Here is an English translation of French Patent laying-open publication 2,684,952 to Giraud, "Aerostats using Steam as Main Lift Gas"..... translated (roughly) by myself...



(Translator's note: the writer has used the expressions "water vapor" and "steam" promiscuously and synonymously throughout; in any case, what is meant is steam, i.e. H2O in the gaseous state, at a temperature below its critical point, which strictly speaking is a vapor, not a gas).

FRENCH NATIONAL INSTITUTE OF INDUSTRIAL PROPERTY

Application Date: 13 December 1991

Publication Date: 18 June 1993

PATENT PUBLICATION NO. 2,684,952

NUMBER OF NATIONAL REGISTRATION: 91/15540

International Class: B 64 B 1/40, 1/02.

Inventor: Giraud Andre

Attorney: Cabinet Chaillot

Applicant: Compagnie Generale D'Innovation et de Developpement, France

Aerostats Using Water Vapor as Principal Lift Gas

Abstract: The system of water vapor production of these aerostats includes a principal section situated upon the ground, capable of ensuring the initial and rapid inflation of the envelope (2), and, to the extent necessitated by the mission, a secondary section, carried on board, having a thermal power less than that of the principal section, and comprising a means capable of re-vaporizing the steam condensed on the interior of the envelope and/or, if this is double layered (3-4), in the intermediate space (5), and/or, by at least a burner immersed in the condensed water, capable of re-vaporizing the same and of supplying the resulting mixture of water vapor and gases of combustion into the envelope. The layer or layers of the envelope is/are formed of a film or a coated fabric, chosen so that the impermeability of the envelope vis-a-vis water vapor, expressed in loss percent per hour, is at most equal to 5% of the weight of its contents.

Aerostats Using Water Vapor as Principal Lift Gas

The present invention relates to aerostats or lightened aeronefs, which includes free or captive balloons and dirigibles, in which water vapor (steam) is used for lift, whether or not they include means of propulsion (motors and propellers) or of direction and stabilization (rudders and empennages).

The choice of gaseous fluid for filling aerostats greatly influences the performance of these devices and their possible uses.

The fluid should be intrinsically lighter than air, which excludes most gases and vapors. Moreover, it should also possess other characteristics, such as ease of provision, safety in use, compatibility with the materials used for the envelopes, and cost price. Non-heated fluids utilized for aerostats all present inconveniencies. Thus, for example, hydrogen, lighting gas, and methane suffer from the inconvenience of being combustible, and their use has led to catastrophes which have become historic. Hydrogen and helium permeate through thin and light envelopes. Helium, which is nowadays the most appreciated solution, requires considerable logistics for supply, and its cost is incompatible with many uses. Ammonia presents inconveniences of the same type, and itself requires safety precautions.

To avoid these inconveniences, for some years now, the use of heated air has been resorted to in order to provide balloons with lift. Most often the heating is produced by direct exchange, using a burner of which the flame is oriented towards the center of the balloon through an opening, so that the hot combustion gases mix with the air in the balloon. The specific mass of this mixture is very close to that of air, and the lift effect is accordingly due only to the expansion of the mixture as an effect of its temperature. Unfortunately, in consideration of the limitations upon temperature imposed by the envelope, the lifting power of craft of this type remains mediocre, and demands very great dimensions. This system cannot easily be applied to dirigibles. Moreover, radiation away of the heat results in a quite severe loss of energy, and imposes a level of heating upon the envelope which is deleterious to its durability.

The employment of water vapor would make it possible to obtain a lift force greater than that of hot-air balloons, without encountering the inconveniences explained above presented by hydrogen, helium, methane, lighting gas, or ammonia.

Because of the lower molecular mass of water with respect to the average molecular mass of the constituents of air, the lift force is approximately double, as the following table shows:

Temperature (oC) Lift Capacity (kg/m3)

Air Steam

100 0.283 0.662

150 0.397 0.736

200 0.492 0.796

Assuming that the structural elements of balloons of the same size are comparable to a first approximation, the usable load in the case of the use of water vapor is accordingly more than twice as great.

