Ignition Process

This section is concerned with the mechanism or the process for initiating the combustion of a solid propellant grain. Specific propellants that have been successfully used for igniters have been mentioned in Section 12.5. The hardware, types, design, and integration of igniters into the motor are described in Section 14.4. Chapters 2, 5, and 6 of Ref. 13-1 review the state of the art of ignition, data from experiments, and analytical models, which have been found to be mostly unreliable.

Solid propellant ignition consists of a series of complex rapid events, which start on receipt of a signal (usually electric) and include heat generation, transfer of the heat from the igniter to the motor grain surface, spreading the flame over the entire burning surface area, filling the chamber free volume (cavity) with gas, and elevating the chamber pressure without serious abnormalities such as overpressures, combustion oscillations, damaging shock waves, hang-fires (delayed ignition), extinguishment, and chuffing. The igniter in a solid rocket motor generates the heat and gas required for motor ignition.

Motor ignition must usually be complete in a fraction of a second for all but the very large motors (see Ref. 13-9). The motor pressure rises to an equilibrium state in a very short time, as shown in Fig. 13-3. Conventionally, the ignition process is divided into three phases for analytical purposes:

Phase I, Ignition time lag: the period from the moment the igniter receives a signal until the first bit of grain surface burns.

Time, milliseconds

FIGURE 13-3. Typical ignition pressure transient portion of motor chamber pressure-time trace with igniter pressure trace and ignition process phases shown. Electric signal is received a few milliseconds before time zero.

Time, milliseconds

FIGURE 13-3. Typical ignition pressure transient portion of motor chamber pressure-time trace with igniter pressure trace and ignition process phases shown. Electric signal is received a few milliseconds before time zero.

Phase II, Flame-spreading interval: the time from first ignition of the grain surface until the complete grain burning area has been ignited.

Phase III, Chamber-filling interval: the time for completing the chamber-filling process and for reaching equilibrium chamber pressure and flow.

The ignition will be successful once enough grain surface is ignited and burning, so that the motor will continue to raise its own pressure to the operating chamber pressure. The critical process seems to be a gas-phase reaction above the burning surface, when propellant vapors or decomposition products interact with each other and with the igniter gas products. If the igniter is not powerful enough, some grain surfaces may burn for a short time, but the flame will be extinguished.

Satisfactory attainment of equilibrium chamber pressure with full gas flow is dependent on (1) characteristics of the igniter and the gas temperature, composition and flow issuing from the igniter, (2) motor propellant composition and grain surface ignitability, (3) heat transfer characteristics by radiation and convection between the igniter gas and grain surface, (4) grain flame spreading rate, and (5) the dynamics of filling the motor free volume with hot gas (see Ref. 13-10). The quantity and type of caloric energy needed to ignite a particular motor grain in the prevailing environment has a direct bearing on most of the igniters' design parameters—particularly those affecting the required heat output. The ignitability of a propellant at a given pressure and temperature is normally shown as a plot of ignition time versus heat flux received by the propellant surface, as shown in Fig. 13^; these data are obtained from laboratory tests. Ignitability of a propellant is affected by many factors, including (1) the propellant formulation, (2) the initial temperature of the propellant grain

10 20 40 60 80100 120 Heat flux, cal/cm2-sec

FIGURE 13-4. Propellant ignitability curves: effect of heat flux on ignition time for a specific motor.

10 20 40 60 80100 120 Heat flux, cal/cm2-sec

FIGURE 13-4. Propellant ignitability curves: effect of heat flux on ignition time for a specific motor.

surface, (3) the surrounding pressure, (4) the mode of heat transfer, (5) grain surface roughness, (6) age of the propellant, (7) the composition and hot solid particle content of the igniter gases, (8) the igniter propellant and its initial temperature, (9) the velocity of the hot igniter gases relative to the grain surface, and (10) the cavity volume and configuration. Figure 13-4 and data in Chapter 14 show that the ignition time becomes shorter with increases in both heat flux and chamber pressure. If a short ignition delay is required, then a more powerful igniter will be needed. The radiation effects can be significant in the ignition transient as described in Ref. 13-11. In Section 14.3 we describe an analysis and design for igniters.

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