As those of you who follow our Facebook and Instagram (@cprocketry) pages might know, we had an exciting night this past Monday.
After several weeks of deliberation on how to ignite, we settled upon a cocurrent jumpstart from a solid rocket motor. Imagine our large engine housing a smaller engine. The heat of combustion of the small engine ignites the rush of fuel traveling through our bigger engine and — POOF. We have lift-off.
Combustion theory is hard. A good assumption to make, one that doesn’t need a Ph.D., is that our oxidizer and fuel will both need to be vaporized before burning. That is: They need to be a gas. This, at least, is handled by the injector.
The second part is the orientation of the starter flame. To summarize many, many studies and papers in one quick sentence: “The igniter should be oriented the same direction as fluid flow.” (Downwards.) Doing so reduces turbulence inside the engine, reducing the chances of the engine exploding. It also orients the igniter flame away from the shower-head face of the injector, which could cause some orifices to close up.
The big question, then, is how to take a solid, put it inside a liquid shower, turn it all into a gas, and light the whole thing on fire.
A few ideas came to mind. Do we hang it from a shepherd’s crook? Adhere it to the bottom of a plate? Stick it to the side of a wood pole? The problem with each of these ideas was two-fold. One: With the relatively small size of our engine, how do we ensure that these bulky geometries don’t increase the turbulence? Two: How do we take the straight flame from a solid motor and fan it out in all directions to ignite as much fuel as possible?
Luckily, a little bit of CAD helped solve all of these problems. We settled on a cylindrical igniter plug. This design allows for a variety of motors to be installed and incorporates a diverter to spread flames in all directions. (You can see the outline of our model in the video below.)
After a watching our first prototype plug print for a few hours, we couldn’t wait to test it out. Even though it was printed from polylactic acid (a plastic which melts at 220°C) and solid motors burn MUCH hotter than that, we figured we could get a few seconds of slow-motion burn time on camera.
And boy, did we get a show.
After slowing five seconds of burn time down to several minutes, we had 10 seconds worth of good footage. You can see in the video that each of the radial flame outlets has even distribution and steady flow. Which is just what we want!
You can also tell when the melting plastic starts to disrupt the flame dispersion — about 7 seconds in. The result of that were the smoking, charred remains of our plastic igniter plug. It gave renewed meaning to the word “pungent.”
Though the test was a success, we began optimizing our design immediately. We inverted the design to allow for ease of access during testing and launch. Loading the motor from the bottom required the flame to shoot upwards, though. This, in turn, required us to reorient the flame outlets.
The team also bought several cylindrical bars of aluminum. After a successful test of our new design, we plan to machine a couple out of aluminum — which hopefully won’t be reduced to a smoky pile of ooze.
Currently, the greatest design hurdle is how to remove the igniter from the engine during testing. Since the rocket is attached to the ground and the current igniter design doesn’t move, we’re back to square one — disrupted Mach flow in the engine. (A result of turbulence.) Luckily, we’ve got a crack team of young rocket scientists working on it.
So that’s where we stand! Proof of concept confirmed, future planned out, and a moveable design in the works. Just in time for our rapidly-approaching testing timeline.