WO2025003666A1

WO2025003666A1 - Aerostat and method of its manufacture

Info

Publication number: WO2025003666A1
Application number: PCT/GB2024/051640
Authority: WO
Inventors: Peter Davidson; Stewart Huxley
Original assignee: Tethercells Ltd
Current assignee: Tethercells Ltd
Priority date: 2023-06-27
Filing date: 2024-06-27
Publication date: 2025-01-02
Anticipated expiration: 2025-12-27
Also published as: GB202309644D0

Abstract

An aerostat having an elongate hollow body having a length defining a local axis of the body, the hollow body being suitable for receiving sufficient lighter-than-air gas, such that the aerostat can become buoyant in air, the aerostat body comprising a sequence of a plurality of joined body portions, each body portion comprising an outer skin and a hollow internal space, wherein the joins are substantially perpendicular to the local axis of the body, the joined body portions comprising two end body portions, and a plurality of internal body portions, so that the joined body portions together form the elongate hollow body; and a method of manufacturing an aerostat according to any one of the preceding claims, comprising the steps of supporting a first body portion to form a first mounted hollow body portion, supporting a second body portion to form a second mounted hollow body portion, bringing the first and second mounted body portions to come into contact such that the outer skins of the first and second body portions come into alignment, followed by joining together the outer skins of the first and second body portions, followed by repeating the process with further body portions, thereby to produce the aerostat having an elongate hollow body.

Description

Aerostat and Method of its Manufacture

Technical Field

The invention relates to an aerostat having an elongate hollow body having a length defining a local axis of the body, and a method of its manufacture.

Background to the invention

Low latency access to mobile communications and general information services is becoming vital for economic and social well-being. The Covid Pandemic has accelerated this process. New applications (for example, autonomous driving, remote medical support) and more generally the Metaverse, require reliable, low latency and high bandwidth mobile communications.

Compared with satellites, aerostat-supported platforms have several advantages, primarily because the distance from a transmitter to a receiver on Earth can be much less, with geostationary satellites typically at 36,000 km altitude and around 1000 km altitude for a “Low Earth Orbit” or LEO satellite.

This relative nearness of tethered aerostat platforms can result in much stronger signals relayed to Earth and avoid the expense of rocket launches as well as providing shorter development times, and allows power and backhaul connection via the tether.

Recently-developed, lightweight, very large-capacity phased array antennas have the potential to transform global mobile and fixed line connectivity e.g. by delivering cellular telephone services, including linking to the internet. This is dependent upon them being positioned appropriately, and at typically between 200m and 2500m over most geographies. Suitable tethered aerostats are therefore needed to support such antennas, that are reliable in all weathers at moderate elevations, with power and fibre optic cables as well as lighting and lightning protection being part of the tether. Such aerostats need to be situated below commercial aviation traffic, but sufficiently elevated to provide links of up to 80 km range for low population densities, up to 30 km for moderate rural densities and 5 km in urban areas.

Similarly, there are a range of earth observation, meteorological data collection, and astronomical data collection systems which could substantially benefit from being supported by a suitable tethered aerostat system that is reliable in all weathers at elevations at typically between 200m and 22000m.

A tethered aerostat has the potential to be far cheaper than satellite systems with similar or improved functionality: power supply and fibre optic cable links can be supported by or be an integral part of the aerostat tether, avoiding the expensive and limited backhaul systems and power systems required for satellites or aircraft. Furthermore, data latency effects are important to many applications (including but not exclusively, augmented reality, autonomous driving, health care, interactive video games, video conferencing, remote control of UAVs etc.); there are significant problems with latencies offered by satellites, even low earth orbit satellites.

The need for improved mobile connectivity has prompted a resurgence of interest in using alternative delivery technologies rather than ever large numbers of mobile communication masts e.g. low earth orbit satellites, stratospheric platforms and tethered aerostats. All of these solutions have issues; respectively: data capacity and latency, technology readiness level, and wind stability.

Current tethered aerostat systems include tethered rigid and non-rigid airships (blimps) and hybrid balloon/kite systems. Airship-type designs are usually inclined to the horizontal to provide both aerodynamic lift as well as buoyancy. The principle of a hybrid balloon/kite system is that the balloon provides lift in low wind speed conditions and the kite provides lift in high wind speed conditions. Hybrid balloon/kite systems are usually less expensive than airships. However, for such balloon/kite systems in high wind speed conditions the horizontal drag on the balloon is high and the kite therefore needs to be large, incurring considerable drag forces to provide sufficient lift to maintain altitude. These large drag forces then require a very strong and hence, heavy, tether, which has its own associated drag that requires a still larger balloon/kite system to support it, thus reducing payload carrying capacity.

Current tethered airship designs and balloon/kite systems do not survive strong winds of more than 50 knots for smaller systems and for very large systems winds of more than 70 knots or exceptionally 100 knots. For continuous operation at useful elevations of typically over 400m, high wind speeds of over 100 knots will have to be sustained. With high winds, existing systems become unstable, moving uncontrollably and are ultimately blown over.

Because of these effects it has not been possible hitherto to design and build a reliable tethered aerostat system to deliver a high availability service because very high wind conditions will be encountered almost everywhere from time to time, particularly at suitable altitudes needed for many different applications. That, in turn, has meant that any potential use of a tethered kite/balloon aerostat carrying a system that requires high availability and reliability has not been possible.

