Solid Propellant Rocket Fundamentals

This is the first of four chapters on solid propellant rockets. It discusses the burning rates, motor performance, grain configurations, and structural analysis. In solid propellant rocket motors—and the word "motor" is as common to solid rockets as the word "engine" is to liquid rockets—the propellant is contained and stored directly in the combustion chamber, sometimes hermetically sealed in the chamber for long-time storage (5 to 20 years). Motors come in many different types and sizes, varying in thrust from about 2 N to over 4 million N (0.4 to over 1 million lbf). Historically, solid propellant rocket motors have been credited with having no moving parts. This is still true of many, but some motor designs include movable nozzles and actuators for vectoring the line of thrust relative to the motor axis. In comparison to liquid rockets, solid rockets are usually relatively simple, are easy to apply (they often constitute most of the vehicle structure), and require little servicing; they cannot be fully checked out prior to use, and thrust cannot usually be randomly varied in flight.

Figures 1-5 and 11-1 show the principal components and features of relatively simple solid propellant rocket motors. The grain is the solid body of the hardened propellant and typically accounts for 82 to 94% of the total motor mass. Design and stresses of grains are described later in this chapter. Propellants are described in the next chapter. The igniter (electrically activated) provides the energy to start the combustion. The grain starts to burn on its exposed inner surfaces. The combustion and ignition of solid propellants are discussed in Chapter 13. This grain configuration has a central cylindrical cavity with eight tapered slots, forming an 8-pointed star. Many grains have slots, grooves, holes, or other geometric features and they alter the initial

Composite solid propellant grain

Mounting flange

Composite solid propellant grain

Mounting flange

Titanium 8-point star, internal case burning grain cavity

Contoured exhaust nozzle with carbon phenolic inner liner

FIGURE 11-1. Cross section of the STAR™ 27 rocket motor, which has been used for orbit and satellite maneuvers. It has an altitude thrust of 6000 lbf, nominally burns for 34.4 sec and has an initial mass of 796 lbm. For more data see Table 11-3. (Courtesy of Thiokol Propulsion, a Division of Cordant Technologies.)

Titanium 8-point star, internal case burning grain cavity

Contoured exhaust nozzle with carbon phenolic inner liner

FIGURE 11-1. Cross section of the STAR™ 27 rocket motor, which has been used for orbit and satellite maneuvers. It has an altitude thrust of 6000 lbf, nominally burns for 34.4 sec and has an initial mass of 796 lbm. For more data see Table 11-3. (Courtesy of Thiokol Propulsion, a Division of Cordant Technologies.)

burning surface, which determines the initial mass flow and the initial thrust. The hot reaction gases flow along the perforation or port cavity toward the nozzle. The inner surfaces of the case (really a pressure vessel), which are exposed directly to hot gas, have a thermal protection or insulation layer to keep the case from becoming too hot, in which case it could no longer carry its pressure and other loads. The case is either made of metal (such as steel, aluminum or titanium) or a composite fiber-reinforced plastic material.

The nozzle accelerates the hot gas; it is made of high temperature materials (usually a graphite and/or an ablative material to absorb the heat) to withstand the high temperatures and the erosion. The majority of all solid rockets have a simple fixed nozzle, as shown here, but some nozzles have provision to rotate it slightly so as to control the direction of the thrust to allow vehicle steering. Chapter 14 describes nozzles, cases, insulators, liners, and the design of solid propellant rocket motors.

Each motor is fastened to its vehicle by a thrust-carrying structure. In Fig. 11-1 there is a skirt (with a flange) integral with the case; it is fastened to the vehicle.

The subject of thrust vector control, exhaust plumes, and testing are omitted from these four chapters but are treated for both liquid and solid propellant units in Chapters 16, 18, and 20, respectively. Chapter 17 provides a comparison of the advantages and disadvantages of solid and liquid propellant rocket units. Chapters 3 to 5 are needed as background for these four chapters.

Applications for solid propellant rockets are shown in Tables 1-3, 1-4, and 11-1; each has its own mission requirements and thus propulsion requirements. Figures 11-2, 11-3, and 11-4 illustrate representative designs for some of the major categories of rocket motors listed in Table 11-1: namely, a large booster or second stage, a motor for space flight, and a tactical missile motor. Reference 11-1 is useful for component and design information.

There are several ways for classifying solid propellant rockets. Some are listed in Table 11-2 together with some definitions. Table 11-3 gives characteristics for three specific rocket motors, and from these data one can obtain a feeling for some of the magnitudes of the key parameters. These motors are shown in Figs. 16-5 and 16-9.

Solid propellant rocket motors are being built in approximately 35 different countries today, compared to only three countries about 50 years ago. The technology is well enough understood and disseminated that many companies or government arsenals are now capable of designing developing, and manufacturing solid rockets in several categories.

Almost all rocket motors are used only once. The hardware that remains after all the propellant has been burned and the mission completed—namely, the nozzle, case, or thrust vector control device—is not reusable. In very rare applications, such as the Shuttle solid booster, is the hardware recovered, cleaned, refurbished, and reloaded; reusability makes the design more complex, but if the hardware is reused often enough a major cost saving will result. Unlike some liquid propellant rocket engines, a solid propellant rocket motor and its key components cannot be operationally pretested. As a result, individual motor reliability must be inferred by assuring the structural integrity and verifying manufacturing quality on the entire population of motors.

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