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BURAN Orbital

Spaceship Airframe


The Doors of BURAN Orbiter Payload Bay

Stepanov A.P.
The paper discusses the problems occurred when designing and building the payload bay (PB) doors of BURAN Orbiter: choosing the material and configuration, making thermal compensation between the KMU composite doors and the aluminum fuselage, developing the technology of composite parts manufacturing. Technical solutions to be used for the first time when making PB doors of KMU are covered. Ways of improving the design are analyzed as well as the possibilities to use the gained experience in developing new air- and spacecraft.

The BURAN payload bay doors are designed to provide access from the bay to outer space with the purpose of gripping and installing the spacecraft which need repairing or returning to Earth, injecting spacecraft into their orbit, mooring to orbital stations, and letting the crew to open space.

The main problem in creating the doors was to secure their operation in the orbit in considerable temperature changes and unequal heating of the surface. The main factor of the external effect to define the choice of material was the band of temperatures in the orbit: from –110˚C to +120˚C. Another important problem was to meet the high strength and stiffness demands with minimal mass and moment of inertia.

The two symmetrical doors (port and starboard) comprise rotated top part of the fuselage. They are attached to the airframe sides via hinge assemblies and are junketed in closed position by the Orbiter’s upper side. When spread sideward, they open the bay from the crew cabin to the engines compartment (18.5 meters wide), and equal in width to the widest part of the fuselage (5.5 meters).

According to the calculations, if made of metal, the doors would make it impossible to build any serviceable locking system of appropriate size and mass due to their considerable heat deformation, since they comprise a big-sized shell structure of unclosed contour. A material with low linear extension factor and high specific strength was needed. So VIAM created a combined material under the specification issued by NPO MOLNIYA for the doors. It was of the KMU-4E carbon plastic armed with the SVM organic canvas. The characteristics sully satisfied the requirements of the Orbiter’s designers: high specific strength and linear extension factor close to zero. The calculations have shown, that when heated, the doors made of this material almost exactly retained their form. But another problem was remaining unsolved: since the fuselage is made of the 1201 aluminum alloy, the extreme temperatures would make it deform considerably, bending the hinge axis to 50 mm at the span between side hinge assemblies with the span itself varying from +40 to –52 mm. This would cause arching forces in hinges and additional stresses in the framework. To make the doors rotate freely a thermal compensation between them and the hull was needed. With this purpose each door was divided by length into four sections with compensation gaps between. Every two neighboring sections were joined together by five rods allowing them to move longitudinally and preventing them from moving separately around the axis. This saves the smooth shape of the fuselage and transceivers the shifting forces of the torque. Besides, each section had three hinge assemblies being firmly attached only to one of them. It could freely travel vertically against the other two. This was the way thermal deformations of the structural members were isolated, fostering the free movement of the doors.

The introduction of the compensatory gaps didn’t mean excluding the doors from the fuselage structure. The doors are a frame composite structure. They sustain the excessive pressure and are loaded by the torque closing the framework. This results in considerable reduction of the airframe’s weight. Besides the gaps prevented the doors from being put into service when the fuselage is bent.

Each section consists of a three-layer panel, a VT-23 titanium alloy frame, and seven π-shaped KMU-4E bulkheads. A panel is a 15- mm thick VK-36-glued structure of one external and two internal shells of KMU-4E + SVM, separated by a PSP 2.5×45 polymeric paper honeycomb block. The 0.4-mm thick shells are built of two layers of carbon ribbon placed longitudinally and a 45˚-tilted layer of SVM canvas between them. In the places of titanium frame and bulkheads the panel thickness was reduced. For this purpose the honeycomb filler and internal shells were replaced on outlining ribbons. Since the installation of these ribbons the final operation of the gluing procedure, the ribbons serve to correct all shape defects occurred when building and gluing the shells and the honeycomb block. This couldn’t provide a high-quality gluing of the pre-polymerized ribbons with the panel. So there was suggested a principally new solution: the outline was formed of soft ribbon and placed to the assembled panel. It was polymerized and glued to the panel simultaneously in the autoclave. The panel and the frame assembled via titanium bolts or bolt-rivet glued on VK-27 glue.

The temperature of the edges and the compensation gaps reaches +250˚C, so these parts are tiled with KMU-8 high-temperature carbon plastic patches.