Furthermore, the lift force of a balloon filled with helium is 1.113 kg/m3, which represents scarcely more that 50% more than that of a balloon filled with water vapor at 150oC, so that the dimensions of the latter would be only 15% greater than those of a balloon filled with helium which had the same lift force.

The implementation of such aerostats, using water vapor, either superheated or saturated, has already been envisaged or evoked. For example, French Patent FR-A-1,527,954 describes a dirigible with a flexible envelope, including a large lift cell filled with water vapor.

The water vapor is generated by an installation situated on board which is used for re-vaporizing the water which is condensed by heat exchange with the surrounding atmosphere. Certain projects have already been described in which water vapor is only one of several fluids utilized for sustenance of aeronefs which include several compartments, certain of these containing a lift gas such as helium, air (which accordingly is heated), or methane.

In any case, the use of steam also entails difficulties caused by the different behavior of water vapor with regard to thermal losses.

In fact, if superheated steam is used, it is necessary to compensate for the thermal losses of the balloon by transferring calories to the steam by recycling, because one cannot allow loss of the high calorific content of the even partially cooled steam. The reheating of a gaseous fluid is a difficult operation to which no lightweight solution has yet been found which is compatible with the load which such an aerostat is able to carry.

If saturated steam is used, any thermal loss is expressed as condensation of the steam. Thus the heat exchange through the skin takes place according to a different mode, via the liquid which keeps the skin at an elevated temperature, and not only by convection. The losses accordingly become much more important, and this phenomenon can annul or even reverse the advantages which steam aerostats should present as contrasted with hot-air aerostats. Thermal transfer through a thin skin upon which condensation is occurring is in fact, at the relevant temperatures, 3 or 4 times greater than transfer by convection.

Accordingly, all things being equal, the fuel consumption for maintaining the lift of the fabric envelope is much increased, and is even greater because steam balloons of the normal type cannot avoid, while re-vaporizing water which itself is at 100oC, releasing smoke [TN: 'smoke' is used throughout to mean 'flue gases'] at a temperature substantially greater than 100oC, and only attain an efficiency noticeably less than 1. On the contrary, the direct heating of hot air by the burner flame can approach this elevated value if that which is necessary is done to avoid losses by radiation.

This is why authors who have advocated the use of water vapor have also proposed the use of a double envelope whose faces are treated so as to reduce thermal transfer as much as possible. In particular there has been proposed, according to French Patent No. 1,527,954, an insulating double-layer envelope, of which the internal layer comprises, from the inside towards the outside, a layer of hydrophobic synthetic material for preventing the inside of the envelope from becoming heavy, a metallic foil impervious to steam, a binding layer, and a layer of fabric of synthetic material, like polyester, or of fibrous matter, on which can be adhered by processing, at least on one of its sides, a thin foil of the same material, in order to establish gas-tight zones. The external layer comprises an interior layer of fabric followed by a binding layer, and then by a metallic foil impervious to water vapor and a hydrophobic layer, the latter permitting flowing off of rain, snow, or dew, thus preventing weighing down of the aerostat. The two layers are connected by transverse connections formed of fabric or foil, for example of polyester, carrying a layer of vaporized aluminum which prevents radiation of heat.

However, these arrangements, although complex, do not appear to be sufficient to prevent the difficulties of employment of steam which have been noted. The use of water vapor for sustaining aerostats does not appear to have known any development up to the present.