Lighter than air balloon/kite systems are described in US 2011/0222077A1 with the following examples: US 2,398,745 and US 2,431,938 to Jalbert, US 4,029,273 to Cristofel Jr., in US 6,016,998 to R. Allsopp, and in U.S.Pat.No.6,499,695 to Talamo. In US 6,555,932 to Mizzi, there is described a combined buoyant aerofoil for use in generating electricity with wind power or for aerial advertising. Such combination balloon/kite systems are also available commercially for surveillance and advertising use, such as the SkyDoc TM Aerostat, supplied by Floatograph Technologies LLC, of SilverSpring Md., or the Helikite, supplied by Allsopp Helikites Ltd of Fordingbridge, Hampshire, UK. Such systems have been used for military and civilian use, and the data generated has been described as being conveyed to and from the ground station by means of wireless, cable or optical fibre. US 2018/0050797 Al discloses a conceptual tethered lighter-than-air unmanned aerial vehicle that has a wing and a fuselage. However, no consideration is given to how to maintain such an arrangement so that it is stable in the atmosphere. KR 10-2016-0081328 discloses a buoyant aerostat that is tethered to ground level. However, no consideration for how such an apparatus can be kept stable in the atmosphere is given. US 2016/0122014 Al discloses a conventional blimp attached to the ground by two tethers. US 3,620,292 discloses a known type of wing balloon.

An aerostat system highly stable in extreme weathers for a wide variety of locations with the ability to withstand meteorological challenges including: extreme gusts, lightning strikes; prevention of snow and ice build-up; and safety features to prevent accidents with low flying aircraft, is therefore likely to be valuable in a whole variety of applications including but not limited to those described above.

However, such aerostats have to be large to provide the buoyancy required to lift suitable payloads. Furthermore, significant buoyancy is required to support the tether which has to contain lighting conductors required for all-weather operation at suitable altitudes (above 200m preferably 1000m and most preferably 1500m). Typical required aerostat volumes are at least 300 m³, more preferably above 1000 m³. An example of a vertical buoyant aerofoil is given in international patent application no. PCT/GB2022/053290.

Conventionally large aerostats are transported when uninflated and folded. However, the foldable materials currently in use suffer from significant helium permeability if they are to be suitably lightweight and strong and such aerostats typically need to be topped up with helium every two to six weeks by winching them down to the ground or some other means, which is highly undesirable since the launch and recovery of large aerostats is a significant undertaking requiring suitable weather conditions (moderate wind gusts) and careful management. The time taken will also lead to significant interruptions to communications service. Aerostat skins are therefore required where the strength to weight ratio is as high as possible and where the skin prevents significant helium or hydrogen permeation.

Such skin properties are also required for liquid hydrogen and liquid helium cryogenic tanks for use in aircraft or drones where light weight, high strength and low permeability are also required but with the added requirement of being able to withstand being cooled, preferably repeatedly, to the relevant cryogenic temperatures: typically, between a few Kelvins, and a few tens of Kelvins.

It is well known that helium or hydrogen diffusion rates through metals are far lower generally than through plastics and many attempts have been made to combine fibre reinforced plastic components with thin skins or foils of metals, particularly aluminium, gold and so forth. Typically, fibre reinforced skins are made from woven resin ‘prepregs’ which are composite materials made from "pre-impregnated" fibres and a partially cured polymer matrix, such as epoxy or phenolic resins. These woven ‘prepregs’ have been placed adjacent to thin films of aluminium foil and the system cured, often in an autoclave. Such systems have been highly disappointing - small defects - even as small as a few microns in diameter - in the aluminium foil or joints in the aluminium foil allow the passage of helium or hydrogen and the resin and woven prepreg is not a sufficient barrier to allow a significantly lower diffusion path to helium or hydrogen as compared to existing aluminised plastic systems.

Fibre composite materials with suitable metal barrier technology and jointing technologies as aerostat skins would allow aerostats to be operated for far longer without helium top-up. Furthermore, with an external metal skin to block UV weathering, such aerostat skins have the potential to last for many years if they could be realised. However, such aerostats would be bulky and unfoldable, presenting significant challenges for manufacture and transport. Summary of the Invention

In a first aspect, the invention relates to an aerostat having an elongate hollow body having a length defining a local axis of the body, the hollow body being suitable for receiving sufficient lighter-than-air gas, such that the aerostat can become buoyant in air, the aerostat body comprising a sequence of a plurality of joined body portions, each body portion comprising an outer skin and a hollow internal space, wherein the joins are substantially perpendicular to the local axis of the body, the joined body portions comprising two end body portions, and a plurality of internal body portions, so that the joined body portions together form the elongate hollow body.

By “buoyant in air” is meant that the aerostat is lighter-than-air, e.g. wherein the buoyancy force is greater than the weight of the aerostat.

By “local axis of the body” is meant a line connecting adjacent centroids of cross sections of the body.

Preferably the hollow joined body portions comprise an outer skin comprising a layer of metal foil, sandwiched with a first layer of unidirectional fibres embedded in a cured resin matrix and a second layer of unidirectional fibres embedded in a cured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres. Preferably the metal foil is the outermost layer of the outer skin.

It has been discovered that having the metal foil on the outer surface of the aerostat skin provides several significant benefits:

(a) Metal foils, particular those of aluminium have a high reflectivity. This high reflectivity reduces the impact of solar radiation on the aerostat temperature. Diurnal variation in aerostat temperature can cause significant pressure variation in aerostats leading to helium loss or increased envelope design pressures in constant volume aerostat systems. An aerostat with a reflective metal foil as the outermost layer such as aluminium foil, compared with conventional plastics can lead to a reduction of a factor of two to four of the aerostat diurnal temperature variation.

