BURAN Orbital

Spaceship Airframe

Creation

Non-Metallic Structural Materials of the BURAN Orbiter

Gutman I.L., Kulikova G.V.
The main usage fields of thermoactive carbon plastics and carbon-carbon materials in the BURAN Orbiter are considered in the paper. Basic requirements to technical solutions are listed. The effectiveness of the composites used is estimated. Prospects of creating new composite materials and constructions of them are shown.

Carbon Plastics, Carbon-Carbon

Weight effectiveness and thermal resistance are basic requirements to reusable spaceships. The widest perspectives to meet these goals are provided by using new materials, primarily composites with high-strength and high-module fiber fillers (carbon-, glass-, organo-, boron plastics etc.).

Modern carbon plastics outclass aluminum alloys about 5 times in specific strength, almost 3 times in specific modulus of elasticity, more than 2 times in fatigue strength. High corrosion resistance of carbon plastics and the ability to use carbon-carbon at up to 1600°C should be added to this.

The use of carbon- and boron fiber armed composites besides a considerable decreasing of construction mass raises strength thus improving a plane’s flutter characteristics and fostering the creation of lift low-thickness surfaces with high-aerodynamic characteristics.

Problems appeared at working with carbon type composite materials (CCM) are described below:
1. Low impact strength. It eliminates using of impact riveting attachment.
2. Electrochemical corrosion when contacting aluminum alloys. This fact excludes aluminum fittings from the carbon-plastics structures.
3. Expansion forces of titanium and steel rivets may cause damage to the holes of the joined elements. It eliminates usage of impact riveting. 4. Low coefficient of linear extension along the fiber. It complicates operation of composite materials in combination with metals at big thermal changes. This fact should be considered when designing structures. Titanium alloys are the most appropriate for use together with carbon plastic elements. The previous problem (corrosion) is also solved in this case.
5. High anisotropy of physico-mechanical properties requires inserting the arming fillers to provide resisting of force in two perpendicular (0° and 90°) directions, as well as resisting of shear loads (± 45° layers). Simultaneous action of equal forces qσx, qσy, and qτ it is needed increasing of covering thickness, thus increases the mass of the structure in comparison with the isotropic metal one. Maximal weight efficiency is achieved in one-axially loaded structures (such as rods, booms), however their weight fraction in the whole framework is unsubstantial.
6. Low shear performance of composites especially at high temperatures, closed to maximum operational temperature (τ/γ = 2 at T = 160°C, for metals τ/γ = 8). The use of electrochemical viscoserization of fiber, nitric acid processing and other actions allow to increase interlamination shear for unidirectional material to 6...8kgf/mm². It allows to raise the compression strength limit to 90...100kg/mm², and to gain the shear strength limit of 25...30 kg/mm² for tiles with stacking of ± 45°.
7. The most complicated problem is jointing of different composite elements and units to each other and to metallic structure elements.
8. The variation coefficient of mechanic properties is considerably higher than that of metals.

The listed problems define the features of construction development, choice of primary schemes, solutions of the members relating. Anisotropy of mechanical properties, depended on fiber’s orientation, gives a broad way to a designer’s creative thought.

Main actions allowing to design a composite construction, mass of which less than a metal one (with features listed above):
1. Titanium alloys are combined with the composites.
2. Rivets are replaced by adhesive-mechanical joint with usage of a bolt-rivets or bolts.
3. Oval head fittings are used with no countersinking of the composite.
4. To raise the bearing strain strength limit of composite materials calibrated nut tightening is introduced. Titanium washers with class three processing to the hole are placed under the nuts. This allows to increase σcm to more than two times.
5. At complicated loading of the composite shells the reduced module of elasticity and consequently critical stresses of stability are decreased due to combination of varied laminas (0°, 90°, ±45°). So three-layer panels with carbon plastic load-bearing skin and with honeycomb filler are most effective. Polymeric honeycomb blocks are used for structures heated to 160°C, glass honeycomb blocks are used for up to 250°C.
6. To lower mechanical properties variability of the initial material (AeLUR-0.08) a coil-by-coil selection is used. It allows to increase the level of average strength and stiffness data and their variation factors. This also eliminates the necessity to introduce additional safety factor which is rather high for modern carbon plastics fsafe = 1,25.

The effect of the production technology on the properties of composites is checked on the witness samples, cut from the checked part, and accompanying samples similar in material, laminarisation, and technology to those of the checked part. The tests of the samples reproduce the loading situation of the material in the structure and allow to estimate the strength of material with appropriate probability using the results of non-destruction inspection.

The different composites: carbon-, organic, glass- armed plastics, carbon-carbon and others - were used in structure Of BURAN Orbiter. Cargo compartment doors, wing leading edge and nose cone of fuselage, pipes of pressurization system, radioparent fairing, hatch doors are made of composites.