To provide the quality of mechanical attachment of the composite parts NPO MOLNIYA in cooperation with TsAGI and NIAT carried out a program of strength- and service life tests of KMU samples with various types of fastenings. The results showed, that that the best qualities are provided when the following conditions are met: no-beat installation of the titanium fastenings with enlarged lenticular heads, enlarged washers with accuracy rated holes, package pressing force of 100...150 kgf. These conditions provided the σcm = 50...60 kgf/mm². Special titanium bolts and bolt-rivets for the doors were developed and standardized by GPKO NORMAL on the technical specification of NPO MOLNIYA.

The doors are fixed in closed position by 33 locks. The first and last sections have four locks each on lateral joints with the fuselage. They are linked with rods and are driven with a single electric actuator. The longitudinal joint of the first section has five locks. Others have four. Common electric actuator also drives them via torsion shafts. The locks have wedge catchers, which in lateral joints act as force-transmitters and accept the fuselage twisting forces. With the same purpose each pair of locks on the longitudinal joint has a wedge connection. To turn the doors each section has an electric actuator on the fuselage side. The doors are open and closed consequently.

8 door panels have radiation heat exchangers (RHE) mounted equidistantly to the doors’ inner surface. Each panel is attached by three hinge units in the lower part and has six additional surface-placed locks actuated by common electric actuator via a torsion shaft and a system of rods. Thermal separation between the RHE panels of aluminum alloy and the doors is made. The RHE panels are unlocked before the doors start moving. When opening the panels rotate together with the doors but a special device makes them go with a lower angular speed, so that when the doors are fully open by an angle of 178˚ the RHE panels are only turned by 143˚. This provides radiation from both sides of the heat exchanger.

All movable joints of the doors are covered with the Ag-Mo-VAP-2 anti-frictional solid lubricant to prevent them from vacuum welding and to provide a friction factor of not more than 0.25.

To provide protection against dust and rain as well as protrusion of plasma inside the fuselage, the doors have double tightening: hermetic and thermal. The hermetic tightening is designed as a thin-walled profile of springiness steel with a silicon low-temperature rubber profile attached. This solution secured constant tightening force almost independent thermally. Thermal tightening is made of high-temperature nap, which fills the growing gaps making for the smoothness of the outer shape.

The module of the payload bay doors with radiation heat exchanger panels incorporates 14 actuators, 57 locks, thermal compensation parts thermal and hermetic tightening. It is a complicated multi-functional mechanic system, which required a huge program of tests from development of separate units of new materials to tests as part of the Orbiter.

A specific role in this program was played by the STVOR-2 semi-actual stand to test the opening of the doors and functioning of the lock system and tightening. The doors together with RHE panels were mounted vertically. On the successful ending of the STVOR-2 tests as well as static-, service life-, tribo-technical and other tests there was issued a conclusion on the readiness to flight tests as part of the Orbiter.

The stage of production preparation of the composite material doors is very time-consuming. It is due to introducing of new technologies and manufacturing of a large-size forming rigs, the manufacturers decided to make a second set of doors of the D-16 aluminum alloy under traditional technology for the BURAN Analogue for horizontal test flights and mockup technological tests. The same strength was provided to the metal doors. But according to the estimations, they appeared to be 620 kg heavier (38%) than the composite ones. The composite doors weighed 1625 kg.

At the stage of manufacturing the designers made a range of proposals further on improvement the doors design, basically aimed at weight lowering by replacing titanium parts with composite ones. On later stages it was proposed to use the KMU-8 carbon plastic with operational temperatures of up to +250˚C, instead the KMU-4E so that to lower the width of the heatproof coating.

The creation of the doors for Orbiter’s payload bay has accommodated the latest achievements in Russia’s applied science, design, materials science, upgraded aviation industry technologies and industrial culture, showed high organizing capability of the industry. To reach the assigned task, many scientists, technologists, workers were involved from the following enterprises: NPO MOLNIYA, TsAGI, VIAM, NIAT, ONPO TECHNOLOGIYA, NAZ-&-Ch, TMZ, MAZ DZERZHINETZ, NIIRP, GPKO NORMAL.

The following people of NPO MOLNIYA greatly contributed in the creation of the doors: Mr.: Gutman I.L., Dautov N.K., Yeliseev U.I., Mrs. Kulikova G.V., Mr. Feygin D.I. Their proposals are confirmed by inventor certificates.

To present time the doors are the largest composite frame construction built in Russia for aerospace vehicles, and this assures the specialists of NPO MOLNIYA of realizing the claimed mass-inertia characteristics of the MAKS Aerospace System. Even now, 8 years after the building this unique structure it seems to be a bold breakthrough of Russia’s aviation industry to the future, and form a reliable basis of building new issues of aerospace vehicles.