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BURAN Orbital

Spaceship Airframe


Strength of BURAN Orbital Spaceship

Dr. Tarasov A.T.
The task of optimization of loading the orbital spaceship (Orbiter) in a joint flight with the rocket-launcher and in autonomous flight is considered. Loads are made closer at these two stages by introducing constraints. The application of finite element method and of modern computers for determining stressed-deformed state and lift capabilities of the OS construction is shown. The issues of provision of aeroelasticity and fatigue strength resistance in a wide range of aerodynamic and acoustic influences at the stages of launch and descent are considered.

External Loads

A definition and standardization of external loads exerted on the airframe construction represented the important component of safety control of flight for strength conditions.

This work included:

  • development of strength standards;
  • calculation of external loads in all cases of operation;
  • development of methods and conditions of ground-based tests;
  • issue of conclusion on the flight of vehicle for the complete set of made calculations and ground tests;
  • preparation and measurement of external loads at flight tests, confirmation of standardized loads.

At development of the strength standards and definition of external loads, the following problems were solved:
1. Determination of external disturbances, in particular, standardization of the speed and wind speed gradients, gusts, deviations of atmospheric density.
2. Choosing of the so-called ‘reference trajectories’ of launch for the standard unperturbed atmosphere.
3. Setting constraints for dynamic pressure parameters of the rocket launcher trajectory: dynamic pressure, axial g-load and the product of angle of attack by dynamic pressure at zonal wind effect.

The loads at the launch stage (in joint flight with the rocket- launcher) exerted on the main units of the spaceship airframe (wing, tail and fuselage) have appeared to be much greater, than in an autonomous flight.

Therefore, the decision was made to decrease the loads at the launch stage. At formation of the time shadow of the rocket-launcher flight the dynamic pressure constraint of q < 3,000 kgf/m2 and axial g-load constraint of nx < 3 was set by throttling engines. Besides, at zonal wind effect, the rocket’s turn on the wind was stipulated to reduce the spatial angle of attack 2…3 times. A special automatic system is installed on the launcher to ensure constraints on products of dynamic pressure by components α and β of spatial angle of attack:

  • α · q < 15,000 (kg/m2)·deg;
  • β · q < 13,000 (kg/m2)·deg - as values (α · q) and (β · q) determine lateral loads on the units of the spaceship.

Imposing of these constraints has multiply decreased loads of joint flight having approached them to a level of autonomous flight.

The second group of external loads includes repeatedly static loads, which alongside with thermal were used for determining the service life of main load-bearing elements of the construction.

Having analyzed the Orbiter’s flight trajectory parameters by method of mathematical simulation on the basis of algorithms implemented in the Orbiter’s control system, the trajectory with mean repeatability of loads was chosen. At the stage of launch and descent, the conditions of flight were varied. As a result, the time shadow of probable changes of Orbiter’s trajectory parameters and deviations of aerodynamic control surfaces was made for such a flight. This time shadow was named a ‘standard flight’.

Then the operational loads were calculated along the trajectory of chosen ‘standard flight’, as well as the random loads, the bulk of which are of dynamic character. In many respects these loads effect on the construction fault probability.

The important stage of solution of this task was determination of reliability coefficients of conditions of the construction fatigue strength.

The service life of construction was determined in two stages:

  • at the first stage it was an initial preset service life;
  • at the second stage after realization of repeatedly-statistical tests and measurement of external loads repeatability at flight tests - the final service life.

The final stage of works was measurement and confirmation of normalized loads and loads of ‘standard flight’ at flight tests. Strain gage indicators after preflight calibration were installed on the vehicle to measure external loads.

Static Strength

The greatest complexity at solving problems of the airframe static strength by theoretical methods was represented by the following issues:

  • effect of great concentrated forces applied to the airframe in points of attachment to the rocket-launcher;
  • existence of irregularities in construction, the of which are a large cut-out in the shell of fuselage for the payload bay and cut-outs for a landing gear in a fuselage and wing;
  • joint operating of construction elements made of high-strength titanium and aluminum alloys;
  • joint operating of heat-protective planes and carbon-carbon elements with the load-bearing structure.

Determination of the stressed-deformed state (SDS) and lifting ability of the airframe and its units has required a wide application of calculations on the basis of finite elements method (FEM) with using of the available park of computers.

