EP2223003A1 - Cryogenic insulating item, method of implementing and using such an insulating item, and launcher equipped with an insulating item such as this - Google Patents

Abstract

The invention relates to a cryogenic insulating item which comprises at least one first layer of insulating material with good mechanical performance and a thickness of less than 5 mm and a second layer of insulating material that is not as dense, is thicker, and is used as a barrier against incoming thermal flux.

Description

 CRYOGENIC ISOLATION ARTICLE, METHOD OF IMPLEMENTATION

WORKING AND USING SUCH ISOLATION ARTICLE, AND

LAUNCHER EQUIPPED WITH SUCH ISOLATION ARTICLE

DESCRIPTION

TECHNICAL AREA

The technical field of the present invention is that of the cryogenic insulation of equipment, for example space or aeronautical vehicles.

The present invention relates to a cryogenic insulation article and a method of implementing such a cryogenic insulation article designed to define a thermal insulation for protecting for example a tank subjected to cryogenic temperatures.

The term "cryogenic temperatures" refers to very low temperatures, such as those of liquid oxygen (90 K) and liquid hydrogen (20 K)

The invention also relates to the use of such a cryogenic insulation article for the cryogenic insulation of equipment or structures for example of space or aeronautical vehicles, such as thin-walled tanks in which there are propellants, such as hydrogen or oxygen. The invention finally relates to a launcher equipped with such a cryogenic insulation article. In the following, to simplify the description is considered, for example, the thermal insulation of a cryogenic tank of a launcher.

STATE OF THE PRIOR ART

To ensure the protection of the cryogenic tank of a launcher, it is coated with a thermally insulating material to maintain the propellants it contains in the liquid phase by minimizing heat losses. Such a material may be of the closed cell rigid foam type polyvinyl chloride, polyurethane loaded or non-glass, polyisocyanurate or phenolic. Such a material, generally used in a thickness of about 20 to 25 mm, makes it possible to respect a given maximum input thermal flux while being able to withstand the mechanical stresses imposed by the thermal contraction, the pressure and other forces applied to the structure of the structure. tank. This material can be glued or sprayed directly onto the surface of the tank.

Such thermal protection must allow the movements of the tank surface and support the acceleration loads subjected to the launcher. Indeed, before and after the takeoff of a cryogenic spacecraft, the loss of a part of insulation, mounted on a tank filled with liquid propellant would involve potential risks of malfunction that could lead to the destruction of the launcher through important thermal flows entering through the poor surface of the insulation. To increase the mechanical strength wall while reducing the level of incoming flow without penalizing the mass, the patent application FR 2,876,438 describes thermal insulation articles which adhere to the thin-walled structure of a cryogenic tank subject to constraints of mechanical and thermal origin which cause the deformations thereof. However, such insulating articles are dimensioned as such, to respond to aerothermal flows. A complementary material can then be used as a high temperature protection material to avoid significant loss of propellants by evaporation.

The object of the invention is to propose an insulation article that makes it possible to optimize the mass of a cryogenic tank by minimizing the thermal protection mass, with obtaining a higher insulating performance, and the mass of the propellants. inconsistent, and therefore increase the launcher payload.

SUMMARY OF THE INVENTION

The invention relates to a cryogenic insulation article for protecting a structure subjected to cryogenic temperatures, characterized in that it comprises at least a first layer of insulating material, which is a closed cell rigid foam layer of polyetherimide and thickness less than 5 mm capable of being brought into contact with the wall of this structure, and a second layer of less dense insulating material, which is a layer of copolymer ethylene vinyl acetate of greater thickness used against incoming heat flux, glued to the surface of the first layer.

Advantageously, this first layer has a thickness of about 3 mm.

Advantageously, the second layer is a copolymer layer of ethylene vinyl acetate. Advantageously, this second layer has a thickness of about 19.5 mm. Such an insulation article makes it possible to offer, with as good a level of thermal and mechanical performance as the devices of the prior art, a considerably lower density. It also allows, while maintaining masses and similar mechanical performance, to improve thermal performance or, while maintaining similar masses and thermal performance, to improve the mechanical margin. The sum of the density of the two materials forming the combined thermal protection according to the invention is less than the currently used insulations for better thermal and mechanical performance. The thermal performances are thus dimensioned with respect to the incoming flows. In addition the mechanical performance responds to the behavior of the cryogenic tank which, given the significant differences in stiffness with the insulating article, imposes its "mechanical" _deformation (ε _meC a) to the assembly.

The invention also relates to a method for implementing a cryogenic insulation article which comprises the following successive steps: a step of preparing the surface on which the cryogenic insulation article is to be applied,

a protective step during which a primer is applied to the surface thus prepared,

a fixing step during which said cryogenic insulation article, in the form of at least one panel made from a rigid closed cell polyetherimide foam, is glued, using a glass fabric impregnated with an adhesive material.

The invention can be used in all industrial fields, in particular the aeronautical and space fields, where thermal insulation, for example in the form of insulating panels, is used to protect a structure subjected to cryogenic temperatures in the purpose of using an insulating material currently unsuitable for these temperatures or simply to reduce the mass of the insulation assembly thus obtained.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The invention relates to a cryogenic insulation article which comprises a first layer of insulating material of small thickness, for example less than 5 mm, having good mechanical performance, so as to perfectly withstand the mechanical stresses on a cryogenic tank at the temperature. 20 K, and a second layer of less dense insulating material, thicker, used against incoming flows.

The invention makes it possible to guarantee the dimensioning of thermal performances with respect to the incoming flows, and the response of mechanical performances to the behavior of the cryogenic tank which, taking into account the significant differences in rigidity with the set of two layers of insulating materials. , impose its "mechanical" deformed (ε _meC a) to the set.

The expression of this "mechanical" deformation of the reservoir imposed on the two thermal protection layers and resulting from the forces of internal pressure and general forces is given below:

- In longitudinal (along the axis of the tank,

_ΔPxRx (0.5-υ) φ

Exe Exe

- In radial 'according to the radius of the tank,

ΔPxRx (l-0.5υ) υxφ ε _r = ⁱ '- + ^

Exe Exe

With:

R: tank radius

: thickness of the tank wall E: Young's modulus υ Poisson's ratio

ΔP differential pressure in the tank

Φ = flow of longitudinal force in the current zone. In addition, the tank wall, in contact with the propellant, undergoes a deformation of thermal origin

(ε _t h _e rm) whose expression for a liquid hydrogen reservoir is the following: ε _t herm = oc (20 -293), with α coefficient of expansion of the metal wall.

This "thermal" deformation of the tank is also imposed integrally with the two insulation layers. Since these two layers have coefficients of thermal expansion ai and α ₂ different from α, the mechanical deformations actually experienced by the set are given below:

- For the first layer: ε _mecα i = ε _mecα + h _therm ^~ <V (20 - 293)] - For the second layer: ε _ffleca2 = ε _meca + [ε _rterm -α _2. (R ₂ -293)] with T ₂ temperature at the interface of the two layers of insulation.

The thermomechanical optimization of the definition of all these two layers consists in verifying the following relations, with either the lowest possible final density or the largest possible break margins: ε _mecal -α ₁ . (20 - 293)] <ε _Λ1 [20i:] ε _meca2 = ε _meca + [ε _therm - α ₂ . (r ₂ -293)] <ε _Λ2 [r ₂ ] with ε _R i and ε _R2 : deformed at break of these two layers of insulation.

For the sake of simplicity, we reason here in terms of uni-axial deformations but the reasoning is of course applicable in bi-axial equivalent deformation.

