In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, the burn rate (or burning rate) is a measure of the linear
combustion
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
rate of a compound or substance such as a candle or a
solid propellant. It is measured in length over time, such as millimeters per second or inches per second. Among the variables affecting burn rate are pressure and temperature. Burn rate is an important parameter, especially in
propellants
A propellant (or propellent) is a reaction mass, mass that is expelled or expanded in such a way as to create a thrust or another Net force, motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid ...
, because it determines the rate at which exhaust gases are generated from the burning propellant, which decides the flow rate through the nozzle. The thrust generated in the rocket of a missile depends on this flow rate. Thus, knowing the burn rate of a propellant and how it changes under various conditions is of fundamental importance in the successful design of a solid rocket motor. The concept of burn rate is also relevant in case of liquid propellants.
Measurement
One device for measuring the burning rate is a V-shaped metal channel about 1–2 feet long wherein a sample is placed, with a cross-sectional dimension of approximately 6 mm or 1/4in. The sample is ignited on one end, and time is measured until the flame front reaches the other. Burn rate (typically expressed in mm/s or in/s) is the sample length over time at a given pressure and temperature. For solid fuel propellant, the most common method of measuring burn rate is the Crawford Type Strand Burning Rate Bomb System (also known as the
Crawford Burner or Strand Burner), as described in MIL-STD-286C.
Characterization
A substance is characterized by a burn rate vs. pressure chart and burn rate vs temperature chart.
* Higher burn rate than the speed of sound in the material (usually several km/s): "detonation"
* A few meters per second: "deflagration"
* A few centimeters per second: "burn" or "smolder"
* 0.01 mm/s to 100 mm/s: "decomposing rapidly" to characterise it.
However, there is a difference in opinion in differentiating the three in the absence of firm numbers at a given
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
or
temperature
Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...
.
See also
*
Detonation velocity
Explosive velocity, also known as detonation velocity or velocity of detonation (VoD), is the velocity at which the shock wave front travels through a detonated explosive. Explosive velocities are always higher than the local speed of sound in t ...
References
{{DEFAULTSORT:Burn Rate (Chemistry)
Combustion
Temporal rates