300 400 600 1000 2000 3000 Chamber pressure, psi
FIGURE 11-6. Plot of the burning rate versus chamber pressure for several typical solid rocket propellants, some at three different temperatures. A particular double base plateau propellant shows a constant burning rate over a fairly wide pressure range.
Example 11-1. Tabulate the variation of burning rate with pressure for two propellants with a\ = 0.00137, = 0.9, a2 = 0.060, and n2 = 0.4, with p expressed in pounds per square inch and r in inches per second.
SOLUTION. Use Eq. 11-3 and solve for several conditions, as shown below.
500 0.367 0.720
1000 0.685 0.95
1500 0.994 1.11
2000 1.28 1.26
From inspection of these results and also from Eq. 11-3, it can be seen that the burning rate is very sensitive to the exponent n. For stable operation, n has values greater than 0 and less than 1.0. High values of n give a rapid change of burning rate with pressure. This implies that even a small change in chamber pressure produces substantial changes in the amount of hot gas produced. Most production propellants have a pressure exponent n ranging between 0.2 and 0.6. In practice, as n approaches 1, burning rate and chamber pressure
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