NDRS-SC-4 scratch Build, Build Thread

Hey all. This is my first scratch built Mid – High power rocket and my first build thread. The NDRS-SC-4 stands for “Neighborhood Dispute Resolution System – Stealth, Conventional warhead – 4”. I am going for a “Stealth” look to this rocket. It is 4” x 32” and about 34 oz sans motor. It is 29mm MMT and being designed to fly on G to baby H. 3 grain max due to size.

Because it is a short fatty, a lot of nose weight will be needed. With 6 – 7 oz, it has a stability of .75 to .86 with the largest motor. Even with 6oz and an angle of attack of 15°, it will still have a stability above .5 according to OR.

I am using a Mad Cow 4:1 plastic NC, 4” Vaughn Brothers BT (Discount Rocketry), and 1/8” plywood laser cut fins, rings and NC bulkhead. My local trophy shop that I do business with had a laser cutter, and he offered to cut these out for me for free (cool to watch it cut). Though OR can’t support it, I am adding ¼” triangular balsa strips to be “fillets”. This should increase the angular, stealthy look of the rocket.

Nose weight. So if I am not doing something new or experimenting, the build is boring. So I am going to cast the lead nose weight. I put the NC in the freezer, then oiled up the inside with canola, and poured melted candle wax inside it. The outside only got a little warm, and there was no warping or deformation. Once it soft-set, I froze it again. I then put the tip in simmering water a few seconds to release the wax impression. I mixed up a batch of plaster of Paris and poured into a tin can. I then placed the wax impression into it. Once set, I placed it into a 200°F oven upside down with a cup to catch the wax. The wax fell out in about 15-20 minutes. I then left it in the oven another 45 minutes or so to help dry out the plaster. I may bake it once more before casting the lead.

I tried to post pics but TRF is freezing up every time I try. I will get some soon

So with Mother’s day and running my son’s baseball tourney, I wasn’t able to get as much done as I normally might have, but I made some good progress, and am now able to post pics.

So I cut the fin slots with an Exacto knife. This was my first time doing this. They turned out ok, but with the balsa strips covering things up, you won’t ever know I have an un-steady hand. I also got the top CR ring glued onto the MMT and made a lot of progress with the NC.

So I did an initial fill on the exterior of the NC. I also cut off the end of the NC. When I had the plywood cut, I had him cut 3 sizes of 1/8” bulkheads (2 each), because at the time I did not know the ID of the NC. I picked the smallest 2 and Gorilla glued them to make ¼”. I then narrowed the radius a bit with a rasp and sandpaper till it fit. I then placed it 2.25” into the NC and drilled 8 holes around to fit 8 2-56 x ½” set screws. I plan to Gorilla glue this as well. I don’t trust any glue on PP completely and I don’t trust these screws completely to not rip through the plastic, but together, I am very confident. I should mention that I will be using 2 chutes. I decided that with a NC coming in at a total of about a pound, it is easier to not have to over engineer the shock cord and mounts. Also, if I over-do the ejection charge a little, no big deal. All the recovery system has to endure is the drag of the chute. I will also be doing shear pins as well. Three 2-56 nylon screws.

So I have done a couple small things. I drilled and installed the shear pins to test fit everything, epoxied in the fore CR, and cast some lead weights.

The casting process went smooth. The only issue was getting exact weights. My first melt was of 6.00 oz, but the final product was 5.7oz. I apparently skimmed 0.3 oz of “scum” off the top. This first one is the ugly pitted looking one. This is the one I will probably use because it is probably right where I want the weight (can always drill out some mass if needed), and the pitting will help the Gorilla glue stick. I then poured a few more, which had much smoother surfaces. I will wait till it’s all built and then check the CG before gluing the right one in. I used a SS soup ladle from Good Will for $0.99, and an Acetylene Turbo Torch from work. It only took about a minute to liquefy each batch.

Next step is to put on the fins. Would love to hear any comments, suggestions or advice.