YikStik

I've always thought the high-gloss red paint job on one of my son's rockets when out on a launch rod in the sun looks a lot like glistening wet lipstick.

Combine that with the fact that my wife who isn't fond of the stuff refers to lipstick as "yik stick"... and the rest should be obvious.

My planned paint scheme is a bright red nosecone, gold tube, and black fin can, which is the mental image I have of what lipstick applicators look like, most likely from a stick my mother or one of my grandmothers had when I was a child. Something like the image by Calliope1 that I found on flickr.com.

YikStik is a fairly simple "three fins and a nose cone" dual-deploy rocket using a 75mm motor mount, 4 inch glass-wrapped phenolic airframe with zipperless fin can, plastic nose cone, plywood fins, and lots of glass and carbon fiber reinforcing. The primary electronics bay will be designed to hold two altimeters, and a distinct payload bay may carry an experimental altimeter, GPS receiver, and downlink transmitter.

I intend to cut the airframe components from two 48 inch lengths of 98mm Giant Leap Dynawind tubing. The 30 inch main bay and 18 inch drogue bay will be cut from one length, while the 33 inches of fin can, 2 inches of electronics bay, and 8 inches of payload bay will be cut from the second.

I intend to use a Giant Leap "Pinnacle" 3.9 inch nose cone.

The fins are designed from scratch, and I intend to build them up from two layers of 1/8 inch birch plywood, three layers of carbon fiber, and two layers of 6 oz glass. The stack will be glass, carbon fiber, plywood, carbon fiber, plywood, carbon fiber, glass. The edges of the plywood will be routed to give a modified airfoil shape to the finished fins. The stack will be laminated using West Systems epoxy products and vacuum bagged. The shape is a compromise between mass, surviving Mach-transition stress, optimal stability margin, and avoiding damage during handling and on contact with the ground during recovery.

The fins will be locked in to milled slots in two of the centering rings, and will be epoxied to the motor mount with glass reinforcing tape. The airframe will be slotted to allow the completed motor mount / fin assembly to be inserted from the rear, with fillets of epoxy applied inside and outside the airframe after insertion.

All centering rings and bulkheads will be custom machined from 3/8 inch birch plywood using my 3-axis CNC milling machine. Some rings will use laminated pairs of 3/4 inch total thickness to enable use of threaded inserts for 1/4-20 rail button screws or deep routing for fin alignment slots.

I will embed three 8-24 T-nuts in the aft centering ring spaced to allow the use of home-made Kaplow clips to retain 75mm motors. The same holes may be used to attach custom motor mount adapters for smaller diameter motors.

This design has been thoroughly analyzed using RockSim with motors ranging from the Cesaroni J285 through the Aerotech M1850W and appears to be unconditionally stable across that range. The lowest margin is around 1.2 seen with the M1297W planned for my level 3 certification flight, albeit with many masses still only roughly estimated.

These simulations will be refined as the build proceeds and as-built stability verified before flight.

The Aerotech M1297W reload should carry this vehicle without ballast to just over 14 thousand feet AGL. It should make over 16 thousand feet AGL on an M1850W, and should fly stably to roughly 2.5k feet AGL on a Cesaroni J285.

Hitting optimal mass on the largest motors may require ballast, depending on final build weight. My plan is to fly without ballast on the certification flight, trading some altitude for a slower and softer recovery. If the cert succeeds, then I might try an optimal mass flight sometime later on an M1850W or equivalent "bigger M" reload to join the "three mile club".

The recovery system will use dual redundant barometric altimeters firing black powder charges. At apogee, a drogue chute will deploy from just forward of the fin can, with size selected for an approximately 100 ft/sec descent rate. At a preset altitude, a main chute will be deployed to achieve recovery of the bulk of the rocket at under 20 ft/sec. The main chute will be packed in a deployment bag, configured as a "freebag" and pulled out of the airframe by a second drogue chute. This drogue will recover the nosecone and deployment bag separately from the remainder of the rocket which will recover under the main.

I intend to sew the parachutes from scratch using a design documented by Team Vatsaas using 1.9oz rip-stop nylon and 550 lb parachute cord. If time runs short, equivalent chutes from SkyAngle, Rocketman, or Giant Leap could be substituted (at significantly higher cost).

