Monday, February 2, 2015
Friday, January 16, 2015
Drilled and tapped the nozzle this morning. I don't think that I mentioned it earlier but the steel is 12L14 steel, commonly refered to as Ledloy because of addition of lead (0.15-0.35%). It's really nice stuff to work with. Here are a couple of pictures.
Thursday, January 15, 2015
Tuesday, January 13, 2015
I tried to take a lot of pictures while making the nozzle for this latest motor and thought I would show the progression and talk about each of the steps a bit. I'm not a machinist and this isn't meant to be a tutorial, just a bit of insight into my thought process. Feel free to comment.
In this image I have the rough stock in the lathe. It's about 0.75" longer than the nozzle needs to be but I will cut it to length later when I turn it to work on the other side. I begin by facing the end, if nothing else I take this as an opportunity to ensue that the cutting tool is at the correct height.
After using a center bit to start drilling out the throat I progress through larger drill bits until I get close to the final dimension.
I then switch to a boring bar and cut the throat to the final dimension using several light cuts. Although the piece is pretty securely held by the chuck I use light cuts are to minimize any deflection of the piece.
Once the throat is to the final dimension, I use the tail stock live center to help keep the piece as rigid as possible and I begin taking the piece down to the correct diameter. I am working on the convergent side of the nozzle at this point.
Once to the final OD, I roughly mark the end of the "shoulder" where the retaining screws will eventually be added and cut the o-ring gland using a cut off or parting tool. This action demands that the piece be very rigid and often I find that this is an operation that is easier if done closer to the chuck. In this instance it was fine though.
Back to the boring bar and then we began working on cutting the convergent angle.
This is looking right down the length of the boring bar.
This a close-up showing the inlet to the throat area, it's currently a pretty sharp angle. Ultimately you want a smooth transition from the convergent angle to the throat rather than a sharp edge.
I will spend some time later sanding this area to get a smoother transition. One thing that I like to do is knock the edge off a bit with the boring bar. The convergent angle is 30, so I reset the tool to 15 and take just a bit off as seen in the above picture. The rest will be smoothed with sandpaper later.
The final step before flipping the piece around is to index the holes for the retaining screws. This particular lathe as an indexing locations on the main gear that allow me to do this. I use a Dremel and center drill to mark out the locations on the piece. I then removed it from the chuck and rough cut it to length before re-chucking it and working on the other side.
It's critical to take your time when re-chucking the piece to ensure that you get it centered correctly. Here I am facing it to the final length needed.
I took one quick cut with a0.625" bit to remove some excess material from within divergent side.
Then I brought the live center back in and roughed out the excess material from the outside of the divergent cone.
Here I have it cut to the max diameter of the divergent section.
I like to mark out some rough locations so I know what I'm working towards.
I then set the tool to 10 degrees and begin removing the rest of the material from the outside of the divergent cone. Sometimes cutting towards the right...
and sometimes cutting towards the left.
All of the excess material around the divergent cone is now removed.
I then add the steady rest for rigidity and bore out the divergent cone.
The finish looks a bit rough in this image but it actually cleans up nicely with a bit of sand paper and time. I do most of that finish detail on my smaller lathe as it's variable speed control makes it a bit easier.
The last step is to remove some excess material from the shoulder area. This is tricky on a piece this small but with a little patience and the right tool it isn't too bad.
Nearly completed nozzle. All that remains is to drill and tap the four screw holes and sand/polish the inside smooth.
Monday, January 12, 2015
The 2.375" rocket motor is ready to test but I haven't had the opportunity to get out the test site. I have been feeling the urge to be doing some rocketry though and with that in mind I decided to build something. I chose something small. While the SStS project was immensely satisfying and a great educational experience it did lack a big part of what I enjoyed from my early days in rocketry. The thing i really miss is taking something from concept all the way through to completion, being involved with every step, and then actually getting out to the desert and launching the vehicle. It turns out that I had a rocket that I had promised to rebuild after it was lost over 8 years ago. That rocket was powered but Richard Nakka's C-400 motor, slightly modified to utilize KNSB, a Sorbitol based propellant versus Sucrose which the C-400 was originally intended to be fueled with. I came across this VCP design printout a while back. At the time it wasn't the largest rocket I had built or flown but it was definitely the highest performance one with a lot of attention to drag, weight, and construction details.
