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Advanced Non-Metallic Materials for the Cryogenic Aerospace Structures

Vulfovich L.V., Kurochka G.M.
It is considered possibility of application of the new structural materials - the reinforced thermoplastics. Their property and prospect of development are discussed. There are indicated brief data on the materials of the external heat insulation of the cryogenic tanks - foam plastics on the basis of polyisocyanes and polyimides.

Nowadays a wide experience in non-metallic structural materials application in the constructions of flying vehicles including the BURAN orbital plane is accumulated in our country and abroad. It allowed to pass to the creation of fundamentally new materials of the second generation for the aerospace systems (ASS). These materials have unique set of mechanical and technological properties. Among them are materials under development from 1980s on the basis of thermoplastic matrixes withstanding increased temperatures.

The composite thermoplastic materials (CTM) could completely realize their advantages in aerospace techniques of the XXIst century.

The major advantages of the composite thermoplastic materials in comparison with the composite materials on the basis of thermosetting plastics which were used in the BURAN Orbiter are:

  • smaller sensibility to impact loads;
  • chemical inertness;
  • multiple forming and welding capability;
  • environmental safety and utilization capability;
  • low-level penetration in corrosive mediums, including liquid hydrogen, naphthyl, oxygen, nitrogen and helium.

Nowadays the composite thermoplastic material range on the basis of glass, carbon and organic fillers and matrixes from polysulphone, polyestersulphone, polyphenilethylenesulphide, polyesteresterketone and liquid-crystal polymers has been already developed.

The most successful property combination was received for carbon plastics based on polysulphone, polyestersulphone and polyesteresterketone. The KTMU-1 carbon plastic based on polysulphone and the ELUR-0,08 tape was certified in VIAM and have been mastered by domestic industry. To synthesize polyestersulphone, polyesteresterketone and liquid-crystal polymer the pilot-scale (for polyesteresterketone) and industrial (polyestersulphone, liquid-crystal polymer) plants were created.

The researches on creation of the carbon (based on liquid-crystal polymer) have been rapidly activated lately. Liquid-crystal polymers can be used in combination with other thermosoftening plastic to provide self- reinforcement and to increase strength in a given direction.

On coordinated request for proposal NPO MOLNIYA, VIAM, NPO Technology and TsNIISM fulfilled pioneer study concerned creation of the CTM on the basis of polysulphone, polyesteresterketone and liquid-crystal polymers. Experts of these enterprises also developed technology of CTM processing for use in the ASS cryogenic tank constructions.

The main material properties were determined. The level of the liquid hydrogen penetration of the CTM, based on polysulphone and carbon composite filler (CCF) braid, equal to 10^-9 sm^3*sm/sm^2*s*atm at 1,5-mm wall thickness was conformed by testing of the experimental tank.

To achieve required level of penetration of tank walls, the technology using polymer liners from modified heat-shrinkage tape based on polyimide and fluoroplastic was tested.

The following capabilities are shown:

  • prepreg production using electron-ion, film and fiber technologies;
  • production of details with complicated shape and double curvature;
  • repair by secondary moulding and welding method, that is especially important at local leakage discoveries in cryogenic tanks or pipes.

The manufacturing methods of rivets of CTM based on liquid crystal polymers, the mechanical processing technology of CTM parts and technology of welding and riveting were worked off.

The ∅400-mm experimental tanks and their fragments were produced using winding and lining methods. The moulding was fulfilled under the press, in autoclave or in heat-treating furnace. The laser-beam heating method also was tested.

The power element of tanks and pipelines (frames, flanges, stringers and other) can be produced from CTM. When designing tank or pipeline it is necessary to optimize wall thickness providing minimal fuel penetration and minimal weight at required strength of the whole construction. The dependence between penetration and wall thickness is obvious.

