LOC Precision Graduator post launch questions

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Jul 18, 2017
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I launched my first Mid powered rocket last weekend, a Loc Precision Graduator, at a club launch. I used an Aerotech F25-9W motor. The igniter was attached a little to well and did not release right away when the motor ignited. It was off the pad before it came loose which looked kinda cool in the video. I learned not to secure it so well with tape is the cure for that. I did cause the rocket to go up at more of an angle than planed though. Because it did not go up at the angle it should have, it was on it's way back down when the chute deployed. I replaced the shock cord with Kevlar rope. When I recovered the rocket, there was a tear in the body tube about an inch long from the shock cord mount. Its a thin rope that is epoxied to the body tube. Any suggestions on how to prevent that from happening again on this rocket? Someone at the launch site said put a small rubber ball over the cord so it will cushion the cord. I am just wondering if there are better ways to fix this issue.
I see a couple of opportunities here.

First, and possibly most important is the delay. The 9 second delay following motor burnout is too long for the Graduator. A 6 second is ideal for most Graduators; I'd venture to say that a 4 second is great in windy conditions and/or if the rocket angles as your flight did.

Second: the 2.56" cardboard airframe is comparatively thin walled to the 2.1" and some of the other mid power tubes. It will definitely zipper if the rocket is hauling the mail nose down when the chute suddenly blossoms. Kevlar cord will exaggerate this. Note: I'm a fan of Kevlar, but not in this application. Kevlar has little-to-no give, so when the body tube is yanked 180 degrees by chute opening, there is no give and it will tear the relatively thin walled cardboard. Though most rocketeers will advise that "underwear elastic" has no place in high power, my experience is that sufficient length elastic in mid power works better IMO than the fancier, heavier, less stretchy tubular nylon and Kevlar products. Though the stock elastic can stretch and cut the airframe, I've found it to be more forgiving on the less-than-optimal delay flights (where the rocket is "coming in hot").

Third, I'm not sure I follow the rubber band over the cord idea mentioned, but along those lines, there are several ways to help prevent zippering. One can reinforce the end of the airframe (wrap of fiberglass, strapping tape, soak with resin, etc.), one can increase the surface area of the cord where it contacts the edge of the tube (wrap a sock or something around the cord, tape in place, etc.). One could build with a "zipperless" design, though that's not overly practical with a stock Graduator.

Above all however, try out the -6 delay for most conditions, -4 for high winds or mega angle, and you'll see that zipper phenomena disappear!

I'll concur with the delay, and the zipper you got. The larger the rocket, the more mass is moving about. And with that, more accidents can happen. Small LPR rocket are not as prone to a long or short delay damage. larger ones are!

In your case, yes, a shorter delay is a must. Do some sims , at least on Thrustcurve.org to get some numbers. 2nd, the Kevlar is a good start. As Eric mentioned, a short (2ft) of stretchy stuff between the end of eh Kevlar & the NC is a good "energy absorber". I've done that a few times. Question here is, how long is your shock cord?

I've hear the ball idea a few times, and it does work. what you're trying to do is give the shock cord a large surface area so that it won't rip thru the wall of the BT. Tape, tubing, etc.. all can be added to help prevent a zipper.
Rocksim says, on my LOC Graduator, for an F25W the optimal delay would be 6.29 seconds. Switch to the 6 second delay and you'll be good to go.

You can also put some thin CA around the inside of the top of the booster tube, 1 or 2 inches worth, to combat zippers. Make sure you sand it down so it's smooth and doesn't hamper the laundry. :facepalm:
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Thanks everyone for the quick responses to my questions.

Sounds like I need to start by going with a shorter delay. I did some figuring with an app I found for my phone after I had launched and I was coming up with a coast time of around 6 seconds. I just went off a recommendation from someone at the launch. I guess I will hang onto the other F26-9 motor I bought. As for the zippering, I was thinking about a 2" piece of fiberglass around the outside of the body tube that I will coat in a thin layer of epoxy and then add something to cushion the shock cord which BTW is about 8 to 9 feet long. Will fiber glassing over the damage part be enough? I thought about cutting off the damaged part which would make the rocket about 1 1/2" shorter. Is that a bad idea?
It won't hurt anything to cut it off, as long as your anchor is far enough down to allow the nose cone to seat. I use elastic in pretty much everything that is cardboard. I think my heaviest cardboard guy is 10 pounds.
I would suggest also that you familiarize yourself with software to predict your altitude. For IOS i really like "Smart Launch." You can also get good runs from www.thrustcurve.org. John Coker (who runs thrustcurve) now has both IOS and Android apps.

If you start with empty weight and diameter, you get a good idea with the defaults. If you send an altimeter along, you can dial in you drag coefficient and confidently launch a wide variety of motors.