Propellant Burning Rate

The rocket motor's operation and design depend on the combustion characteristics of the propellant, its burning rate, burning surface, and grain geometry. The branch of applied science describing these is known as internal ballistics; the effect of grain geometry is treated in Section 11.3.

The burning surface of a propellant grain recedes in a direction essentially perpendicular to the surface. The rate of regression, usually expressed in cm/ sec, mm/sec, or in./sec, is the burning rate r. In Fig. 11-5 we can visualize the change of the grain geometry by drawing successive burning surfaces with a constant time interval between adjacent surface contours. Figure 11-5 shows this for a two-dimensional grain with a central cylindrical cavity with five slots. Success in rocket motor design and development depends significantly on knowledge of burning rate behavior of the selected propellant under all

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Solid Rocket Booster Cross Section

FIGURE 11-2. Booster rocket motor for the Pegasus air-launched three-stage satellite launch vehicle. It has a cylinder grain cavity with fins. The 50 in. diameter case has structural reinforcements to attach the Pegasus vehicle to its launch airplane and also to mount a wing to the case. It produces a maximum vacuum thrust of 726 kN (163,200 lbf) for 68.6 sec, a vacuum specific impulse of 295 sec, with a propellant mass of 15,014 kg and an initial mass of 16,383 kg. (Courtesy of Orbital Sciences, Corp. and Alliant Tech Systems.)

FIGURE 11-2. Booster rocket motor for the Pegasus air-launched three-stage satellite launch vehicle. It has a cylinder grain cavity with fins. The 50 in. diameter case has structural reinforcements to attach the Pegasus vehicle to its launch airplane and also to mount a wing to the case. It produces a maximum vacuum thrust of 726 kN (163,200 lbf) for 68.6 sec, a vacuum specific impulse of 295 sec, with a propellant mass of 15,014 kg and an initial mass of 16,383 kg. (Courtesy of Orbital Sciences, Corp. and Alliant Tech Systems.)

Rocket Nozzle Vector Control

FIGURE 11-3. Inertial upper stage (IUS) rocket motor with an extendible exit cone (EEC). This motor is used for propelling upper launch vehicle stages or spacecraft. The grain is simple (internal tube perforation). With the EEC and a thrust vector control, the motor has a propellant fraction of 0.916. When launched, and while the two lower vehicle stages are operating, the two conical movable nozzle segments are stowed around the smaller inner nozzle segment. Each of the movable segments is deployed in space and moved into its operating position by three identical light-weight, electrically driven actuators. The nozzle area ratio is increased from 49.3 to 181; this improves the specific impulse by about 14 sec. This motor (without the EEC) is described in Table 11-3 and a similar motor is shown in Fig. 16-5. (Courtesy of United Technologies Corp., Chemical Systems.)

FIGURE 11-3. Inertial upper stage (IUS) rocket motor with an extendible exit cone (EEC). This motor is used for propelling upper launch vehicle stages or spacecraft. The grain is simple (internal tube perforation). With the EEC and a thrust vector control, the motor has a propellant fraction of 0.916. When launched, and while the two lower vehicle stages are operating, the two conical movable nozzle segments are stowed around the smaller inner nozzle segment. Each of the movable segments is deployed in space and moved into its operating position by three identical light-weight, electrically driven actuators. The nozzle area ratio is increased from 49.3 to 181; this improves the specific impulse by about 14 sec. This motor (without the EEC) is described in Table 11-3 and a similar motor is shown in Fig. 16-5. (Courtesy of United Technologies Corp., Chemical Systems.)

Free Standing Propellant

FIGURE 11-4. Simplified cross section through a typical tactical motor. The blast tube allows the grain to be close to the center of gravity of the vehicle; there is very little movement of the center of gravity. The nozzle is at the missile's aft end. The annular space around the blast tube is usually filled with guidance, control, and other nonpropulsive equipment. A free-standing grain is loaded before the aft closure is assembled.

