My first mid-powered rocket was the North Coast Big Brute. I love that stubby rocket and when it came time to decide how to use some of the 5.38" tubing that I won from LOC, an upscale Big Brute was the natural decision. I decided to incorporate several features I'd been wanting to try, including: air starts, electronics mounted in the rocket's fin unit, a window to allow me to see the G-Wiz altimeter's LEDs, and an ejection cannon based on ¾" PVC fittings. My upscale has a central 38mm mount with two 29mm and two 24mm motor mounts. It also features a adjustable nose weight assembly designed by TRF member Ibeblip.
Here are the parts used for this build:
- One LOC 5.38" tube, 24" long
- One LOC 5.38" coupler, for the inner lining of the electronics bay
- One LOC 38mm MMT tube, 12" long
- Two LOC 29mm MMT tubes, 6" long
- Two LOC 24mm MMT tubes, 6" long
- 10.5mm tubing, 6" long, for the air start igniter conduit
- One LOC 5.38" LONG nose cone
- Three LOC 38mm-5.38" centering rings, two of which were custom drilled for the outboards by Performance Hobbies
- Four 3/16" plywood fins, custom cut by Performance Hobbies
- One LOC ½" launch lug
- Two 1500-series Delrin rail buttons from railbuttons.com (Matt's Railbuttons)
- One 38mm Slimline motor retainer from Giant Leap
- One ¾" PVC threaded adapter
- One ¾" PVC male plug
- Four small pieces of coat hanger wire, used to insure the PVC coupler is affixed solidly
- Misc. nuts, bolts, and washers for retention of 29mm and 24mm motors
- Misc. 3/16" plywood for in the electronics and ejection bays
- Misc. plywood, basswood, hardware for mounting electronics, connectors, etc.
- Clear plastic from a suitably sized plastic jar for electronics hatch window
- Audio quick connect terminal for ejection and air start igniters
- One multi-pole key switch (Aerocon Type 2 switch)
- One Aerocon 'Remove Before Flight' ribbon
- Eight socket head sap screws for hatch door retention
- Ten small nuts and bolts to attach the clear plastic view port and the key-switch plate
- Recovery system from other projects: 15 feet of ¾" tubular nylon, Giant Leap Kevlar® cord protector, 24" diameter chunk of a car air bag, Aerocon 66" chute, two large quick-links
- Adjustable Nose Weight Assembly with Spanner Driver
- 2-inch mailing tube (20.5") with telescoping tube (21") from The Container Store
- ¼-20 threaded nylon rod from Fastenal
- Four ¼-20 nylon nuts
- ¼" aircraft plywood, enough for two 5 3/8" bulkheads, and three 2" bulkheads
- Stainless steel U-bolt, ¼-20 thread, 1.5" x 2", with four nuts and four washers
- ¼" wooden dowel, ¾" long x 2
- 3/8" brass tubing, 2" long
- 2-part expanding foam from Giant Leap
- 1 lb, No. 7 ½ lead shot
This photo shows the components of the motor mount assembly (top and bottom rings are dry-fit). As you can see, the 38mm motor mount extends to the third centering ring and the other tubes, including the 10.5mm wiring conduit only run between the lower two rings. These rings sandwich the fin tabs of the fins. I added small bolts to the lower ring to serve as attachment points for outboard motor retention. Since I plan to use SU motors for the outboards, I probably will not use positive motor retention, but I included them in case I ever want to use reloadable motors. The Slimline retainer will be installed after the rear centering ring is installed, which will itself be installed after the rest of the motor mount, fins, and associated internal fillets.
The PVC coupler (i.e. the ejection cannon) and U-bolt have already been installed in the upper ring. Initially, I was worried about two things: the cannon blowing back into the bay, and the cannon twisting when the ejection plug is screwed in. The first item should not be an issue because the coupler has protrusions, presumably to allow a wrench to attach. Although epoxy alone might have solved the twisting issue, the plugs do get pretty tight and I'd hate for it to twist rather than unscrew. So, I drilled four small holes into the side of the coupler, ahead of where the plug would seat. I also cut corresponding notches into the centering ring. Four small pieces of coat hanger wire fit into the holes and then down into the notches. I still epoxied it in, using JB Weld on one side and 30 minute Bob Smith on the other (leftovers from other jobs). That coupler isn't moving now!
The key switch consists of a metal key latch that is mounted to the side of the airframe with a retaining screw. The actual switch pops onto this key latch from the rear. Because the switch itself is larger than the opening for the key latch, it would not be easy to remove after the two parts are snapped together. Therefore, I mounted the switch assembly to the small piece of tubing that was removed to form the view port in the electronics bay door. A hole large enough to accommodate the switch was cut in the airframe and the switch plate was bolted to the airframe. The wiring was all soldered, installed, and fully verified before the motor mount/electronics bay assembly was glued into the airframe.
I was going to use both threaded inserts and small socket head cap screws to hold the hatch doors on. However, after I drilled the holes and test fit the doors, they seemed to be attached soundly with only the screws attached. So, I treated the holes with thin CA, re-drilled them because of swelling, and reinserted them. I think they will be fine and, if the loosen over time, I will add the threaded inserts.
