Aerostat
Introduction to Aerostats
An aerostat is an aerodynamically shaped body that is tethered to the ground. Aerostats are filled with a ‘lighter than air’ gas which is helium in most cases and thus generates lift due to buoyancy. During the Second World War, aerostats provided a new tool for defensive warfare called the balloon barrage system. Steel wires were raised by these balloons in order to deter enemy aircraft from flying low and thus bringing them into the range of ground artillery. The steel wires posed a hazard to pilots as they could shear off the wings of a plane. These days, aerostats are used mostly as an aerial platform of airborne sensors for communication and aerial surveillance.
Figure shows an aerostat developed by TCOM at mooring platform.
The primary requirements of an aerostat are
i) High payload
ii) Low blowby
iii) Sufficient stability and fast response to winds
Payload: Payloads in modern day aerostats are usually radars, cameras or communication equipment. In order to deploy more sophisticated equipment on the aerostat, it is always desirable to increase their payload capacity. The total lift that is produced by buoyancy and aerodynamic forces is balanced by the weight of the aerostat, the tether force and the payload. The buoyancy depends solely on the volume of LTA gas contained in the envelope. To increase the payload that can be carried by an envelope of fixed volume, either the weight of the aerostat has to be reduced or the tether force has to be reduced.
The weight of the envelope depends on its total surface area and the density of the material that is used for manufacturing the aerostat. Thus, to reduce the weight of the aerostat, its surface area should be reduced. Selection of the proper geometry of the hull can reduce the surface area of the hull for the same volume. However the surface area alone does not decide the shape of the envelope as there are other considerations such as stresses generated and drag produced in the aerostat. When stress is low, thinner fabrics with lesser density can be used for manufacturing the aerostat. Another method to reduce the weight of the envelope is to use patches of thicker and denser material in highly stressed regions and thinner materials for other regions. Fins are provided to impart stability but constitute a major portion of the weight of the aerostat. Hence, in order to increase the payload of the aerostat, the size of the fins should be made as small as possible through proper selection of fin geometry and location.