Saturday, January 15, 2011

Reviews: Two Birdhouses (the original ones)

After seeing David Jarkey's flying birdhouse, I grabbed several coupons and went to Michael's Crafts. I bought a couple, one hexagonal and one round, on a 2-for-1 sale (and used the coupons for some motors). This post will describe V1, which flies on 29mm G motors, and V2, which flew on 6 D12s.

Birdhouse V1

The parts list:

* One hexagonal birdhouse
* 29mm motor mount
* Spent 29mm casing
* Two 6-32 bolts with nuts and washers
* 3/16 plywood scraps
* ¾" pine strip
* Two rail buttons
* Lexan
* 3/8" elastic
* Kevlar® twine

Construction started by carefully prying the roof of the birdhouse off. This naturally serves as a nosecone, however, it lacks a shoulder. I decided to run the motor mount to the top of the "cone" and use a chunk of a spent 29mm motor as the shoulder. The chute would reside beside the motor tube and would be protected from the top by a chute pad.

I drilled a hole in the base using my Harbor Freight drill press and a suitably sized wood bit. The center was easy to find by drawing lines from opposite corners. I also drilled holes for the bolts that would serve as mounting points for the motor clips.

Next I glued the mount in place, checking that it was vertical by measuring the distance to each corner. I used 5-minute epoxy throughout so the build would go fast. The Kevlar® leader was inserted between the motor tube and the body and was wrapped around the back of the tube.

The base is thick (3/8") but the motor tube still needs some additional support. I cut two vertical braces that fit between the mount and wall of the body, and which are about half the depth of the cavity.

To attach the piece of casing (bottom 2"), I inverted the cone and filled the tip with epoxy. With the casing inserted all the way into the motor tube, I placed the body on the cone and made sure it was seated securely. I then used a dowel to push the motor casing down into epoxy puddle. This ensured its alignment. I also epoxied a piece of elastic onto the tip of the cone (actually 2 pieces tied together--this was a "use what ya got" project).

I recently found that with proper care and a diamond cut-off disk in your Dremel, you can cut Lexan pretty easily so I whipped out three fins that extend below the body by about 5 inches. This overhang was based on what I remembered about the proven designs I had seen earlier. The bottom rim of the birdhouse, which protrudes from its base, was notched to accommodate the fins. The fins are also attached and heavily filleted with epoxy.

A thick piece of pine was required for the rail buttons to clear the cones overhang (I guess this overhang would technically be the eaves.) This could have been narrower, but since the overhang mostly occludes it and I'm not going for altitude, it sufficed.

I modeled the hexagonal structure in Rocksim 8 using circular parts whose diameter is the average of the short and long dimensions of the actual rocket. I also plugged in the actual weight and CG. My ace in the hole was knowing someone who had proven rockets based on the same product. I added nose weight and ended up with a static margin of 2.0. This seemed tight, but the Birdhouse will have a ton of base drag.

Finishing consisted of a couple of coats of Minwax Polycrylic. I also brushed on some Helmsman spar urethane over the fillets so exposure to UV wouldn't yellow them.

I used a 36" nylon parachute, which I attached to a loop in the shock cord. It was placed in the nose and covered with dog barf wadding. I flew the Birdhouse on a F52-5T. It weathercocked a little and ejection was just past apogee.  The 36" chute spared the Lexan fins!

For the next flight, I installed an ornamental bird and a G64-5.  This flight was much more satisfying that the first.  It ejected at apogee and, again, the big fins survived.  However, the bird was no where to be found.

What's there to say? This is a unique, inexpensive rocket that is sure to get people’s attention.

Birdhouse V2

This is my second birdhouse rocket. The fins are attached the end of a thick dowel and six 24mm motor tubes are mounted in the body around the dowel. The original rocket had canted mounts. However, the original rocket had a recovery failure and turned itself into toothpicks. When I rebuilt it, I had trouble drilling the canted mounts so I settled for parallel ones. This review covers both the original and re-built rockets.

The parts list:

* One cylindrical wooden birdhouse (Michael's crafts)
* One 24mm LOC motor mount tube (21" required)
* 1"x 34" dowel
* Four light ply rectangles (Michael's)
* Small section of 4" mailing tube with telescoping inside tube
* Kevlar® twine
* 36" nylon chute
* 1st unit only: 3/4" pine strip and two rail buttons (
* 2nd unit only: 3/8" brass lug

Construction began by removing the top of the birdhouse. The conical top mounted to a flat circular plate that was in turn attached to the cylindrical body. While I was trying to remove the plate from the body, the cone spontaneously popped off. I eventually removed the plate and decided to leave it off, since I wanted the cone to be open for parachute storage anyway.

