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200 300 400 500 600 700

Freestream velocity, m/sec

200 300 400 500 600 700

Freestream velocity, m/sec

FIGURE 11-10. Effect of gas velocity in the perforation or grain cavity on the erosive burning augmentation factor, which is the burning rate with erosion r divided by the burning rate without erosion r0. (Reproduced with permission of the AIAA from Chapter 10 of Ref. 11-3.)

Other Burning Rate Enhancements

Enhancement of burning rate can be expected in vehicles that spin the rocket motor about its longitudinal axis (necessary for spin-stabilized flight) or have high lateral or longitudinal acceleration, as occurs typically in antimissile rockets. This phenomenon has been experienced with a variety of propellants, with and without aluminum fuel, and the propellant formulation is one of the controlling variables (see Fig. 11-11). Whether the acceleration is from spin or longitudinal force, burning surfaces that form an angle of 60 to 90° with the acceleration vector are most prone to burning rate enhancement. For example, spinning cylindrical interal burning grains are heavily affected. The effect of spin on a motor with an operational composite propellant internal burning grain is shown in Fig. 11-12. The accelerated burning behavior of candidate propellants for a new motor design is often determined in small-scale motors, or in a test apparatus which subjects burning propellant to acceleration (Ref. 11-8). The stresses induced by rapid acceleration or rapid chamber pressure rise can cause crack formation (see Refs. 11-9 and 11-10), which exposes additional burning surface.

The burning rate of the propellant in an end-burning grain at a location immediately adjacent to or near the propellant-to-insulation bondline along the case wall, can, depending on the propellant formulation and manufacturing process, be higher than that of the propellant elsewhere in the grain.

The embedding of wires or other shapes of good metal heat conductors in the propellant grain increases the burning rate. One technique has several silver wires arranged longitudinally in an end-burning grain (see Ref. 11-11). Depending on wire size and the number of wires per grain cross-sectional area, the burning rate can easily be doubled. Aluminum wires are about half as effective as silver wires. Other forms of heat conductors have been wire

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