Abstract:
Unmanned aircraft for telecommunicative or other scientific purposes, which is stationed at a determined height, in particular in the stratosphere. The aircraft includes a gas-filled balloon carrying a platform and equipment that maintains the platform in a position relative to the ground. The balloon carrying the platform is disposed in the interior of an external balloon of aerodynamical shape, particularly in the stratosphere. At least one low or high pressure insulation chamber filled with a medium is arranged between them and encircles the inner balloon. The medium used in the insulation chamber is a gas having a low thermal conductivity. The negative effects of the temperature differences are largely compensated so that the inner balloon can be produced from a lighter and cheaper material, thereby increasing durably its longevity.

Description:
FIELD OF THE INVENTION 
     The invention relates to an unmanned aircraft for telecommunications or other scientific purposes, to be stationed at a predetermined height in the stratosphere, that includes an outer balloon which is provided with an aerodynamic external shape, and a gas-filled balloon arranged inside the outer balloon, which in combination support a platform, and means for maintaining the position of the platform with respect to the Earth. 
     BACKGROUND OF THE INVENTION 
     The use of gas-filled pressurised balloons to station diverse telecommunications and/or surveillance platforms in the stratosphere is known, for example, from U.S. Pat. No. 5,104,059. One particular problem of such pressurised balloons arises from the variations in temperature to which they are exposed, firstly throughout the day and secondly at night. In the daytime, the balloon&#39;s surface is exposed to direct solar radiation, and the gas in the balloon&#39;s interior is heated by the solar radiation, causing the gas pressure to rise. At night, on the other hand, both ambient and gas temperatures fall and therefore also the gas pressure in the pressurised balloon. This imposes even more demands on the material and the construction of the pressurised balloon. It also makes it more difficult to maintain the platform&#39;s altitude and position with respect to the earth. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention is based on the problem of creating an unmanned aircraft of the aforementioned type in which the gas-filled pressurised balloon supporting the platform can be kept at the desired altitude and position in optimal fashion, and additionally has a long lifetime. 
     This problem is solved according to the invention by an aircraft wherein between the outer balloon and the inner balloon, at least one low- or high-pressure insulating chamber is formed, where a low thermal conductivity gas is used as medium for the insulation chamber(s). 
     Further preferred embodiments of the aircraft according to the invention form the subject matter of the dependent claims. 
     In the aircraft according to the invention, in which the pressurised balloon is arranged inside an outer balloon which inflates in the stratosphere into an aerodynamic external shape, and in which at least one low or high-pressure insulation chamber filled with a medium is formed between this outer balloon and the inner balloon, the medium used for the insulation chamber being a gas with low thermal conductivity, the negative effects of the temperature variations on the gas pressure in the pressurised balloon are largely avoided, so that it can be produced from a lighter and cheaper material, and its lifetime is durably increased. 
     The platform&#39;s position with respect to the earth can be kept as stable as possible over long periods due to the largely constant gas pressure in the pressurised balloon and the electrically-driven propellers outside the outer balloon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will next be explained in more detail with the aid of the drawings, which show in purely diagrammatic form: 
         FIG. 1  is a first embodiment of an aircraft according to the invention in lateral view; 
         FIG. 2  is a part of the aircraft according to  FIG. 1  in cross-section; 
         FIG. 3  is a second embodiment of an aircraft according to the invention in lateral view, and 
         FIG. 4  is a further variant of an aircraft according to the invention in diagrammatic longitudinal section and partially in plan view. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic view of an unmanned aircraft  1 , in particular a platform  10  for wireless communication and/or for other scientific purposes, a so-called “high altitude platform”, in the stratosphere. This aircraft  1  can hereby be controlled in such a way that it adopts a stationary position with respect to the earth or it can also be arranged to be movable relative to the earth, if for example it is to be positioned flying stationary with respect to a satellite in space. This aircraft is suitable, not only as a transmission station for telecommunications, but also for scientific measurement purposes, as a transmission station for TV or radio stations, for photographic purposes, as a weather station and much more. It is equipped with a GPS and other control devices, so that automatic on-board guidance of the aircraft is enabled, with an electronic connection more or less remotely controlled by a control centre on earth. 
     According to  FIG. 1 , the aircraft  1  is already at the desired altitude of 20 to 30 km, which is advantageous in terms of wind conditions. The platform  10 , equipped with corresponding devices (“payload plane”) is supported by a pressurised balloon  11  filled with gas, preferably helium. As a variant, it is possible for this platform  10  to be supported by support elements  17  extending around the balloon  11 , for example belts or suchlike. 
     The pressurised balloon  11 , which advantageously takes the form of a pumpkin or other shape (“pumpkin balloon”) sits within an outer balloon  12  which has an aerodynamic outer form, which is filled with a medium and inflated into the aerodynamic outer form only once the platform  10  has been brought through the troposphere with ease by means of the pressurised balloon  11  to the desired altitude, in particular of 20.7 km. 
