Thursday, September 10, 2009

Motor Design, Casing

The material that I've chosen for this design is the same as the original; steel. Specifically, fence post material available at the local home improvement store. It has an OD of 2.375" and a wall thickness of 0.049". I've had guys from the high powered rocketry community tell me that steel is dangerous and that it's a poor choice for a rocket motor casing. I think this stems from the fact that the governing bodies for hobby rocketry forbid the use of steel. I'm not sure how this rule came to be, but in commercial/military applications steel is a perfectly acceptable casing material. I think that the argument against steel goes something like this: Steel is denser than aluminum so for a given fragment size the steel contains more energy and will travel farther. Another way to think about it is if an aluminum fragment from a motor over pressurization were to travel 10 feet, then a similarly sized fragment would travel, lets say 15 feet. Therefore steel is more dangerous. I think the argument for steel would be I'd rather not stand near any pressurized vessel no matter what material it's make of, I'll be 50 feet back in the bunker thank you very much. Since my intent is to only test and fly this motor at the RRS, which isn't run under the same rules as hobby rocketry, steel my choice.
The other thing to consider is that not all steel is the same. This material is highly ductile and in my experience doesn't fracture. It tends to split, but stay in one piece. Take a look at this steel casing failure of an early SStS motor test that I conducted.

Others have experienced the same results. Look at this image of a ruptured casing that Richard Nakka experienced during the development of his epoch motor. You can read his thoughts on steel casings here.
Steel casings have been a favorite of mine for a while. Here is an image of some of the motors that I've made that utilized steel casings.

Motors one and two are an A-100 and B-200 made to the specs found on Richard's site. Motors 3-5 are variations on the B-200. Number three was an attempt to make as light weight a B-200 motor as possible. I used the base of a touchier lamp that I found in the trash for the casing. It was an incredibly light weight steel tube with a wall thickness just under 0.020". That was a great motor back when I was flying it. Motors six and seven were good motors, I discussed them in an earlier post here. Number eight is the motor that I'm updating now and number nine was a workhorse of a motor that used 2.5" emt (electrical metallic tubing) with an actual OD of 2.875". You can see that motors performance in an earlier post here. To get back to this design, lets look at the numbers I came up with using Richards Motor Casing Design software.
This is a very handy tool. You enter in the dimensions and material properties of the proposed casing. It then tells you the the design pressure, burst pressure, elastic deformation under pressure, and design and burst safety factors. If I use 01 as the design safety factor, I find out that at around 1800psi the materials yield strength would be exceeded and I could expect permanent deformation to occur. Knowing this I will design my closures (a radial bolt pattern) to fail at around 1800 psi. That way if I want to add a burn rate enhancer to the propellant, as I did here, and run the motor at a higher chamber pressure, I'll have the flexibility to do so. Also there is no point in running the motor at a chamber pressure that would damage the case, at least not for this design.
Next I will draw out the desing in CAD so that I can start to visualize it better.

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