The invention makes it possible to use thermally insulating materials, which, in the known art, could not be used to protect cryogenic tanks. Indeed, the material used as the second insulating layer is isolated from the surface of the reservoir, in contact with the liquid propellant, by the first insulating layer of small thickness in contact with the cold wall can reach the temperature of 20 K and capable of undergoing total deformations of mechanical and thermal origin (ε meca + ε therm) • Thus, the material of the second insulating layer is subjected to higher temperatures resulting from the thermal gradient making it possible to minimize the mechanical strain ε _meCA 2 •

Example of realization

In an example of thermal insulation applied to a cryogenic tank intended to equip an Ariane 5 launcher, a first rigid polyetherimide closed cell (PEI) foam layer is used on which a second layer of copolymer foam is adhered to. ethylene vinyl acetate (EVA).

The first polyetherimide layer is known, for example, under the R82 domination available from the AIREX Company. Such a rigid closed cell polyetherimide foam may have a density of between 55 Kg / m ³ and 60 Kg / m ³ , for example of the order of 60 Kg / m ³ . This rigid foam fairly low density has a low thermal contraction of 0.73% at 20 K and a good tensile strength of 1.7 MPa.

The second ethylene vinyl acetate copolymer layer is known, for example, under the EVA25 domination available from the ZOTEFOAMS Company. Such a closed-cell cross-linked ethylene vinyl acetate copolymer foam can have a density of 25 Kg / m ³ and a thermal conductivity value of 0.035 W / mk at room temperature.

Placing an adhesive between the two layers provides the desired insulation.

Table 1 at the end of the description makes the comparison between this embodiment and a thermal insulation known under the H920A domination and available from the Air Liquide Company. This known thermal insulation is made with 21 mm rigid closed-cell polyvinyl chloride foam characterized by a density of 55 Kg / m ³ , a thermal conductivity of 0.033 W / mk at room temperature, a thermal contraction of 1% at 20 K and a tensile strength of 1.0 MPa.

This table 1 shows an equivalent density of the embodiment of the invention significantly lower than the thermal insulation of the prior art. A low p _equ i of 0.67 Kg / m ² is obtained with the example of the invention, for as good thermal and mechanical performance as the insulation of the known art. To account for the excess adhesive required at the inter-layer interface, one can add the necessary mass of adhesive used in its manufacture which is between 0.08 and 0.1 Kg / m ² , hence a value of p _equ i of 0.77 Kg / m ² .

The adhesive, chosen so as to retain its properties up to a cryogenic temperature, may advantageously be a polyurethane consisting of a polyol and an isocyanate.

The gain on the mass of a cryogenic tank, mounted on an Ariane 5 launcher, is about 120 Kg (knowing that it is isolated on a surface that represents not less than 300 m ² ).

According to this embodiment, it is possible to insure the thermal insulation of the cryogenic tank with the aid of insulating panels, previously made, directly bonded to the external wall thereof, using an epoxy adhesive and a vacuum bag.

The following steps can thus be carried out: 1. A step of preparation of the surface In a first step, the metal surface of the tank is prepared which it is desired to protect thermally. This surface preparation is intended to give said surface a sufficient surface tension to ensure optimum adhesion of a primer. It can take various forms well known to those skilled in the art. It usually consists of an alkaline degreasing operation and a suifochromic pickling operation for aluminum alloys, followed by a demineralized water rinse or a grinding with fine-grade abrasive paper. , or corundum sandblasting of small particle size, to avoid any degradation of the metal structure.

2. A step of protection

As quickly as possible (generally less than 8 hours) to avoid further pollution of the surface, a projection operation of a primer is carried out. This operation improves the adhesion of the adhesive used subsequently to fix the insulation and protects the structure against corrosion.

The primer, for example PZ 820 / HZ820 from the company Sodiema, is advantageously an epoxy chosen mainly to maintain its properties up to a cryogenic temperature of about 2OK. In order to avoid any pollution of the surface thus protected, it is isolated within a maximum of 30 days after the polymerization of the primary and at least 36 hours after the end of its projection. The thickness of the insulation film is advantageously between 10 and 50 microns, knowing that its optimum thickness is about 20 microns.

