2.375" Nozzle Update

I finished the back cut on the shoulder area. In an attempt to get really clean flow into the throat area, the convergent starts at a steep 45 degrees that leads into a shallower 22.5 degree section leading into the throat. This can be clearly seen on the back of the convergent side in the photo. I also chose a shallower divergence than the "standard" 12 degrees. A couple more hours and I should have a finished nozzle.

2.375" Motor, Nozzle Progress

I'm making pretty good progress on the nozzle. The black line in the top few photos is about where it'll get parted off. I still need to cut the 0-ring and insulation grooves, do the back cut behind the shoulder, and index the retaining screw locations. Then I'll part it off, flip it over, and finish the convergent side.

2.375" Motor, Starting Nozzle

I managed to start the nozzle. I'm starting with 2.75" diameter stock which is less than ideal; I would prefer to start with some thing closer to the OD of the finished part, but it's what I have on hand. It shouldn't be too bad though since I added a new, larger motor to the lathe a while back. I've only done some light turning with the new motor, so I can't wait to find out how aggressive a cut it'll handle. The starting piece is about an inch and a half longer than the finished part will be and weighs nearly nine pounds. I would guess that the finished nozzle will come in at around 1/2 lb. The general plan is to machine the divergent section, throat, shoulder, and index the retaining screw locations. Then part off the nozzle, flip it, and finish the convergent side

2.375" Motor Casing Completed

I finished the casing for the motor. I realized that I made a mistake in the CAD drawing that I posted earlier today. There should be six retaining screws for both the nozzle and the bulkhead versus twelve. I also made a slight change to the length of the casing, opting to make it 3/4"shorter which allowed me to true the ends on the lathe. I hadn't planned on being able to do that but by turning the tool post backwards I was able to extend it's capacity by just enough. Since just a very light cut was needed this worked fine. The design is pretty generous in the propellant segment spacing, so I'll make up the deficit by reducing the spacing slightly.

The bottom photo shows the new extended casing next to the older motor design. The new casing still has it's rust preventive zinc coating. The coating doesn't add any structural strength and has a fairly low melting point so I'll remove it with a quick dip in some muriatic acid.

2.375" Motor, Completed CAD (nearly)

I'll need a few more dimensions, but I'm pretty much ready to start. I'll start with the casing , then I'll be able to determine exactly what the OD of the nozzle and bulkhead needs to be.

2.375" Rocket Motor Design, Nozzle

I haven't done a lot of additional work on the design. I'm not sure that I'll remove the material indicated in red in the first image, back cuts like that can prove to be pretty time consuming. I should be able to shorten the shoulder a bit once I add the bolts into the design; that would make it a bit easier. I'm not too concerned about making as lightweight a motor as possible, but the aft end of the motor is a good place to remove weight from since additional weight in this area pulls the center of gravity aft. This isn't necessarily what you want since the likely result is larger fins to compensate. I'll wait and see how it goes while I'm machining it; I can always revisit the nozzle at a later date.

Cadillac Launch

I don't think of myself as someone who is easily influenced, particularly by advertising, but the other day I saw a car commercial that totally convinced me that I wanted to have this car. I've been considering getting a new car, but a Cadillac. Never! But it seems as if GM geared this commercial directly at me (or at least anyone interested in rockets, aviation, etc). After seeing it once, I decided I really wanted this car. That has to be good advertising!

Here is the commercial:

Rocket Motor Design, Initial CAD

Here is the first pass in CAD. I like to get to this point and then live with the design for a while and see how I feel about it after a day or two. I like the overall look of this motor, so I'll probably try to finish the drawing tomorrow. After I finish it, I'll point out some of the features as well as some of the things to consider during the design phase. Then I'll create a 3D model, at least of some of the components.

NASA ARES-1 DM-1 Rocket Motor Static Test

NASA ARES-1 DM-1 Rocket Motor Static Test at ATK, originally uploaded by S. Hurley.

I saw this image on Dick Stafford's rocket dungeon blog. Sorry for using the same image Dick, but you picked a pretty good one :)

Given my post on steel castings yesterday, I thought it might be worth noting that this motor uses a steel casing. Shuttle boosters use a four segment steel casing, and this motor was essentially an extended version of a shuttle booster utilizing five segments. I also read on the Arocket list this morning that the casing sections used for this test were actually flown on STS-1 and have 48 missions on them. I was a little sceptical but I found this article on the NASA site which states "... the cases used in this Ares I first stage ground test have collectively flown on 48 previous shuttle missions, including STS-1, the very first flight.

The video of this test is awesome and definitely worth watching!

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.