I'll start by defining what CATO stands for. I'm not an authority and in my research I found two different definitions. The first is CATastrOphic failure. Rather simple but accurate. The one I like better is Catastrophe At Take Off. I'll let the judges decide which is correct. To understand how CATOs happen you need to understand rocket motor construction. If you have built a reload, I doubt that you'll need to review the construction of a rocket motor. If you do need a refresher in motor construction, click here. Just close the new window when you are ready to return.
How do they happen? Well, I've never suffered one but I have seen several. Most often the owner will admit that the motor was stored in a garage, car or some other non environmentally controlled location. With Estes (I only pick on them because they are the largest manufacturer of model rocket motors in the US) motors, I've heard several theories. First one I like the best.
Because the motor and the case are made of different materials (paper and black powder) they expand and contract at different rates with changes in temperature. When the motor is cooled and warmed the paper contracts and expands at different rates than the propellent. This action tends to separate the case from the propellant. So when you light the motor, the pressure builds in the combustion chamber and it can be enough to push the propellent out the forward and / or aft end of the motor case. I've heard some rocketeers swear that a 30 degree temperature change will cause a motor to CATO.
Another theory is that during temperature cycles, cracks form in the propellant. This exposes more surface area in the motor and so it burns more rapidly than it was designed to. The case can't take the pressure and either splits or an end blows out. Here is a picture of a case that split. Just close the new browser window to return to this page.
This one can happen to RMS (Reloadable Motor Systems) as well as SU (Single Use) motors like Estes. An object (like the ignitor or a chunk of propellant) can lodge in the nozzle of the motor. This can cause back pressure of an unacceptably high level resulting in failure of the case. Be sure to use the correct ignitor for the motors you are using.
RMS failures are due to different reasons. Sometimes the motors are not correctly assembled. Other times the case gets worn and the threads on the forward or aft closure can give way. Sometimes it isn't due to the propellant but due to gases leaking by the delay element and setting off the ejection charge too early. If you can find enough parts of the motor and rocket a post-mortem can be performed. What this really means is that a bunch of us old time rocketeers get to argue about what might have gone wrong and swap stories about the worst failures that we have witnessed. If the RMS is largely intact, you can normally figure out what went wrong. When you disassemble the RMS look for tell tale burn marks around the delay element and liners. Just a word of warning to those of you thinking about going for your level one certification. The delay elements in those 29mm H reloads are often very loose. You need to tighten them up with a wrap or two of tape so that this very failure does not occur to your rocket.
Although I haven't had a CATO, I have had a failure. I think it was at the December 1998 LUNAR launch. I put my NCR Phantom 4000 on the pad with a G64 locked, loaded and ready to go. Shortly after lift off, I knew that something was wrong. I thought that I had lost a fin but I didn't see one fluttering down to the ground. The tail of the rocket was swirling. It was going pretty much straight up, but you could see the rocket rotate about it's longitudinal axis and it was doing it in a most peculiar way. The rocket seemed as if it were out of balance. After recovering the rocket I could see that my 29mm 40-120NSec case suffered what I'll call a O-ring failure. At the point where the case and aft closure join, a hot jet of gases ruptured the case. It acted as a second motor nozzle and attempted to push the rocket end over end. Fortunately the hole was not as good a nozzle as the one designed by AreoTech, so it only caused some instability in the fight path. If you would like to see the case, I have it mounted on a display board that I bring to every launch.
There are stories out there of motor that are nearly guaranteed to CATO in you rocket. Among the most notorious motors are the Estes E-15. I was not in rocketry when these rocket motors existed. But I'm told that is why they do not exist any more. Most of them popped.
Estes D12's are the motors I have personally witnessed that CATO the most. I saw one D12 that was in a Mean Machine. When we lit the motor both forward and aft closures failed. The aft nozzle must have been second because the motor case came out of the end of the Mean Machine with such a force that it punched a hole in the blast deflector. BayNAR used an old hub cap that had plenty of holes already in it from AeroTechs burning holes in it. So it wasn't quite as spectacular as I allude to, but none the less it was quite a bang.
I read a lot on the news group RMR (rec.models.rockets) about a lot of D12 that were supposedly shipped on a semi truck that broke down in a very cold area of the country. As the story goes, the shipment of D12's got very cold. Once the truck was repaired, the D12's got sent to their normal distribution channels where the CATOed nearly 100% of the time.
There was also a batch of C5-3's a few years ago that were bad. Every one in the pack that Sean McAndrew had from then cato'ed. And this was right after he bought them.
Aerotech have a cheaper line of motors called Econojets. I don't know how old the line is, but when I was born again as a rocketeer two years ago, they seemed to be the new thing on the street. I watched 5 of them at BayNAR. Of those 5, three popped. I swore that I would never fly and econojet. I went straight to RMS. Do you think they planned it that way? I'm sure that many people now fly them at LUNAR without problem.
Rocket Flite F104 Silver Streaks, especially the March of 95 batch are extremely prone to cato's. The other silver streaks, G160's and H220's, are cato prone as well. Just about any FSI core burning motor is cato prone. F100's are the most common FSI cato's.
All of these can potentially destroy your rocket. Please enjoy the pictures and thank the Rocket Gods that it wasn't your rocket in these pictures.
P.S. A special thanks goes to all the people who read RMR for supplying me with these pictures. Keep them coming and I'll keep the page growing.