3. A fixing step

Then adhered to the surface thus prepared and coated primary insulation panels at previously defined locations with an adhesive capable of withstanding the cryogenic temperatures without losing its integrity, and compatible materials which it ensures the bonding.

The adhesive film can be produced by means of an adhesive of the epoxy family such as "Araldite" (registered trademark) AY1O3 / HY953F of the VANTICO company. Thus this adhesive adheres to the structure at ambient temperature as well as at cryogenic temperatures while being chemically compatible with thermal protection. The adhesive is applied using a glass cloth impregnated with glue, preferably with rollers, and then wound directly on the tank. The mass of adhesive to be deposited is advantageously between 150 and 200 g / m ² . After placing the panels on the structure, they are then maintained under a vacuum of 0.7 bar using a vacuum bag for a period of about 36 hours, time required for the glue to polymerize.

After this polymerization, the vacuum bag is removed to ensure good adhesion of the insulation panels and the presence of glue at the inter-panel joints. A seal without glue could lead to a phenomenon of cryopompage of the air during the cooling of the tank, possibly damaging the insulation. The allowed play between two neighboring panels should be around one millimeter. The mass of the glass fabric is advantageously between 30 and 40 g / m ² and its width is about one meter for ease of implementation. At the temperature of 2OK, the glue guarantees a shear strength of 3 MPa.

The insulation thus made allows the movements of the skin of the reservoir due to the hyperstatic forces (thermal contractions and pressurization of the reservoirs) during the ground filling, as well as the general efforts supported by the launcher in flight.

The insulation panels thus made with a first layer of closed cell polyetherimide foam of 3 mm thickness and a second layer of ethylene vinyl copolymer foam of

19.5 mm thick have excellent impact resistance but also a high peel strength, which is essential for this type of dynamically loaded structure.

Table 1 P2 P2 Pequ Reserve Reserve

Configuration e ₂ (mm) φ (W / m ² ) T ₂ (K) (mm) (kg / m ³ ) (kg / m ³ ) (kg / m ² ) εi (%) ε ₂ (%)

monolayer

21 55 0 0 286 1.16 - 0.60 - H920A

R82-60 / EVA

3 60 19.5 25 288 0.67 99 3.07 (*) 1.52 (**)

25

(*) (ε _Λ1 [20 *] - [ε _Λerff1 -Cc _1. (20 -293)])

(**) (ε _Λ2 [r ₂ ] - [ _εrtβ111 -α _2. (r ₂ -293)]) with: e _± : thickness of the layer i,

P ₁ : density of the material of the layer i, φ: heat flow entering the tank on the ground, peqm: equivalent density isolating the complex in kg / m ² .

8i and £ .2 '• deformation reserves of the two available materials before mechanical stress via

Claims

A cryogenic insulation article for protecting a structure subjected to cryogenic temperatures, characterized in that it comprises at least a first layer of insulating material, which is a layer of rigid closed-cell polyetherimide foam and of lower thickness. 5 mm to be brought into contact with the wall of this structure, and a second layer of less dense insulating material, which is a layer of ethylene vinyl acetate copolymer of greater thickness used against the incoming heat flux, glued on the surface of the first layer. The cryogenic insulation article of claim 1, wherein the first layer has a thickness of about 3 mm. The cryogenic insulation article of claim 1 wherein the second layer has a thickness of about 19.5 mm. 4. A method of implementing a cryogenic insulation article according to any one of claims 1 to 3, which comprises the following successive steps: a step of preparing the surface on which the cryogenic insulation article is to be applied, a protective step during which a primer is applied to the surface thus prepared, a fixing step during which said cryogenic insulation article, in the form of at least one panel made from a rigid closed cell polyetherimide foam, is glued, using a glass fabric impregnated with an adhesive material. The use of at least one cryogenic insulation article according to any one of claims 1 to 3 in the form of an insulation board for the insulation of a thin-walled structure of a spacecraft. or aeronautics. 6. Launcher, which is equipped with at least one cryogenic insulation article according to any one of claims 1 to 3, in the form of an insulation board.