High values of n can produce large changes in burning rate with relatively small changes in chamber pressure, with potentially catastrophic consequences, as higher burning rate leads to even greater chamber pressure. Another explanation is that the condensed phase combustion products may "pool" and retard heat transfer to the surface at elevated pressure levels.īurning rate can be particularly sensitive to the value of the pressure exponent, n (the slope of the log-log curve in Fig.3). Plateau and mesa effects may be the result of different rates of surface regression (as a function of pressure) of the binder compared to the oxidizer particles. Both the KNDX and KNSB propellants exhibit this behaviour, the former plateau, and the latter, mesa (see KNDX & KNSB Propellants - Burn Rate Experimentation). These type of propellants are termed plateau or mesa propellants, as illustrated in Figure 3. When plotted on log-log scales, the Saint Robert's function is a straight line.Ĭertain propellants (or with additives) deviate from this behaviour, and exhibit sharp changes in burn rate behaviour.
#Fuel cell constant surface area when burning shape rocket full#
More than one set may be necessary to accurately represent the full pressure regime of interest, as illustrated in Figure 2.įigure 2 - Saint Robert's model of burn rate v.s. It is important to realize that a single set of a, n values are typically valid over a distinct pressure range. Various means may be employed to determine these parameters, such as a Strand Burner or Ballistic Evaluation Motor (BEM). The values of a and n are determined empirically for a particular propellant formulation, and cannot be theoretically predicted. Where r is the burn rate, r o is a constant (usually taken as zero), a is the burn rate coefficient, and n is the pressure exponent. The usual representation of the pressure dependance on burn rate is the Saint Robert's Law (a.k.a. However, at 68 atmospheres (1000 psi), the burn rate is about 15 mm/sec., a four-fold increase. For example, KNSU has a burning rate of 3.8 mm/sec. Burn rate is profoundly affected by chamber pressure.Velocity of the combustion gases flowing parallel to the burning surface.Initial temperature of the propellant grain.
Propellant burning rate is influenced by certain factors, the most significant being: These two events are, in fact, directly related, as will be discussed shortly. Also note that the surface regression rate (burn rate) is not constant. Note that the burning surface area (represented by the arc length of the red lines in this figure) is continually increasing. Burning commences along the length of the central core, with the burning surface receding radially outward (shown at arbitrary times t1, t2, t3). This web page discusses the factors that influence burn rate, how it may be modified, how the burn rate can be determined experimentally, and the physical processes that occur at the burning surface of a propellant that governs the burning rate.Īn illustration of the concept of burning surface regression is given in Figure 1, for a section of a hollow cylindrical grain, with an inhibited outer surface ( "inhibited" means that the propellant surface is protected from the heat of combustion and as such, burning does not occur). Knowing quantitatively the burning rate of a propellant, and how it changes under various conditions, is of fundamental importance in the successful design of a solid rocket motor. This rate can differ significantly for different propellants, or for one particular propellant, depending on various operating conditions as well as formulation. The rate of regression, typically measured in inches per second (or mm per second), is termed burning rate (or burn rate). The burning surface of a rocket propellant grain recedes in a direction perpendicular to this burning surface. Richard Nakka's Experimental Rocketry Web Site Once all the fuel is burned in the first stage, the stage is disconnected from the rocket.Richard Nakka's Experimental Rocketry Site You need a big container to store the fuel.
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