Minuterman-1 1st Stage Engine Technology

The motor casing is made up of six welded cylindrical sections (wall thickness 3.7 mm) and two bottoms (the top bottom is welded to the cylindrical part of the casing and the bottom is screwed on multistart threads). The material for the casing is D6AC high strength steel, obtained by double fusion in vacuum. Enclosures are checked before entering the production line, during which the dimensions, location of holes for nozzles and igniter installation, as well as quality of welds are checked. The entire inspection process takes about three hours.

After the inspection, the bottom bottom is removed and an insulating coating consisting of a layer of asbestos and a layer of fibreglass reinforced phenolic resin is applied to the inside of the bottom. Both layers of coating are manufactured as four preformed segments. The same coating is applied to small exhaust pipes connecting the combustion chamber to the nozzles. These pipes are then fitted with 25 mm thick graphite inserts. To the inner surface of the upper casing and the upper bottom, a preformed insulating rubber gasket about 9.5 mm thick is glued. The insulating gasket of the lower part of the casing is a glass fibre-reinforced resin sheet and its thickness increases from top to bottom, reaching 50 mm at the point of attachment of the lower base. This insulation thickness is necessary so that the motor exhaust fumes with a temperature of ~3000°C do not burn the bottom of the case. The first insulation gasket is covered with the second one consisting of two layers of rubber. The outside of this gasket is connected to the first insulating gasket, and the inside layer is connected to the fuel charge. All remaining unprotected inner surface of the casing is sprayed with a layer of rubber material about 2.5 mm thick. This layer of insulating coating compensates for different expansion coefficients of steel and fuel and promotes tight fuel fit to the hull. After the insulation coating is applied, it is polymerised for 18 - 24 hours at 60°C. During the polymerisation process, the housing rotates around its longitudinal axis in a special fixture in order to ensure uniform heating of the coating.

After the internal insulation has been cured, the outer casing is covered with a protective cover and transported to the place of fuel preparation. To fill the fuel, the housing is placed in a vacuum chamber and lowered vertically into the pouring well.

Before pouring the fuel, an aluminium rod with a six-pointed star cross-section is placed in the housing. This rod is used to form an inner cavity that runs along the longitudinal axis of the fuel charge. The rod is coated with a Teflon emulsion before being placed in the casing and then placed in a 400°C furnace to polymerize this coating. The Teflon coating prevents fuel from sticking to the rod and allows free removal of the rod from the fuel charge after it has hardened.

After installing the rod in the housing, the vacuum chamber lid closes and the pressure in the chamber decreases to 100 mm Hg. Fuel enters the engine housing through a special valve in the vacuum chamber lid.

By the time the housing is installed in the pouring well, the mixing process of fuel components is completed.

Before entering the mixers, ammonium perchlorate is pre-treated (milled and mixed) to ensure maximum perchlorate density in the blended fuel. The ammonium perchlorate mixture is composed of four parts of ungrinded perchlorate, one part of finely ground perchlorate and one part of coarsely ground perchlorate; the size of the particles in this mixture varies between 10 and 10,000 microns.

Polybutadiene acrylic acid and aluminium powder are also pre-mixed and the content of each component is controlled with an accuracy of approximately 100 g.

The final mixing of the pre-mixed components and the epoxy resin takes place within approximately two hours at a temperature of 60±2°C, which is controlled by hot water circulating in the heating jacket of the mixer.

Before being fed into the mixer, the fuel components are weighed and these scales are transmitted to the control panel of the fuel mixture manufacturing process. If the data presented on the panel are within the tolerances (±0.9 kg for 1380 kg of ammonium perchlorate, ±0.45 kg for 540 kg pre-mixed with polybutadiene acrylic acid aluminium powder and ±0.045 kg for 45 kg of epoxy resin), the signal for loading the components into the mixer is transmitted. In case of deviation from the set weight tolerances, the cycle of weighing and feeding components is automatically interrupted.

After mixing, air is extracted from the fuel, which ensures a higher density of the fuel, and then the fuel enters the loading container equipped with a heating jacket. In this container, fuel of soft oil consistency is fed to the pouring well, where the engine housing is filled with fuel. The fuel is mixed and filled into the casing in individual portions; 13 portions are required to fully fill the casing.

At the end of the pouring process, the pressure in the vacuum chamber is raised to normal and the curing and curing process begins. For this purpose, air heated to 63°C is supplied into the chamber from below, which heats the engine housing and exits through the chamber lid. Fuel polymerization takes place within two days, followed by a 48-hour period of gradual cooling of the fuel charge to 27 ± 10 ° C to avoid shrinkage cracks in it.

The aluminium rod is removed from the fueled engine and the cast charge is x-rayed, which takes 16 hours to complete.

After checking the end of the fuel charge, the excess fuel is cut off and the housing is screwed on to the lower bottom. The outer surface of the engine after the equipment is sprayed with an insulating layer of synthetic material. This coating is polymerised within 16 hours. At the same time, the engine is checked for tightness by injecting nitrogen into the inner cavity of the charge. The outer insulation layer protects the motor from overheating during starting (in the starting shaft the projectile is exposed to temperatures up to 2700°C) and burns during starting and flying in the atmosphere.