Posted: Wed Sep 05, 2007 7:00 pm Post subject: Case Compatibility - Is It a Good Thing?
At first blush, the answer seems like a pretty simple, "Yes!" It allows more motor options for a single case configuration, which is a good thing.
But, there's more to it than that.... Some things to think about, that were raised by another rocketeer in a discussion we were having recently:
Can dealers afford to stock a full line from multiple manufacturers, where there are six to eight loads for the same case? Or will they have to cut back, in order to keep inventory costs under control?
What happens when a Brand A reload suffers a cato in a Brand B case? Who's responsible? Assuming the reload manufacturer Brand A takes responsibility, what will they provide you with for hardware? Brand B, or the equivalent in Brand A?
Joined: Jul 01, 2007 Posts: 543 Location: Fullerton, CA
Posted: Thu Sep 06, 2007 12:43 pm Post subject:
Myself, I don't think that it is necessarily a good idea, and here's why:
Everyone has their own idea of how to do things, Minimums, tolerances, safety margins and the like. Company A devises a case, and an appropriately designed load (or three) to fit said case. This entire design seems to be VERY simple, but if you dissect it a bit further, you have manufacturing tolerances, tolerance stack-up, you can hold 0.002 on metal, but can only get .030 on paper ang grain length, or sleeve lengths, and molded nozzles have different shapes due to flashing, expansion, etc. All of these elements take up tolerance, and if designed correctly, 99% of the production will fit and function 99% of the time. There's always something that gets through Q.C.
If Company C comes up and says "Hmm, I'd like to manufacture a load for Case Z from Company A" He then has to either explicitly copy the devices that make Company A's load work with Company A's case, or reverse engineer the sizes and tolerances for his load from ONE sample case, or a spot-check of sample cases, etc. Company C has a different idea of what is safe, and safety margin is larger, but then he can't hold the same .030 on paper, liners and grains that Company A can, so he needs to fudge somewhere else, etc.
I realize that there are only so many schools of thought about what's important in a motor, and that two completely independent companies can do the same test and come up with the same result and all, but still...
I have made my own motor mount tubes for two rockets - one is a 54mm, the other is 38mm. I have used three different cases in the 54, and 3 different cases in the 38.
The first case I tried to use in the 54 wouldn't fit. I borrowed another (from the same MFR) and it did fit. I then reamed the ID to fit the first case, and then it fit. I tried to use another case (same MFR) and again had a fitment issue. Reamed again, and NOW it fits anything I have tried to put in it.
The first case I tried in the 38 fit perfectly. Then I tried another case, and it didn't fit. Reamed it, and then it fit. I tried another case - of the same designation by a different _certified_ manufacturer, and it did NOT fit. I now have to ream it again.
So Company A contracts to Company B to make "Hardware Set J" from the same blueprints, same tolerances, by the same machine shop, and you get that much of a deviation? Just imagine what's going on inside that Company A case when Company C came up with the parameters for it.
What I want to know is whats keeping company A that uses a grafite nozzle , to change the nozzle throat , and not tell anybody. Then when somebody trys putting a load from company L or G into that case , the propellent will fit , but the nozzle may be to small. I would hate to see that happen and the motor over pressurize. While this hopefully wont happen , theres no way to keep manufactures from swaping cases and loads. They could patenent there cases , but all themanufacter has to do is change 5 percent of the design so there not the same.
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum