Monday, December 21, 2009

Static Test Video

Here is video of the test that my friend James took. Nice quality footage. Apparently I laugh like Santa Clause when rocket motors fail.

Sunday, December 20, 2009

"Test Complete"

One of the very first large solid rocket motor static tests that I witnessed was out at the RRS nearly ten years ago. That motor was huge, 12 inch diameter if I remember correctly. A couple of seconds into an 8 or so second planned burn the casing failed... spectacularly! The first thing I heard was Dave Crisalli on the PA system calmly declare "test complete!" I was hooked, this amateur rocket thing was for me.
Ok, so yesterdays failure was not nearly as spectacular, but a fraction of a second after ignition I heard Dave's voice in my head declare "Test Complete". I have a couple of theories on what went wrong and a ton of video that I will post. The likely cause of the failure was probably a combination of things, but I think the leading cause was not the motor design, but how we retained it to the "test stand" and the pressure port....more on that later.
In the first image you can see the motor is up to pressure. In the second image (separated from the first by 1/30th of a second) the bulkhead has been blown out of the casing. The third image shows one of the two camera's close to the motor and the pressure gauge in the lower right hand corner on their way out. The fourth image shows the casing and nozzle "landing" after a full ten seconds in the air.
I'll need to make a new casing and bulkhead, and with a few small adjustments to the design I should be ready to test again soon. There were a couple of interesting observations concerning the alloyed sugar propellant that I'll post once I get the videos in order. Overall, I think the KN/ER/SB has advantages over KNER or KNSB and I'm going to continue down this path for a while.

Monday, December 14, 2009

Test Date

Looks like I'll finally get a chance to test this motor this Saturday the 19th at the FAR site. Thanks for the suggestion Ben.

Here's the starter grain. Easily ignitable, fast burning KN/SU/RIO. For this test a standard BP igniter strung through the nozzle will be used to initiate ignition since the port in the bulkhead will be used to measure chamber pressure. This is approximately 15 grams of propellant.

Sunday, November 29, 2009

2.375" Motor

Well November went by and I didn't get much done on the new motor. I did do a little testing with Erythritol based propellant. My first attempt was a little surprising given the fact that others have expressed that the combination is really easy to work with and cast. I found that the Erythritol based propellant "froze" quickly once removed from the heat source. Casting was challenging at best. I think there may be at least a couple of different factors at play. In my initial attempt I used a stainless steel pot which has thin walls and not much ability to retain heat. The other possible factor is the brand that I used. In contrast to the pure white Erythritol that other experimenters are using the brand that I have seems to have a hint of color which means that it may have some contaminant that is affecting it. I don't think that the quick freezing of the propellant is something that could not be overcome, but since I tend to cast out at the RRS facility where it's difficult to escape the wind and sometime surprisingly cold weather. Since I have around 50 lbs. of Sorbitol I decided to try an alloy of fuel consisting of 50/50 Sorbitol/Erythritol. This combination cast very well and was cured enough to remove from the mould after about 1.5 hours. A test strand had a burn rate that fell right in the middle of KNSB and KN/ER, so I'm expecting motor performance to be roughly the average of the the two fuels. Expected chamber pressure is expected to be around 1250psi.
The first image at the bottom is a sample segment of KN/ER, the middle image is a segment of KN/ER/SB, and the top image is a frame of the test strand burn test of the KN/ER/SB.
The RRS Christmas party/launch where I was expecting to test this motor was canceled this year so I'm hoping to find some time between Christmas and New Years to test.

Wednesday, November 4, 2009

Casting Tools

I haven't had much time lately, but the 2.375" motor project is moving along. The 2.375" motor project...hmmm. I have never a given a motor or rocket project a name. I usually refer to them by color, some key feature, or size like the orange and black rocket, or the camera rocket, or the 2.375" motor. I think I'll try and come up with a name for this motor, maybe something from Norse mythology since the high L/D of the design makes me think of the motor of one of my favorite sounding rockets the Loki dart.
I should be ready for the first test firing which is tentatively scheduled for early December. I modified the one existing casting set-up that I had for the original 2.375" motor to accommodate the slightly smaller diameter casting tube size. That set-up worked well in the past. The t-handle is removed during casting so it doesn't get in the way. Once the propellant is cured the handle is used to unscrew the mandrel from the base which pushes it part way out out of the propellant segment making mandrel extraction pretty easy.
The other two are simple hardwood bases coated with epoxy to maintain dimensional stability and keep them from absorbing the grease which will be used to keep the propellant from sticking. The coring rods are Delrin, which I haven't used before. Apparently Delrins thermal expansion properties allows them to basically fall out of the propellant once fully cooled. Basically the coring rod takes longer to return to ambient temperature than the propellant allowing the rod to shrink a bit more than the propellant.

Wednesday, October 28, 2009

Ares x-1 Launch

There is some good video of this morning Ares launch. The second video as some cool camera angles. Everything apparently went as planned, I guess the payload seperation wasn't as dicey as it looked.

Sunday, October 25, 2009

2.375" Motor Updated

I changed the design again. I decided to optimize the it for KNER propellant by adding two additional segments. The additional propellant brings the motor up to a solid L-class motor and the chamber pressure sims. at right about 1000psi. I'm hoping to have an opportunity to test fire it for the first time in early December
Update: I updated the SRM output to correct a mistake. I had been using the wrong grain OD.

Friday, October 23, 2009

2.375" Motor Update

I've completed a couple of the wooden pyrogen canisters. I also ran a new SRM simulation with KNER propellant. The result of switching from KNSB to KNER is that the burn time basically doubles and the chamber pressure is cut to about half. The drop in chamber pressure results in a bit of a drop in Isp, but for my first KNER test I'm fine with that. A sugar motor this size with a four second burn time should be pretty cool and at this point the purpose of this test is to get some experience with KNER and test the pyrogen design.

Tuesday, October 20, 2009

2.375" Motor Update

I've been talking with Magnus from AIR recently about this motor and he has convinced me to use erythitol based propellant. I first heard about erythitol on Scott Fintel's site and the biggest attraction seemed to be ease of casting as well as a decent Isp even at relatively low chamber pressures. One of the other attractive things that Magnus has pointed out to me recently is that it functions well in a motor with a high L/D. Click here to see Magnus' latest motor that utilizes nine segments and has an L/D of around 15. One slight drawback is that it's hard to ignite, but from a safety stand point this is actually a good thing.
I'll keep the length of the motor the same for now, but in the future I could conceivably lengthen the casing and add two propellant segments without any additional changes and increase the total impulse by about 36%. The resulting increase in Kn would increase the chamber pressure, but it should still be well within the design limitations of the casing and the closures.
Rather than using a hotter igniter, I have decided to add a simple pyrogen cannister to aid in ignition. The design is a small wooden cup that will have a KN/SU/RIO propellant cast into it which will be ignited by a "standard" BP igniter. For added safety, the igniter will be inserted through a NPT plug fitting that can be screwed into the bulkhead just prior to testing. The cup will have a burst diaphragm attached to help ensure a quick ignition of the pyrogen grain.
Wood may not seem like the best choice, but I used a wooden disk in the original design to insulate the bulkhead and it was reusable a surprising number of times with just a bit of refurbishing each time.

Friday, October 16, 2009

Reaction Research Society, 1958

I just noticed that the RRS website has a pdf of a 1958 Popular Mechanics article reporting on the activities of the society. It's fun to think what it would be like to have been alive at some point in history, whether it be in the midst of the "space race" or some other time. It's nice to see that while much has changed since 1958, the essence of what the RRS and other similar organizations stood for then still exists today. Whether it's medium or small size companies competing with big ones, or organizations that not only encourage but facilitate education and experimentation, or small organized efforts like SStS, the spirit of competition, exploration, and experimentation is alive and well. No need to travel back in time.

Wednesday, October 14, 2009

2.375 inch Motor Update

I finished drilling and tapping the nozzle and bulkhead. I also got the o-rings, so I'll test fit everything soon. I need to start thinking about casting bases and mandrels for the propellant; I may have some from the previous motor that I can just modify slightly. If not, I'll make some new ones.

Interesting Rocket Motor

I saw this motor on EBay, the other day. I bid on it, but unfortunately didn't win. The description said that it was 13 inches long by 4 3/8 inches in diameter. There was another (really blurry) photo looking down the nozzle that showed a graphite insert.

Sunday, October 11, 2009

New Motor, Finished Bulkhead

I finished the bulkhead. I varied from the drawing a bit by adding a pressure port so that I can measure the chamber pressure during testing. I need to order some o-rings and then I'll be able to test fit everything.

Friday, October 9, 2009

New Motor, 3D Model

I'm working on making some animation to show how this motor goes together. I did a couple of cross sectional views that pretty much show just that. In the close up view you can see the nozzle and the insulation (the reddish colored stuff). The original version of this motor didn't use any insulation. With the short burn time and steel casing , it's really not necessary. I added insulation to this motor as the plan is to eventually epoxy fins directly to the casing in order to construct the rocket

Sunday, October 4, 2009

2.375" Motor, Nozzle Update

I pretty much finished the nozzle. I used a parting tool to cut the o-ring groove and the groove where the insulation fits into. I then parted it off, flipped it, and cut the convergent side. I still need to drill and tap the retaining screw holes and I'm planning on adding some "gun blue" or similar rust resistant finish.

Sunday, September 27, 2009

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.

Friday, September 25, 2009

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

Sunday, September 20, 2009

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.

Thursday, September 17, 2009

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.

Wednesday, September 16, 2009

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:

Saturday, September 12, 2009

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.

Friday, September 11, 2009

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!

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.

Wednesday, September 9, 2009

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.