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Val1 lawn dart. R.I.P

Posted: Wed Jun 27, 2018 10:36 am
by retrotec29
Couple months ago my daughter and I hit the park to fly. Conditions were good and we were ready for some fun. My favorite flyer Val1 was first up. As with everything chance reared its ugly head and we had a no deploy on the recovery. Flight was well over 400 feet. The impact was quite dramatic. We were so stunned both of us forgot to take a picture. The altimeter survived but only a few other few parts were salvageable. The bent and twisted carcass sat on the shelf for some time. With a couple late nights of limited sleep due to a cold I decided to rebuild the rocket. I had saved all the original build templates so the fabrication time was quite short. Improvements in materials and deployment were added but other than that I stuck to the original design. Here are some pictures from the build.
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Re: Val1 lawn dart. R.I.P

Posted: Wed Jun 27, 2018 10:38 am
by retrotec29
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Re: Val1 lawn dart. R.I.P

Posted: Wed Jun 27, 2018 10:40 am
by retrotec29
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Re: Val1 lawn dart. R.I.P

Posted: Wed Jun 27, 2018 10:42 am
by retrotec29
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Re: Val1 lawn dart. R.I.P

Posted: Wed Jun 27, 2018 10:43 am
by retrotec29
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Re: Val1 lawn dart. R.I.P

Posted: Wed Jun 27, 2018 10:44 am
by retrotec29
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Re: Val1 lawn dart. R.I.P

Posted: Wed Jun 27, 2018 2:35 pm
by anachronist
Nice work rebuilding.

I wish I had a slow-motion video of my "anomaly" (NASA speak for "explosion), in which my rocket shredded itself in the air about 50 feet off the pad. It was spectacular, especially since the payload part continued up to 170 feet according to the altimeter. Fortunately it landed in tall springy grass and all the expensive bits were safe.

Subsequent experiments traced the cause of the problem: after the water was all expelled, and the residual pressure expelled, the rocket was moving about 200mph, creating a low-pressure zone at the nozzle. This created sufficient negative pressure in the bottle to buckle it. A bent rocket body combined with high speed ripped off the nosecone and parachute.

Your 1.25L bottles don't have that problem. They can take a good amount of negative pressure without buckling. Your 1.25L bottles also have a nice taper to minimize the low pressure zone.

A big 2L bottle, however, collapses easily with negative pressure. Older 2L bottles were made with a groove molded around the circumference, stiffening it. Newer bottles don't have that, at least in the US. When I traveled to Asia I found a couple of long and slender 2L soda bottles with the stiffening groove. My wife objected to the luggage space of two empty 2L bottles, but relented when I filled them with all the prawn crackers she wanted to take home. I'm saving the bottles for a future design.

Last summer I was between jobs and it was a great opportunity to pursue this hobby. But now I barely have weekends anymore. Not only do I need to build a new rocket with my son, but my launcher leaks somewhere. I dislike making launchers, but it has other problems (like needing 30+ lb of pull to release the collar), so I guess I need to grit my teeth and make another one. The worst part is living in a condo with no workshop... and I grew up with a workshop always available in every house I lived in. I don't have a good way to bore out a PVC pipe to fit a copper pipe for the o-ring groove. It takes many tries and a lot of wasted tubing before I ream out a straight 5/8" hole by hand.