Lighter-than-air vehicles, particularly the non-rigid type, have been used for a variety of applications including rescue, cargo transportation, and meteorological research. In a typical case, an airship is launched with the hull fully and stiffly inflated, ascends to a target altitude, performs its mission, and returns to the ground with the hull still fully inflated. To enhance intelligence-gathering, surveillance, reconnaissance, and communications relay missions, it would be useful to have unmanned aerial vehicles available that can operate at very high altitudes and that have a high endurance. The capability to operate at very high altitudes is desirable to make the platform survivable against anti-aircraft threats, to maximize the line-of-sight radius for sensors and communications equipment, and to place the aircraft above the effects of atmospheric weather system.
The 265,000 feet of airspace (60,000 feet to 325,000 feet) holds great potential for providing enduring support directly to the Warfighter. This area is called high altitude. It's an area ofoperations spanning from just above the ceiling of most aircraft to just below near earth orbit. On Sept. 4, 2007, Army Regulation 10-87, made USASMDC the Army specified proponent for High Altitude. There is no estab-lished Service lead for High Altitude within the Departmentof Defense.
A number of concepts for high atmospheric altitude platforms already exist, such as high altitude balloons, large dirigibles or blimps, unmanned heavier-that-air aircraft (drones) of traditional configuration or of flying wings configuration. Free balloons or tethered balloons would not tend to be suitable: a free balloon is not tethered, and will tend not to stay in one place; a 40,000-60,000 ft tether is not practicable (a) because of the weight of the tethers themselves; and (b) because of the danger to aerial navigation. Heavier-than-air aircraft tend not to have the required endurance, and any aircraft that relies on airflow over a lifting or other control surface must maintain sufficient velocity to maintain control, a problem that worsens when the density of the atmosphere is reduced.
Very large volume gas envelopes are needed to achieve neutral buoyancy at high altitudes. Therefore, lighter-than-air vehicles are large and heavy, which limits the altitude at which the vehicles can operate. Traditional airships, whether blimps or having a rigid internal skeleton tend generally to be low altitude aircraft, seldom being used at altitudes above about 5,000 ft above mean sea level. Modern airships that rely on the buoyancy of a lifting gas may tend to suffer from a number of disadvantages, such as (a) poor low-speed manoeuvrability; (b) the need for relatively large ground-crews for take-offs and landings; (c) the need for relatively large fields from which to operate; (d) complicated and expensive infrastructure for mooring (parking); and (e) susceptibility to damage in turbulent atmospheric conditions. In the view of the present inventor, many, if not all of these disadvantages appear to stem from the fundamental shape and configuration of traditional airships--that is, the characteristic elongated, finned hull.
The manoeuvrability of traditional airships tends to be related to the design and structure of their fins and control surfaces. Below 10 to 15 km/h (6-10 mph), there tends no longer to be sufficient airflow over the fins' control surfaces, making them ineffectual. When the pilot slows down, as when landing, a ground crew of up to 20 people may be required to assist the pilot. The same size of crew may also be required for take-off.
Conventional scientific balloons allow expansion of internal helium gas since the balloons do not have air for venting when ascending, and the ascension occurs with an usable buoyancy by preventing a rise in internal pressure rise as compared with the atmospheric air outside of the balloons. In horizontal flight, the balloons are flown by the wind by dropping ballast to cope with the loss of buoyancy. When the balloons have to descend, the balloon skin is ripped in order to vent the buoyant gas, and the payloads descend by parachutes.
On the other hand, in the low stratosphere, namely at an altitude of about 20 km above ground level, the weather is fine throughout the year and the wind is relatively weak, so that it is effective to make a large-scale LTA platform fly in such a space for a long period of time for the purposes of environmental observation and telecommunication relay. In this case, it is difficult to apply a means used for the above-described scientific balloons, but it is necessary to have a propulsion power to keep a position of the platform against the wind in the stratosphere, and at the same time, a means is necessary for ascent and descent of the vehicle which enables high speed shuttling between the stratosphere and the ground.
In such a high-altitude airship, the volumetric increase/decrease caused by the temperature fluctuations, such as the super-heat of the buoyant gas affected by the solar radiation at high altitudes, should be absorbed by air-filled ballonets (i.e., air chambers) allowing air flow from/to the outside atmosphere, which are usually positioned fore and aft of the airship hull, and thereby the trim function of the pitch attitude is provided. For example, if a temperature fluctuation band of the buoyant gas is 70.degree. C., and the mean temperature of the atmospheric air is about -53.degree. C. (220.degree. K in absolute temperature), the ballonet volume of not less than 70/220 of the hull volume is necessary, and as a result, the buoyancy of the ballonet space is lost and a weight is added by ballonet fabric materials, and the gas bag skin of the ballonet forms a free surface of the fluid boundary, namely between ballonet filled air and the buoyant gas in an envelope (airship hull body), which causes problems that the ballonet skin is exposed to the sloshing of the gas, and material fatigue of the fabric is apt to be caused by this kinetic movement.
Traditional cigar shaped blimps may also tend to present other disadvantages when viewed in the context of an aircraft having a high altitude service ceiling. Conventionally, cigar shaped airships employ fore and aft balloonets that can be inflated, or deflated, as the internal gas bags expand or contract with changes in altitude or temperature. Differential inflation of the balloonets can also be used to adjust airship trim. The balloonet operation between sea level (where ambient pressure is about 14.7 psia) and 5000 ft (where ambient pressure is about 12.5 psia) may involve balloonets of roughly 20% of the internal volume of the aircraft. To reach a service ceiling of about 60,000 ft (where the ambient pressure is about 1.0 psia), the volume of the lifting gas used at lift-off from sea level may be as little as about 1/18 of the volume of the lifting gas at 60,000 ft. This may present significant control challenges at low altitude for a cigar shaped aircraft.
Further, conventional airships tend to rely on airflow over their control surfaces to manoeuvre in flight. However, at high altitude the density of the air is sufficiently low that a much higher velocity may be required to maintain the level of control achieved at lower altitude. Further still, blimps and dirigibles are known to be susceptible to "porpoising". At 60,000 ft there is typically relatively little turbulence, and relatively light winds, or calm. In a light or "no-wind" situation, it may be difficult to maintain a cigar shaped dirigible "on station", i.e., in a set location for which the variation in position is limited to a fixed range of deviation such as a target box 1 km square relative to a ground station. Although 1 km may seem like a large distance, it is comparatively small relative to an airship that may be 300 m in length.
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