(b) Metal foils, particularly aluminium have robust atmospheric lives being particularly insensitive to moisture and UV promoted degradation. Conventional aerostat skins typically have operational lives measured in months. Aluminium foils have lives measured in decades or more.

Such an arrangement has been found to provide for a rigid outer skin that is highly impermeable to hydrogen and/or helium, whilst also being of relatively low weight. The use of unidirectional fibres allows a greater fibre-to-resin content, which gives a greater str ength-to- weight ratio.

Preferably the outer skin comprises an outermost layer of metal foil and an innermost layer of metal foil, sandwiching between them the two layers of unidirectional fibres embedded in a cured resin matrix.

To manufacture the vessel from the skin panels, a jointing system is needed to suitably join the body portions. It has been discovered that it is possible to make joints that can provide minimal hydrogen and/or helium gas leakage and be extremely strong without significant weight increase of the skin, by having butt joints with skin elements on one or both sides of the butt joint. Thus, preferably at least one pair of adjacent body portions butt together to form a contiguous region of skin with a joining line therebetween, wherein at least one further skin panel is placed to cover both the innermost and/or outermost facing of the joining line.

Such an arrangement provides an effective seal against hydrogen and/or helium leakage, as the join is covered by one or two further skin panels. The main principle of such joint design is that gas diffusion or flow through the join encounters one or more metal foil surfaces, or must travel longitudinally through the further pair of skin panels (as opposed to through the thickness thereof), which provides a much longer route and therefore much greater sealing. Preferably the adjacent skin panels have a joining region, adjacent to the joining line, wherein there is no layer of metal foil. This is so that, in the joining region, a continuous resin seal may be made by the placement of the further skin panels leading to greater strength of the joint.

Preferably, the further pair of skin panels do not have an innermost layer of metal foil. This is for similar reasons, to allow a continuous resin seal in the joining region.

It has been found that each joining region extends for a length from the joining line that is from 5 to 150 times the thickness of the outer skin outside the joining region, provides an effective hydrogen and/or helium seal.

Alternatively, joints may be formed as overlap joints, wherein at least one pair of adjacent body panels overlap each other to form a contiguous region of outer skin with an overlap region, wherein at least one further skin panel is placed to cover the overlap region.

In such overlap joints, one or both of the adjacent body portions may have some metal foil removed in the overlap region adjacent to the joining overlap panel. At least one further skin panel covering the joint may not have an continuous layer of metal foil so as to allow better load transfer between one body panel and another.

This arrangement of outer skin provides a very effective helium sealing with minimal weight and thickness of skin. Therefore, preferably each skin panel has a thickness of from 0.1 to 5mm, preferably from 0.1 to 3mm, more preferably from 0.2 to 2mm.

Preferably the metal foils are any metal that is malleable and stable in air, such as aluminium or gold, but preferably aluminium. The metal foils are typically 0.5 to 200 microns thickness, preferably 5 to 40 microns thickness, most preferably 8 to 25 microns thickness. Such foils are manufactured by rolling and often have small holes offering flow paths through their thickness. It has been found that plugging such holes by resin is desirable but not essential, and this process happens naturally if sufficient curing pressure and resin is used. Too much resin is undesirable since to achieve a high strength to weight ratio, a low volume fraction of resin compared with fibre is to be preferred, consistent with having minimal gas voidage in the resin/fibre system.

Therefore, preferably the volume of cured resin matrix is from 15 to 100%, preferably from 30 to 70%, more preferably from 45 to 70%, of the volume of unidirectional fibres, to provide good strength to weight ratios and good barrier properties.

The resin may be any suitable thermally curable thermoplastic resins suitable for use in a fibre composite material, such as an epoxy resin.

Preferably, the angle between the first layer of unidirectional fibres and the second layer of unidirectional fibres is at least 10°, preferably at least 25°, more preferably from 45 to 90°, which allows the strength of the skin to be tailored to the magnitude of the maximum stresses requirements in different directions.

Preferably, the unidirectional fibres are carbon fibre or Kevlar™. Prepreg carbon fibre or Kevlar weights of 30 grams per square meter to 500 grams per square meter can be used.

Gas voidage, e, is defined as the fraction of the total volume of resin / fibre system which is free space available for the flow of fluids, in this case helium or hydrogen. Preferably the cured layers of unidirectional fibres embedded in a cured resin matrix have a gas voidage of less than 5 vol%, preferably less than 1%, most preferably less than 0.1%.

It is can also be advantageous for a plurality of the internal body portions to be essentially identical, i.e. being of the same geometry. This produces a manufacturing simplification. The aerostat may comprise from 5 to 500 internal body portions, as desired. Each internal body portion may have a dimension in the direction perpendicular to the length of the body of from 30 to 4000 cm. For example if the body portions are cylindrical they would have a diameter of 30 to 4000 cm.

The aerostat may be located at ground level ready to be launched, or it may be located buoyantly in the atmosphere at an elevated location. Preferably the elevated location is at an altitude of from 100m to 10,000m, more preferably from 200m to 5,000m, most preferably from 250m to 3,000m.

Preferably the aerostat comprises a tether connecting the aerostat to a substantially ground level location.

The local axis of the body in use (i.e. when buoyant in the air) may be substantially horizontally or vertically oriented. If vertically oriented, the length extends between an upper end and a lower end in use, the two end body portions providing both the upper end and lower end of the body. However, whilst being manufactured, the local axis is preferably horizontal, although a vertical orientation is also feasible.

It has furthermore been found to be particularly advantageous when the aerostat is shaped as an aerofoil. Therefore preferably the elongate body, has a horizontal cross-section at each point throughout substantially the entire length that is an aerofoil, providing a leading edge and a trailing edge, and defining between them, for each horizontal cross-section, a chord line between the leading edge and the trailing edge of the cross-section, having a chord length.

