This chute release is airspeed-sensitive: as long as the rocket is moving fast enough, the chute remains tucked safely away in the nose, but when the rocket's airspeed falls low enough, the chute is released. Obviously, this scheme only works on near-vertical launches, where the rocket's airspeed falls to near zero at the top. If you launch at an angle, and the airspeed of the rocket never gets low enough, the chute won't release.
The basic idea is to have a flap that lies flat along the side of the rocket and is hinged at the top. This flap is gently tensioned with a rubber band, so it will spring open if nothing holds it down. As long as the rocket is traveling reasonably fast, the air rushing past the rocket will hold the flap flat against the rocket. Once the force of the moving air isn't enough to hold it down, the rubber band pulls the flap open, releasing the nose and the chute.
We also need a way to hold the flap closed while the rocket is sitting motionless on the launch pad. Although you can do this in several ways (including just leaning a stick against the flap), I chose to use a second, smaller flap that's rigged to hold the main flap in place. This second flap sticks straight out from the rocket when it's at rest: at launch, the small flap (called the trigger flap) is pushed down out of the way, freeing the main flap.
Lastly, the nosecone containing the parachute is spring loaded, so when it's released it springs away from the rocket, delivering the chute into the airstream for deployment. This really helps deploy the chute fast.
There are about a zillion ways to make this thing, but here's one good one. First, here's a side view drawing of the whole thing, to acquaint you with the parts. There are also good drawings at a recently mounted French water rocket site (worth a visit!). See the page in French or English (note: as of 5/98, the english page isn't up yet, but it should be soon.)
*2 small, flat nylon hinges, the type used to attach elevators and ailerons 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 like that clear, tough package sealing tape).
*Rubber bands of various sizes.
*A latex glove or large balloon (optional).
Cut the main flap from the body of a spare bottle. Mine is roughly triangular in shape, with a flat spot at the top, sort of a truncated triangle with rounded corners, and it's 4 inches long and about 3 inches wide at the base. I find that the plastic wants to curl up after being cut free, so you'll need to flatten it a bit to fit closely to the curvature of the rocket body.
Bend 2 short pins from wire as follows: bend a small triangle (about 1/4" on a side), letting one of the ends extend another 1/4" or so beyond the triangle (see inset in drawing above). Bend the extending bit of one of them up perpendicular to the plane of the triangle; leave the other one flat. (The idea here is to have a short pin and then some supporting surface area - the triangle - to attach it to the flap, so however you want to accomplish that is fine.)
Poke or drill a small hole in the flap, centered 1 1/4" or so down from the top end. Insert the "perpendicular" pin from the back so the free end sticks out from the surface of the flap, letting the triangle hang below the hole, behind the flap. 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 the free end 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.
The nylon hinge itself has enough surface area 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 line, like this:
Note: The hinge provides plenty of area for "full bore" launches, but if you're using a narrower nozzle, and thus have slower initial acceleration, you may want to attach a larger flap to the hinge to make sure it gets pushed down out of the way.)
Many people have been confused about the operation of the trigger, so here's a detail drawingthat will help (I hope). Drawing 1 is the "locked" configuration on the pad, the next two drawings show what happens at launch.
See Nose Cones elsewhere on this site for ideas on how to make a good one. Here I'll just tell you how to "spring load" it.
Cut a 1" strip from the latex glove or balloon, and place it across the mouth of the nose cone, not tight but without much slack. 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 ensures that the chute doesn't stay in the nose (I had a problem with the chute getting stuck in the nose in my early attempts).
Another method that works well (Bruce Berggren came up with this one) is to attach a long rubber band to the top of the nose cone on the outside. Then attach the other end near the bottom of the rocket. The chute is attached to the inside of the nose cone. This will also pull the nose away from the rocket when released, and is a little easier to rig.
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 VERY IMPORTANT to prevent the nose cone from "wedging" on to the bottle at liftoff. Glue the ring in place with a little CA or Goop around the edges.
(The following assumes you will attach the flap mechanism directly to the rocket body, but I prefer to attach it 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. Make sure the flap swings very freely. 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. It's very important that the trigger flap can be blown down easily, disengaging the trigger pin. If you can't blow it down with a moderately strong breath, then you should keep working on it, either changing the position of the hinge relative to the pin, or shortening the pin, or whatever it takes.
Cut a long( 12" - 14"), narrow (1/8") strip of plastic (the "retainer strip") from a spare bottle. I had to cut it in a spiral to get it long enough. Tape one end to the rocket body on the opposite side from the main flap and below the nose, then run it up and over the nose cone, ending a couple inches above the main flap hinge. Bend the end under to form a hook for the rubber band (see drawings). Put a smallish rubber band (#16 is good) in the hook and tape it closed so the band can't come off.
Lastly, tape a piece of string a foot or two 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 chute to the nose cone would be better, so that when it springs away it unfurls the chute. (Note: if you used the external rubber band to the nose, then just attach the chute to the inside of the nose cone.)
Here's a photo of the finished system on a rocket (there's no chute in the nose here, but otherwise it's ready to launch). The main flap is outlined in black to help you see it.
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, if you have one), 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 lock down the flap. If the rubber in the nose is too tight, the strap won't hold to nose cone down well: you'll have to adjust it depending on how big your 'chute is.
I use a "Breath Test" to make sure everything is tuned properly: Holding the rocket with the nose near your face, you should be able, with a really big breath, to blow the trigger flap down, but keep the main flap down as long as you can keep blowing hard. By the way, speaking from experience here, it's a good idea to keep your face far enough away that you don't get slapped when the flap lets go :-) If you can't blow down the trigger flap, or you can't keep the main flap down with your breath, you need to do some tuning. Try different sized rubber bands, etc., until you get it right.
I like to have a small piece of tape holding down the main flap while I'm getting the rocket ready to launch, then I 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 chute, two rectangles sewed together like a plus sign. The nice thing about X-form chutes is they don't drift much. However, if you're going for duration, you'll want a big, round, conventional chute.
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 getting stuck in the nose, so decided it would be better to have the nose detach completely and spring away from the rocket.) And you don't necessarily need the trigger hinge: you could have something attached to the pad that holds the main flap in place, or even just lean a stick against it. These are just a few thoughts...be creative!
Some contests prohibit metal in the rockets. To eliminate metal in this design, you need to get rid of the hinge pins in the nylon hinges, and the wire pins attached to the main flap.
I've tried replacing the metal hinge pins with short pieces of heavy fishing line (mine is 60 pound test) with knots tied in the ends to keep it in place, and it works great! Alternatively, you can just make hinges from tape, though they're not quite as good as the nylon hinges.
The pin that sticks out from the main flap to hold the rubber band can be replaced with a small tab cut in the center of the main flap and bent out slightly to hook the rubber band over. To get rid of the trigger pin, the hinge can be notched like this:
Then it can fit into a corresponding notch in the bottom of the main flap. So when it's locked it looks something like this (sorry about the crude drawing, but you get the idea):
In my experiments with this configuration, I've had to round the corners of the notch in the main flap a bit, so that the hinge can slip out of the notch easily.
This is just a starting point: as I said, there are a zillion ways to make this kind of release, so experiment! Please let me know how it works (or doesn't!), and keep me posted on any refinements you make, chute packing methods, good stories, etc.
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