W. Boccius

The advantages of applying solid propellants in rockets and the progress in the field of solid. propellants with regard to impulses and manufacturing techniques have led to designs of larger propulsion units in recent years. It may be noted that Lockheed Missiles and Space Division is actively engaged in the development of such rocket vehicles. Since the rocket velocity at burn-out depends very much on the mass ratio (take-off weight to weight at burn-out), a high structural efficiency is imperative. The inert weight of rocket motors representing a large percentage of the total structural weight, therefore is an important factor; the range and the size of a rocket depend to a great extent on the efficient utilization of the structure and the materials. The configuration of the motor case has an influence on the inert weight. A comparison indicates the relative weight penalty by not using the spherical vessel which for a given volume results in the smallest weight. The combustion pressure will mostly determine the critical design condition. The burning process results in a great variety of pressure-time histories. Additional load conditions occur during ground handling, storage, launch, and in flight conditions. Since the motor cases have extremely thin walls, buckling problems are associated with rocket supports in areas of discrete load actions. Generally, membrane stresses prevail in the walls of the motor cases. The shape of the domes should be designed with no hoop compression present during the burning process. Holes and enclosures represent structural interruptions which produce additional bending and shear stresses. The transition regions between cylinder and heads and around the vertex of the domes need special care in design and analysis. Stability problems, except in the local support areas, can be eliminated by keeping the rocket vessels pressurized. Minute cracks in joints and defects in the basic material represent zones of danger which might lead to unexpected failures. Failure criteria for the elastic and plastic region are available, but much depends on actual test results. The proper selection of the motor case materials is of primary importance in developing light weight motors. Factors like stress/weight ratio, elongation, notch sensitivity, formability, and weldability enter into this decision. High strength steel alloys are favored, however, titanium alloys and plastic filaments should be considered for more advanced designs. Special fabrication techniques are utilized to form large, thin-walled cases. Conventional methods of rolling and welding sheet material to cylinders have been applied extensively. The forming of end sections requires special techniques. An innovation in the manufacturing is the wrapping of small metal strips of high srength over a mandrel for building motor cases. Another promising technique is the power shear forming process by plastically deforming the material into the required shapes during rotation. An advantage of this method is the elimination of longitudinal weld joints. In the plastic field, the method of spinning fibres, like glass and other efficient materials together with resins to cylinders and complete vessels offer excellent strength.

WGL-Tagung, Hamburg 1959

Conference Paper

englisch

A4, 12 Seiten

Jahrbuch der Wissenschaftlichen Gesellschaft für Luftfahrt, 1959; S.331-342; 1959; Braunschweig : Vieweg

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1960