Optionally, the elongate body may take a swept-back or swept-forwards arrangement to provide appropriate mass distribution and stability. Thus, preferably the aerostat comprises regions that are oriented at an angle to the vertical. A swept-back or swept-forwards arrangement can be provided for when the leading edge and trailing edge, in the regions that are oriented at an angle to the vertical, are located within the single vertical plane of symmetry, parallel to the wind direction in use.

A low drag aerostat is highly preferred for such an aerostat to operate in high winds without an excessively strong and therefore heavy tether. If the aerostat has a higher drag then the aerostat needs to be larger to carry the weight of the heavy tether and pay load at high winds. A larger aerostat is more costly and less economic. It has been discovered that for practical utility in being able to carry substantive payloads in high winds of more than 30 metres per second, a drag coefficient of less than 0.35, preferably less than 0.2, more preferably less than 0.06 is required.

In this application, the drag coefficient of a tethered aerostat is defined as CD = F / (’A u² A) , where F is the horizontal aerodynamic drag, p is the air density , u is the horizontal component of wind velocity and - as is conventional in aerofoil theory, well known to those skilled in the art, A is the plan area when seen looking at the aerostat horizontally perpendicular to the wind. The leading edge at any vertical level, is defined by the point which first meets oncoming air, and the trailing edge at any level, the point which last meets oncoming air.

Preferably the aerostat has a length of from 5 to 500m.

In a second aspect, the invention relates to a method of manufacturing an aerostat as described herein, comprising the steps of supporting a first body portion to form a first mounted hollow body portion, supporting a second body portion to form a second mounted hollow body portion, bringing the first and second mounted body portions to come into contact such that the outer skins of the first and second body portions come into alignment or overlap, followed by joining together the outer skins of the first and second body portions, followed by repeating the process with further body portions, thereby to produce the aerostat having an elongate hollow body. The systems therefore allow the mounted body portions to be raised from the ground and positioned correctly with their weight and shapes supported, so that the joins can be performed conveniently.

It is important that the spacing between systems is controlled so that the outer skins line up to within under three skin thickness, preferably under two and most preferably under one to ensure good joints. If the skins are not lined up wrinkles or other defects will ensue giving rise to poor joint quality and loss of gas containment.

The hollow body portions are preferably made by laying-up metal foils and prepreg comprising uncured resin coated unidirectional fibre layers on a solid former with a release film if necessary and vacuum bag surround before being cured at the appropriate resin curing temperature and time, as is well known to those skilled in the art of fabrication of composite components. The former can be flat or curved to form elements of the aerostat surface or tank.

Each hollow body portion may be cured as a single formed item, or cured in smaller components and assembled accordingly. This makes it easier for each hollow body portion to be cured in an autoclave, which can then reach very high pressures, in order to reduce the voidage to acceptable levels. Curing in an autoclave with pressures of up to 8 Barg provide minimises hydrogen and/or helium permeability.

In the “laying-up” or placing process it is important that the new layer is laid down without crinkles or inclusions or air as is well known to those skilled in the art. If a surface is curved in only one direction, so the minimal principal curvature is zero (see Gauss, Theorema Egregium), and the maximum principal curvature is non-zero, thin surfaces can be laid down easily. However, if the surface has two non-zero principal curvatures, the surface will wrinkle unless small elements are used with modest size compared to the minimum principal curvature. It has been discovered that the tolerance of line up shall be preferably less than three times the skin thickness, more preferably less than two times the skin thickness and most preferably less than the skin thickness.

In a first embodiment of the second aspect of the invention, the first body portion is placed in a first support frame to form the first mounted hollow body portion, and the second body portion is placed in a second support frame to form the second mounted hollow body portion. Such support systems may be external frames supported on a floor, ceiling or wall. To achieve a good edge line up and edge quality, the spacing between the supporting frames must be sufficiently close to provide suitable support, but sufficiently far apart to allow for the placement of the jointing strips.

In this embodiment, internal tension ties may be attached to the interior of the outer skins of the first and second body portions, before further body portions are joined thereto. In this manner, easy access is gained to the interior as the aerostat is built-up portion by portion.

Preferably the support frames each comprise at least one support rim that is parallel to the join, the support rim being in contact with a perimeter of the outer skin of the mounted body portion in the vicinity of the join. Thus, the body portion can sit mounted within the support rim and firmly held in place. Preferably the body portions are initially held loosely by the support rim, to allow their position to be adjusted, before being held tightly when jointing is being undertaken.

In a second alternative embodiment of the second aspect of the invention, the first body portion is placed around a first support mould portion to form the first mounted hollow body portion, and the second body portion is placed around a second support mould portion to form the second mounted hollow body portion. Such support moulds are interchangeably referred to as mandrels in the context of the present invention. Such a method may involve a mandrel providing an internal support surface for the placement of a body portion. Such mandrels may preferably have a centroid on the local axis. Each support mould portion may joined together, can be separate or portions of a larger single mandrel.

Preferably a means of providing external pressure with hoops or external pneumatic tubes is provided to put pressure on the skin joints as well as heating arrangements to make joints whose axis is at right angles to the local axis or locally close to the skin surface.

It has been discovered that a mandrel support system is particularly suitable for the manufacture of aerostat envelopes of circular axisymmetric cross - sections with a common local axis. Circular cross sections are to be preferred since they allow the internal pressure of the aerostat to support the envelope skin without the need for internal structure such as ties to form the aerostat envelope into a desired non-circular shape.

