CIJ 2020 Build - Darkstar Nemesis

[NateB]

I'm getting started on the kit I was given as a Christmas in July gift from my secret santa. I have a Darkstar Mini which will be built as a 38mm minimum diameter dual-deploy rocket. It will be painted black and renamed Darkstar Nemesis after the fabled twin star to our Sun.

I laid out the parts, which still need to be washed, trimmed, and weighed. Pictured is the Loki 480 case, I'm not sure the 740 would fit with enough room for recovery. Of course I will plan on adapting down too, this should be fun to fly on a Loki G as well.

 

[Rocketjunkie]

The recovery goes in the nose cone. Use the longer coupler as the nose cone base for your avbay. Look for Punisher build threads for details. https://www.rocketryforum.com/threads/cjs-punisher-build.120615/ A LOT of comments, etc. One of Blackjack2564's builds. This allows a motor almost the whole length of the body tube.

 

[NateB]

I'm not sure I can fit the recovery in a 38mm nosecone, has anyone tried? If a chute release will fit in the thin wall body tube, I wouldn't need to worry about anything in the booster. I could also take the retainer out and use an eye bolt for recovery on the threaded clure and friction fit the motor. There would be room for a shock cord and drogueless at apogee.

 

[cwbullet]

No, but I might enjoy watching someone try.

 

[blackjack2564]

In case you had not noticed he has one of the new NC's with the shoulder "ground" on.Cannot install a coupler into NC.
I have a work around for larger diam. rockets,but in this case you are best served with standard build.
You would first have to install recovery point in NC

Here it is for the curious.

 

[NateB]

I made a little more progress today by cutting the fin tabs off the 6 fins. I haven't shaped the leading and trailing edges yet.That is the easy part.

Over the weekend, I thought about how to best attach the fins with the slots already cut. The fins are the standard 1/16" stock from Wildman. Even the Mini Mach uses 1/8" find, so I will be reinforcing them. Rather than full tip to tip, I think I am going to sandwich each fin with 4oz fiberglass, attach and fillet the fins, then add a strip of fiberglass tape along the fillet and the body tube. This will give 2 extra layers of fiberglass along the joint and an extra layer to reinforce the fins.

 

[NateB]

Back at this project, some of the supplies I ordered took a wild ride from Salt Lake City to Louisville, then to Wichita before returning to Louisville and making their way to Indiana to be delivered. Now that everything has arrived, I can continue.

Today, I beveled the leading and trailing edges of the fin sets using the small guide from NCR. It didn't leave as sharp of an edge as I expected, but the fins are all even. Next up is laminating the fins with a layer of 1.5oz fiberglass cloth and carbon fiber.

 

[NateB]

I didn't shelve this project, but haven't worked on it in months. I bought a generic food saver from a Black Friday sale and decided to give vacuum bagging a shot when I laminate these 6 fins.

First, I traced some templates just in case I need them later.



I have worked with fiberglass cloth before, but have never used carbon fiber. I have also never tried to vacuum bag anything, so these are new techniques to me. I decided to start on a single lower fin and learn as I go.

No pictures, because the process was messy. After sanding the fin with 220 grit and cleaning with alcohol. Using US Composites epoxy, I brushed some on the fin and layered each side with the carbon fiber, more epoxy worked in, the fiberglass, more epoxy, then the release and breather material. I used a piece of flat melamine board covered with wax paper as a base in the vacuum bag.

At the moment, everything is sealed in the bag waiting for the epoxy to cure to the leathery stage. I'll post some pictures once I peel off the release and trim the excess cloth away. Hopefully it cures nicely and I will move on to the rest of the fins.

 

[NateB]

The fin took longer to cure than I expected and I missed the leathery stage after I went to bed. I trimmed off the excess cloth this morning and still have some sanding to do. I'm happy with the results, I will just start the other fins earlier in the day. There is a texture to the fin from the release cloth. I'll probably wait to sand that down until the fins are mounted on the rocket and if I do a tip to tip layer.

 

[OverTheTop]

I made a little more progress today by cutting the fin tabs off the 6 fins. I haven't shaped the leading and trailing edges yet.That is the easy part.

Why did you cut the fin tabs off?

 

[NateB]

Why did you cut the fin tabs off?

To fit a 38mm motor.

 

[OverTheTop]

OK. So you are just mounting the fins on the surface of the airframe now?