The preliminary estimation shows that a tank made of CTM on 20…25% lighter than of aluminum. Due to high heat-resistance CTM allow to reduce weight of the external thermal heat protection of cryogenic tanks by decreasing the heat insulation thickness. Nowadays, operating temperature range for CTM based on the polyesteresterketone is -253°С...+230°С, based on polysulphone is -253°С... +160°С, material density is 1,5...1,6 g/sm3, tensile strength is 100...110 kg/mm2 (with EPUR-0,08 tape as a filler) and 160 kg/mm2 for carbon composite braid.

A wide application of CTM based on polyesteresterketone are limited nowadays by the difficulty of material processing as far as moulding temperature increases up to 380°С at pressure up 25 bar. It requires using of expensive equipment. However, there are already several ways leading to the considerable reduction of the processing parameters. And by 2000…2010 it is possible to create thermal carbon plastic with 200...250 kg/mm2 strength and processing temperature 200°С...250°С, using carbonic fibers with 700...800 kg/mm2.

In this connection it is necessary to continue study on creation of technological thermoplastic bindings based on polyesteresterketone, high-modulus fillers and composite materials, on optimization of their processing technology with reference to large-dimensional tanks with consideration of connection methods (welding, glue-welding, riveting an so on).

For the next-generation of CTM it is necessary to provide their certification considering features of application in different products (filler type and orientation, forming and thermal-processing methods). It is worth to draw attention to the development of material processing technology. It will allow to use initial strength of the reinforced fillers as much as possible. That will lead to the reduction of margin of safety and the increment of weight construction perfection on the whole.

The cryogenic fuel tanks and pipelines construction provides external thermal heat protection. The major requirements for materials of the thermal heat protection are:

  • apparent density not more 35 kg/m3;
  • thermal conductivity (at temperature from -253°С to +20°С) not more 0,03 W/(m*degree);
  • tensile and compressive strength (at temperature from -253°С to +20°С) not less 2 kg/sm2.

The domestic foam plastic based on polyisocyanurats meet all produced requirements. The new foampolyisocyanurat of ISOLAN-type was tested. The ISOLAN examples with ~ 30 kg/m3 density, strength up to 2,5 kg/sm2 and percent elongation at tensile (at -225°С) 6…8% were received. The materials are applied by spraying and have good adhesion to aluminum alloys and carbon-plastics through glue precoat. The table shows results of breaking test of glue connections (VK-11 glue) of cryogenic foam plastics: earlier developed RIPOR foam plastic and new ISOLAN plastic to the D16an aluminum alloy.

The destruction in every case was appeared on foam plastic.

The indicated data confirms ISOLAN preference. The maximum ISOLAN operating temperature is up to 160°С, in the short run (up to 5 minutes) material withstands temperature up to 200°С.

The ISOLAN material included as component in ∅400-mm experimental tank made of CTM based on polysulphone and UKN-P/2500 carbonic filler was tested on cooling by liquid nitrogen and hydrogen . The play separation and breaking damage of the heat insulation weren’t found.

The further development should be directed to the detailed material study, its certification, technology of application and development of special equipment to produce actual tank.

Application capability to use the closed porosity elastic foam polyimide as a cryogenic heat insulation is being considered. Its apparent density is ~ 10 kg/m3, continuous heat resistance is up to 300°С. So application of such material reduces tank mass. The first examples of the closed porosity polyimide with 10…18 kg/m3 density and thermal conductivity of 0,028...0,031 W/(m*degree) were received. However, its application is limited by presence of considerable number of open freckles in material structure that is not permissible. That’s why it is needed to continue studies on further improvements of this promising material, on investigation of properties in accordance with requirements specification of the NPO MOLNIYA. The estimation of possibility to use elastic foam polyimide as an internal thermal insulation for the cryogenic tank or pipeline walls should take place.

The experimental researches of the new structural and thermal heat protection materials should be performed on models of tank verifying construction and technology with the purpose to create actual tanks for aerospace industry.

The results achieved in development of the new-generation composite thermoplastic materials and technologies of its production create an actual basis for the further development of aerospace systems.

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Breaking strength of glue connections of RIPOR and ISOLAN foam plastics to the D16an aluminum alloy (kg/sm2) at 20°С testing temperature