In the category of EPIC FAILures few things are as dramatic as a failing rocket motor! We (in the hobby) call this a CATO.
If you follow my other Rocketry related Instructable.com articles (such as the one on making rocket nozzles, or the one on characterizing rocket propellant), you will read and re-read my pleas for safety in all things rocketry. Fortunately for this failure, safety protocols were followed, nobody was injured, and no property (other than the motor and test equipment) was damaged. From the video, you will quickly see that this would NOT have been the case if safety precautions were not followed. If you have an interest in rocketry, especially in EX (Experimental) High Power Rocket Motor Construction, please join the Tripoli Rocketry Association and first learn how to do it safely and legally with others that have experience! If you can't do those things, this article and video demonstrate what could be a catastrophic failure that could put an end to life, limbs, and happiness. PLEASE keep our sport free, by always practicing safety!!! This can't be said too strongly. As you will see in the subsequent steps, the pressure produced by this failure could easily have maimed or killed if precautions were not taken!
If you enjoyed the video, or appreciated the information on this particular Epic Failure please vote for this entry in the Epic Fail Contest! Also, don't miss the zoomed in video in the next step.
It is important to note that this is the first CATO I've had after hundreds of single-grain motor tests on the test stand. In most cases we are able to tell by previous burns when case pressure is rising to the dangerous level and we stop before equipment can be damaged. In this particular case, the CATO occurred on the first test of this formula, using the widest (generally safest) nozzle opening.
Step 1: What Went Wrong...
At the start of this video is an indication of what really led to the failure. If you are not new to EX rocketry you will spot the problem right away...
Notice that in our characterization testing we always record variables such as the density of the propellant grain being tested. This particular formula used Copper Acetate in an attempt to get a nice blue flame. If you've seen copper acetate burn, you will know that the flame takes on a very nice hue of intense blue.
The problem here is that the density is very low. The formula has a slightly lower theoretical density than most formulas I've tested, and so I thought the low density might be okay. The theoretical density calculated by ProPEP3 is 0.0576274 lbs./cubic inch. This grain (which used with the widest nozzle opening -- a number #19 nozzle which has a 0.296875" throat -- of the nozzles we generally test with) only had a density of 0.05194 lbs./cubic inch! This means it likely had several large or many small air bubbles trapped in the cured APCP grain. This increases the burn surface area which produces gases more rapidly as the propellant burns. The presence of the Copper Acetate may have also sped up the burn rate substantially -- as copper compounds are almost all burn rate accelerants in Ammonium Perchlorate based propellant. The result as you can see was an over pressurized chamber leading to an unfortunate (and very expensive) CATO.
Because this test was performed on the test stand, a tiny amount of data was recorded prior to the destruction of the load cell (used for measuring thrust) and the pressure transducer (used to measure pressure in the motor chamber.) Though it is completely useless other than to laugh at, an image of the graphed data is attached to this step.
Step 2: The Cost...
This was a costly failure. As you can see in the images attached to this step as well as the images at the end of the last video, the load cell and pressure transducer were completely destroyed. A replacement load cell costs around $20 on e-Bay -- not too bad, but the pressure transducer (good to about 2,500 PSI) costs about $65.
As you can see from the photos, a few of the brass fittings were also destroyed. There was minor damage to the test stand that was easily fixed. The "nozzle" (not truly a nozzle, just a 1" thick graphite disk with a 19/64" hole drilled in the center and 45 degree half-angle entrance cone) was found in pieces within a few hundred feet of the test site. Part of the propellant grain was also found unburned.
The snap ring on the nozzle side of the motor case ripped through the snap ring groove destroying that end of the case. The snap ring, thrust washer and case are beyond repair. (The case mostly because it isn't long enough for another snap ring groove to be cut while remaining serviceable for its designed purpose.)
Failures like this are common in EX rocketry. It is always better to have a CATO on a test stand than to destroy a rocketry mid-flight or on the pad. Rockets themselves can often cost many times what a new load cell and pressure transducer costs, and it is not uncommon for a CATO on the pad to damage or destroy the launch pad. A single 54mm grain of propellant also costs far less as a loss than a larger flight motor with several grains.
The moral of the story: If your grain density is low -- especially less than 0.052 lb./cu. in., throw it in the scrap-burn pile. Don't waste your equipment. And ALWAYS follow all safety precautions!
The only good thing about a CATO on the test stand -- with a vertical test stand, is that the projectiles go straight up in the air, and are less of a danger than is possible with a horizontal stand or even a mid-flight CATO in a rocket.