November 07, 2005
The motor was a Contrail Systems L1222 "sparky" (none of us know if it really was, by the way, we were kinda too busy to notice). The motor itself is about 3" in diameter, it's 54" long, of which the bottom 12" is the combustion chamber where all the flamey zoomy stuff happens. The oxidizer tank holds 3200cc's of nitrous oxide, which comes out to .85 of a gallon. Doug estimated that about half of that had been vented when the ignition happened.
The burn time for that motor is listed at 3.1 seconds, but I would guess that it ran out of nitrous (oxidizer), and hence the oxygen need to burn, long before that, so the thrust would've fallen way off from the specs.
But at ignition, well, there was plenty of oxidizer for that, and that sucker lit up with a peak thrust of 2892 newtons/second, which works out to 650 pounds of thrust right off the pad.
I've heard it said more than once at a rocket launch: even our failures are entertaining to watch.
Posted by: Ted at
12:29 PM | category: Rocketry
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November 05, 2005
I only flew two rockets myself, but there's a story to be told, and we all know how much I love that. So first, the details, then the good stuff.
My first flight was my Centuri Groove Tube upscale. 2.6" diameter tube-fin design, launched on an H165-Redline motor. Typical great boost from this rocket, and she arced over at apogee and just after going nose down the chute ejected. Recovered about 150 yards from the pads, undamaged.
Second flight, Barenaked Lady on an F24 with a seven second delay. This rocket is ultra-light, and the F24 seriously overpowers the rocket, which is fun as hell and why I do it. Waaaaaaaaay up there in a hurry and recovered about 100 yards away undamaged.
So that was all I flew. I had a few other rockets, but I had a great time anyway, picking a friend's brain for altimeter bay ideas (his always work, and I've never been completely satisfied with my designs), and shooting the breeze with fliers I haven't seen in awhile (frequent commenter Russ was there).
"I've done everything I know how to do, so if this doesn't work then we'll learn something." -- Doug Pratt
Later in the afternoon, Doug Pratt readied the rocket he's going to eventually fly for his Level 3 certification. Twelve foot tall, six inch diameter, all fiberglass, he was going to use a hybrid L-something motor.
They had the rocket on the pad (very big motor, so it was loaded on the "away" cell, much farther than normal), and I headed out to ask if there was anything I could do to help. Doug said something about giving them good luck with the flight.
Ha! That'll teach him.
Filling the nitrous tank for the motor seemed to take an unusually long time, and after the countdown there was no ignition. Ivan (another friend) started to vent the nitrous back out of the motor, and while that was going on Doug, Ivan and I walked back to the pad to see what was wrong. There was smoke coming from the pad, and we saw that the igniter wires were smoking. This put us all on guard, and we started visually checking the setup.
Doug switched off the power to the pad, making it safe. Moving over to the base of the rocket, he lifted the ignition wires and the motor instantly ignited! I was farthest away of the three of us, maybe 10-12 feet. Ivan dived away, Doug wound up with all the hair on one arm singed off, and I twisted and turned my back to the roar of this big honkin' motor going on right next to us.
Summary so far: big motor, too close, accidental ignition.
We were busy making sure that Ivan was ok (he hit the ground and rolled) when people started yelling "heads up!" at us. I confess that I had two thoughts before looking up:
1. Uh oh, the chute didn't open and it's coming in ballistic.2. This was a weird motor ignition, so the sucker coming down on top of us is probably on fire.
When I did look up, I was relieved to see it descending normally under chute. Even better, it was going to miss us. Then came the second bit of excitement.
Fire.
Ivan started yelling, and we ran about thirty feet downrange to start stomping out a brush fire caused by... well, we're not sure what caused the fire. The motor ignition, certainly, but why or how... no idea. Anyway, the three of us were stomping and stepping, holding the fire at bay more or less, until folks with water buckets made their way out to where we were and saved the day.
Ok, failure analysis. While the rocket was being loaded, the igniter wire insulation were chafed or otherwise broken. This caused the ignition wires to short out when they touched bare wire to the metal launch pad. That was problem number 1.
Next, when the ignition button was pressed to light the motor, the short prevented the current from reaching the business end of the igniter, but the relay in the circuit welded itself open. What that means is that although Doug shut off power to the pad, the relay had enough juice in it to fire the igniter, which happened as soon as Doug moved the wires, which unshorted them.
Whoosh! A helluva lot closer than I ever want to be ever again.
Nobody was hurt (beyond that singed arm hair), which was the main thing. The relay box is being disected this evening to figure out why it stuck open and how to prevent it from ever happening again.
Just to give you an idea of the power of the motor: even with only half a tank of nitrous to work with, the motor lofted the forty-plus pound rocket over four hundred feet into the air.
Hanging out with Doug always makes for an interesting day. Afterwards, we hit Country BBQ for some excellent ribs and fixin's and then I headed home; happy, tired, and with another cool story to add to my collection.
Posted by: Ted at
09:49 PM | category: Rocketry
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November 03, 2005
Perfect.
Posted by: Ted at
04:18 PM | category: Rocketry
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