Lighter-than-air craft (aerostats) have been progressively neglected by the main stream research in Aerospace Engineering during the second half of the past century after having made remarkable technological progress that culminated in the 1930’s with the construction of over 200m long airships (Dick & Robinson, 1992, Robinson, 1973). There have been some developments of historical interest (Kirschner, 1986) but little of significance.
However, in the last few years, aerostats have attracted a renewed interest. Their typical market niches (scientific ballooning, surveillance/reconnaissance (Colozza & Dolce, 2005)) are expanding and more researchers have proposed several different applications, ranging from high altitude aerostats as astronomical platforms (Bely & Ashford, 1995) to infrastructures for communication systems (Badesha, 2002).
Amongst the most recent achievements in scientific ballooning are the Ultra-High Altitude Balloon (UHAB) developed for NASA (launched in 2002 with a volume of nearly 1.7 million cubic metres and reached an altitude of 49 km) and the ultra-thin film high altitude balloon constructed by the Institute of Space and Astronautical Science (ISAS) of Japan, which successfully carried a 10 kg payload to a world-record altitude of 53 km.
Tethered aerostats are limited to lower altitudes due to the weight of the tether, which increase linearly with height. Commercial aerostats fly up to 8km but various studies have been conducted to prove that considerably greater altitudes can be reached. For example the Johns Hopkins University Applied Physics Laboratory (JHU/APL) has conducted a successful feasibility study (although not experimentally demonstrated) on a high altitude (20 km) tethered balloon-based space-to-ground optical communication system (Badesha, 2002). The US Airforce has made extensive use of aerostats as a surveillance system, and there are aerostats available on the market like the Puma Tethered Aerostat (www. rosaerosystems. pbo. ru) or the TCOM’s 71M (www. tcomlp. com) that can fly up to approximately 5 km tethered with payloads of 2250 kg and 1600 kg respectively. These aerostats have a mooring cable (i. e. their tether) that supplies the aerostat onboard systems and the payload with electric power, and they are designed to be able to withstand lightning strikes and strong winds.
Concerning the size, today’s airships are considerably smaller than those constructed in the 1930’s and that is mainly due to the economics of their typical functions. However, from a technical point of view, the state of the art in the relevant technologies would allow the construction of aerostats much larger than those currently in operation.
The possibility of using solar power as source of energy for the airship propulsion and/or to supply energy to on board systems has been investigated by Khoury and Gillett, 2004. The "sunship" that he proposed was a very simple and conventional envelop design, filled with helium, with thin film solar arrays covering appropriate areas of the external surface. The electrical power produced by the cells was then used for the propulsion and on board electrical system, with part of the energy stored in suitable units with high storage energy to weigh ratio. Notwithstanding the quality of the case made by the author, the "sunship" was never built.
However changes in the economy, driven by politics and/or technical factors (limitation of resources or scientific advances) transform the markets and the viability of certain technologies may change as a result. A typical case is that of wind turbines, whose technology has been available for decades, but only in the last few years have become a viable method to produce large quantities of electric energy.