In order to expedite creation and development of computational model (FEM), the spaceship construction was divided into the following units:

  • nose part of fuselage with cabin (NPF);
  • medium part of fuselage (MPF);
  • rear part of fuselage (RPF);
  • door of payload bay (DPLB);
  • wing (Wg);
  • internal elevon (El. int.);
  • external elevon (El. ext.);
  • balance flap (BF);
  • vertical tail (VT);
  • rudder - air brake (R-AB).

The unit calculations for operational external loads were fulfilled independently. To determine the interaction force produced by a unit towards a separated part of construction, a rigidity simulator was introduced into calculation model. This allowed to determine SDS of the units and loads applied at their joints with the adequate accuracy.

By the stage of finish designing on the basis of created MEF models of separate units the integration of sub-constructions in a unified model of the whole orbiter was made. As a result, the updated outcomes of IDS were obtained, which allowed to design cross-section of load-bearing elements and to reduce structural mass.

Application of such technique provided designing reliably and with good weight efficiency, and also permitted to solve the problem of strength at static and thermal-strength tests for separate units and bays.

The example of finite-element model of the BURAN Orbiter is shown in the Figure.

Some Specific Features of Compliance with the Requirements of Aeroelasticity

The fulfilled calculated-experimental studies of aeroelastic stability of the BURAN Orbiter have confirmed the necessity of integrated approach to solution of these tasks. Such approach consisted of both improving the available calculation methods, and experimental confirmation of the obtained characteristics using of dynamically and structurally similar models and full-scale vehicles.

Specific features of construction and operation conditions of the Buran Orbiter (lifting fuselage, large cut-out for the payload bay, interference on lifting surfaces from units of the launcher, large angles of attack and control deflections, wide range of Mach numbers) have needed correction the traditional calculation method for determination of aeroelasticity parameters. For this case the local lift coefficients resulted from testing of vented aerodynamic models in wind tunnel are used. The analysis of features of the BURAN Orbiter’s operation has shown that the transonic range of flight velocity (M = 0,8 … 1,3) is the most unfavorable.

Estimation of dynamic characteristics (frequencies and oscillation modes) with application of MEF was conducted to specify the preset elastic scheme of the Orbiter at the stage of finish designing. The calculations showed that separate oscillation modes are present with the low-frequency range, which are not detected by conventional computational methods (oscillation of vertical empennage in chords plane, low-frequency oscillations of doors of payload bay, etc.).

This, in turn, required determining the critical flutter speed at presence of the indicated oscillation modes (panel flutter of doors of payload bay and oscillation of vertical empennage oscillations in chords plane).

Horizontal frequency tests of the Obiter in different complete sets (appropriate to horizontal flight tests and nominal flight) were worked out to achieve an experimental confirmation of orbiter’s dynamic characteristics necessary both for determining reserves on flutter stability, and reserves on the control system allowing for structure flexibility. Special frequency tests for supports and frequency tests for the complex components are conducted for the article in a nominal complete set, too.

The results of frequency tests have confirmed good concurrence of frequencies and oscillation modes with calculation, obtained using the finite elements method.

A wide research of aeroelasticity parameters was conducted in T-109 wind tunnel of TsAGI both for dynamically similar, and for structure similar models. The principal target of researches was:

  • confirmation of the calculated flutter critical speed, the effect of construction flexibility on aerodynamic coefficients, confirmation of the requirements to structural and technological parameters of the Orbiter at different modes of operation, study of flutter stability, determination of buffet parameters in case of the open flappers of air brake, research of effect of the launcher units interference on aeroelasticity parameters.

The results of studies have shown that the using of comprehensive approach at determination of the dynamic scheme parameters and critical flutter speed (clarifying the analytical model on the basis of more perfect computational methods, full-scale and model experiment) showed a good concurrence of calculation with experiment.

Fatigue Strength of Construction under Acoustic Loading

The provision of structural strength of The Buran’s airframe exposed to acoustic loading is a complicated problem connected with a number of specific features of Orbiter:

  • presence of heat-protection insulation (HTI) on the surface of Orbiter;
  • reusable of usage;
  • high level of acoustic loads (up to 168 dB within the frequency range of 16 … 4000 Hz).

The complicity of the problem was aggravated by the fact there was no experimental base or calculation methods to estimate the construction response to acoustic loading at the initial stage of works.

