Erosive Burning in a Rocket Motor

MPa s When a negative catalyst such as LiF or SrCOj was added to the AP composite propellant, combustion interruption occurred. As shown in Fig. 7.27, the pressure deflagration limit is lowered by the addition of the negative catalysts. Detailed design work of a dual-grain dual-thrust motor and the associated combustion test results are shown in Ref [5]. 

Since the initial port area of the propellant grain is small, the flow velocity becomes large because the velocity at the nozzle throat is always the sonic velocity. Furthermore, Kn is kept constant, and then both Ay and At increase simultaneously as L/D increases. Since the port area of the propellant grain is kept constant, the flow velocity increases as At increases, in accordance with the mass conservation low given by Eq. (1.49). Therefore, the heat flux transferred from the gas flow increases and the erosive burning ratio increases. However, as the port area increases, the flow velocity decreases and the erosive burning then diminishes. 

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Fig. 13.21 shows another example of oscillatory burning of an RDX-AP composite propellant containing 0.40% A1 particles. The combustion pressure chosen for the burning was 4.5 MPa. The DC component trace indicates that the onset of the instability is 0.31 s after ignition, and that the instability lasts for 0.67 s. The pressure instability then suddenly ceases and the pressure returns to the designed pressure of 4.5 MPa. Close examination of the anomalous bandpass-filtered pressure traces reveals that the excited frequencies in the circular port are between 10 kHz and 30 kHz. The AC components below 10 kHz and above 30 kHz are not excited, as shown in Fig. 13.21. The frequency spectrum of the observed combustion instability is shown in Fig. 13.22. Here, the calculated frequency of the standing waves in the rocket motor is shown as a function of the inner diameter of the port and frequency. The sonic speed is assumed to be 1000 m s and I = 0.25 m. The most excited frequency is 25 kHz, followed by 18 kHz and 32 kHz. When the observed frequencies are compared with the calculated acoustic frequencies shown in Fig. 13.23, the dominant frequency is seen to be that of the first radial mode, with possible inclusion of the second and third tangential modes. The increased DC pressure between 0.31 s and 0.67 s is considered to be caused by a velocity-coupled oscillatory combustion. Such a velocity-coupled oscillation tends to induce erosive burning along the port surface. The maximum amplitude of the AC component pressure is 3.67 MPa between 20 kHz and 30 kHz. - ...

Burning, Erosive(in Propellants)(Erosive Effect of Gas Flow or Erosion of Propellants). Mansell (Ref 1) was one of the first to observe that the rate of burning inside tubular proplnts was faster than that on the outside. A similar phenomenon was observed later by Muraour(Ref 2). No importance was attached to this phenomenon until it was observed also(but on a larger scale) in rocket motors during and after WWII(Ref 11)... 

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