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Hi All.
As promised, here's a detailed description of the chute release. This is actually the second-generation, much sleeker and fewer parts than the first. Let me know if anything's unclear.
I enclosed two JPEG files, one a drawing, the other a not-very-good photo. I hope the encoding works for everyone. If not, I posted the same files, and the text description pasted below, on the ftp site (ftp://ftp.apple.com/pub/davejohnson). There's also a pict version of the drawing for you Mac folks.
Feel free to spread it around wherever you like, post it on your web sites (or just link to the ftp site) or whatever. And above all, please build it and try it, I want to hear stories from the field!
----FUNCTIONAL DESCRIPTION----
The basic idea is to have a flap, hinged at the top to the body of the rocket, that is held flat against the rocket by the air as long as the rocket is traveling reasonably fast, but springs open once the airflow slows down enough, releasing the nose. A second, much smaller flap is rigged to hold the main flap in place while the rocket is on the pad: at launch, this small flap is pushed down out of the way, freeing the main flap to do its thing. The nosecone containing the parachute is spring loaded, so when it's released it springs away from the rocket.
----CONSTRUCTION DETAILS----
There are about a zillion ways to make this thing, but here's one good one:
Required parts:
*2 small, flat nylon hinges, the type used to attach elevators and alierons to model planes, from a hobby store. (You could also just make tape or cloth hinges, but these nylon ones are great, and cheap, about $1.25 for 6)
*A bit of wire (paper clip is fine, I used .025 music wire because it's nice and stiff).
*A spare bottle or two to cut up.
*Some glue or tape (I used mostly CA glue, and that clear, tough package sealing tape).
*Rubber bands.
*A latex glove or balloon (optional).
MAIN FLAP
Cut the main flap from the body of a spare bottle. Mine is roughly triangular in shape, 4 inches long and about 3 inches wide at the base. I find that the plastic wants to curl after being cut free, so you'll need to flatten it a bit to fit the curvature of the rocket body.
Bend 2 short pins from wire as follows: bend a small triangle (about 3/16" or 1/4" on a side), then let one of the ends extend another 1/4" or so beyond the triangle (see inset in drawing). Bend the extending bit of one of them up perpendicular to the plane of the triangle; leave the other flat. (The whole idea here is to have a short pin and them some surface area - the triangle - to attach it to the flap, so however you want to accomplish that...)
Poke or drill a small hole in the flap, centered 1.25" or so down from the top end. Insert the "perpendicular" pin from the back so it sticks out from the surface of the flap, letting the triangle hang below the hole. Tape or glue in place from the back. Attach the other, flat pin (the "trigger" pin) to the bottom of the flap, again from the back, so it extends down from the bottom edge in the center.
Tape or glue one of the nylon hinges to the top center of the flap, from the back, so the hinge pin is just above the top edge and parallel to it.
SMALL FLAP
The nylon hinge itself has enough surface to serve as the small trigger flap, so the only preparation is to drill a small hole in the center of one hinge flap, close to the hinge pin (perhaps 1/16" away).
NOSE CONE
I'm using the bottoms of the "old fashioned" bottles, the kind with a hemispherical bottom and an attached plastic base taht can be pulled off. These are getiing hard to find, but there are still some 1-liter ones around, Schweppes brand drinks and mixers are currently replacing them, but there are still some on the shelves around here as of this writing (Feb. '97). They'll be gone soon, though. Bruce Berggren has reported success in smoothing the bottoms of bottles with a heat gun - PET shrinks when heated. Another excellent choice is to use the top of a bottle, cutting off the neck and covering the hole with tape or something.
Cut a 1" strip from the latex glove or balloon, and place it across the mouth of the nose cone, not tight but not too loose. Tape the ends in place on the outside of the nose cone. (This is the best method I've found so far for attaching it, but there might be a better one.) Alternatively, you could use rubber bands, but the latex glove is ideal. The point of this is twofold: when the nose is pushed on over the packed chute, the rubber wants to push it back off, so when the nose is released it will spring away from the rocket. Secondly, it pushes the chute out of the nose (I had a problem with the chute geting stuck in the nose in my first attempts).
ASSEMBLY
Cut a narrow ring from the spare bottle (about 1/4" wide, or a little thinner). Put this over the top of your rocket, so that it acts as a "ledge" for the nose cone to sit on. THIS IS IMPORTANT to prevent the nose cone from "wedging" on to the bottle at liftoff. Glue the ring in place with a little CA around the edges.
(The following assumes you will attach the flap mechaism directly to the rocket body, but I prefer to attach the flap mechanism to a strip cut from a spare bottle instead, then the strip can be taped to the rocket. This lets you move the flap assembly to a different rocket if desired.)
Attach the main flap to the rocket just below the nose ledge, gluing or taping the free hinge flap to the rocket. Attach the other hinge under the bottom edge of the main flap so that the bottom pin can *just* be caught in the hole when the hinge is bent at 90 degrees to the rocket, but the hinge flap can be swung down to release the pin without any binding. This is of course important.
Cut a long, narrow (1/8") strip of plastic from the spare bottle. I had to cut it in a spiral to get it long enough. Tape one end to the rocket opposite the main flap and below the nose, then run it up and over the nose, ending about 3" above the main flap hinge. Bend the end under to form a hook for the rubber band. put a rubber band in the hook and tape it closed so the band won't come off when released.
Lastly, tape a piece of string a couple feet long to the inside of the nose cone, and tape the other end to the top of the rocket, so you don't lose the nose. I also tie a loop in the end near the rocket to attach the parachute. I'm still experimenting trying to find the best configuration: perhaps attaching the chut to the nose cone would be better, so that when it springs away it unfurls the chute.
TESTING
This is probably obvious by now, but you roll up your 'chute, hold it on top of the rocket, push the nose down over it, catching the 'chute under the latex band, pull the retaining strip over the nose, hook the rubber band over the pin in the main flap, and bend up the bottom hinge to catch the bottom "trigger" pin of the flap. Holding the rocket with the nose near your face, you should be able, with a big breath, to blow the bottom hinge down, releasing the main flap, but keep the main flap down as long as you can keep blowing hard. Keep your face far enough away that you don't get slapped when it lets go :-)
----LAUNCH!----
I like to have a small tab of tape holding down the main flap while I'm getting the rocket ready to launch, then remove it just before I pull the trigger (so far I haven't forgotten to remove it, but I'm quite sure it will happen some day :-) The chute I've been using is an X-form one, two rectangles sewed together in a plus sign. (This is actualy a military surplus "pilot" chute I got from American Science and surplus, much heavier duty than is required.) The nice thing about X-form chutes is they don't drift much.
----VARIATIONS----
There are an endless number, of course: The nose can be hinged to the rocket, so you don't need the retaining strap, just a rubber band from the nose to the pin. (This was my original configuration, but I had trouble with the chute staying in the nose, so thought it would be better to have it detach completely and spring away from the chute.) The small flap can be notched to fit a notch in the bottom of the main flap, doing away with the trigger pin. For that matter, perhaps you could have something attached to the pad that holds the main flap in place and stays behind. These are just a few thoughts...
Please let me know how it works, and keep me posted on any refinements you make, chute packing methods, good stories, etc.
Dave Johnson
silverfish@aol.com