The goal of any rocket motor designer is to pack as much propellant into the motor casing as possible. After all, you’ve only got so much room for motor — might as well make the most of it. One popular way to increase volumetric loading is to step the cores of the grains as they go down the length of the motor, putting a large port diameter near the nozzle throat and a smaller port diameter up near the head end of the motor, where the mass flux and port velocity is low. As the gas accelerates down the length of the grain, opening up the port lowers the mass flux to hopefully mitigate the effects of erosive burning.

But what if the aft grain is small? It’d pack more propellant in the motor, but bad things could also happen. Common industry wisdom says keep a port to throat throat to port (thanks James!) area ratio of 0.5; McCreary is a bit sportier in “Experimental Composite Propellant“, going for a diameter ratio of 0.75 (and thus an area ratio of ~0.56). Of course, many rocketeers have been known to push this limit, even so far as to have a port the same size or smaller than the throat. And it works, sometimes. So what happens as the port size is increased? Read on »

After more research, discussion, and testing, the linear erosive model (see this post for more info) doesn’t seem to hold 100% true. Sometimes. This post is more of me thinking out loud about the subject in general, and attempts to tie up most of the theories into a neat, easily accessible post. So, here goes. Read on »

There has been a lot of discussion as of late regarding erosive burning and its relation to mass flux. Many have suggested that propellants with certain additives (notably, Oxamide) have a better response to high mass flux conditions, and have proven with ample results that it in fact does. While the effect these additives have is noticeable, it does not seem to scale linearly with pressure, as I might have expected thanks to the basic interpretation of Vielle’s law. After doing some thinking and some research, I happened upon some useful information on how to characterize the effect mass flux has on erosivity. Read on »