I glued the motor mount/electronics bay assembly into the airframe using West epoxy. This allowed plenty of time to align the hatch opening. I also poured some epoxy onto the top centering ring to create a hefty fillet between that ring and the airframe and main motor tube. Originally, I was going to use epoxy fillets inside the fin can area. Instead, I ended up adding dowels and wood glue. The thick outside fillets are epoxy. This photo shows a bottom view of the rocket after the rear CR was glued in.
At this point, I weighed the rocket and updated my sims. The refined sims said that I needed up to a pound of extra nose weight for a J350 with 2 G80's. Since I want to be able to re-use this fairly expensive nosecone on a future project, I decided that I didn't want to glue in a fixed amount of weight. About that time, I learned of an adjustable nose weight assembly designed by Steve Pasquier. He provided me with the plans and it fit my needs well.
I will refer the reader to Steve's article, and will merely outline some of the changes I made and provide some lessons-learned. The figure above shows the parts before assembly. The next photo shows the assembly installed. The parts numbering is per that review. Now, the changes:
I made a ½" bulkhead from two ¼" bulkheads epoxied together. These were cut with my RotoZip.
A 2" hole was cut in these rings with a circle cutter attachment for my hand drill- an added perk for using the 2" tubing. These smaller circles represented two of the five required bulkheads. Three additional 2" bulkheads were cut with the circle cutter.
I used a piece of brass tubing in place of a dowel for the bar that is used to attach to a drill chuck. In my nose cone turning activities, I have found this to be a better material.
The Adjustable Nose Weight Assembly and Spanner-Driver tool were easy to make, here are some notes:
Read all the instructions and understand the operation of these items before you start .
Care must be taken to make sure all the holes in the smaller bulkheads (2" in my case) are perpendicular to minimize eccentricity when the tool is turned with a drill. This is pointed out in the instructions but I want to reiterate it. Despite using an attachment on my drill to make sure the holes were true, a couple were off and took some adjustment (sanding, filling, etc.). A drill press would be best of course.
When I bought my nylon all-thread, I got a 6' length from Fastenal. They were cheaper than McMaster-Carr, and they had a local store. However, they must have shipped it rolled up as it arrived looking like a piece of limp spaghetti. I wasn't happy, but was anxious to proceed so I accepted it. When I glued the bulkhead with all-thread (items #4 & #5), I centered it with both of the other bulkheads (#6 & #7). It still was bent, so I left about ½" extending out of the tube, which makes it easy to start the threaded bulkheads and insert the spanner-driver. If this was down in the tube and bent, it would be almost impossible to do so.
The airframe was not glassed so the spiral lines were filled with Fill 'n Finish. Somehow, I messed up slightly on about half of the outer fillets and had to fill them with a mix of epoxy filler and Fill 'n Finish. I used Krylon primer as the base coats and then painted the body using purple/green Duplicolor Mirage, and the nose cone with Rustoleum Hammered Gold. The body, and hatch doors were painted separately. The hatches and other holes were all covered with masking tape during the painting process to keep paint out of the bays. I also masked the motor tubes and retention hardware on the business end. Finally, the switch-plate is painted with Krylon chrome to make it stand out. One caution with the Mirage paint: heed the warnings about humidity! I was impatient to get the rocket done and there are a lot of small spots in the paint. This didn't happen the first time I used it, so I surmise it was due to the ambient conditions.
This photo shows the ejection charge holder (3/4" PVC plug) with a Daveyfire match installed. To seal the hole, I first inserted the leads, but left the head of the match hanging out. I plopped some hot glue on the hole, and pulled the match through. This made sure the channel was filled with glue. I then put a large fillet of hot glue around the leads in the back.
Prior to going to the launch site, I installed the G-Wiz, and the batteries, and re-verified the connections. I was going to take no chances and am separately powering the computer and the ejection charges, and will also use dual batteries on the latter. I found that I couldn't get the dual batteries assembly in once the altimeter was installed, so I had to remove and reinstall it after the batteries were mounted. Of course, the batteries were disconnected after continuity was verified.
At the launch site, I installed the shock cord with its Kevlar® sleeve. I used a humongous 24+" protector than I made from a car air-bag and a 66" chute that I got from Aerocon.
The flight was relatively 'slow and low'. Both airstart G80's lit, and there was some minor wobble at about that time. I suspect maybe the G80's may not have lit exactly at the same time. According to the G-Wiz, the flight reached 1734 feet. This may not have been accurate since I don't have pressurization holes in the bay (the G-Wiz staging and apogee deployment is accelerometer-based). Rocksim 6.05 says 2400 ft.
The 66" Aerocon chute deployed nicely and it came down nice and soft. However, Murphy was at the site and the nose cone found the one rock-encrusted access road at that end of the field. No real damage, but I left some Rustoleum behind!
I have since flown the Big Brute on a CTI J285, again with altimeter ejection. This was followed by two flights on Loki I405-8's. The ejection on both flights was a tad late, but not too bad. The last flight to-date was on a Loki I700. This was a manufacturer's test and, since the delay formula was not finalized, this also used altimeter ejection.