The engine housing of the third stage is made of spirill material, which is a fiberglass reinforced epoxy resin. The casing is made by multi-layer spiral winding of glass fibre. The fibres of each layer work only on stretching, and resistance to cutting is provided by epoxy resin of a binding fibre; resistance to cutting of each layer is small, but as a whole the case possesses high strength and weight characteristics.
Bottoms, manufactured in conjunction with the cylindrical part of the body, have the shape of a semi-ellipse.
Fiberglass is wound on a plaster mandrel, placed on a metal rod. Before winding, the mandrel is fitted with auxiliary parts required to form various holes, and then an insulating rubber gasket with the addition of silicon oxide is applied. The thickness of the gasket is determined by the temperature regime of different places of the housing when the fuel burns and increases from 3 mm at the top to 25 mm at the bottom. When winding the glass fiber passes through a bath with epoxy resin and, entering the rotating mandrel, is glued to the rubber gasket. During the winding process, the mandrel and all parts installed on it are covered with glass fibre.
For the formation of skirts at the ends of the body, necessary to connect to the adapters, on the rod of the gypsum mandrel are installed special metal mandrels, which are also covered with fiberglass. After cutting the second bottoms obtained by winding, skirts are formed (upper length about 200 mm and lower length about 100 mm).
The thickness of the layer of winding glass fibre has a different size along the length of the body, increasing from top to bottom, and on average is about 6 mm. After the glass fibre winding is complete, the housing is removed from the rod. The holes for fixing the nozzles and shutdown plugs are drilled out and the edges are finished manually. The plaster mandrel is broken and removed from the housing.
The finished casing is checked and hydrostatically tested. Aluminium rings are then attached to the ends of the skirts using epoxy resin and holes are drilled for the attachment bolts to the adapters.
A rubber insulation coating is applied to the inner surface of the bottom bottom and an elastic gasket is applied on top of it, which is glued to the rubber coating only at the point of transition of the bottom to the cylindrical part. The rubber coating and gasket have holes in the nozzle mounting points and in the central part. The gasket is glued to a fuel charge and does not lag behind during shrinkage. This creates cavities between the rubber coating and the gasket, which are filled with polyurethane.
The same gasket, installed on the top bottom. Tubes for the engine shutdown plugs are covered with rubber insulation to protect the phenolic coating of these tubes from the nitroglycerin, which is part of the fuel. Before filling up the fuel, the openings of all the tubes are covered with rubber plugs.
The outer surface of the casing is sprayed with a blazing coating made of synthetic material, which thickness is about 5 mm on average, and in the lower part of the casing reaches 9.5 mm.
Immediately prior to fuel loading, the inner surface of the casing is coated with an epoxy resin layer, after which a second coat is applied, which is polymerized to an adhesive state to ensure reliable adhesion to the fuel charge.
To load the fuel, the engine housing is first fitted with a special device that ensures that it rotates around its longitudinal axis and tilts in different directions. Such a device is necessary to ensure that the first portion of fuel weighing about 34 kg is evenly distributed over the entire inner surface of the hull with a sticky layer of epoxy resin.
Before loading the main part of the fuel into the casing, a rod is inserted through the nozzle mounting holes and the central hole of the bottom bottom bottom to form a charge in the inner cavity. The rod consists of a conical axis with longitudinal slots. In which four radially diverging plates are fastened. As assembled, the rod is a pyramid with a four-pointed star in the base. The rod is held in place by special rings and does not reach the upper bottom by about 0.4 m.
The housing prepared for loading is installed vertically in the vacuum chamber of the filling manhole; fuel is loaded through the central hole of the top bottom.
The third stage engine fuel consists of a mixture of dry components: ammonium perchlorate, aluminum powder, nitroglycerine, nitrocellulose, triacetine, nitrodiphenylamine and a component of unknown composition in the form of small cylinders and a solvent from nitroglycerine with the addition of a component that promotes rapid dissolution of the mixture.
First, a mixture of dry fuel components of the main composition is poured into the engine housing, which fills the housing for three quarters of the volume, then a mixture of slightly modified composition with a higher rate of combustion is poured. For compaction of the mixture vibrators are used.
After full backfilling of the dry fuel components in the vacuum chamber, a low pressure is maintained for 24 hours in order to remove moisture and gases from the mixture. Then, at the same pressure in the chamber, the solvent filling process begins, which is fed from below and fills the body. The engine is then removed from the filling well and an additional amount of solvent is pressurized for two days to ensure that the mixture is completely impregnated. The next stage is fuel polymerization, which is carried out in the chamber, where for 18 days the engine is kept at a temperature of about 50 ° C. The polymerization is followed by a five-day cooling period.
Permissible deviations in weight of the engine equipped with fuel are 5 kg.
The rod is removed from the cooled engine. The cavities between the elastic gasket and the rubber insulation resulting from fuel shrinkage are filled with polyurethane and the centre hole of the top bottom is closed with an aluminium plug.
A rubber insulating gasket is glued to the outer surface of the bottom bottom bottom to prevent it from overheating when the engine is running.
The last step in the manufacture of the third stage motor consists of installing four tilting nozzles and plugs in the shutdown openings for the motor.