The deployment bag will probably be purchased from Giant Leap. The recovery harness will probably use tubular kevlar, also from Giant Leap.

The recovery system attachment points will all use 1/4 inch u-bolts with nuts, washers, and backing plates through bulkheads except for the fin can. The fin can will be equipped with a loop of 3/16 inch stainless steel aircraft cable bonded to the motor mount tube during construction since there is insufficient room to install screw-eyes or a u-bolt in the design. If available, a screw-eye attached to the forward motor closure may be used instead of or in addition to the aircraft cable as the fin can recovery harness attachment point.

The tubing for the airframe, couplers, and motor mount was all cut using a carefully aligned and adjusted power mitre saw, and the ends lightly sanded to remove rough spots. The main and drogue bays were cut from one 48 inch length of Giant Leap 98mm Dynawind tubing, the fin can, electronics bay, and payload bay were cut from the second. The three couplers for the fin can, electronics bay, and payload bay were cut from Giant Leap 98mm phenolic coupler stock. And the motor mount was cut from Giant Leap 75mm phenolic airframe stock. This photo shows the airframe tube on the left, the motor mount in the middle, and the coupler sections on the right. Note that the motor mount is the longest piece because of the zipperless design with full-length motor mount.

The airframe tubing selected includes a wrap of 10oz glass in epoxy over the base phenolic tubing (visible in the previous photo as a shine on the outside of the tubing), but the coupler stock is unreinforced. To ensure the couplers can handle the anticipated loading, I reinforced each with one layer of interior carbon fiber, using the "kitchen vacuum bagging" technique documented by John Coker.

This was my first hands-on experience working with carbon fiber. The end of the coupler nearest the unit during bagging experienced some crushing of the fibers right at the end. It doesn't matter for this project because each of the couplers will have at least one end fitted with a bulkhead or centering ring, but in the future I'll be tempted to cut the coupler stock a bit long before bagging and trim to length after reinforcing to get "perfect" ends. The technique worked marvelously otherwise, and the resulting couplers look and should work great!

Six pieces of 1/8 inch birch plywood were stacked, edge-aligned on what would be the fin root edge, and clamped. The outline of the fin design was marked in pencil, and three 1/8 inch holes drilled through the stack inside the fins to allow using 4-40 screws and nuts to hold the blanks together while making the initial cuts, so that they would all be matched in size. The clamps were removed to avoid interference during cutting. The fin outline was then cut using a radial arm saw.

A router table with 1/8 in roundover bit was then used to round over the outer edge, 3 blanks on one side and three on the other. This edge might have been left square, but I prefer the look and feel of rounding. The router table with a 1/2 inch diameter straight cutting bit and a fin beveling jig was used to impart a 10-degree bevel on the leading and trailing edge of each fin blank, again 3 on one side and three on the other. The resulting 6 blanks thus form 3 pairs of fin components with a modified airfoil shape.

The fin assembly started with a simple lamination of two layers of ply sandwiching a layer of carbon fiber. Each fin used "one pump" of West Systems epoxy and the stack was vacuum bagged using the Foodsaver with wide bagging material. To keep everything flat while the epoxy cured, the stack of fins was sandwiched between two unused extra shelves for a storage cabinet I had on hand (particle board covered in laminate, very flat and smooth, nearly inflexible at this loading), and stacked with about 75 lbs of loose barbell weights.

On one of the three fins, the plywood layers are out of alignment by 1-2mm in the longest axis. The other two are nearly perfect. Light sanding should allow me to match them before laminating the outer layers of carbon fiber and glass.

Pairs of 3/8 inch birch plywood blanks were laminated using Titebond wood glue and clamped while curing to form 3/4 inch blanks for centering rings. From a strength perspective, 3/8 inch should suffice, but there are two reasons for going with thicker blanks in some places. The first is that the rail buttons chosen use 1/4-20 mounting screws, and threaded inserts in that size are nearly 3/8 inch outside diameter (and thus would tear up a ring only 3/8 inch thick on insertion). The second is that I like to mill slots in the centering rings on each end of the fins to "lock" the fins into position. Doubling the blanks used to cut those rings will allow me to cut 1/4 inch deep fin slots and still have a half inch of unmolested wood in the rings for strength.