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Author: Konstantin FROLOV, Vladimir RACHUK, Nikolai MAKHUTOV, Mark RUDIS
By Academician Konstantin FROLOV, Director of the RAS Institute of Mechanical Engineering named after A. Blagonravov; Vladimir RACHUK, Dr. Sc. (Tech.), Chief Designer, KHIMAVTOMATIKA Design Bureau; RAS Corresponding Member Nikolai MAKHUTOV, Head of Department, RAS Institute of Mechanical Engineering; Mark RUDIS, Dr. Sc. (Tech.), Head of Department of the above Design Bureau
The central achievement of the staff of the KHIMAVTOMATIKA R&D Bureau (Voronezh) over the past few years has been a sustainer engine for the second stage of the ENERGIYA-BURAN space booster complex. This is the RD- 0120 oxygen-hydrogen engine with a thrust of 2,000 kN.
Its operating parameters (chamber pressure, temperature in gas generator, rate of gas flow from nozzle and RPM of the turbine of the turbopump as well as component dimensions) are really impressive. Also impressive are the strains and stresses in its components, especially those associated with fuel supply-in the turbine, high-pressure hydrogen pump, chamber, etc. -all of these are much higher than of the most home- made and foreign analogues. In recognition of this fact the engine designers attached particular importance to the strength and dependability of its junction points and components.
In dealing with all these problems, Voronezh scientists established close cooperation with their counterparts from the RAS Institute of Mechanical Engineering (named after A. Blagonravov) who possess a wealth of experience in methods of calculation of the mechanics of objects under deformation, boosting the durability and service life of different kinds of machines and equipment. One of the most "strained" components of RD-0120 is the running wheel (rotor) of the high-pressure hydrogen pump. It represents a complex geometric structure which operates at spin rates of up to 600 m/s at cryogenic temperatures and high dynamic loads. These components are produced from titanium alloys with 5 percent of aluminum and 2.5 percent magnesia by methods of powder metallurgy and isostatic pressing. The design parameters of these key components rest on theoretical solutions ensuring their high strength.
Technological tests of these running wheels were conducted for each of them separately at room temperature and the spin rate was 3 - 7 percent higher than the operational one. The destructive spin rate was determined at a cryogenic test stand, and it should be noted at this point that, according to calculations and experiments, this value largely depends on the ductility of material. It is therefore very important that this parameter be the same in different zones of the rotor and retain its high value at the liquid hydrogen temperature (20 K). This problem was solved by choosing the appropriate kinds of materials and production technologies thanks to which the destructive spin and static strength values are within the optimal limits.
Our experts have also been able to cope with the problem of fatigue failure of the impeller wheels which is caused by "pulsating" pressure and which depends on the wheels design. Positive results were achieved by a transfer to powder technologies and certain design improvements at the junction points of blades and disks.
Deserving of special attention are problems of increasing the service life of turbine blades of the second stage. Mechanically these form a single whole with the disk and wheel band * and are made of heat-resistant nickel alloy containing 7 percent of tungsten, 15 percent of cobalt and 3 percent of molybdenum and produced by methods of powder metallurgy. In order to increase
* Metal band connecting tips of blades for increasing their strength and reducing wear and vibration. - Ed .
the service life of the vane wheel its designers increased the number of vanes, their radius, the thickness of the wheel band and also resorted to powder (granule) technology. The static strength of the vane wheel meets the whole range of operating requirements, as has been proved by stand tests.
However in cyclic-mode operation of RD-0120 at regimes exceeding the nominal local strains proved to be greater than the yield strength of the material at the base cross-sections of the blades and points of their attachment to the wheel band. Because of that fatigue cracks developed at these points after only 5 to 6 working cycles.
The blades are also subjected to pulsating pressure. And although strain changes of this kind are relatively low, at frequencies of the order of 20,000 cycles per second the summary number of strain cycles reaches 10 7 - 10 8 , causing fatigue failure of the blades. And it has been possible to increase their service life through a "redistribution" of stresses and reducing their concentration by 20 percent.
Another important aspect of increasing the dependability of RD-0120 has been the choice of structural (construction) materials. The main criteria here were their rates of deformation and "behavior" in hydrogen medium under high pressure at cryogenic, room and increased temperatures.
The stability of a range of metals and alloys under such conditions was investigated by experts of the KHIMAVTOMATIKA Bureau in conjunction with specialists of the RAS Institute of Mechanical Engineering, the Physico-Mechanical Institute of the National Academy of the Ukraine and other centers.
Rated stresses in a turbine blade.
The scientists established the regularities of their deformation and "hydrogen friability" which have to be considered in designing engine components.
As has been established, gaseous hydrogen at room temperature produces the greatest impact on the plasticity and strength characteristics of materials (relative transverse narrowing, destructing stress), low-cyclic fatigue, rate of cracks propagation. As for "hydrogen friability", it reaches its maximum at 173 - 473 K. And the danger is the greatest in parts of complex configuration and in zones of stress concentrations, because the plasticity of material is reduced by 2 to 10 times.
Tensile tests have proved that in hydrogen gas medium and at high pressures and temperatures close to room ones, the strength and plasticity of alloys on the basis of iron and nickel are significantly reduced. Many of them are subject to appreciable "hydrogen friability".
Our studies have made it possible to map out some "preventive measures". In designing engine components it is first of all necessary to "rule out" zones of plastic deformations and other "foci" of tensions. Alloys should be used which are resistant to "hydrogen friability" and protective coatings of copper, silver and gold. The effect of these recommendations has been proved in practice. The service life of engine parts was substantially increased by copper coatings of fuel supply components and the use of weld seams of steels resistant to gaseous hydrogen environment in the zones of welding joints.
The effect of hydrogen on the parameters of structural materials manifests itself in the fact that at certain pressures and temperatures it can be "transferred" into the metal, that is the concentration of hydrogen in it is increased. This process takes place according to the diffusion and dislocation mechanisms at plastic deformation. An important role belongs to the level and gradient (drop vector) of stresses, and also time.
On the basis of our experimental and theoretical data we have drawn up the equations for: dependence of material plasticity on pressure, temperature, initial plasticity, diffusion factor, time, etc. for the hydrostatic pressure in the concentrator zone and an analysis of material hardening. Using the above ratios and such a characteristic as the true strength of materials in hydrogen medium, one can determine the maximum stresses upon engine components at the stage of breaking point.
All of the above equations and models provided the basis of the methods of calculation of the durability and longevity of RD-0120 components which are in contact with hydrogen gas and made it possible to work out theoretical assessments of their service life. The results of test and practical use of the engine have confirmed the viability of this method.
At the present time we are modelling the "exhaustion" of the RD-0120 service life taking into consideration the main factors of damage and the ultimate conditions. This should make it possible to increase the accuracy of our calculations of durability, service life and operational risks associated with oxygen-hydrogen engines of new generations.
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