FWIW I am building a 38mm Go Devil and I still slot the fins into the airframe. If you mount on the surface you are only using the epoxy to hold the fin to the surface and are relying on the peel-strength of the epoxy, which is not really great. If you slot into the airframe you are relying on compression/tension forces where the airframe and fin meet, likely resulting in a stronger joint, that will be less reliant on fillets or tip-to-tip for surviving flight forces, IMHO. I also usually put a small piece of f/g cloth on the inside along the joint, just to reinforce the area. There is usually plenty of clearance between motor and airframe to allow for that.

 

[boatgeek]

If you didn't get an answer to this already, you can fit a JLCR into a 38mm tube with some care in packing technique. What you want to do is fold the chute into a long skinny bundle at least twice as long as the JLCR. Fold the chute over the JLCR, wrap the rubber band 1.5 times, and test fit. You want the chute sliding on the inner surface of the body tube since the JLCR sometimes sticks if it rubs the body tube.

 

[NateB]

OK. So you are just mounting the fins on the surface of the airframe now?

I was planning to mount the fins as you describe. The body tube is already slotted and I can sand the slots wider if I need to after they are laminated. I have some mylar I can wrap around motor to make sure I don't epoxy the motor case into the rocket. A layer of cloth on the joint sounds like good insurance .

 

[Tobor]

oops, sorry Nate. Too many threads open while posting..

 

[NateB]

I finished laminating the rest of the fins today. They aren't perfect and require some sanding once they are fully cured. I figured out I need to cut the carbon fiber a little bigger to account for some of the weave pulling loose. I'm sure the next time will be easier. Now to sand and work on getting them mounted to the body tube.

 

[NateB]

I spent an hour wet sanding the edges of the 6 fins. The 5 that I was able to trim before the epoxy was fully cured were a lot easier than the first one. I forgot to weigh each set before laminating with carbon fiber and fiberglass. Compared to the other DS Mini I built using the foam and fly, the upper set gained about 1.5 grams on each fin and the lower set gain about 0.75 gram each. I'm going to attempt tip to tip once they are mounted and then sand them smooth.

 

[NateB]

I also mounted the MD retainer in place for a 38/480 case. It is easy to drill and tap the holes when you can see through the fiberglass.

 

[NateB]

I made a little more progress on this one today. The lower fins are tacked into position and curing. Once both sets of fins are in place, I'll reinforce the joint on the inside of the body tube with some lightweight fiberglass tape and then the external fillets.



Edit: I really need to change the title to Christmas in July 2020-2021 for this build. At the pace I'm going, I might be ready to launch it by Labor Day.

 

[NateB]

I used a motor casing wrapped with a few turns of wax paper to ensure the fins did not go too deep when tacked on. I had a nice surprise when I removed the case. I was worried the US Composite epoxy would be too thin an run in places I didn't want it to, so I used rocketpoxy. I used the rocketpoxy immediately after mixing and it was just thin enough to run in the tiny gap between the motor case and the body tube. Now there is a very thin fillet to help reinforce the fins on the inside and my cases still fit.

 

[OverTheTop]

Looks good!

 

[NateB]

It is beginning to look like a Darkstar now!

 

[NateB]

Today, I marked the area for the fillets and started to mask it off.



I mixed up 30g of rocketpoxy and a few drops of black dye. Let it sit for about 15 minutes, then apply to the rocket. I'm bad at estimating how much I will need, but I'd rather waste a little epoxy than come up short.



Let it sit for another 15 minutes, then remove the excess and shape with a fondant tool.



After the tape is removed, the fillets are cleaned up with my finger and a paper towel dipped in alcohol.



Once all the fillets are on and cured, I'll tidy them up with sandpaper and work on further reinforcement with fiberglass tape or tip-to-tip.

 

[NateB]

I put the last set of fillets on this evening and I'm happy with the results. I also ordered decals from Stickershock this week and will be getting them soon. Graphics are silver and everything else will be black, fins will just get clear if I'm happy with the finish.

 

[NateB]

Still working on this build a year later, but it is not forgotten. I had some epoxy mixed up for something else today and decided I would further reinforce the fins with fiberglass tape.

After prepping the surfaces, applying and wetting the fiberglass, I just wasn't happy with the results. I ended up pulling the extra fiberglass off and cleaning up the mess. I'll finish the build with the fins mounted as is, with the slotted tube, fin roots the thickness of the body tube mounted with US Composites epoxy, and the Rocketpoxy fillets. I'm confident the fins are secure and everything else was overruling in an exercise in new-to-me techniques.



Edit: the thread title should now be Longest Christmas in July 2020- ??? Build. Who knows when I'll get this done at my pace.

 

[aerostadt]

Looks good! Time to finish up and fly.