Recovery systems - Nose Separates at Apogee

This is the simplest method of deploying a parachute or streamer. Unfortunately, it is also has a reputation for not being particularly reliable unless you are careful to make it so. This will be discussed here. Mounting the nose with parachute onto the rocket is exceptionally easy in the field with this method.

Design

This device is designed to be a separate unit that can be attached to any suitably sized bottle. It is made from one standard 2 litre bottle and will house any of the parachutes or streamers.

The parachute can be packed and the rocket mounted on the launcher as two separate procedures as the parachute is connected to the rocket via a bottle top which can be simply screwed on at the right time.

Manufacture

All that you need to build this is . . .

  • One 2 litre pop bottle;
  • Some gaffer tape;
  • A soldering iron;
  • Some sand paper; and,
  • A pair of scissors.
As this procedure requires increased air pressure to inside of the bottle, it must be done before any cutting operations are performed on it.

To modify the nose cone (the bottom of a pop bottle) in order to make is more aerodynamically sound, you first of all need to have the connector described on the connectors page. Attach this to the bottle (as described on that page) and pump up a fair pressure - between 1 and 3 BarG.

Unscrew the pump, leaving the hose (with its one-way valve) in place and start to rotate the bottle, holding the nose (a) approximately 9" above a gas ring (for those without gas rings, I have seen a barbecue mentioned as an alternative although I have not used one myself). After a while, the plastic will have softened sufficiently to form a hemispherical shape by itself (b), at which point, you should cool it down under a cold tap or in a bowl of cold water.

Remember that unlike the body of the rocket, this structure does not have to withstand any extreme pressure.

Carefully Holes in rocket nose cone mark out two rings on the pop bottle (about 1 cm apart, near the bit where the sides become parallel). I have found a convenient way of doing this by trapping a felt tipped pen in a drawer and then holding the bottle at either end whilst rotating it. Cut along the lines with a pair of scissors. You now have all of the bits that you will need.

Make a series of around 30 holes in the nose cone, on the straight portion just where the curve of the nose finishes. There is a low pressure area here and making around 30 holes in 2 lines around 1 cm apart will keep the pressure inside the nose low enough to keep it on the rocket until it gets to apogee. Note that the PTFE sheet, if you are going to use one goes up the inside almost as far as this but make sure that it doesn't cover the holes.

Now cut the ring and tape it so that there is about 1 cm of overlap. Tape this to the neck of the bottle top as shown in the diagram on the left. (I have found that in most cases, this ring is not needed but you can include it if you expect to have problems with the nose falling off on the launcher.)

Then, using the soldering iron, cut an inverted 'U' (as shown in the picture on the left) thus creating a tab. The position of this tab should be such that the nose cone can rest on the tab without having to be forced into position - in other words, the tab stops the nose from sliding back over the rocket during thrust phase and allows it to fall off at apogee.

Then, slide the tip of the soldering iron across the base of the tab lightly so as to soften the plastic at this point (don't heat it up enough to cut or melt the plastic) and then, while flexible, bend out to around 30 from the side (as shown in the diagram).

Allow it to cool in this position. Repeat at equal intrvals so that you end up with either 3 or 4 tabs making sure that they are positioned acurately so that the nose cone is both supported evenly and is straight. If you put too many tabs on, you run the risk of increasing the pressure inside the nose cone.

You now have the base unit for the nose cone.

Slide the base unit over the end of the bottle and then tape it to the bottle, making sure that the nose cone will sit straight on it.

Then, make a hole in the bottle top with the soldering iron so that the elastic shock cord of the parachute will go through it. Note that this hole should only be large enough to take the cord that you are going to use for the drogue chute - during flight, this is a high pressure area and if the hole is too large, it will allow air into the nose cone which will cause early deployment. Once you have packed your parachute, the job is complete (see the photogaph at the bottom of the page).

