Labyrinth Smoke Grain Illustration

Labyrinth Smoke Grain

"For every problem, there is a solution."

You've heard that, right? Well I believe it, for one.

But I am also convinced that for every simplistic statement like that, the converse is equally true. It's the law of conservation of fiction.

Question: Do your pages always have to start with a long, preachy narrative?
Answer: Yes.

Problem 1: No smoke trail. When I first launched my Sugar Rush with the Loki 54mm motor, it simply vanished. I was thrilled by that act a couple of times, then though it might be nice to track the rocket to apogee. Someone suggested a smoke trail, so I started thinking about it.

Solution 1: End-burner smoke grain. I started making end-burner smoke grains that would continue burning after the primary propellant had burned out, at least long enough for the rocket to reach apogee. These worked well on the average, some a little better than others.

Problem 2: The end-burner uses up a lot of propellant space. Since the first 5/8 inch will burn at warp speed (the speed of the propellant grains, since it is the same stuff and burning at the same pressure), it takes at least a 2-inch smoke grain to burn all the way to apogee. Since the 3-grain Loki casing has 9.5 linear inches between nozzle and header, this smoke fixture uses about 1/4 of the available propellant space. My first launches without smoke averaged about 4700, subsequent launches with smoke averaged about 3500 feet.

Solution 2: The moon-burner grain idea started this ball rolling. It provides a long burn time compared to Bates grainsWhat about making a moon-burning smoke grain, inhibited on both ends, and sitting it on top of a Bates grain motor? It would burn from one side to the other. The first web-thickness would burn at high speed while the propellant grains were going, but the remaining propellant would burn at 1 atm speed, providing a few seconds of smoke trail.

Problem 3a. A few seconds are not enough. In the 54mm motor, a moonburner grain with a 1/4 inch core* would have a web thickness of 1.5 inches. The first 5/8 inch would burn in 1.3 seconds or so as the propellant grains are doing their thing. The remaining 7/8ths inch would continue to burn for perhaps 10 seconds, and this rocket will take maybe 15 seconds to reach apogee. That leaves it smokeless at the top.

(* A small core is acceptable here, as the grain will be only about an inch long. 1/4 inch is plenty of area for gasses to exit, pressure inside the grain will be low.)

Problem 3b. I also want to use this smoke grain with moonburner propellant grains, so the web thickness will be almost completely used up during the propellant burn.

Solution 3: The Clock grain. At first I placed a divider inside the smoke grain, a small chunk of 1/4 inch plywood
(Click the small picture or here for a better description)

Problem 3a: It didn't work. In my one static test, the smoke grain did not make smoke. At least not after the propellant grains had fired. I assumed that the flame had "leaked" through past the top inhibitor or maybe somewhere else and burned the smoke stuff along with the propellant stuff. Not bad for thrust, not good for its intended purpose.

And yes, there is plenty of safety margin in this motor casing to burn the smoke grain simultaneously, no significantly increased danger of CATO.

I believe that the "clock" solution has promise, and with a little more development could be made to work. That would be good, because this grain would only take up perhaps 1/2 inch of space in the casing. But now I am onto a different solution, less efficient but maybe effective.

Solution 3b. The labyrinth grain. Instead of having the propellant burn around a corner, I have it go zig-zag from one layer to another. The first layer is catalyzed, to prevent extinguishment at pressure-drop. The second layer is uncatalyzed, to provide a longer burn.

Note that the illustration is "upside down." Meaning that the propellant grains and nozzle will be to the top of the illustration, the head-end closure will be to the bottom.

I drew it this way because this represents the view while assembling the grain.
A short section of inhibitor tube is cut, in this case one inch in depth.

A solid plywood disk is epoxied into the bottom end.

Topside of the solid disk is coated with epoxy, as well as the inside of the inhibitor tube. 5-minute epoxy works fine, as long as I work quickly.

A blob of warm, uncatalyzed rcandy is pressed into the tube, enough to almost half-fill it.

The first perforated disk is pressed firmly onto the propellant. It had been coated with epoxy first. Enough pressure is applied to remove any bubbles around the edges, and to extrude propellant slightly through the perforation.

The process is repeated with another layer of rcandy, this time catalyzed with a little red iron oxide. I find that catalyzed propellant does not extinguish easily when the case pressure drops, uncatalyzed propellant sometimes goes out.

As before, all surfaces are coated with epoxy, to fill any air gaps and to ensure that the propellant sticks to the surfaces well.

Wanting to save all my grains for launch, I forwent static testing. This may seem radical to some, and did make me a little nervous. But since the only likely failure mode is "no smoke trail," it seemed worth the risk.

These motor casings have plenty of safety margin to burn the smoke grain simultaneously with the propellant grain.

We will just have to make do with flight tests. Life is so hard sometimes...

Jimmy Yawn
5/31/05
jyawn@sfcc.net
Recrystallized Rocketry