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 »

I was browsing the catalog at one of my favorite chemical suppliers (Darren originally told me about them — Mach 1 Chemicals), looking to submit a few quotes for new bonding agents, as my supply of HX-878 is rapidly aging and I’m looking for a new, possibly more useful, compound. In the middle of my browsing, I happened upon the following entry: Read on »