A circular parachute should be packed by holding it by the centre in one hand whilst holding the ring where the cords join either in the other hand or under your foot. Make sure that there are no tangles in the cords and then make sure that the shroud has all of the loops of farbic away from the axis (where the cords should be). Now, gather up the shroud and fold it into a three or four part zig-zag such that the edge of the chute (where all of the joints with cords are) protrudes a little. The cords should be zig-zagged in the same way. This means that the parachute occupies a minimum volume and when the chute is deployed, there are no tangles. If you were to scrunch the chute up, there would be a distinct lack of space and if you wound the cords around and around the folded chute, it would never unwrap itself in time. Using a zig-zag strategy means that the chute acts like a spring when it is deployed.

Just add fins (see the aerodynamic fins page for construction details) and you have your rocket.

Now all you have to do is tie all of the bits together in some sort of logical order and fold and pack the cute correctly to make sure that the parachute deploys at the right time.

Note that by having a separate nose cone that is secured to the base unit only by one bottle top, it is possible to back the parachutes into the nose cone, fill the rocket and mount it on the launcher and then take the nose cone, gently screw the top onto the thread on the base unit (you don't need to do this tightly as: it is not going to get a chance to unscrew; and, it only has to hold onto a hundred grammes or so of rocket for a few seconds) and put the nose cone in place.

Here are some different strategies:

  1. As in the diagram on the left, melt a hole in the end of the nose cone and push the drogue shock cord through it and tie a knot to secure it. Pack the main chute as described above, then pack the drogue ensuring that the drogue's chute will deploy before the drogues strings (this means that the drogue will come out quicker). When the nose cone separates, the drogue deploys, pulling the nose cone off the main chute and they both float to the ground separately.

  2. As in the next diagram, the drogue is tied to the centre of the main chute which is then secured to the nose base unit using the bottle cap. The nose cone itself is secured via a short cord which is passed through two holes melted in the side of the nose cone (for the sake of clarity, the diagram shows this cord on the other side - in reality it would be on the same side as the main chute shock cord) with a soldering iron (make sure that you remove any burrs with a sharp knife and sand any rough bits). When the nose cone separates, the drogue deploys and drags the main chute out of the nose cone and they both float to the ground together.

  3. Another option is to tie the drogue to the nose cone (as in the first diagram) and the main chute to the rocket but to have a line that connects the nose cone to one of the main chute cords via a loose knot that can slide up or down the main chute cord. This line has to be longer than the radius of the main chute. When the nose cone separates, the drogue deploys and pulls the nose cone off the main chute which then deploys. The two float to the ground joined by the line. The ony disadvantage this has is that there is a greater probability of the line snagging the other chutes. An amount of practice is needed to get the packing right if you are going to use this in a right first time situation such as a science olympiad.

One distict improvement for this method is to allow for the easy deployment of the main chute.

One serious problem is the chute catching on any burrs on the nose cone - simply cut off and sand any burrs so that the chute will fall out freely.

Another serious problem is the amount of force required to get the main chute out. The easy solution is to sprinkle talcum powder over the chutes and cords and inside the nose cone - the talcum powder acts as a dry lubricant.

One alternative is to use a PTFE sheet inside the nose cone. The sheet is cut so that it lines the side of the nose cone (there is no need to cover the end) so that whereever the parachute needs to slide across the inside of the nose cone, it only comes into contact with the sheet. These sheets are available in supermarkets and are allegedly for lining oven trays but their primary purpose of making parachutes deploy easily is enough to justify their existence.

An additional solution is to make the nose tapered slightly - this has the effect of making the sides of the nose cone face the open end instead of the other side. As a result of this and the chute's own springiness, the chute is pushed out to some extent.

To make the nose tapered, shrink it slightly during the nose rounding stage (as described on Body section of the 1 litre 'Egglofter' page).

So, to use this device:

  1. Pack the chute(s) (according to your strategy) in the nose cone USING PLENTY OF TALC or a PTFE sheet and put it to one side;
  2. Fill the rocket with water and mount it on the launcher;
  3. Lightly screw the chute shock cord bottle top onto the nose base unit (there is no need to make it tight);
  4. Pressurise; and,
  5. Release.

Note that the order of steps 1 and 2 can be reversed without consequence.


For Talc assisted deployment, look at pictures with the circular Bin-Liner and Nylon parachutes.

For PTFE assisted deployment, look at some pictures of a 2 litre rocket.


Back to the Water Rocket Recovery Systems Index
Back to the Water Rocket Index

 
Site Map