More testing on thermite. I knew it wasn’t going to light, but I needed some photos to prove it. So, bust out the compound, a sample tray, a blowtorch, and a camcorder…

Quite boring, really. But that’s a good thing. Working in energetics, I’d much rather have a “boring” day than an “overly exciting” one.

Mixing propellant can be a chore, mostly due to one thing: cleaning the beater and bowl. Resodyn of Butte, MT produces a line of “resonant acoustic mixers” (RAMs) that simplify things greatly. The RAM is essentially a glorified paint shaker, with a system of accelerometers and driver masses that automatically tune the mixture to vibrate at its natural frequency, thus imparting as much energy as possible into the mix. What’s neat about it is that the mounting table is designed such that you can actually pour all your propellant chemicals into the casting sleeve, put the sleeve right on the mixer table, and mix right there — no cleanup needed! (They were demoing this technique at the ARL review meeting.) It’s also a tremendously fast way to mix propellant (~10 minute cycle time), and gives tons of data on what is happening during the mix cycle; I just finished working on a Phase I project that used this data to parameterize a model to determine how mixed the propellant is for a given cycle energy input.

Anyways, the RAM mixers were brought up a few months ago on the Tripoli list, to some interest. I made a quick video of what the mix cycle looks like to hopefully answer some of the questions that were asked. This mix cycle is of an 83% solids propellant based on AP and HTPB, mixed under vacuum. I think it’s really cool how the propellant kneads and folds itself during the mix cycle, going from a system of discrete ingredients to one big well-mixed mass of propellant. It looks like a superball during the mix, but as soon as the mixer powers down, you see the propellant settle out against the bottom of the mix jar and remember that it is, in fact, a really viscous solid.

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 »