The Binder Cobra (or the rocket formally known as Raptor) is a 4" x 84" rocket that comes stock with 54 mm motor mount. The rocket can be flown with a hybrid motor with additional 54 mm motor mount tube (not supplied) as it comes stock with about 14" of tube. Fins mount through the wall to the motor mount tube and are secured by centering rings made of 1/4" masonite hardboard. Airframe is kraft paper, chute is a 48" ripstop parasheet, nose cone is an Ace polystyrene shape.
The fins are 5 ply 1/8" thick baltic birch which need some sanding to achieve a nice airfoil and uniform shape. There are 3 sets of fins on this rocket which give the rocket it's nice lines and attendant complexity, so be prepared to do some sanding. (see section on using the Belt Sander below)
Binder supplies a fin mounting template over which the rocket is centered for fin alignment. The trick here is to use a couple of drops of 5 minute epoxy to tack the fins in place then fillet them securely with a higher quality epoxy once they are positioned correctly.
The glassine covered kraft paper body tube is marked from the factory for fins and launch lugs while the fin slots are left uncut for the enjoyment of the rocketeer. There are a lot of slots to cut so be prepared to spend some time with a utility knife cutting slots. On a 12 finned rocket there are lots o' slots so I highly recommend the Dremel slot cutting method. (see section on using the Demel below)
The centering rings are made from masonite as mentioned above. The fit is a bit loose but this can be taken care of with proper filleting technique. Binder (and I) highly recommend the use of a 50/50 mixture of milled fiberglass and epoxy. Use a minimum of 30 minute epoxy and try to avoid the one to one mixes as these are not as strong (due to the addition of cheap fillers) as four to one or five to one mixes. Milled fiberglass allows the epoxy to be mixed to the consistency of peanut butter which gives the rocketeer freedom to do something else than tend runny epoxy as it sets up. Plus this thixotropic mixture can be smoothed with a gloved fingertip dipped in denatured alcohol to give a super smooth finish to the fillet without all that hateful sanding.
Once the entire body tube/centering ring/fin unit is epoxied into place the airframe is very stout and stable, it makes for a robust flier. Launch lugs are 1/2" kraft paper tubes epoxied to the airframe.
The nose cone is a quality polystyrene Ace 4 to 1 ogive NC. It seems to be a very common nose cone in rockets. It will even hold your favorite beverage when fitted with the proper screw cap.
There is a provision in the instructions for the addition of a simple payload or electronics bay. This rocket is two standard 34" kraft body tubes long. One coupler is supplied with the kit so an additional coupler is necessary should a payload or electronics bay be used. I have only flown this rocket using motor ejection but I built it with a bay anyway, just in case. If a bay is installed remember to allow for proper upper airframe and nose cone retention.
The motor mount tube as supplied did not fit a LOC adapter as I wanted the versatility of a 38 mm and 54 mm motor mount. A 54 mm motor slides nicely into the tube though the adapter needed significant sanding to fit easily. I swapped it out for LOC 54 mm motor mount tube.
Tip on using a Belt Sander:
I used a belt sander chucked into my bench vise as a flat bed sander of sorts. Be careful and use proper eye protection when using a belt sander. Always make sure the direction of travel of the belt is away from you which will attempt to pull the fin from your fingertips rather than embed it in your palm. Use double stick tape to stick all similar fins together for sanding as a unit to ensure uniformity. Bevel the leading and trailing edges of the fins using the laminations in the plywood as a guide. Mark where the fins enter the body tube and do not bevel edges below this mark otherwise the gap will have to be filled later on.
Dremel Body Slotter:
Take your Dremel tool with the threaded cut-off blade attachment chucked into the collet. Start with one fibered cut-off blade. Stack one, two, or three (depending on thickness of fin) of the brick colored composite cut-off blades on top of this blade and install an additional fibered cut-off blade. This will make a sandwich of sorts, more similar to a stacked dado cutter than anything else. Secure this into the threaded attachment so that the tool is held by at least 5 or 6 threads. Center the middle of the blade unit over your slot line and go to town. It is an unbelievably fast and precise way to slot kraft, phenolic and fiberglass tube. Use with caution at slower speeds at first and check for tightness of screw periodically. It makes a bit of smoke and dust so be prepared.
Finishing the rocket was pretty straightforward on my rocket. I painted the fin can and nose cone satin black. The remainder of the airframe I wrapped in a cast vinyl normally used for making vinyl signage. I found a very cool faux carbon fibre vinyl and installed on the rocket which looks like the real thing at first glance. The material is strong, it takes some abuse and lends strength to the airframe as well. I suffered one descent without a chute from 3,500' on this rocket (on a J350) and the airframe and fins were undamaged. The enclosed "Cobra" decal with swoosh that is supposed to look like a snake is completely and totally lame. It should be thrown away as soon as the kit is opened.
Lose the enclosed chute as well. It is a simple 48" parasheet and it does not do this rocket justice since it is only 1 or 2 ounce ripstop with the suspension lines sewn into the hem and not over the top. Any type of high speed deployment and this type of chute has a very high chance of failure. I usually fly this rocket on a 60" 12 gore surplus chute I picked up and have also flown it on a reefed 52" SkyAngle chute.
I employed Stu Barrett's anti-zipper design (Click <HERE> then goto "Anti-Zipper Design" to see the concept.) building my Cobra and included a stuffer tube and baffle assembly. This is a bit more work but the retention system is bulletproof. Remember to toss the enclosed elastic shock cord and install 20 or 30 feet of tubular nylon and you will never suffer a separation as long as your retention points are stout.
I have flown my Cobra on an I 161 for a level one certification flight, an I 211, an I 357, and a J 350 for my level 2 cert. All these motors pushed the rocket along nicely. Tom Binder of Binder designs recommends a minimum of an I 161 or I 211 though he generally flies his on a Hypertek J or K motor.
It is a very nice rocket to launch and watch. Boost is generally very straight with little weather cocking. This was the first high power rocket I built. I probably also overbuilt it as well but that's why they make bigger motors, right? You can tell I made some mods as I went along but what rocketeer builds a rocket box stock, not me.
I learned a lot building and flying this rocket. While Tom Binder's selection of kits is excellent as well as varied, his construction technique does not change from rocket to rocket. Essentially, if you build one Binder kit you can build them all. Though this is typical for most kitters (Estes, Quest, LOC, PML) it's fun for me to move to another manufacturer, let's say U.S. Rockets or THOY, and see how they do things. I believe this is how one gains skills and knowledge in a real hurry, for what you are often getting is the culmination of the manufacturer's talent and experience all wrapped up in a plastic bag. And that is worth more than the price of the kit alone. LOC/Precision and Binder share the same simple design-build philosophies except that Binder fins go to the motor mount which was important to me since the fins become part of the entire structure and not a liability. I would rate this rocket a 4 on Essence's scale of one to five.
Brief: The Binder Design Raptor is a single stage 4 inch diameter kit. It has a 54mm motor mount and comes with high quality paper body tubes and laser cut fins. Construction: The Raptor came with two 36" body tubes and an Ace nose cone. The body tubes are paper and BSD style with a glassine coating. It comes with one paper coupler, one bulkhead, 3 plywood centering ...