At the stage of finish designing, the so-called “global” estimations of construction response to acoustic loading (by simplified methods) were conducted, and selection of fasteners for the skin was made to provide the strength of the regular construction. Creation of experimental base for simulation of acoustic loading of separate construction fragments in running waves chambers and for testing full-scale OS’s units in reverberation chambers was stipulated at that time.

The experimental base was created in the headquarters institutes: TsAGI and SibNIA. Calculations of construction response to acoustic loading were executed both by simplified methods and using the finite elements method.

Calculation showed that the construction response was determined by the lowest panel frequencies for this zone. The frequency range of separate construction panels was from 100 to 300 Hz, the maximum root-mean square values of vibration acceleration levels induced by acoustic effect reached 100 units, the maximum of root-mean square values of stress on stringers was 40 MPa, and on the skin the stress level was 3…5 times lower, than on stringers.

Manufacturing and testing for over 200 construction fragments of area of 1 … 2 m2 from 40 zones of the Orbiter were fulfilled to obtain an experimental confirmation of construction strength against acoustic loading at the stage of autonomous tests. From the indicated quantity of fragments, 30% represented the construction of internal bearing structure without HTI, and remaining 70% were panels supplied with HTI, and special attention was focused on irregular zones of construction (cut-outs, hatches, doors, radio-transparent inserts, glazing, and others). During the tests of a number of fragments, the acoustic and thermal loads were simultaneously applied. As a result of autonomous tests, a satisfactory concurrence of experimental and calculated loads (for SDS and level of g-loads in regular zones of a fragment) was obtained. The following specific locations of damage were detected in the airframe construction in the form of cracks:

  • compensators of redocked construction load-bearing elements (frames and ribs with the skin);
  • brackets for fastening stringers to frames;
  • stringers at rivet holes of their attachment to the skin;
  • tips of corrugation (in corrugated panels) and tops of stiffening ribs (for milled panels) of internal bearing assembly.

By results of autonomous tests, the design updating of separate construction locations with subsequent experimental checkup and their insertion into the nominal vehicle were proposed. It is necessary to note that the autonomous tests (except for fragments with thermal loads) were carried out to provide the complete service life at nominal operation of the Orbiter with a five-fold reserve. Some fragments were continued under tests up to occurrence of construction failures.

Besides the autonomous tests of fragments, a number of complex acoustic strength tests on the whole full-scale units of the BURAN Orbiter were worked out: for external elevon, balance flap, vertical empennage, wing, and on a section of a wing leading-edge. The calculated estimations of construction loading with exposure to external acoustic effect, the results of autonomous acoustic tests of separate construction fragments and complex strength acoustic tests of full-scale units have shown that the construction strength of the airframe is sufficient for the first operation phase.

In the process of provision of the BURAN Orbiter strength many fundamental problems were solved:
1. The strength sdtandart of reusable space systems, such as ENERGIA-BURAN were developed. Allowing the limitation and using special system of rotational displacement of the rocket-launcher ‘on the wind’ the loads exerting on the Orbiter at the launch were considerably reduced. The rated conditions were determined at operation of the repeatedly-static loads together with temperature, and vibration and acoustic effects. Experts of organizations-developers of the Orbiter and Launcher and of the branch research institutes created of strength standards. The author of the article directed these works on behalf of NPO Molnya, Mr. Gladkiy V.F represented NPO Energia, Mr. Selikhov A.F – TsAGI, Mr. Karmitin A.V.- TsNIImash.
2. The technique of subsystems calculation with application of finite elements method and ‘stringing’ the unit models in a unified system of the spaceship was developed and successfully applied.
3. Wide-scale researches of aeroelasticity on dynamic and constructively similar models approved the calculation methods at the initial stage of development and excluded essential adaptations of the construction after field tests.
4. The high level of acoustic loads required wide-scale theoretical researches and great number of experiments with samples, fragments and full-scale aggregates. To provide the acoustic tests of bulky objects for a high level of effect the RK-1500 acoustic reverberation chamber was constructed in TsAGI under the requirement specification of the NPO MOLNIYA.

The majority of these studies were conducted in domestic practice for the first time, and their results as well as the created techniques of strength provision can be successfully used at creation of space-rocket and aerospace systems of the future.

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Example of finite-elemental model of BURAN