Briefly, it is based on the locking of the nose cone by a rubber band. This one is hold by an aerodynamic flap. This flap is kept along the body of the rocket by the air speed, or by another small flap before the launch. When the speed becomes low (thus near the apogee), the tension of the rubber band in stronger than the pressure of the air speed on the flap which can open and free the rubber band. So the nose cone is ejected and the parachute can deployed.
You can see below a picture of my realization :
The nose cone is fit on a ledge made with a sleeve of PET glued (the part yellowish on the picture). The nose cone is hold by a set composed of a PET strip (you can see it on the left side of the cap) and a rubber band(the yellow line on the right side). The rubber band is hooked onto a little pin glued on the flap (the pin is juste under the piece of black adhesive tape).
The flap which goes down to the second black adhesive tape, is fix on its top by a hinge (white on the picture. I buy it in a model shop) and kept along the body, before the launch, by a small flap (this is another hinge), that you can distinguish just bellow the second black adhesive tape, and which is closed by the air speed during the launch, so the main flap is free.
Note the two rubber band (red and green) which are tight under the parachute to make easy the ejection of the nosecone
To make easier and more reliable the ejection of the nosecone, I made a cross with 2 rubberbands which are held by handles. these handles are made with PET and staple on the nosecone (see the opposite pictures)
To compute its size, we have to remain that the drag force i given by :
R = 1/2*r*Cd*S*v2,
where r is the air density (1,293 kg/m3), Cd is the coefficient of drag which depends of the shape of the object, S is the frontal area of this object and v is the speed.
The forces applied on the parachute are :
The weight of the rocket (with its parachute) P = m*g
The drag R = 1/2*r*Cd*S*v2
To have a constante speed (that is no acceleration) we have to get the balance beetween P and R thus m*g = 1/2*r*Cd*S*v2 . According to the speed we expect, the calculation of the area is :
Exemple : With a rocket's weight of 150g, if we expect a speed of 2 m/s ( 7,2 km/h), given that 0.5*r*Cd is about 0,9 (r = 1,3, Cd = 1,4) for a parachute, we have S= 0,15*9,81/0,9*22 = 0,41 m2 thus a diameter of 0,72 m
The making of a parachute is easy. Take a sheet of light plastic, as a garment bag of dry-cleaner's or a garbage bag. Cut a square with a side equal to the diameter of the parachute and fold it along its diagonal, then beetween the top of the triangle and the middle O of its hypotenuse. The point O will be the top of the parachute, fold again the plastic sheet 3 or 4 times beetween the point O and the middle of the opposite side (as shown on the picture). The triangle that we obtain have to be cut in such a way that the 2 longest sides are equal to the radius of the parachute ( line AB on the picture). Cut also the top of the triangle ( line CD on the picture) to have a hole at the top of the parachute ( This is necessary to avoid instability of the parachute). Now we can unfold the sheet, so we have the canopy.
For the shroud lines, cut 4 length of 3 times the diameter of the parachute, either with nylon thread or a fine string. Each of these shroud lines are folded in 2, so we have 8 ends that we have to tied on the canopy. For that, we draw 8 marks on the canopy, where we will tie the schroud lines. At these place, put a piece of strong rubber band on each side of the canopy, then we make a little hole, so we can pass one end of a schroud line and tie it.
When we have tied the 8 ends of the schroud lines, we knot the other ends of the shroud lines with a string. Now we can tie our parachute on the rocket.
I know at least 2 others methods :
The "Helicopter" recovery which consist of a deployment of blades. With the effect of speed, theses blades turn and so the speed decrease. See the Hervé Brégent site
The "gliding" recovery (like the space shuttle), with 2 ways, the fixed wings (as Dave Johnson had made) and the spreading wings. I test the second solution and I describe it with my results in the page glider.