Typically, a mandrel system consists of one or more mandrels of circular cross section with a common axis. Cylindrical or conical or tapered mandrels can be used to achieve the desired aerostat shape.

Preferably the support moulds or mandrels, are adjustable, such that they are expandable and contractable to provide a variable size of support mould. Such a mandrel system, with adjustable diameter, allows the forming of a body portion adjacent to a series of previously formed body portions by sliding the mandrel inside the previously formed body portions, allowing excellent line-up of adjacent body portions.

Any cylindrical or tubular series of body portions will need tapered closures at both ends of the cylinder or variable diameter tube. Such an arrangement requires that at least one of the mandrels shall be constructed in a fashion that allows disassembly and extraction through an open-end cap that provides envelope pressure containment when in place.

The invention will now be illustrated, by way of example with reference to the following figures. Figure 1 shows a plan and a side sectional view of an aerostat according to the present invention.

Figures 2a and 2b show plan views and side sectional views of two further aerostats according to the present invention.

Figures 3a and 3b show plan views and side sectional views of two further aerostats according to the present invention.

Figure 4 is a perspective view of an end body portion in a support frame, for use in the method according to the present invention.

Figure 5 is a perspective view of the end body portion in a support frame according to figure 4, with a second support frame in place, for use in the method according to the present invention.

Figure 6a is a perspective view of the arrangement shown in figure 5 wherein an internal body portion has been placed within the second support frame, and figure 6b shows the detail of the joining process between the body portions.

Figure 7 is a perspective view of the arrangement shown in figure 6 following joining of the body portions wherein a number of ties have been placed within the hollow internal space of the body portions.

Figure 8 is a perspective view of the body portions shown in figure 7, after a number of additional internal body portions have been added and joined together, according to the method of the present invention.

Figure 9 is a perspective view of two completed halves of an aerostat produced according to the method of the present invention, prior to being joined together to form the aerostat. Figure 10 is a perspective view of an aerostat according to the present invention formed by joining together the two halves shown in figure 9.

Figure 11 shows a number of views of an axisymmetric aerostat envelope formed by the present invention with a straight-line local axis. It shows a 3D perspective without joins on the skin visible, and the key body portion assemblies.

Figure 12 shows a number of views of another axisymmetric aerostat formed by the present invention and more detail on possible jointing arrangements.

Figure 13 shows a perspective and end view of a compressed mandrel for forming cylindrical body portions.

Figure 14 shows the uncompressed mandrel for forming cylindrical body portions shown in figure 13 with a wedge section inserted.

Figure 15 shows views showing the formation of a cylindrical body section on such a mandrel.

Figure 16 shows side views of a process of formation of a number of cylindrical body sections.

Figure 17 shows perspective views of the formation of a tapered body section joined to a cylindrical section.

Turning to the figures, figure 1 shows an aerostat having an elongate hollow body of a substantially constant aerofoil cross section and a substantially straight leading edge 108 and a substantially straight trailing edge 109, according to the present invention. The aerostat has two end body portions 101 (the length of the body being defined between these end body portions) and a plurality of essentially identical internal body portions 103 with parallel sides 105, which are joined together with joins 102 such that the sides 105 and the joins 102 are substantially perpendicular to the length of the body. In the centre are two central body portions 104.

The aerostat skin 106 is slightly undulating, by virtue of the internal structure holding the aerostat skin, however a smooth exterior is also preferred. As the internal body portions 103 are identical, the aerostat has a uniform cross section 107, shown as sections AA and BB, aside from the end body portion 101, and to a limited extent in the central wedge sections 104.

The skin of the hollow body is made from an outermost layer of metal foil and an innermost layer of metal foil, the metal foils sandwiching between them cured resin and fibre layers, or alternatively a one or more metal foils and cured resin and fibre layers.

Such an aerostat may be filled with hydrogen and/or helium, and launched to adopt a position at an elevated location in the atmosphere, and may be tethered to a ground level location. Such an aerostat may comprise an antenna, e.g. a phased array antenna, if it is to be deployed for telecommunications purposes.

Figure 2a shows a plan view (and side sectional view) of a further aerostat according to the present invention that is similar to that shown in figure 1 but with some curvature on the leading edge and trailing edge. Figure 2a shows an aerostat 200 with pronounced undulations in the skin. Figure 2b shows essentially the same aerostat 203 as shown in figure 2a but wherein the skin is smooth with minimal undulations in the skin. As the internal body portions are not identical, the cross sections 201, 202, 204, 205 vary along the length of the aerostat. Cross sections 201, 204 refer to sections at AA and cross sections 202, 205 refer to sections at BB respectively.

The advantages of such a system over the system shown in figure 1 , is that the aerodynamic drag for such an aerostat can be beneficially reduced for a given aerostat area. However, manufacturing such a system is more expensive with variable size of joints or edges 206 requiring more sophisticated and expensive support systems during the process of manufacture. Furthermore, each section 207 is slightly different from its neighbour requiring skins to be made with non-parallel edges, with different skin circumferential lengths, the section edges 206 not being parallel.

Figure 3a is a plan view (and side sectional views) of a further aerostat 300 according to the present invention with a variable cross section and curved leading edge and trailing edge. In figure 3a the aerostat 300 has pronounced undulations in the skin. Figure 3b shows the same aerostat 303 but with minimal undulations in the skin. Cross sections 301, 304 refer to sections at AA and cross sections 302, 305 refer to sections at BB respectively.

The advantages of such a system over the system shown in figures 1 or 2, is that the aerodynamic stability can be improved for a smaller horizontal aerostat size which can make transport easier, but it has further significant drawbacks over the systems shown in figures 1 and 2 in having a much greater manufacturing complexity.

The aerostats according to the present invention may be manufactured in the following exemplified manner.

Figure 4 is a perspective view of an end body portion 500 in a support frame 501, at the start of a method of manufacture according to the present invention. The frame 501 comprises a support rim 502 into which the end body portion 500 snugly sits and an open lattice structure 503. The frame 501 can be disassembled and slid out from the aerostat once manufacture is completed.

The support rim 502 tightly engages with a perimeter 504 of the end body portion 500 to within an accuracy of less than 1 mm, preferably less than 0.3 mm, which results in a region of the end body portion 500 projecting out of the plane of the frame 501. This is to allow a joint to be made all along the edge 504, as will be described. The end body portion 500 projects from around 10 mm to 50 mm from the frame 501, depending on the detailed design of the edge of the end body portion. As shown in figure 5, the next step in the process is to place a second support frame 600 (which comprises two rectangular frame portions 601, 602) adjacent to the first support frame 501. Second support frame 600 comprises two support rims 603, 604, to receive an internal body portion, as shown in figure 6.

In figure 6 the end body portion 500 is held in position by support rim 502, which is attached to frame 501, in turn in contact with the second support frame 600. Placed within the two support rims 603, 604 is an internal body portion 704 made from a cured composite of fibres and resin, in this case from a number of portions 706 joined at joins 705. The internal body portion 704 is a continuous smooth strip with the minimum principal curvature being close to zero at any point on the strip surface and having a constant cross section in planes parallel to an axis, in this case shown as AA and a constant width.

A butt joint is shown in which further skin panels 708, 707 are placed on the inside and outside to cover the joining line of the butt joint (see figures 6b and 6c). The spacing between frames 501 and 601, 602 is controlled so that the skins of the internal body portion 704 and end body portion 500, align with each other to within three skin thickness, preferably under two and most preferably under one to ensure good joints. It can be appreciated that with a large aerostat where the chord length may be over 10m, the line-up of skins to within one skin thickness where the skin thickness may be under 0.5mm thickness, such line up poses challenges.

If the skins are not lined up, wrinkles or other defects will ensue, giving rise to poor joint quality and loss of gas containment. To achieve a good skin alignment and join quality, the spacing between the support rims 502, 603 must be close enough to provide suitable support, but sufficiently far apart to allow for the placement of the further skin panels 707 and 708.

Appropriate means of attachment of the skins 704, 700 should allow an initial placement where movement of the skin relative to its support or slip can be arranged, followed by a means of reducing slip to negligible levels. This can be achieved by pulling a vacuum between the frame and the sheet surface, by magnetic attachment, or by adhesion with tapes or glues that can be removed later in the process.

Once the join is completed, internal “curtains” 801 of tension ties within the hollow internal space between opposite faces of the skin of the body portions are introduced, as shown in figure 7. The curtains 801 are attached to the internal surface of the skins normally in lines parallel to the axis of the aerostat AA, and normal to the long chord of the aerostat to provide control of the skin shape under pressure when the aerostat is in use. The relatively thin section 802 allows access for the insertion of the curtains through the open side of 800.

Additional support frames and internal body portions can then be brought into contact and joined, to produce the arrangement shown in Figure 8, which is almost one half of a completed aerostat. The partially completed aerostat skin assembly 900 is supported on a number of frames 901 with a number of joints 902 regularly spaced.

Figure 9 shows two half aerostats 1000 and 1001 (of dissimilar lengths in this case, but they could be similar) immediately prior to final jointing of edges 1003 and 1004, supported on frames 1002. The final joint has an additional inward protrusion like a small flange so that a good joint can be made without internal access, the flange providing extra stiffness to the surfaces allowing an external skin to be pressed onto the surface and the joint sealed by thermal curing or by gluing. Additional strength can be achieved by gluing the flanges together.

Figure 10 shows the final aerostat skin or envelope jointed assembly, and all that remains is for the final support frames to be removed. Such a resulting aerostat is then suitable for being filled with hydrogen and/or helium gas so that it becomes buoyant in air, so that it can adopt a position at an elevated location in the atmosphere with the upper end substantially vertically positioned above the lower end, and preferably has a tether attached connecting the aerostat to a substantially ground level location. Figure 11 shows a details of an axisymmetric aerostat envelope 1100 formed by the present invention with a straight-line local axis 1116. Figure I la shows a cut away diagram of the envelope skin made by the present invention without detailing the individual skin joins.

Figure 11b shows a side elevation of the axisymmetric aerostat detailing the shape of transition body portions 1110, 1112 (not shown in Fig 11b) and the position of tails 1102 for aerostat stability, and winglets 1106 if needed and the position of a pay load attachment 1105. Figure 11c shows a 3D exploded illustration of the axisymmetric aerostat skin 1100. The tails 1102 are preferably manufactured as a separate pressurised hollow vessel according to the present invention.

The essence of this aerostat shape is to have the majority of the shape volume and hence buoyancy provided by the central cylindrical section 1101 made up of cylindrical body portions, and end conical body portions 1109 and 1114, all of which have surfaces curved in only one direction, so the minimal principal curvature is zero and the maximum principal curvature is non-zero and can be formed from flat sheets of skin. These are examples of shapes where thin surfaces can be laid down easily and is easier to make by the method of this invention than continuously variable cross sections of conventional aerostats. However, to achieve low drag coefficients which are desirable in high - performance aerostats, suitable transition body portions are manufactured to ensure minimal boundary layer separation occurs when the external wind direction is substantially parallel to the straight-line local axis. In this example that is achieved by transition body portions that are part of a spherical surfaces 1103, 1104 as shown in the side elevation of Fig 1 lb. For low aerostat drag transition body portions can also be of ellipsoidal shape or where one axis of the ellipsoid is along the local axis of the asymmetric body 1116 which is substantially parallel to the external air flow direction 1117.