FIGURE 11-4. Simplified cross section through a typical tactical motor. The blast tube allows the grain to be close to the center of gravity of the vehicle; there is very little movement of the center of gravity. The nozzle is at the missile's aft end. The annular space around the blast tube is usually filled with guidance, control, and other nonpropulsive equipment. A free-standing grain is loaded before the aft closure is assembled.

TABLE 11-1. Major Application Categories for Solid Propellant Rocket Motors

Category

Application

Typical Characteristics

Large booster and second-stage motors

High-altitude motors

Space launch vehicles; lower stages of long-range ballistic missiles

Upper stages of multistage ballistic missiles, space launch vehicles; space maneuvers

Tactical missiles 1. High acceleration: short-range bombardment, antitank missile

2. Modest acceleration: air-to-surface, surface-to-air, short-range guided surface-to-surface, and air-to-air missiles

Ballistic missile defense

Gas generator

Defense against long- and medium-range ballistic missiles

Pilot emergency escape; push missiles from submarine launch tubes or land mobile cannisters; actuators and valves; short-term power supply; jet engine starter; munition dispersion; rocket turbine drive starter; automotive air bags

Large diameter (above 48 in.); L/D of case = 2 to 7; burn time t = 60 to 120 sec; low-altitude operations with low nozzle area ratios (6 to 16) High-performance propellant; large nozzle area ratio (20 to 200); L/D of case = 1 to 2; burn time t = 40 to 120 sec (see Fig. 11-3) Tube launched, L/D = 4 to 13; very short burn time (0.25 to 1 sec); small diameter (2.75 to 18 in.); some are spin stabilized Small diameter (5 to 18 in.); L/D of case = 5 to 10; usually has fins and/or wings; thrust is high at launch and then is reduced (boost-sustain); many have blast tubes (see Fig. 11 —; wide ambient temperature limits: sometimes minimum temperature -65° F or -53°C, maximum temperature +160°F or +71°C; usually high acceleration; often low-smoke or smokeless propellant Booster rocket and a small upper maneuverable stage with multiple attitude control nozzles and one or more side or divert nozzles Usually low gas temperature (< 1300°C); many different configurations, designs, and propellants; purpose is to create high-pressure, energetic gas rather than thrust

TABLE 11-2. Classification of Solid Rocket Motors

Basis of Classification

Examples of Classification

Application

Diameter/Length

Propellant

Case design

Grain configuration

Grain installation

Explosive hazard

Thrust action

Toxicity

See Table 11-1.

0.005-6.6 m or 0.2-260 in.; 0.025 to 45 m or 1 to 1800 in. Composite: Heterogeneous (physical) mixture of powdered metal (fuel), crystalline oxidizer and polymer binder Double-base: Homogeneous mixture (colloidal) of two explosives (usually nitroglycerin in nitrocellulose) Composite-modified double-base: Combines composite and double-base ingredients Gas generator and others: See Chapter 12 Steel monolithic: One-piece steel case

Fiber monolithic: Filament wound (high-strength fibers) with a plastic matrix Segmented: Case (usually steel) and grain are in segments which are transported separately and fastened together at launch site

Cylindrical. Cylindrically shaped, usually hollow End-burning: Solid cylinder propellant grain Other configurations'. See Figs. 11-16 and 11-17 Case-bonded: Adhesion exists between grain and case or between grain and insulation and case; propellant is usually cast into the case Cartridge-loaded'. Grain is formed separately from the motor case and then assembled into case Class 1.3: Catastrophic failure shows evidence of burning and explosion, not detonation Class 1.1: Catastrophic failure shows evidence of detonation Neutral grain: Thrust remains essentially constant during the burn period Progressive grain: Thrust increases with time Regressive grain: Thrust decreases with time Pulse rocket: Two or more independent thrust pulses or burning periods Step-thrust rocket: Usually, two distinct levels of thrust Toxic and nontoxic exhaust gases

TABLE 11-3. Characteristics of Missile Motor and Space Motor

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