The present invention has as its objective to permit the effective implementation of aerostats using the lifting force of water vapor. It is notably applicable to an aerostat or a non-rigid aeronef of novel construction, thanks to an envelope improved over what has been proposed up to the present, and to a new system for functioning of the sustaining power. This objective is attained by an aerostat, free or dirigible, which utilizes, as main lift agent, water vapor which inflates an envelope of one layer or of two layers delimiting an isolating intermediate space, characterized by the fact that the system of water vapor production comprises a principal section situated upon the ground, capable of assuring initial and rapid inflation of said envelope, and, to the extent necessitated by the mission of the aerostat, a secondary section carried on board, having a lesser thermal power than that of the principal section and comprising a means capable of re-vaporizing the vapor condensed on the internal surface of the envelope, said means being constituted at least by a re-boiler or exchanger, in which case the combustion or exhaust gases which are produced are supplied directly into the heart of the envelope and/or, in the case that the latter is double layered, into the intermediate space, and/or by at least a burner immersed in the water of condensation, capable of re-vaporizing the same and of supplying, into the heart of the envelope, the mixture of water vapor and combustion gases which is produced, and/or in that the layer or layers of the envelope is or are formed of a film or a reinforced fabric, chosen so that the impermeability to water vapor of said envelope, expressed in percentage of loss per hour, is at most equal to 5% of the weight of its contents.

The present inventor has observed that the thermal content of a balloon filled with water vapor is very high, due to the high value of the latent heat of vaporization of water. Accordingly the balloon itself constitutes a powerful reserve of energy.

As has been indicated above, the envelope of the aerostat according to this invention is made from a single layer, or preferably double layers. By comparison with known balloons, the aerostats according to this invention are characterized by the fact that the single or double layer envelope is made from a reinforced film or a fabric whose impermeability properties are as defined above. Thus, a skin porosity which only causes a loss of 5% of the weight of the gas contained in the envelope on a classical montgolfiere {TN: hot-air balloon} would on the other hand involve a very important loss of water vapor of 215 kg/h, which would be too great. This would be equivalent, in fact, to a net loss of fuel, and would thus diminish the capacity to carry a useful load to a disastrous extent.

As reinforced fabrics which are usable for manufacture of the layer or layers of the envelope of the aerostat according to this invention, one may mention polyamide gauzes reinforced upon at least one face with poly(ethylene terephtalate) and with aluminum, of which an example is mentioned hereinafter.

As films usable for the same goal, one may mention polyester films which are then metallized, such as the metallized polyester films commercialized by the "ICI" company under the title "MELINEX" and the polyester films commercialized by the "Du Pont" company under the title "MYLAR".

It has already been proposed to endow the exterior surface and the interior surface of the envelope of an aerostat with a hydrophobic covering, in order to avoid water accumulating over the entire surface and weighting down the craft. This is certainly desirable, but may perhaps not be sufficient, because water can remain upon the layer, condensed into droplets. Moreover, wherever water is present, the transfer of heat is seriously increased as a consequence. Thus, the present inventor has conceived the idea of printing, at least upon the internal surface of the envelope, a network of lines covered with a hydrophilic product. The aerostat according to this invention is accordingly advantageously further characterized by the fact that at least the internal surface of the envelope comprises a covering of a hydrophobic product, the entire internal surface being thus covered except for lines covered with a hydrophilic product which are arranged sloping with respect to the horizontal, in order to assure rapid guiding of the water towards the base of the envelope.

The hydrophobic covering mentioned above may be based upon a polysiloxane (silicone), such as that represented by the formula:

CH3

|

(HO)3Si(OSi)nOSi(OH)3

|

CH3

or upon a fluoridated polymer such as poly-tetra-fluor-ethylene in the pure state or mixed with other polymers.

As for the hydrophilic material, it may be chosen from the polysaccharides and acrylic polymers having free carboxyl groups, obtained in the classic manner by polymerization and reticulation, by example by peroxides or UV irradiation. As acrylic polymers of this type, one may mention methacrylate copolymers of methyl - acrylic acid (50-50 for example). According to a particular mode of implementation of the double layered envelope according to the invention, the two layers are joined together by lines of adhesion arranged so that the space between the two layers is divided into compartments which communicate with one another, which makes it possible to reduce the turbulence that can prevail in this space. Advantageously, it may be so arranged that the lines of adhesion and the lines of the internal layer of the envelope which are destined to be covered with the hydrophilic product are substantially coincident. In a particular mode of implementation, the lines of adhesion form such a pattern that, between the two layers, there are constituted alveoli communicating with one another by conduits which are oriented towards the bottom of the envelope.

In the case of a double layered envelope, it may be advantageously contemplated for the confronting surfaces of the two layers to be covered with a thin reflective coating, for example silvered, in order to minimize transfers by radiation between the two confronting surfaces.

Moreover, it may be contemplated for a single layered envelope or at least the outer layer of a double layered envelope of the aerostat according to the invention to comprise reinforcement by fibers which are included or are adhered against at least one face of the layer in question, and disposed for example following lines or in the form of a grid or a network. The arrangement of the fibers may be such that the outer surface remains smooth, particularly in the case of a dirigible, in order not to increase the friction in air. As fibers which may be used, there may be mentioned aramid fibers, for example the fibers commercialized under the name "KEVLAR" by the "DU PONT" company.

Moreover, the aerostats according to this invention comprise a system for water vapor generation which is planned in order to obtain the best estimate for the weight of the combination of the generator and the fuel, in view of the mission which should be fulfilled.

1/:- The steam generator is composed of two sections: the ground section, intended to furnish the initial filling for the aerostat within the desired time limit, and the airborne section, intended to ensure the maintenance of lift in combination with the jettisoning of condensed water, including an adequate safety margin.

This structure makes it possible only to carry a lightweight generator, and thus greatly to reduce the dead weight, to the point that, for certain missions, it would be possible to have no airborne section at all. For a balloon functioning at 100oC, consisting of a sphere of 8 meters in radius, filled only by the ground section, and not comprising any on-board means of heating, it is possible to make a flight which lasts 4 hours, with a load of 300 kg, under current conditions. In this case, the maintenance of lift is ensured by simply jettisoning the condensed water. The energy necessary for the lift is carried not in the form of fuel, but in the form of the latent heat of the lifting fluid itself.

2/:- The present inventor has also conceived of the idea that the characteristic of high calorific content of the balloon itself allows the injection of the combustion gases into the envelope while only slowly modifying its lift. In fact, according to the approximate data given later, 1 kg of fuel contains the necessary calories to re-vaporize 18 kg of water, and at the same time provides an equivalent quantity of smoke of an average molecular weight close to that of air. The reconstituted mixture has characteristics intermediate between those of air and those of steam. The volume which these 36 kg of mixture occupy is the same as than which, previously, was occupied by 29 kg of steam. For the lift of the balloon to remain the same, it is sufficient to jettison 9 kg of condensed water, which is possible because the new mixture obtained from 1 kg of fuel and 18 kg of water replaces 29 kg of water. The ballast even increases by 2 kg. This mechanism thus allows the condensation of the steam present at the start in the envelope to last. [sic]

However, with the mode of operation which has been explained above, the steam is progressively diluted by the combustion gases, and the envelope will behave more and more like that of a hot-air balloon.

This operational mode allows a re-boiling system which is particularly light in weight to be used as the airborne section: an immersed burner may be employed, such as schematically shown in FIG. 2C of the appended drawing and described in more detail later with reference to that figure.

3/:- For longer missions, a mixed system may be employed. During the first part of the flight the condensed water will be re-boiled at the base of the balloon, either by a classic re-boiler of the type heated by a gas or gasoline burner, or by a catalytic re-boiler, such as schematically shown in FIGS. 2A and 2B of the appended drawing and described in more detail later with reference to those figures. The catalytic system has the advantages of providing gentle combustion at low fuel temperature (as in catalytic stoves), and of avoiding the formation of a flame.

In the second part of the flight the combustion gases are supplied into the envelope itself according to the mode of operation described in paragraph 2/- above, which accordingly prolongs the lift, as the steam is progressively replaced by hot air and the excess condensed water is jettisoned in the form of ballast.

4/:- During the entire phase of the flight in which the [TN: combustion] gases are not supplied into the balloon, there may be a loss of energy if the gases, which are still at a temperature near 100oC (see above) after having passed through the re-boiler, are simply discharged into the atmosphere. Thus it is contemplated, as a means for recovering the calories contained in the gases from the re-boiler, to select a double layered envelope such as has been described above, and to allow the gases to discharge into the double layer, the calories being accordingly recovered, not entirely by direct transfer to the interior of the balloon, but at least partly by reduction of the thermal loss through the double layer. This means may also be resorted to when the balloon comprises a source of hot gases not directly linked to the re-boiling of steam. The construction of a balloon or a dirigible using water vapor for lift becomes practicable, thanks to the gains in lift and fuel consumption which result from the arrangements which have just been described.