     The outer balloon  12  is equipped at its rear end with an elevator and rudder unit  13 ,  14 . There are also means to maintain the position of the aircraft and the platform with respect to the rotating earth. These include electrically-drivable propellers  15  for the forward propulsion of the aircraft or also for aircraft stabilisation, located outside the platform  10 . In this case, the propellers  15  can be driven at individual speeds, in order always to keep the aircraft in the same axis with respect to the surface of the earth. The propellers  15  can also be disposed pivotably on the platform  10  and thus serve both said purposes. The aircraft  1  according to the invention is also equipped with a controller and with an electronic autopilot system. 
     According to the invention, the medium used for filling and inflating the outer balloon  12  is a gas with low thermal conductivity, preferably xenon or krypton. The thermal conductivity of krypton is 0.00949 W/m·K, and that of xenon 0.00569 W/m·K. A low- or high-pressure insulation chamber  20  is formed about the inner balloon  11 , by which the balloon  11  is, so to speak, protected from the temperature differentials which arise for example during the night and in the daytime and its temperature and gas pressure respectively remain as constant as possible. 
     The gas which is notable for being a poor thermal conductor, preferably xenon or krypton, is delivered according to  FIG. 2  by means of a pump  21  from a tank  24  via a feed line  23  into the insulation chamber  20 , where the pump  21  also enables the gas to be fed into a separate balloon  28  forming a compensation chamber, which ensures constant pressure and constant volume in the low- or high pressure insulation chamber  20  and thus also maintains the aerodynamic outer form of the outer balloon  12 . The gas is cleared of any moisture before going into the insulation chamber  20 . A pressure and a temperature gauge  26  and  27  respectively are also provided, which are connected with a control unit, not shown in more detail. The inner balloon  11  is—as already mentioned—preferably filled with helium (but this could also be a different gas, e.g. hydrogen). According to  FIG. 2 , a helium tank  43  is linked via a pipe  49  with the interior of the balloon  11 . A pump  47  allows the helium to be fed either into this pressurised balloon  11  or into an additional helium-filled balloon  58  serving as compensation chamber for the whole aircraft. A pressure gauge  48  which can deliver a signal to the control unit is also provided in the pipe  49 . The helium is delivered under pressure into the inner balloon  11  supporting the platform  10 , for which a compressor, not shown in more detail, is provided. 
     As can be seen from  FIG. 2 , all the equipment is contained in the platform  10 . Obviously, additional instruments and aggregates, not shown in more detail, are also housed in this platform  10 , for example all the electronics, accumulators, control devices and much more. 
     According to the invention, the pressure conditions in the inner balloon  11  are controlled such that the temperature in its interior remains as constant as possible and preferably corresponds to the night temperature of the outside air. The insulation chamber  20  filled with a low thermal conductivity gas ensures that the temperature differentials of the outside air during the day and at night have as little effect as possible on the inner balloon  11 . If, however, the pressure gauge  48  detects a rise in pressure in the balloon  11  during the day, some of the helium is allowed to escape into the additional balloon  58  via a pressure reducing valve. At night, on the other hand, if the pressure gauge  48  displays a pressure below the desired value, the helium is pumped back into the inner balloon  11 . 
     The outer balloon  12 , the base material of which is polyethylene, is provided on its surface with a solar collector film  40 . The electrical energy produced during the day by solar radiation is stored by batteries. 
     The outer balloon  12  is also provided with an infrared collector film  41 , with which the infrared re-radiation from the earth during the night is exploited. The infrared collector film  41  on the inner side of the solar collector film  40  is preferably made of a dark, approximately 12 μm thick aluminium film, a colour coat or similar. Both the outer balloon  12  and the pressurised balloon  11  are advantageously made from a transparent plastic material, with the infrared collector film  41  being attached on the inner side of the outer balloon  12  facing towards the earth. The infrared radiation can then penetrate through both balloons from below and so helps to compensate, in temperature terms, for the cooling which otherwise occurs during the night. The infrared collector film  41  preferably covers a larger area of the outer balloon  12  than the solar collector film  40 . 
     Both on the outside and the inside, the solar collector film  40  and the infrared collector film  41  are covered by a layer of synthetic foam, for example polystyrene, or by another insulation material, so that no excessive heating of the balloon surface occurs. 
     It is, however, also possible to produce both the outer balloon  12  and the inner balloon  11  from an aluminised plastic, this being a multilayer material, in which a layer of aluminium is applied to a plastic, preferably polyethylene, base, said aluminium layer being in turn covered by a layer of plastic. The aluminium layer firstly effects a reflection of radiation and secondly improves the properties relating to gas impermeability, i.e. less gas can escape through the balloon material. Due to the reflection of radiation, its thermal effect, which is intended to be “shielded” by the insulation chamber, is reduced. A solar collector film can, in turn, be attached on the surface of the outer balloon or an area thereof. 