Cargo compartment doors are most interesting. Their length is 18,5 m, width by the arc is 3,8 m, and sheathing area is 144 m².

Three versions of the structural primary schemes of fuselage were considered:
1. Cargo doors are not included into the airframe operation. In this case the torsion rigidity of airframe decreases in three times, and its mass increases.
2. Cargo doors are included into the airframe operation. Such scheme provides minimal construction mass and high stiffness. But it requires installation of special locks on the cross-joints of cargo doors and fuselage. These locks allow not only to open and close the doors, but also to transfer the forces in all the three directions. The developed design showed that the creation of reliable locks satisfied requirements would substantially increase the airframe mass and bring about a large number of construction, mounting, deformation and other problems. As result this version wasn’t accepted for production.
3. Cargo doors accept pressure difference and work only against torsion. This scheme was accepted for production.

In the highly heated areas of cross-joints and hinge units the KMU-8 carbon plastic straps are placed. The KMU-8 is based on the AeLUR-0,08P carbon ribbon and the PAIS-104u (u stands for improved quality) polyamide resin. The KMU-8 carbon plastic was designed by VIAM under the specification of NPO MOLNIYA and is now widely used in devices operating at up to 250°C.

The use of composites decreased the weight by 620 kg compared to aluminum analogue. The doors of traditional aluminum alloy were made for the first flying version and were under all the tests. The mass measurement confirmed the estimated decrease.

Full set of strength, climatic and other tests at both construction choice and proof-of-compliance stages verified all the decisions of the designers. Samples, structure parts, and the whole airframe were under tests. The materials were designed by VIAM (Mr.: Tumanov A.T., Shailn R.E.). Technological development and production of composite parts (panels, bulk heads, booms, diaphragms) were made by ONPO TECHNOLOGIYA with direct participation of the structure designer.

Stringent requirements to the prepreg were met by organizing a coil-by-coil checking with limits imposed on the stretching and pressing strength of one-directed plastic being not less, than 90 kgf/mm². Witness samples were formed and glued simultaneously with the parts. Designing of a special type of fittings solved the problem of the mechanical joint for parts of different materials. This fittings use of increased-diameter and simultaneously reduced heads of bolts and bolt-rivets and calibrated-hole washers. The introduction of this new technology allowed to solve two problems: good gluing of the heatproof layer and lowering the parts overlapping zone when a carbon plastic part is in the package.

The wing leading-edges and nose cone of the fuselage heating up to T=1250°C are of the interest not smaller than cargo compartment doors. None of now existing constructional materials can not work at such temperature providing simultaneously the minimal output of heat to the metal construction on which they are located and also the deformations commensurable with the deformations of much less heated up units of the wing and the fuselage. For manufacturing of the specified configuration items there was developed and made the essence new material - the GRAVIMOL carbon-carbon composition satisfying all the listed requirements. Having the above mentioned properties the given material at the same time has got not so high mechanical characteristics: σв = 3…5 kg/mm2, τв = 1,5…2 kg/mm2.

That is why the configuration parts made of GRAVIMOL were with overestimate of mass. As the technological process of their manufacturing is labor-consuming (one detail is made almost half a year) now together with VIAM the activities on creation of new carbon-carbon materials with the simplified technology of manufacturing of details are being conducted:

• the increased strength with σв = 1520 kg/mm2;
• high-strength with σs = 80 kg/mm2.

The creation of high-strength carbon-carbon will allow to make of it the units of the airframe working in conditions of high temperatures and which are not having thermal protection covers. The activities on creation of the deflected wing of high-strength carbon-carbon now are being worked out. The application of new materials in the design of the units of the space vehicle required the realization of the large volume of experimental-research works for provision and acknowledgement of their strength from which it is necessary to note the following:

• all-round researches of cargo compartment doors three-layers panels with load-bearing layers of KMU-4e carbon plastic and with PSP honeycomb filler (tested about four thousand various samples and fragments of the construction);
• the definition of permissible bearing stresses of carbon plastic in joints in view of torque loads fastening elements (about 50 samples);
• all-round researches of carbon-carbon wing leading-edges and nose cone of the fuselage (tested about 500 samples and full-scale fragments).

The proof-of-compliance tests of the payload bay doors were carried out on the number 0.04 Orbiter intended for static tests.

At tests in the area of the attachment of the most loaded fourth lock, the frame made of KMU-4e composite material collapsed at the load making 90% of the design one. The reason of the destruction was the large eccentricity while the transmission of load from the fitting to the belt of the frame because of the installation of large thickness gaskets and also poor-quality installation of the jointing bolts. In the area of destruction the strengthening of the frame belt is carried out. The efficiency of the carried out adaptation is confirmed by the comparison tests of the frame belt samples with the adaptation and without it. In the whole, the tests confirmed the sufficient static strength of the aggregate and allowed to give the conclusion on the static strength of the nominal door modified taking into account the results of the tests.

