Internal burning

In the case of heavier-than-air purge gas, there is no buoyancy mechanism causing air entry into the stack, and there is thus no incentive to include a dry seal. Unlike a water seal, a dry seal cannot prevent a flashback from traveling upstream if a combustible mixture has been formed by the entry of air into the safety valve or flare headers. It only protects against internal burning flashback... 

When combustion instability occurs for an internal burning grain of a rocket motor, the burning rate of the grain varies with time and so does the pressure in the rocket motor. The pressure versus time curve shows oscillations of a certain frequency. When the propellant burning mode is not in harmony with the pressure oscillation mode, the combustion instabiUty tends to decay. However, when the burning mode is in harmony with the oscillation mode, the pressure oscillation is amplified. 

Fig. 14.3 Pressure versus burning time curves for a wired burning, an external-internal burning, and an end burning.

Fig. 14.13 Cross-section of an internal-burning propellant grain used in a dual-thrust motor.

The combustion gas of an internal burning of a propellant flows along the port of the propellant If the nozzle attached to a rocket motor is removed, the pressure in the port becomes equal to atmospheric pressure and no sonic velocity is attained at the rear-end of the port. Then, no thrust is generated by the combustion of the propellant However, if the mass burning rate of the propellant is high enough to choke the flow at the rear-end of the port, the pressure in the port is increased and the flow reaches sonic velocity. The increased pressure in the port is converted into thrust. The thrust F is represented by... 

Let us consider a gas flow in a port of an internal-burning propellant. The cross-sectional area of the port is assumed to be constant throughout the port from the head-end to the rear-end, i. e., the port is one-dimensional along the flow direc-tion.I l The mass discharge rate from the rear-end of the port, m, is given by... 

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