From SPIRAL to MAKS

Features of the MAKS Structure

Dr. Tarasov A.T.
There are considered the design features of the External Fuel Tank and the Orbital Plane of the MAKS system. Due to these features the MAKS structure is of minimum weight and acceptable technology and cost are needed for development and creation. The concept of the system with the External Tank has allowed to receive the high payload at a allowable weight of all system.

It is obvious that conception of partially reusable aerospace system with an external tank considerably reduces construction mass if comparing with completely reusable system. Such mass reduction is achieved thanks to the following factors:

• lack of oxidizer and combustible agent tanks on the orbital plane;
• reduction of plane construction mass caused by reduction of its dimensions;
• reduction of the orbital plane heat protection mass caused by reduction of its area;
• reduction of landing gear mass caused by reduction of landing mass;
• reduction of communication mass caused by reduction of the orbital plane dimensions.

The developmental work of NPO MOLNIYA showed that at the same perfection of construction and at the same used materials, the payload reduction depending on orbit altitude for completely reusable aerospace system is about 50…100% if comparing with a system with external tank.

The External Fuel Tank (ET)

Depending on tank diameter, external loads and inside pressure, tank envelope construction can be of three different types: smooth, honeycomb and stringer-bulkhead. If tank diameter is small and external loads are negligible, the optimal variant in weight, technical and expense terms will be smooth tank envelope. The envelope of hydrogen tank in its nose part has this particular structure. In the middle part of tank compartment the bending moments are quite significant. The optimization in accordance with weight parameter results in the chose of honeycomb envelope for the middle part. The calculations for different sectors of the honeycomb envelope were conducted to choose optimal envelope width, height and width of honeycomb edges.

At tank’s compartment development the second problem was to choose required constructional materials. The conducted researches on weight efficiency of different materials application for building external fuel tank working at cryogenic temperatures showed that the application of 1460 new aluminum material reduced tank mass under 700 kg. This alloy if comparing with widely used 1201 aluminum alloy has smaller density (on 9%), reduced modulus of elasticity (on 14%) and increased breaking point (on 35%) and yield point (on 60%).

Laborious study was conducted to optimize high-strength connection assemblies to fasten external tank and the orbital plane placed on the carrier-plane. The maintenance of stresses appeared in the tank elements during transportation on the carrier-plane at the level not exceeding those stresses appeared during injection was provided.

As the result of performed optimization of the external fuel tank a high level of mass characteristics was received – the average mass of one square meter of surface is 8,3 kg/m2.

The application of the advanced composite materials, for example, of KTMU-1 material made on the basis of polysulfone and UKN carbonic composite braid results in the reduction of external tank mass by 1...1.5 tons. These composite materials have the breaking point of σ = 160 kg-force/mm2 at density γ = 1.4...1.6 g/sm3. The Orbital Plane (OP)

The orbital plane construction is completely made of high-temperature KMU-8 composite materials. The metal is used as less as possible, only for high-loaded assemblies, undercarriage and bolts to connect composite materials between them. KMU-8 material is quite thermally stable (operating temperature is 250С), so that it allows to reduce the thickness of heat protection tiles and mass. Besides, heat protection mass is reduced in addition due to increase of tile dimensions up to 250…300 mm (for the BURAN system – 150150 mm). This action is caused by closer values of linear expansion factor of composite covering and heat protection tiles.

The orbital plane scheme has hinged wing panels. This constructive feature allows to solve two problems:

• to maintain balancing and acceptable stability and controllability characteristics at high angle of attack when entering atmosphere and at low angle of small attack when making pre-landing maneuver;
• to reduce considerably temperatures on wing panel to the level permissible for quartz tiles.

The analysis of wing heat protection showed that the weigh expenses spent on wing panel turning system appeared to be less than constructive development of ‘hot’ leading edge made of heat-resistant ‘carbon-carbonic’ materials at static wings. Besides, there are several technical problems required to create such ‘hot’ construction with small radiuses of wing dulling. It is obvious that it would require to develop new heat-resistant material designed to be used at higher temperatures or considerably increase leading edge sweep as it had been made on the HERMES orbiter. It is necessary to indicate that reduction of temperatures for wing leading edges with higher dulling radius was not such a problem in the BURAN project.

At consideration of fuselage, wing and tail panels construction three types of edged panels and honeycomb panels made of KMU-8 composite material were considered.

The researches results have shown that the most profitable variant if considering weigh term was honeycomb panels with specific mass of 3...4 kg/m2 that is on 5...15 % less than for edged panels.

The three-layered honeycomb panels were used for shifting elements of construction (payload bay folds, spar webs and other) as far as honeycomb panels have maximum advantages when they are used under high shifting loads.

The edged panels were approved for the wing, fuselage and fin panels exploited under mono-axial loads. This choice was caused by technological and operational conditions.

To minimize fuselage panels mass it is obvious to consider only longitudinal junctions. Their maximum length is 14 meters and maximum width b is 4 meters. The latter value was chosen thanks to limitations of the existing equipment used in ULYANOVSKY Industrial Complex.

Farm constructions with minimal mass are used for creation of walls with higher structural depth. They were used in the BURAN project.

It is necessary to indicate a multi-functionality of separate elements of the MAKS orbital plane. The typical example is a fuselage ring N. 20 separating rear part from payload bay. The connection assemblies of external fuel tank, vertical fin, hinged wing panels and main engines as well as longitudinal stringers of main gears are placed on it.

Indicated constructional features and new materials used for the external fuel tank and the MAKS orbital plane guarantee minimal mass of construction and as a result high performance characteristics of the system.