     It would certainly be possible to form two low or high-pressure insulation chambers around the inner balloon  11 , in that the outer balloon would have an outer sheath and an inner sheath between which the one, first insulation chamber, preferably filled with xenon or krypton, would be formed. The other low- or high-pressure insulation chamber formed between the inner sheath and the balloon could then be filled with outside air and the air could be released from the insulation chamber via an outflow, in order to keep the pressure constant in this chamber. Accordingly, the pressure and also the height above sea level could then be measured and transmitted to the control unit. 
     Two further possible embodiments of the aircraft  1 ′ according to the invention are indicated in  FIG. 3 . 
     In these variants, firstly, a chamber  20 ′ arranged between the inner perimeter of the outer balloon  12  and the outer perimeter of the balloon  11  and extending helically around the balloon is shown, which is delimited by transversal sections  50 . 
     Secondly, a chamber  20 ″ arranged at the inner perimeter of the outer balloon  12 , again helical, extending around the balloon  11  at a distance, can be formed, which is made from one or more envelopes  50 ′ with an approximately rectangular cross-section. 
     In both cases, these chambers  20 ′,  20 ″ are filled with a gas, for example xenon or krypton, with a low thermal conductivity and thus the low- or high-pressure insulation chamber is formed at least partially around the balloon  11 . 
     These chambers  20 ′,  20 ″ are only shown over part of the entire perimeter of the balloon. Obviously, either one or other chamber would be provided over the entire, or almost the entire, perimeter. 
     Similarly to the variants according to  FIG. 1  or  2 , the outer balloon  12  can in turn be provided with the solar collector film and the infrared collector film with which the solar radiation during the day and the infrared re-radiation from the earth at night are energetically exploited. In turn, the transversal sections  50  of these envelopes  50  or  50 ′ preferably then advantageously consist—as with the two balloons  11 ,  12 —of a transparent plastic material. 
     In the case of the embodiments indicated in  FIG. 3 , however, both the outer balloon and the inner balloon  11  could be made from an aluminised plastic. 
     A further possibility lies in the arranging, instead of helical chambers  20 ,  20 ″, of a number of connected pocket- or cushion-shaped chambers which could be filled with a gas having low thermal conductivity, preferably xenon or krypton, around the balloon  11 , over its entire perimeter or at least over most of it. These could in turn at least partially fill the space between the inner balloon  11  and the outer balloon  12  or be arranged on the inner perimeter of the outer balloon  12 , at a distance from the inner balloon  11 . A suitable material for these pocket- or cushion-shaped chambers is the aluminised plastic already mentioned, preferably polyethylene. 
     Since the gas pressure in the balloon  11  of the aircraft  1  or  1 ′ according to the invention is kept largely constant and/or can be effectively regulated and is not exposed to the extreme day/night temperature differentials, the aircraft can remain in operation for substantially longer, and can better maintain its position with respect to the earth (or with respect to a specific area on the earth) than is the case with ordinary balloons. 
     The aircraft  1  is obviously equipped with a complete control system, so that it automatically places itself in the desired position with respect to the surface of the earth. It is also linked to a control centre on earth, so that data transfer and control options can be conducted from the earth. 
       FIG. 4  shows an unmanned aircraft which is designed, per se, identically to that in  FIG. 1 . For the unaltered parts, therefore, the same reference numbers are used. The outer balloon  12  and the gas-filled balloon  11  arranged inside this, supporting the platform, are present. In the inner balloon  11  there is at least one additional balloon  31  with an inlet and an outlet valve for letting gas in or out, preferably air. With this additional balloon  31 , a constant pressure is generated in the balloon  11  enveloping it. To this end, a corresponding pressure regulator is provided in the balloon  31 , not shown in more detail, in which a pressure measurement is taken in the inner balloon  11 . The air can be let out of the additional balloon  31  by means of a controllable outlet or inlet valve or let in via a pump, with the result that the pressure in the inner balloon  11  is kept constant or can be adjusted as required. 
     As a further feature of the invention, the inner balloon  11  and the outer balloon  12  are held together on their underside by connecting means  34 . This produces optimal stability of the aircraft. Also, the additional balloon  31  in the inner balloon  11  is also attached to the latter on its underside. Advantageously, on the underside of the outer balloon, an anodised aluminium layer is provided as outer sheath with which the infrared radiation is intended to be absorbed at night, in order to generate heat in the insulation chamber. 
     The platform  10  is connected, within the scope of the invention, by a connecting element  30  with the underside of the outer balloon  12 . The platform  10  is hereby articulated by a link  33 , indicated, to the outer balloon  12  and detachable from this outer balloon by a coupling, not shown in more detail. As already mentioned, this allows the platform  10  to be brought back to earth following decoupling, while the balloons rise and are destroyed. Advantageously, an electromagnetic coupling is used, enabling release without expensive mechanical devices. 
     It is also shown that, for the purpose of gas circulation, the insulation chamber  20  on the underside of the outer balloon  12  is provided with one or more inlets  36  and on the upper side with one or more outlets  36 ′. This allows optimal cooling of the aircraft during the day.