ANy electronic deployment gurus at NARAM?

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Well, that didn't work out very well :confused:...

Chatted this up at the sport range a bit Friday, got a few tips. Tried a couple test flights in a big clunky sport model that I thought would have a similar flight profile to the pretty nice looking/detailed contest model I'd built, based upon a mediocre boilerplate flight that didn't get high enough to trigger launch detect in an altimeter. First test--low and slow (I'm talking 50 foot apogee), but no deploy. I figured it wasn't triggering the 2g/half second launch detect on the timer, though I'd think it's hard to do that while clearing the rod. Put same model up on twice the impulse, and the charge fired but not enough to blow open the BT-80 tube. No big deal--I'd use more powder and switch to slightly more powerful motors for the contest model.

Contest model turned out to boogie off the pad, getting up to 300 feet or so very quickly. Came crashing down, no deply, model destroyed. Post mortem observations and assessment:

1) Model certainly exceeded 2g for more than half a second. No doubt whatsoever. The g-switch could have been faked out, though. I mounted it somewhat loosely in a BT-50 stuffer tube (padding for and aft to prevent damage). Still, if the switch slid back I can see how the G-switch could have been faked out. Shouldn't have been much of an impact, though--there's a solid bulkhead that would have prevented a slide of more than an inch.

2) Inside of body tube where the charge was appears to be clean--no signs of burning, smoke, etc.

3) plug-in connector for xmas bulb charge shows signs of charring

4) Crash site--didn't see the heat shrink tubing holding the charge laying around, but didn't have time to look too closely. Definitely no longer on the bulb base, and it was on there pretty good.

In terms of figuring out what went wrong/what to do differently, I'm really stewing over not knowing what happened.

  • timer slid around, loosened wire/faked out G-switch (need to mount on board)
  • xmas bulb failed (timer beeped continuity check OK)
  • Powder slipped out of sleeve (unlikely since sleeve no longer attached)

I think I'm going to build another copy of the model, but this time will design in the electronics rather than slide a timer in as an afterthought. Definitely mount on a board which ends in a bulkhead, with timer aft and feeding charge through a hole in the bulkhead to the forward-mounted chute. This is opposite of what I did the first time.
If I were you I would upscale it... Probably double, to give yourself more room to work with.
Believe it or not, the scale I worked to is about the only one that's practical unless I want to start getting into rolling my own tubes and/or glassing. It also drives up the cost of the balsa components substantially, something an unemployed bum whose son heads off to first year of college in a couple of weeks is a bit sensitive to avoiding.

Besides, one of the cool things about that particular model is that it was dead-on scale, at least to the degree that there can be such a thing for this type of model. Most of the other models out there I'd heard about used tubing and parts that were roughly right, but not that tight. I think all my diameters and lengths would measure out to within 2-3% at the worst compared to Jon Rogers' drawing in the Haggerty reference. That's part of what made it such a high-scoring competition model. A rebuild would mainly be a sport model though, since this event is hardly ever flown anywhere.

I do think in the scale I built, there's plenty of room, but I did it backwards. I'm used to LPR recording-only altimeters--just slide 'em in below the nose, pad them for/aft, and they work great. I took same approach here, especially wanting the weight forward if possible--altimeter/battery below the nose, bulkhead then chute then wadding then deployment charge down below, pushing the stuffer tube out when it blows. What I'll do on the rebuild is make a small board for the timer that ends in a bulkhead on the forward end, run the xmas bulb socket up into it, then plug in the charge on the foward side of the bulkhead. Chute pops out front just like a normal rocket, pulled out by the nose. No stuffer tube ejection.
Really sorry to hear about the crash. Isn't that always the way, the ones with the most heart and soul invested in are the ones that crash.

I've never used timers, but I think you're on the right track with hard mounting the timer. You want the g-switch to feel the g forces of the rocket. The padding is great for baro types, but defeats the purpose of the g-switchs.

As for using xmas tree bulbs, be careful. I've had great success with them, but I don't use a socket.
  1. I break the bulbs and use a tweasers to pull the wire out that is wrapped around the filament posts. This is what keeps the string of lights lit when a bulb burns out and will give you a false reading for continuity if the filament is broken. (Check continuity)
  2. I then wire two bulbs together for redundency. (Check continuity)
  3. I use hot glue to glue the bulbs into the bottom of a rolled piece of paper. (Check continuity)
  4. Solder on as long of wire leads as you need. (Check continuity)
  5. When it comes time to use them, pour in your powder, fold over the paper, tape and connect the leads.

This has worked very well and the long leads allows putting the charge on top of the motor while connected to the e-bay in the center of the rocket.

Of course the theme for electronic deployment of any kind is: Ground Test, Ground Test, Ground Test, and Ground Test. If you are just trying to determine the size of the charge, use the full recovery system (shock cord and chutes) but you can use a launch controller to set them off. If you are trying to verify the electronics will fire the charges, you don't have to use a full charge. The bottom line is that when you're done, you should know that the electronics will fire the charges and that there is enough charge for reliable ejection.

Good Luck with the rebuild
A friend at one of launches had a similar problem and it turned out the battery he was using was not up to the task. When he used the one recommended by the timer mfg everthing worked as planned.