In figure 11c the body portions the external wind passes over, are, in order, the nose cap 1108 fabricated as a single small element in a suitable mould, which is joined according to the current invention to a conical end body portion 1109 manufactured from suitably cut and joined flat skin elements curved along one principal axis, which in turn is joined to a 1 front transition section 1110 made up from cast sheets 1111. The cylindrical body portions form 1101 which is in turn joined to the rear transition body portion made up of curved sheets 1113, the rear transition section being joined to a rear conical body portion 1114 which is joined to a small rear end cap 1115. The conical sections can be of several included angles, for example the body sections between the front-end cap 1108 and the transition section 1110 may be assembled from several cones with transition sections in between to allow for a better approximation to a spherical nose profile. Such a spherical nose profile can allow for better aerostat drag performance.

Figure 12 shows a 3D example of such an aerostat skin 1200 and a plan view 1201, with a front elevation 1202, and rear elevation 1203, and one of several frames 1204 used in the manufacture. Individual cylindrical body portions 1205 are shown, and the overlap joints of these 1206 where relatively narrow bands of skin are used to provide strength and permeation barriers. Overlap joints in the transition 1207, and conical 1208 body portions are shown.

Figure 13 shows a mandrel acting as a support mould for forming cylindrical body portions. 1300 is a 3D view, and 1301 a view along the local axis 1302. The mandrel is mounted on rails 1303 parallel to the local axis. The mandrel surface 1304 on which skin elements are laid down and formed is expandable and contractable to provide a variable size of support mould, shown here with an open slot 1306, into which a moveable wedge can be inserted allowing the mandrel external surface to become a complete cylinder on which the skin can be formed. When the wedge is withdrawn the mandrel surface can relax or contract and withdraw from intimate contact with a previously formed skin surface. Such lack of intimate contact allows the mandrel surface to be moved along the local axis without causing significant shear forces on the skin surface.

Figure 14 shows similar views as figure 13 but with the wedge section inserted.

Figure 15 shows the formation of a cylindrical body section on such a mandrel. 1501, 1503, 1505 and 1507, are sections through the mandril - and where appropriate skin - and match respectively the 3D views 1500, 1502, 1504 and 1506. 1500 and 1501 show a mandrel in a contracted form, 1502 and 1503 show the same mandrel in the expanded form ready to receive a skin element. The next diagrams 1504 / 1505, show the placement of the body portion 1508. The body portion would preferably be manufactured in a flat form of layers of foil and fibre reinforced composite as previously described. The length of the body portion - which could be manufactured by the assembly and jointing of a number of elements on a flat surface - would typically be the circumferential length of the cylindrical body portion of the mandrel or aerostat - plus an overlap length. Alternatively, a double overlap joint can be provided with suitable indentations on the mandrel surface. 1506 and 1507 show a compressing band 1509 being applied and tightened with tensioner 1510. Heating systems for the overlap joint are not shown but are well known to those skilled in the art. Alternative means of providing external compressive force on the skin are pneumatic systems, hydraulic systems and other mechanical arrangements. A key feature of the mandrel is that it should have suitable strength to prevent excessive movement or bucking when subjected to the appropriate external compressive loads to make good joints.

Figure 16 shows the formation of a number of cylindrical body sections. 1600 shows two mandrels 1694 and 1605, 1601 shows a skin element 1606 placed on mandrel 1604, 1602 shows the same skin element 1606 now having been formed now as body section 1607 and an adjacent body elements 1608, and the body element 1609 being compressed onto the mandrel 1605, which has been moved along the rails by a process of forming, contraction, re-siting, expansion, forming and so forth.

Figure 17 shows the initial formation of a tapered mandrel to allow firstly - as shown in 1701 - a transition body section to be formed, and then a conical tapered section in 1702. In this system the left-hand mandrel for forming the conical section - 1604 in figure 16 has been removed - either through the large mandrel 1605 or through the smaller aperture in the tapered mandrel.

Claims

1. An aerostat having an elongate hollow body having a length defining a local axis of the body, the hollow body being suitable for receiving sufficient lighter-than-air gas, such that the aerostat can become buoyant in air, the aerostat body comprising a sequence of a plurality of joined body portions, each body portion comprising an outer skin and a hollow internal space, wherein the joins are substantially perpendicular to the local axis of the body, the joined body portions comprising two end body portions, and a plurality of internal body portions, so that the joined body portions together form the elongate hollow body.

2. An aerostat according to claim 1, wherein the length extends between an upper end and a lower end in use, the two end body portions providing both the upper end and lower end of the body.

3. An aerostat according to claim 1 or claim 2, wherein the outer skin comprises a layer of metal foil, sandwiched with a first layer of unidirectional fibres embedded in a cured resin matrix and a second layer of unidirectional fibres embedded in a cured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres.

4. An aerostat according to claim 3, wherein the metal foil is an outermost layer of the outer skin.

5. An aerostat according to claim 3, wherein the skin comprises an outermost layer of metal foil and an innermost layer of metal foil, the metal foils sandwiching between them the cured resin and fibre layers.

6. An aerostat according to any one of the preceding claims, wherein at least one pair of adjacent joined body portions butt together to form a contiguous region of skin with a joining line therebetween, wherein at least one further skin panel is placed to cover the innermost and/or the outermost facing of the joining line, the position of the at least one further skin panel providing a joining region.

