significant component of natural gas in some situations. Throughout most of the twentieth century, helium was regarded as a strategic resource for military and industrial purposes in

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