This launch was to be the first launch of my digital camera. I watched the weather forecast the night before, and it was calling for sun, warm temperatures and little wind. Perfect conditions for a launch. So I decided to prep a rocket for the camera. I needed to get electronics module 1 functioning again, as I had robbed one of the timers for the new EM-2. I built a new timer and tested it, and mounted it into EM-1. Then I needed to cut out a hatch in the body tube for the camera to mount in, and install a battery mount for the camera into the module.
By the time I had the rocket ready, it was getting late, and I wanted to have everything ready to go in the morning, so I started getting in a bit of a hurry. And that's where problems start, with getting in a hurry. I loaded the T-2 motor with a couple of previously made grains, and inserted the nozzle and screwed in the retaining bolts. What I didn't do, was measure and weigh the grains. When I loaded the grains into the motor, it looked like they were plenty long, in fact, I wasn't even sure the nozzle would make it all the way in. But the nozzle went in and I went on my merry way...
Now to the of the launch. Bill came over to help with the launch. As Bill set up the pad and controller, I went about getting deployment charges set, the camera mounted and the motor installed. Once the rocket was on the pad, I got a safe distance away, Bill was at the controller and started the countdown.
5,4,3,2,1 LAUNCH!
Here is the moment of ignition.
Here we are .033 seconds later. It's called a cato. Notice the dark gray smoke.
Here is the view .166 seconds later. I enlarged this one a bit. You can see the upper body tube, and the white spot below and to the left is probably a fin, or maybe the camera. Again, notice the dark gray smoke. That is a characteristic I've noticed when grains fracture and you have rapid burning. It may be due to the high pressure, some how changing the burn characteristic and causing darker smoke.
All I can say is I'm glad I was as far away as I was. It was impressive, although totally unexpected. The T-2 motor has proven very reliable, and this motor has been fired 12 to 14 times without a problem. I'm sure the grains cracked because I had them in too tight. I need to leave about 1/2" of free length in the chamber to make sure the gases can get behind the grains, and I knew this. Previous tests had shown the need for extra space in the chamber, and I didn't do it. I got in too much of a hurry.
Here are the remains of the T-2 motor.
It happened so fast, all failure points occurred. Both the nozzle and the top closure were blown off, and the casing suffered hoop stress failure. The nozzle end is to the left, all the retaining holes are elongated and the bolts pulled through the holes in the casing. The forward closure sheared all of its bolts without damage to the holes in the casing. The blown open casing is typical hoop stress failure. I weighed the casing, and it was exactly the same weight as before the cato. So the good thing is, the casing didn't expel any fragments. That was as I expected. I had been considering doing a test of the EMT casing, to deliberately over pressure it and cause the hoop stress failure. While this wasn't the way I waned to do it, I did get the information I wanted. The failure point was not at the welded seam.
I have heard people say steel casings will fragment, and aluminum won't. Then I've heard people say they both will. I suppose it depends on the alloy. But from my experience in high pressure air cylinders, steel will generally tear and not fragment, while high strength aluminum alloys are more brittle and will tend to fragment some. I'm not going to say steel won't fragment, but in this case at least it didn't. For God's sake don't use cast iron pipe for a motor, could you imagine the shrapnel it could throw? Cast iron is very brittle. You know how to remove rusted cast iron fittings? Whack them with a hammer, once, twice, nothing. But after 7 or 8 blows they break apart.
What happened to the rest of the rocket?
Well, the camera survived. The electronics I made to power and control the camera took some damage, but it's repairable. The camera took 11 pictures before it stopped.
Here is the last image the camera on the rocket recorded. I was kind of hoping for something exciting once I found the camera still working. But the cato happened so quickly, you can't even see any smoke.
The forward closure went up into the rocket, it took out the thrust ring, then the lower ring on EM-1. Both timers had the IC chip gone, and one screw hole broken. Both timers still worked after replacing the IC. EM-1 had 1/2" plywood running the length between the two 1/2" plywood disks. That plywood running the length saved the electronics mounted on it. The plywood was severely damaged at the lower end, crushed for about 2", the module also broke the retaining ring above it, or rather pushed it up into the parachute bay area. The PVC body tube was cracked where the screws went into it. The g-switch board was cracked, but the switch and the brackets are useable.
The parachute and nose cone were undamaged.
The lower body tube is gone, many small fragments. The fins and the aluminum brackets are a mangled mess. The only usable part of the motor is the forward closure.
The launch pad suffered a bent blast deflector, and a bent launch rod. The blast deflector was damaged when the nozzle was blown downward. The launch rod was bent from the force of the casing rupture.
In summary: I was disappointed. What a great day it would have been to get those digital images. A lot of hours of work were destroyed in an instant. Total cost of the destroyed parts, maybe $7 or $8. Not a great investment other than time. This day was another lesson in taking your time, and being meticulous in everything you do. Sometimes, the lessons learned the hard way last the longest, I hope so!