I first found the center of the base plate and drilled the hole for the dowel using a drill press and suitable wood bit. I marked lines to position the motor mounts, but how was I to drill the slanted holes for the motor mounts? The base of the Harbor Freight drill press will tilt to perform angled drilling, but it didn't have the clearance to do this with the birdhouse. I found a spacer that when placed under the edge of the plate, tilted it to an angle of about 15 degrees. So, I proceeded to drill the holes with my makeshift spacer. The drilling went fairly well. The inside of the base chipped as the drill went though but it retained its integrity. After the first two holes, I started drilling on one side, then flipped the birdhouse over and finished the drilling from the inside.

While the body of the birdhouse was supported on the dowel by a nylon rafter angle square (which was taped to the dowel), I glued it in place with 5-minute epoxy. I cut the motor tube into 3.5" sections and also mounted them with epoxy.

I couldn't scrounge any suitable scrap fin material, so I resorted to spending a couple of bucks on some prefab ply rectangles from Michael's. Michael's used to have a huge assortment of pre-cut shapes and I was hoping to get birds, or birdhouses, or some other theme shape. However, their stock was low so I settled for the rectangles. To facilitate their attachment to the round dowel, I mounted them across the dowel and butted up against one another.

I installed a piece of 4" mailing tube in the body to provide a smooth internal surface and to mate with the coupler tube that formed the nosecone shoulder. The Kevlar® shock cord was affixed to the tip of the roof with epoxy and was run through the bottom plate and attached to the dowel.

The rail buttons were mounted on a rectangular pine spacer so the rail would clear the roof.

The final step was to wrap some scrap wire from the base of the body to the bottom fins. This allows me to hook the clips below the fins. Having clips above moving fins is not a good thing. Been there, done that!

I made a RockSim model and it appeared that it was highly stable. Of course, being a cluster there is plenty of opportunity for this design to get ignorant. So, it was off to the 'C' rack of high power pads for this baby! (Hence the buttons and rather large lug.)

This rocket was, is, and forever will be nekkid.

Flight #1:
For the maiden flight, I loaded her up with a chute protector, a 36" nylon chute, and six D12s. The motors were ignited via quickmatch and ejection was to be performed by a single D12-7. I should have computed the volume involved or just used more D12-7 motors to handle ejection. The upshot was the laundry didn't come out and the flight ended in a spectacular lawn dart on the gravel access road. Anyone need some toothpicks? You can see a video of the flight on MDRA's photo page. In the video by Bob Utley, you can see the ejection charge go off just before apogee. The lawn dart also came too close for comfort to one Jerry O. who was arriving late. We were careful with the orientation of the rail, made sure nobody was walking down range, and checked the sky...we just didn't check for moving vehicles. Lesson learned.

The Rebuild:
I replaced the body with a new birdhouse but reused the dowel and fins.  The wood on the base of the new bird house was softer than before and it splintered and broke up when I was drilling it. Luckily, the plate I removed from the top was the same size as the bottom that I had just destroyed. I didn't risk the slant drilling on this one and drilled it straight through, providing parallel mounts.  The other difference with the original is that I used a 1/4: lug instead of rail buttons.

Flight #2
This flight was on the same complement of motors, however, this time I "enhanced" the ejection charge on the D12-7. This time, the boost was cool and it recovered fine. The in-line motors did char the dowel and fins a bit.

Flight #3
Prior to the 3rd flight, I sanded the dowel down and added a conical baffle just above where the charring started (see above).  I coated the baffle with metal duct tape for some added protection.

I used two D12-5s with ejection charges and plugged the other D12-0's.  The boost was great and ejection went as planned.  However, the supposedly protective baffle broke loose and its metal tape burned off. Worse still, it didn't do its job - the dowel was badly barbecued.  One of the fins also broke off so Version 2.5 was retired and scavenged.

This unique odd-roc gets everyone's attention. There are not many rockets with their motors mounted so far forward...and there aren't that many flying birdhouses! As noted in the flight report, parallel mounts take their toll on the dowel, and in general don't look so cool. If you try something similar, I strongly recommend using canted mounts.