7. An aerostat according to claim 6, wherein the adjacent joined body portions have some or all of the layer of metal foil removed within the joining region.

8. An aerostat according to claim 6 or claim 7, wherein the at least one further skin panels do not have an innermost layer of metal foil.

9. An aerostat according to any one of claims 6 to 8, wherein each joining region extends for a length from the joining line that is from 5 to 150, preferably 5 to 50 times the thickness of the outer skin outside the joining line.

10. An aerostat according to any one of the preceding claims, wherein at least one pair of adjacent joined body portions overlap each other to form a contiguous region of outer skin with an overlap region, wherein at least one further skin panel is placed to cover the overlap region.

11. An aerostat according to claim 10, wherein one or both of the adjacent body portions has some metal foil removed in the overlap region.

12. An aerostat according to claim 10 or claim 11, wherein the at least one further skin panels does not have an innermost layer of metal foil.

13. An aerostat according to any one of the preceding claims, wherein each joined body panel has an outer skin thickness of from 0.1 to 5mm, preferably from 0.1 to 3 mm, more preferably 0.2mm to 2mm.

14. An aerostat according to any one of the preceding claims, wherein a plurality of the internal body portions to be essentially identical.

15. An aerostat according to any one of the preceding claims, wherein at least 70% of the surface area of the body portions have a minimal principal curvature of zero, and the maximum principal curvature is non-zero.

16. An aerostat according to any one of the preceding claims, which comprises from 5 to 500 internal body portions.

17. An aerostat according to any one of the preceding claims, wherein each internal body portion has a dimension in the direction perpendicular to the length of the body of from 30 to 4000 cm.

18. An aerostat according to any one of the preceding claims, positioned at an elevated location.

19. An aerostat according to any one of the preceding claims, which comprises a tether connecting the aerostat to a substantially ground level location.

20. An aerostat according to any one of the preceding claims, wherein the elongate body, when the upper end is substantially vertically positioned above the lower end in use, has a horizontal cross-section at each point throughout substantially the entire length that is an aerofoil, providing a leading edge and a trailing edge extending between the upper end and lower end, and defining between them, for each horizontal cross-section, a chord line, between the leading edge and the trailing edge of the cross-section, having a chord length.

21. An aerostat according to any one of the preceding claims, wherein the drag coefficient as defined by the ratio of the aerodynamic drag compared to the dynamic pressure over the aerostat plan area when viewed horizontally normally to the wind direction is less than 0.35, preferably less than 0.2, more preferably less than 0.06.

22. An aerostat according to any one of the preceding claims, wherein the aerostat’s length is from 5 to 500m.

23. An aerostat according to any one of the preceding claims, wherein the body portions are formed from strips of material that are joined together at their ends.

24. An aerostat according to any one of the preceding claims, wherein the elongate body, when the upper end is vertically positioned above the lower end in use, comprises regions that are oriented at an angle to vertical.

25. An aerostat according to claim 24, wherein the aerostat body is an aerofoil and wherein the leading edge and trailing edge in the regions that are oriented at an angle to the elongate axis are located within a single vertical plane.

26. An aerostat according to claim 25, wherein the trailing edge is further from a vertical line extending between the highest and lowest point than is the leading edge.

27. An aerostat according to any one of the preceding claims, which comprises tails manufactured as a separate pressurised hollow vessel.

28. A method of manufacturing an aerostat according to any one of the preceding claims, comprising the steps of supporting a first body portion to form a first mounted hollow body portion, supporting a second body portion to form a second mounted hollow body portion, bringing the first and second mounted body portions to come into contact such that the outer skins of the first and second body portions come into alignment, followed by joining together the outer skins of the first and second body portions, followed by repeating the process with further body portions, thereby to produce the aerostat having an elongate hollow body.

29. A method according to claim 28, wherein the outer skins are in alignment such that the outer skins are aligned to within under three skin thickness, preferably under two and most preferably under one.

30. A method according to claim 28 or claim 29, wherein the outer skin of the body portions before joining comprises a layer of metal foil, sandwiched with a first layer of unidirectional fibres embedded in a cured resin matrix and a second layer of unidirectional fibres embedded in a cured resin matrix, wherein the second layer of unidirectional fibres are oriented at an angle to the first layer of unidirectional fibres.

31. A method according to claim 28, comprising the steps of placing the first body portion in a first support frame to form the first mounted hollow body portion, placing the second body portion in a second support frame to form the second mounted hollow body portion.

32. A method according to claim 31, wherein internal tension ties are attached to the interior of the outer skins of some or all of early body portions, before further body portions are joined thereto.

33. A method according to claim 31 or claim 32, wherein the support frames each comprise at least one support rim that is parallel to the join, the support rim being in contact with a perimeter of the outer skin of the mounted body portion in the vicinity of the join, preferably spaced from 10 to 50mm from the join.

34. A method according to claim 33 where body portions are initially held loosely by the support rim, to allow their position to be adjusted, before being held tightly when jointing is being undertaken.

35. A method according to claim 28, comprising the steps of placing the first body portion around a first support mould portion to form the first mounted hollow body portion, placing the second body portion around a second support mould portion to form the second mounted hollow body portion.

36. A method according to claim 35, wherein the support moulds are axisymmetric about the local axis of the body.

37. A method according to claim 35 or claim 36, wherein the support moulds are mounted on rails parallel to the local axis.

38. A method according to any one of claims 35 to 36, wherein the support moulds are adjustable, such that they are expandable and contractable to provide a variable size of support mould.