Wednesday, April 24, 2013

Rear Ejection

The latest build I'm working on is Pemberton's Little Bucky Jones, and I think it is going to be a slick looking model. Last week I was silently lamenting the fact that it will have a seam where the nose cone detaches, and I thought about the idea of making it, or really any other model, a rear-ejection system. It wouldn't be too difficult: seal on the nose cone and construct an engine mount that is blocked in the front by some sort of engine block, but can slide out the back and drag the parachute with it when the ejection charge fires. Should work.

But if it is so obvious, why isn't it done? The best reason I could think of concerns what would happen if your parachute failed to open for any reason. Since the node cone would still be on, you'd have an aerodynamically stable rocket in the sky, making a very ballistic return to earth. 

Ironically, I was just talking about this very scenario to my father-in-law on Saturday as I prepped my US Rockets Dual 18mm Rear Eject  (also linking to Apogee since they have a much nicer presentation and pics of the rocket) for a flight with two C6-5 engines. The rocket took off, flew high, arced downward, and as soon as the ejection charges fired it was clear things were not good. Two separate pieces were visible, and the big one was headed downward fast.

We recovered the rocket with the nose cone and crumpled front of the airframe buried six inches in the soft ground. At that point it broke off, and the rest of the rocket was intact, except of course for the missing parachute, which had detached as a result of a broken shock cord.

(I should point out that on a previous flight the Kevlar shock cord that came with the kit actually burned all the way through. I replaced it with a cloth-elastic cord mounted in the end of the tube with a paper mount, Estes-style.)

So from what I can tell, one, or possibly two things went wrong:

First, the delay was too long. From what I observed, the rocket had fully arced over and was starting to head down with some speed when the ejection charge blew. I'm sure this put a lot of stress on the chute as it popped out, and it likely led to the snapped cord.

Second, I had set up a camera to get a movie of the take off from the pad. Watching this frame-by-frame clarified what seemed like a not-quite-right launch. Despite the parallel hookup of the two engines, the video clearly shows that they ignited at different times, maybe 1/15th of a second apart. Not a big difference, but I wonder if this led to the first ejection charge pushing out the wadding and chute, but the second charge burning the shock cord on its way out and weakening it. Though I didn't see much evidence of this on what we recovered (we never did find the chute and the end plug to which it was attached).

  1. Verify the proper ejection delay with flight simulation software whenever possible.
  2. I'm done with the idea of rear-ejection. I've had my share of chute failures in other models, but they've always had enough drag after popping the nose that they are recovered with little or no damage.

And here's a semi-related epilogue:
About fifteen to twenty minutes after that launch, I was at the launch base preparing another rocket while my father-in-law was out still looking for the parachute. A police officer drove up and approached, saying that he saw the launch from the road and came to check things out. I told him that although this was my first time launching there (at the Northwest Open Space in Northglenn, CO) I had checked the Northglenn fire code (which is basically the IFC) and verified that model rocketry was permitted. He responded that "it may be allowed by fire code but prohibited by municipal code," and added that there was currently no fire ban in effect - a big issue at the end of last summer around here.

The municipal code issue is something I'd never thought of. But he said that in a very shrugging-it-off sort of matter, then just asked a few general questions about my rockets (how high, do I ever use payloads like an altimeter) and casually looked over my gear. His last question was whether I planned to launch any more, so I figured I was in the clear. He checked my ID and wrote down the info, then left with a not-too-stern "be safe." Another police car rolled up as he was leaving, and my father-in-law overheard him say to the other officer "just someone launching model rockets." 

I've since looked up Northglenn municipal code and can't find anything about rockets. I've really never heard of low power model rockets being illegal to launch anywhere. They're certainly sold all over the metro area.

I still wonder if what he saw from the road was a scary ballistic return - though he never mentioned that.

Update - 06-May-2013
I attended the CRASH launch yesterday (my first time - it was pretty great actually) and talked to one guy briefly as he was preparing to launch his Estes Gemini DC, sharing my experiences. Guess what happened next? Lawn dart.

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