This post is about the propellant parameter, but I had to put a picture in of the world’s most awesome N motor, too, since it’s burning CTI’s C* propellant. (That’s in James Dougherty’s 1/2 scale Patriot — click through and scroll down for the video.) Continuing in the theme of previous theory posts, this one will be about the wonderful, beautiful term called “characteristic velocity”, or C*. C* is one of the terms that is extremely helpful in correlating theoretical and delivered performance, and in discussions I’ve had with lots of rocketeers over the past year or so, it also seems to be somewhat misunderstood. So let’s start at the beginning and see how useful it really is.

C* and are related, and both are found in various forms of the equation for thrust:

where is the thrust force, is the chamber pressure, is the throat area, is the thrust coefficient (provided by the nozzle), is the propellant mass flow rate, and is the gravitational constant. Some quick rearranging shows that

which is also pretty handy, since we know all the terms in the rightmost equation, or at least can measure them directly. This means that we can calculate C* from test data – hey, another reason to build a test stand!

The true beauty of C* comes out, though, when we look at it from the other direction, ahead of time. As motor designers, we get to pick and , so all we need to calculate is , and we can calculate C*. And isn’t that bad to derive, either. So let’s give it a go, yes? Read on »

The 46th AIAA Joint Propulsion Conference happened a few weeks back down in Nashville, and things have finally stabilized around here long enough for me to write about it! I had a great time attending all sorts of presentations on cool state-of-the-art chemical rocket propulsion technologies of all flavors (solid, liquid, and hybrid) — the problem was that there just wasn’t enough time to head to all of the presentations I wanted to see. Some of the highlights of the event included meeting Dr. Ken Kuo and Dr. Luigi DeLuca (above), both of whom are titans of the field, the Moog party on Tuesday night and running into Luke Colby from Scaled (it’s a small world!), giving my presentation, seeing my old compressible flow professor as session chair of TWO sessions (go Dr. Marcu!), seeing two of Martin Summerfield’s students battle it out during a presentation (Herman Krier and Luigi DeLuca, like watching lions fighting on the savanna — you step back and watch in awe), and being herded into the hotel basement with all the other attendees for a tornado warning. More details on the sessions I attended and more photos after the jump. Read on »

One of the best ways to get really high burning rates with propellants is to introduce a much finer oxidizer particle into the binder matrix. Finer AP particles also help out tremendously with combustion, breaking down large fuel molecules much more effectively than coarse AP particles do. The end result is a propellant that burns smoother, produces a cleaner flame, and is nice and fast, to boot.

While I had the 200 micron AP under the microscope, I also pulled out some ground 20 micron to photograph, just for fun. This 20 micron AP is just above the limiting particle size for posting on the list of explosives. AP is a monopropellant, and can also undergo deflagration-to-detonation transition (DDT); these problems are exacerbated when the particles are extremely fine and in the neat state. Any milling operation involving energetics should be treated with the utmost of caution. But the end results are totally worth it. For preparing the ultimate in multimodal propellants, 20 mic can’t be beat.

Just for Scott:

(thanks to Darren@Loki for the hookup!) Wires can be used to increase the bulk burning rate of a propellant by conducting heat deep into the burning surface. I did a literature review and gave a talk on the historical applications of this technology a while back, so I’ve got no excuse not to try it. Paper and presentation video after the jump.
Read on »

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 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 »