The activity of the vehicle in conditions of heating up to high temperatures and large vibration and acoustic (up to 168 decibel) loads required the realization of the ground improvement of the resource strength. The door of payload pay made from the composite material passed the tests for all the effects in the structure of the compartments of the middle part of the fuselage.

Cyclical thermal resistant tests have not revealed any essential defects of the construction, however on panels of the payload bay there were found out the local swellings of the outer skin after 25 cycles of thermal and vacuum effects. The carried out autopsying (cutting) of the skin has shown that at assembly of the panels in some places laminar paper or polyethylene film from the VK-36 film glue connecting the skin from KMU-4e with the honeycomb filler was not removed.

NPO MOLNIYA in cooperation with ONPO TECHNOLOGIYA, NIAT, VIAM and TsAGI carried out research of the additional inspection methods of the payload bay three-layers panels allowing to find out the specified inclusions. The inclusion effect on the structural strength was studied. As a result the technique of the inspection including X-ray checking, ultrasonic and intra-red imaging was adopted. The tests showed that the inclusions of the size up to 64 cm2 (8x8) do not render essential effect on the static strength of the panel. That is rather important for the 1.01 Orbiter as any of the investigated methods does not allow to inspect the panels with the pasted TPS. The carried out inspection of the 1/02 Orbiter’s panels didn't reveal the defects which sizes exceed the specified permissible values.

The successful flight of the BURAN Orbiter verified the adopted design solutions and methods of their calculation. After the first flight there were not revealed any damages of the details of the composite materials.

Using the experience accumulated when creating the doors of payload bay, with the purpose of the further perfection of the mass characteristics of the space vehicles the broad introduction of carbon plastics into the construction is planned. The improvement the manufacturing technology of the fuselage, elevon, the balancing flap, the wing and vertical tail of the MAKS’s Orbiter is now worked out. The new materials, integral panels from the KMU-8 composite material will find their use in these constructions.

On taking place in 1989 in VIAM the exhibition the payload bay’s doors and the BURAN Orbiter’s units made of the composite materials were awarded with a gold medal of VDNH USSR.

The KMU-4e and KMU-8 carbon plastics applied for manufacturing of BURAN Orbiter’s units have the tensile and compressive strength σв = 90 kg/mm2 and modulus of elasticity E = 12500 kg/mm2. The operation temperature for KMU-4e is 160°C, for KMU-8 - 250°C.

The requirements on minimal mass to advanced aerospace vehicles, developed at the NPO MOLNIYA, are even more stringent. That is why the activities on creation of new carbon plastic has already conducted:

• high-strength with σв = 210 kg/mm2;
• high-module with Е = (22…25)·10 kg/mm2;
• heat-resistant with the operation temperature up to 350°C;
• carbonized carbon plastics with the operation temperature up to 650°C;
• heat-resistant glues and honeycombed plastic.

Creation and fast development of the quantity production of the materials allowed to embody in life courageous plans of the designers and scientists and successfully to bring off the flight of the first Soviet reusable spacecraft. The huge merit in it alongside with the workers of the aircraft industry belongs to the specialists of institutes and enterprises of Academy of Sciences of USSR, Ministry of chemical industry, Ministry of ferrous metallurgy, Ministry of non-ferrous metal, Ministry of light industry and other industries.

The advanced achievements of the science and the industry are concentrated in BURAN Orbiter. Practical value of these achievements has a stimulating influence of the scientific progress in various fields of the national economy.

The high-temperature polymeric composite materials, glues, synthetic felts etc. can be widely used in the national economy: in the automobile and machine-tool construction, medicine, agriculture, radio engineering and other fields providing the decrease of material usage and labor input and increase of the work productivity and quality of the products.

The creation of the composite materials on the base of carbon opened broad capabilities of their application for high-resource heaters, electrical furnaces, melting pots for smelting and pouring of the refractory metals, for thermo stable constructions of the space vehicles. These materials can be applied also for replacement of the skeleton parts injured at various traumas and illnesses as the elements having the unique compatibility with living tissue. Especially it is necessary to mention the use of this material for braking devices of the planes such as TU-154, AN-124 etc. that resulted in rising the resource of the brakes in 2,5 times.

High accuracy of the request and the reproducibility of the parameters of the new materials developed for the BURAN Orbiter, complex technological processes of their manufacture and complex tests in terms of the service conditions required the sharp improving of the manufacture culture and qualification of the experts. The use of these materials and technological processes in the national economy will cause of the appropriate qualitative changes in many industries that will promote the technological progress.

Reference

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