Moreover, in the case of a dirigible, the maintenance of the lift of the balloon can be obtained by recovery of the thermal energy which is lost as a consequence of the low efficiency of explosion motors. This lost energy - which represents around 75% of the calories furnished by the combustion of the fuel - is present in the exhaust gases, which can be used first for re-boiling the condensed water, and then for feeding, either the inside of the envelope (a balloon whose lift progressively diminishes), or, this failing, the interior of the double layer.

For such dirigibles, according to the definition of the mission with which they are entrusted, it might even be found advantageous, as indicated above, to install a central energy system composed of a turbine or an internal combustion motor which drives an electro-generating group [sic]. The gases exiting from this central system are used to operate an exchanger in order to re-boil the condensed water, and then to feed the interior of the envelope and/or, this failing, the intermediate space between its two layers. As for the electricity, it is used for feeding the motors which drive the propellers, or for supplementing the lift. This supplementation may itself be of two different natures: it may be employed in the re-boiling of the condensed water, and it may also be used for superheating the steam by convection or radiation, thanks to appropriate means placed at the exit of the re-boiling circuit, or even in the interior of the balloon. The combination of these means thus assures optimal utilization of the calorific power of the fuel and reduction of the thermal transfer through the envelope.

In order further to illustrate the objective of the present invention and the numerous possible realizations thereof, several modes of implementation, which are purely illustrative examples and are not limitative, will be described hereinafter and shown in the appended drawings.

In these figures:

- FIGS. 1 and 3 are schematic views, partly elevational and partially cut away, respectively of a balloon and a dirigible which conform to particular modes of implementation of the invention;

- FIGS. 2A and 2C each show, on a larger scale, a particular mode of implementation of the base of the balloon of FIG. 1 and of the airborne section associated with the steam generator;

- FIG. 4 is a partial view of the internal layer of the double envelope of the balloon of FIG. 1 or of the dirigible of FIG. 3; and

- FIG. 5 is a sectional view cut along V-V in FIG. 4.

In FIG. 1 there is shown the diagram of a balloon, represented as a whole by the reference numeral 1, which comprises an envelope 2 suitable to be filled with superheated or saturated water vapor and presenting two layers 3, 4 delimiting an interior space 5, and a nacelle [sic] 6 suitable for carrying loads and some ballast and suspended from said envelope 2 by classical means, not shown.

The steam generator comprises a principal section 7, situated upon the ground, and a secondary section 8, carried on board, and arranged at the base of the envelope 2. The section 7 ensures rapid initial inflation of the envelope 2 with steam, and the section 8 ensures compensation for thermal losses by re-vaporization of the water condensed due to the effects of said losses and/or by reduction of thermal transfer through said envelope 2, said section 8 being calibrated so as to assure compensation of thermal losses by the minimum amount necessary for the mission of the balloon.

The section 8 can take the form of a classic re-boiler 8a (FIG. 2A), of a catalytic re-boiler 8b (FIG. 2B), or of an immersed burner (FIG. 2C), it being understood that sections 8 may be contemplated which comprise several of these means or combinations thereof.

The re-boiler 8a comprises a lower compartment 9 in which a burner 10 is provided, and an upper compartment 11 in which the condensed water accumulates and in which a tubular exchanger 12 is provided which allows the re-boiling of the condensed water by the combustion gases to be guaranteed.

The re-boiler 8b comprises a tubular exchanger 13 in which the water to be re-vaporized circulates, and whose external surface is covered with a combustion catalyser, the fuel (arrow FC) and the combustion air (arrow FA) flowing upon this surface in counter-current fashion.

