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Friday, January 2, 2015

L3 rocket update

I haven't done much fabrication. I installed the AeroPack 98mm retainer on the rear centering ring. This was kind of a pain in the ass. I tried the method in the instructions on friend's centering ring last year. It suggests holding the retainer aligned with a motor casing, marking the holes, then drilling. The problem is that getting the marks for directly in the middle of the holes is pretty much impossible. This time, I CAD'd the AeroPack retainer and the centering ring, assembled them in CAD, made a drawing of the assembly with hole center marks, printed that out, cut out the center, and taped it to the centering ring. I then punched the center of the holes and drilled them. It came out much better this time, so I did my aft centering ring next.

The holes match up very well.

Screwing in the thread inserts.

Tapped over retainer to prevent epoxy from gluing the retainer to the centering ring.
My friend has a fiberglass aft centering ring (the one I messed up the first time around). The instructions suggest tapping the holes, but I don't think the fiberglass will hold threads well. We'll be using steel nut plates glued to the back of the centering ring to provide threads.

I also cut the motor tube to length and sanded it. Because phenolic tends to fray after awhile, I treated the ends with CA. I did the same to the two tube couplers (I did this to the body tubes earlier).

I did some modelling Modelling and Design updates. I took out the 12" section of body tube and one of the couplers from the stretched version of the rocket (which is the one I'll be using for my L3). The 12" section will be used instead of the 36" section for the scale (non-stretched) version.

Updated CAD
Because my Dad wanted to buy a CTI 5G 75mm case, I've chosen either a CTI M1400 or M1401 as the cert motor, and updated the OpenRocket simulations accordingly.

1/2 Scale
It predicts about 7500ft on the M1400.

Avionics bay design: The avionics bay end caps will have an airframe bulkhead and a coupler bulkhead glued together. Each end cap will have an eyebolt for recovery system attachment (see below). Two 1/4" threaded rods will run through the avionics bay and end caps to link the two end caps together and provide rails to mount the electronics sled. Two altimeters will be mounted on the electronics sled. The end caps will have high temperature screw terminals and charge holders glued to them. The vent hole(s) will be drilled through the upper body tube section and into the avionics bay. The av bay will be secured to the upper body tube section with 4-40 stainless steel screws. Overall, a pretty classic design.

Nose cone design: While running OpenRocket, I realized that, if I wanted to be able to use the same nose cone for both the stretched and non-stretched versions, I would need to make the nose weight adjustable. I'll show pictures of this later when I actually make it, but the plan right now is to epoxy a 1/4-20 threaded rod into the tip of the nose cone with a eye-nut on the end of it to attach the shock cord. I'll mix steel BB's and chopped carbon fibers in with the epoxy to increase weight and strength respectively. I'm also planning on drilling through the tip of the nose in a few places and running wooden dowels through in order to help anchor the tip/threaded rod in place. It'll make more sense once I post pictures of the process. Weight can be added to the 1/4" threaded rod as required.

The recovery system will be as follows: All connections are done with 1/4" quicklinks unless otherwise stated. A Kevlar Y-harness connects to two 1/4" U-bolts in the forward centering ring. The Y-harness connects to a ~20ft long Kevlar or tubular nylon shock cord. The drogue parachute (~48") is attached to a loop in the shock cord about 1/4 of the way from the nose/payload section. The other end of the shock cord attaches to a 1/4" forged, galvanized steel eyebolt in the avionics bay. There is an identical eyebolt on the other side of the avionics bay. This connects to a shock cord similar to the previously mentioned one. The main parachute (Sky Angle Cert-3 XL) attaches to the shock cord about 1/4 of the way from the nose cone. The nose cone will have 1/4" forged, galvanized steel eyebolt in it that attaches to the other end of the shock cord. Nomex cloth protectors will be used to shield the parachutes and shock cords from the ejection charge gases.

Hardware acquisition: I got two PerfectFlite altimeters for Christmas, so that covers the electronics. I managed to find some of the hardware I needed from Home Depot (U-bolts, threaded rod, etc.). The rest I'll get from McMaster.

Time outlook looks abysmal...I kinda have a thesis I need to do. This rocket may not get finished for another year :( .

Left to do:
  • Purchase a bunch of stuff
  • Sand centering rings
  • Make motor mount
  • Install fins
  • Filleting 
  • Build altimeter bay
  • Minor stuff
  • Final primer coat, sanding, then painting
  • L3 paperwork

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