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.

New Motor Design, SRM

Using Mr Nakka's Solid Rocket Motor Design spreadsheet, I came up with the basic design of my new motor.
The dimensions of the motor casing are determined by the casing material. For this motor I have chosen to use the same material that I used in the original motor. The material is steel fence post material available at most home improvement stores. It has a 2.375" diameter x 0.049" wall thickness and I paid around $10 for 5 feet of it. (More on the use of steel in a later post) The next step is to determine the propellant grain geometry. The grain geometry offers a lot of room to play around with. For this motor I know that I will be using two inch mailing tubes to cast my propellant segments into, so that sets the outer diameter of my grain at two inches or 54mm. Then I played with the individual segment length and core diameter in an effort to produce a nice neutral burn profile as seen in Graph 1. The next decision I made was to increase the number of segments to six versus the four segments of the original motor. This essentially increases the total impulse by 50%. The resulting motor is on the long side with a L/D of around 10, but it shouldn't present any serious issues.
The next thing that I did was to lower the grain density ratio from the default of .95 to a more likely .093 given the fact that I don't want to spend a lot of extra time processing propellant for this motor and the performance lose is small and in this case perfectly acceptable.
The last thing that I did was to determine the Kn range that I wanted this motor to operate at. I chose 375 as the starting Kn which results in a max chamber pressure of around 1100psi.

The spreadsheet uses the grain geometry and Kn information to determine the nozzles throat diameter. It then takes all of that info, plus assumed combustion efficiency, nozzle efficiency, and other data and gives us our first glimpse of the designs performance.

It also gives you a nice summary of the key parameters of the designs performance.

Next step will be to take a closer look at the casing and come up a bolt ring design to retain the nozzle and bulkhead.

Designing a New Rocket Motor (step by step)

I have decided to design, build, test, and fly a new rocket motor. Why not just fly one of my existing rockets with any one of the existing motors that I've already constructed? The reason is simple. I really enjoy going out to Mojave and flying rockets but the really fun part for me is the design and construction. The process is the cool thing, the fact that the culmination of that process is a rocket flight is just the icing on the cake. Plus it's been at least a couple of years since I've designed a new motor and I miss it. The one thing that I plan to do differently than in the past is to document the entire process right here.

The motor that I've been thinking of is an updated version of one of the earliest designs that I made. Large enought to loft a good sized rocket and but not too big to be a chore to prep for flight. Simplicity and ease of use are my goal. The orginal motor was my first step away from the venerable B-200 motor designed by Richard Nakka and was influenced by Richard's Kappa motor development. It was in the same class as the Kappa motor with a similar propellant configuration. I chose to use steel for my motors casing to simplify the design. I also utilized a retention groove in the nozzle and bulkhead which provided a recess for the retention screws which engaged into threaded holes in the casing. This was a holdover from the B-200 design and though simple to implement, in this larger diameter motor didn't prove as hardy as in the smaller diameter motors. While I never experienced a failure, the screw holes would get torqued pretty well and the casings would need to be replace frequently.

Anyway here is a picture of that original motor as well as the nozzle drawing that I did. I love that it has oil splatters on it from hanging behind that lathe while I made the part.




Next step will be to crunch some numbers in SRM and come up with a working design, and some basic dimensions.

SStS Success!

The Sugar Shot project reached a significant milestone this weekend with the first successful firing of the flight configuration dualphase motor. This is a design concept that many "experts" have said would never work. Congratulations to the team and particularly to Richard Nakka for never losing faith in the concept and working tirelessly to get the project to this point. This is a truly unique project that involves folks from all over the globe; to learn more about it please visit the website by clicking Here. The goal is to put an amateur rocket into space and the only incentive is to simply prove that it can be done (with a propellant which is made up largely of sugar!!). If you've ever wanted to be a part of something like this please consider making a donation. Video provided by Rick Maschek.

AMATEURS AND ROCKETS-1

I was asked recently if I had lost interest in rocketry. Nothing could be further from the truth; I love rocketry as much as ever and can't foresee ever losing my interest in it. Time constraints have kept me from actually doing much, but I'm always keeping my eyes open for anything rocket related. That leads me to this post. A couple of weeks ago someone on the Arocket email list mentioned a long out of print book called Amateurs and Rockets by Humphreys. I was surprised because with as few books that have been written on the subject, I thought I knew them all. I immediately went to Alibris and found that they had three copies, one seller was asking $280, one $180, and one seller was asking just $20. I ordered the twenty dollar book and forgot about it. When I got home yesterday I found that it had arrived and after a quick flip through I was a really happy. The book was written by two members of the Pacific Rocket Society and contains numerous photos and technical drawings from the 50's and 60's. Many of them from the Reaction Research Society of which I am a member. One of the coolest images that I found was of the RRS's newly constructed launch tower. The photo was taken before I was born! The other photo is of me at least 35 years later using the same launch tower. There are a ton of other interesting photos and stories that I hope to share here.

More Old Photos

These are pictures of the very first rocket that I launched at the RRS back in 2002. I hadn't bought a lathe yet and the motors were constructed primarily out of PVC pipe and fittings. It was the largest PVC motor that I have ever constructed and the last before moving on to fully machined motors made of steel and aluminum.

Old Photos

This is a rocket I flew back in March of 2004. My friend Peter and I got a lot of ribbing from the guys as the vehicle was constructed almost entirely from sewer pipe. The motor utilized an unrestricted burning grain, which basically means that it lept off the ground like a bat out of hell. That's me holding a propellant segment, Peter with the rocket, leaving the ground with a bit of an arc, straightening out, and the landing site. Fun Day!