In the case of the re-boilers 8a and 8b, the circuit for the combustion gases is schematically denoted by the arrow FG, with the water vapor formed (arrow FV) being supplied into the heart of the envelope 2. It can be seen that the combustion gases are advantageously supplied into the space 5, according to the operational mode which has been described above in paragraph 4/-.

The immersed burner 8c consists of a burner itself termed 14, into which the fuel and the combustion air are conducted as shown by the arrows FC and FA, and of which the flame is located right in the heart of the water 15 to be vaporized. In this case, the vapor produced and the combustion gases are supplied, mixed together, into the balloon (arrow FV+G). This corresponds to the operational mode described above in paragraph 2/-.

In FIG. 3 there is schematically shown a dirigible 101, which comprises an envelope 102 made of double layers 103, 104 delimiting an intermediate space 105, as well as propulsion means constituted by a motor 116 and a propeller 117. From the envelope 102 there is suspended a nacelle 106 comprising the airborne section 108 of the steam generator (exchanger), whose ground section is not shown in this figure. An internal combustion motor 118 which powers an electrical generator 119 is also installed in the nacelle 106. The exhaust gases which are exhausted from this central unit are used for re-boiling the condensed water in the section 108, and then for feeding, either the interior of the envelope 102 (dirigible whose lift reduces gradually), or the intermediate space 105, or both (gas circuit as shown by the arrows FG). The electricity produced is used for feeding the motor 116 which drives the propeller 117 and, this failing, for supplementing the lift by being used in the re-boiling of the condensed water and/or to superheat the steam by convection or radiation, thanks to appropriate circuits placed at the exit of the re-boiling circuit (not shown) and/or means, such as a resistance 120, placed in the interior of the envelope 102. The exhaust gases of the motor 116 could also be used for re-boiling the steam and/or for maintaining the temperature of the double layer.

FIGS. 4 and 5 illustrate an example of an advantageous structure for the double envelope 2 (or 102). The layers 3 and 4, of which the respective confronting surfaces 3a and 4b are silvered, are each constituted by a polyamide fabric weighing around 37 gm/m2 and comprising a double faced coating of poly(ethylene terephtalate) and of aluminum, which brings its weight to about 62 gm/m2, which is commercialized under the name "PTL", reference BE 499, by the company "PLASTIQUES ET TEXTILES LYONNAIS". Moreover, these layers 3 and 4 are connected by lines of adhesion 21, which form a network of beehive cross-section of which the parallel sides are situated along the arrow F which indicates the downwards direction, and which are staggered in such a manner that the alveoli 22 thus defined can communicate between one another by conduits 23, and at the base to the exit of the exhaust or combustion gases which are to maintain the temperature of the double layer. The lines of adhesion 21 are interrupted along the conduits 23 in order to permit communication between adjacent alveoli 22 transversally to the arrow F.

The outer layer 4 is smooth on its external surface 4a and is reinforced on its internal surface by the incorporation of a network of fibers 24, for example consisting of KEVLAR aramid.

As for the internal layer 3, it comprises, upon its internal surface 3b, a hydrophobic covering of silicone such as PTFE, and, at the positions where the lines of adhesion 21 are located, a continuous coating of a hydrophilic product such as a methacrylate copolymer of methyl - acrylic acid (50/50), of which the role has been described above.

Naturally the modes of implementation described above are not by any means limiting, and could be subjected to any desirable modifications without thereby departing from the scope of the invention.

CLAIMS:

1 - Aerostat, free or dirigible, which utilizes, as main lift agent, water vapor which inflates an envelope (2; 102) of one layer or of two layers (3-4; 103-104) delimiting an isolating intermediate space (5; 105), characterized by the fact that the system of water vapor production comprises a principal section (7) situated upon the ground, capable of assuring initial and rapid inflation of said envelope (2; 102), and, to the extent necessitated by the mission of the aerostat, a secondary section (8; 108) carried on board, having a lesser thermal power than that of the principal section (7) and comprising a means capable of re-vaporizing the vapor condensed on the internal surface of the envelope (2; 102).

2 - Aerostat, free or dirigible, which utilizes, as main lift agent, water vapor which inflates an envelope (2; 102) of one layer or of two layers (3-4; 103-104) delimiting an isolating intermediate space (5; 105), characterized by the fact that the layer or layers of the envelope (2; 102) is or are formed of a film or a reinforced fabric, chosen so that the impermeability to water vapor of said envelope (2; 102), expressed in percentage of loss per hour, is at most equal to 5% of the weight of its contents.

3 - Aerostat according to Claim 1, characterized by the fact that the means capable of re-vaporizing the steam condensed upon the internal surface of the envelope (2; 102) comprises at least a re-boiler (8a; 8b) or exchanger (108), in which case the gases of combustion or exhaust which are produced are supplied directly into the heart of the envelope (2; 102) or, in the case that the latter is of double layers, directly into the heart of the envelope (2; 102) and/or into the intermediate space (5; 105).

4 - Aerostat according to Claim 3, characterized by the utilization of a catalytic re-boiler (8b), notably of the type in which the water to be re-vaporized circulates in a tubular exchanger (13) of which the external surface is covered by a catalyzer for combustion, on which surface the fuel and the combustion air flow in counter-current fashion.

5 - Aerostat according to any one of Claims 1, 3, or 4, characterized by the fact that the means capable of re-vaporizing the steam condensed upon the internal surface of the envelope (2; 102) comprises at least a burner (8c) immersed in the condensed water, capable of re-vaporizing the same and of supplying, into the heart of the envelope (2; 102) the mixture of water vapor and combustion gases which is produced.

6 - Aerostat according to any one of Claims 1 through 5, characterized by the fact that at least the internal surface (3b) of the envelope (2; 102) comprises a covering formed from a hydrophobic product such as a polysiloxane or a fluoro polymer; the entire internal surface (3b) being thus covered with the exception of lines (21) covered with a hydrophilic product such as a polysaccharide or an acrylic polymer with carboxyl groups, arranged so as to slope with respect to the horizontal in order to ensure rapid guiding of the water towards the base of the envelope (2; 102).

7 - Aerostat according to any one of Claims 1 through 6, characterized by the fact that the envelope (2; 102) is double layered, the two layers (3-4; 103-104) being joined together by lines of adhesion (21) arranged so that the space (5; 105) between the two layers is divided into compartments (22-23) communicating with one another.

8 - Aerostat according to Claims 6 and 7, taken together, characterized by the fact that the lines of adhesion and the lines of the internal layer destined to be covered with the hydrophilic product are substantially coincident.

9 - Aerostat according to any one of Claims 7 and 8, characterized by the fact that the lines of adhesion (21) form such a pattern as to define, between the two layers (3-4, 103-104), alveoli (22) communicating between one another by conduits (23) oriented towards the bottom of the envelope (2; 102).

10 - Aerostat according to any one of Claims 1 through 9, characterized by the fact that the envelope (2; 102) is double layered, the confronting surfaces (3a-4b) of the two layers (3-4; 103-104) being covered with a reflecting coating.

11 - Aerostat according to any one of Claims 1 through 10, characterized by the fact that the single layered envelope or at least the outer layer (4) of the double layered envelope (2; 102) comprises a reinforcement by fibers.

12 - Dirigible aerostat according to any one of Claims 1 through 11, characterized by the fact that the secondary section (108), carried on board, of the system for production of steam is constituted by an exchanger which functions on the exhaust gases of a turbine or of an internal combustion motor (118) driving an electro-generating group (119), the exhaust gases coming out from said exchanger (108) being re-injected into the heart of the envelope (102) or, in the case that the latter is double layered, into the heart of the envelope (102) and/or into the intermediate space (105), and the electricity produced by the electro-generating group (119) being utilized, as needed, for propulsion of said aerostat (101); for re-vaporization of condensed water and for heating of the lift liquid by convection or radiation by at least an appropriate means of heating placed at the exit of the re-boiling circuit or in the interior of said envelope (102), such as a resistance (120) arranged in the heart of said envelope (102).

- o O o -


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