Ahh yes... erosive burning... yet another exciting topic to discuss...
When the nozzle throat diameter is equal to the core diameter of the grains, hot gases can flow out the nozzle at the speed of sound or greater. These gases also flow down the core of the motor at the speed of sound or greater, since they can exit the nozzle faster than they can exit the core. Really hot, flaming gases moving over a burning surface can cause nasty side effects.
So some motor designers make the core just a little larger than the nozzle throat. That way, they can fit the most amount of propellant in the motor without causing this disruptive flow. But when the diameters of the throat and core are really close, you get
erosive burning. It's basically when these gases are flowing fast enough to decompose the oxidizer and propellant faster than the usual burn rate and thus causes an increase in chamber pressure. This pressure increase is maintained until the core becomes a little larger, the gases are slowed, and the disruptive flow stops. Some people design motors this way to give a thrust spike off the pad and then taper off for the duration of the burn.
Erosive burning is usually seen in longer motors like the J570W because the gases have a longer core distance to cover before exiting the nozzle throat. They just gotta travel faster to get there. To combat this, a slower propellant can be used, and the previous example of P<sub>c</sub> increase until dropoff is effective (notice the violent initial thrust spike and resulting oscillations in the
J570W thrust curve -- very cool). Some experimenters use a stepped core to reduce the erosive burning, which seems to work quite well. Mike Fisher of Binder Design has a 38mm K motor with a stepped core which works extremely well. The AMW Green Gorilla load for the 75/7600 case is also designed this way.
Hmm, this is turning into a propulsion theory thread
HTH.