Here's a photo of its only launch. My brief blog entry of the launch can be found by clicking here.
As you can see by the photo, my daughters paint scheme won out over my own preference. I hope to document this build to a much greater extent than the first time around. With that in mind here are my initial design thoughts. The hope is to create a motor/rocket that is simple to construct, durable, and easy to prep for launch. I'm initially designing towards a Dextrose based propellant but am hoping to be able to use several of the other sugars available. Actually my notes indicate that the above launch used a propellant that was comprised of 50% Sucrose and 50% Sorbitol.
Here is my initial CAD:
Friday, October 24, 2014
I've had a hard time finding time to post, but the good news is that I have been finding little bits of time here and there to continue working on rocket stuff. I went through and organized my garage/work space a while back which was good. I also when through my bins, range box, the stuff I haul out to a lunch and organized, took stock, and replenished on the things that I was out of. I also recently rebuilt a laptop and geared it towards rocketry so that all of the design, drawing, spreadsheets, photos, etc are centrally located. I have made a new motor case to use with my 2.375" motor to test Dextrose with. The motor was originally designed as an 8 segment KNER (erythritol based propellant). That test cato'd half way through the burn. I do still want to redo that test, but this time with a stepped core. After that test I reduced the segments to 6 and it was successfully flown, though the recovery left a little something to be desired. The motor got so hot that the while floating down on the parachute the lower half of the aeroshell (made of heavy cardboard) caught fire. It eventually fell away from the 'chute and came in ballistic. Fortunatly that nozzle didn't sustain any damage. Any way, the next time I manage to get out to FAR, I plan to fire this motor as a 4 segment and as a 6 segment motor, with some luck I may get the 8 segment motor ready as well.
Saturday, October 18, 2014
I suspect that the switch from sorbitol to dextrose based propellant was the reason the TR-1 motor experienced a cato. More specifically the fact that most Dextrose is the monohydrate form which contains molecularly bound water, Even more specifically, the fact that I didn't remove all that water is the reason. I believe that the extra water lead to grain slumping which was the ultimate demise of the TR-1 motor. Fortunately I cast an extra segment and as that one wasn't used by the end of the day of testing it had been exposed to the ambient temperature of the day which was in the upper 90s. It had been in the same location as some similarly sized KNSB segements cast by Rick. The KNSB segments looked completely fine while the KNDX segment had the consistency of taffy and the core had all but disappeared. I had some concerns after casting the segments. There were a number of observations made during the casting process that were likely a result of the water content. It melted quickly and poured easily, but the most obvious and concerning indicator was that the burn rate couldn't be measured because burning was so unstable that it kept self extinguishing, I assumed that wouldn't be the case when fired in the motor burning under pressure. I didn't consider at the time that the water content may effect the dimensional stability of the segment.
I assumed that most of the water would be driven off during casting but that was apparently not the case. More on that in a bit.
As far as dextrose based propellant (KNDX) goes I decided I needed to get more experience with it. The first step was to remove the water from the dextrose and from anhydrous dextrose. This was done (as suggested by Richard Nakka's site) by dessicating the dextrose in an oven at 175 f for two hours. The sample weighed 901gr prior and 825gr after, approximately a 09% reduction.
The dextrose came out of the oven caked together. The clumps were pretty tough; initially I thought I could push them through a sifter but ultimately had to use a coffee grinder to break them up. I cast a small sample to measure the burn rate from. Melting took much longer to commence and the resulting propellant was noticeably more viscous. The resulting propellant sample burned well and the resulting burn rate was in the expected range.
Back to the TR-1 grain segment. Those segments we approximately 5lbs each and took about an hour to melt and cast. One hour at a nearly twice the temperature used in the desicating process seems like enough to remove the water. The problem though is that not all of the mixed dry ingredients were added at the same time. Typically small amounts are added and once melted more dry mix is added. The result is that the last amount added spend much less being heated. I'm sure some water was driven of those initial TR-1 segments but clearly not all.