Water Rockets by Yoram Retter

1.	Introduction
	The design's basic principle is the ability to force a skewer into a fin so that it creates a "stabilizer set", which in turn can be easily connected to a bottle. The specific design described in this page is optimized for minimal required materials and minimum construction time. Typically a class of up to 20 students (ages 8-15) can each finish a rocket within one class period and, since the design is very tolerant to "not-so-exact-work", everyone has a shot at launching. The design is also surprisingly robust and will require only minor adjustments after hitting the ground straight on.

2.	Materials and equipment
	Note: some materials have different names in different countries. If you recognize a material from its description and it has a commercial/other name in your country let me know, I will add it to the description.

2.1	Plastic bottle that fits your launcher: Best results are obtained with bottles that have a straight contour line both at the center section and the neck section.

2.2	3 Wood (bamboo) skewers: Normally used for roasting marshmallows or barbeque. These are wood (bamboo) sticks about 30 cm (12 inch) long with a 3.2 mm (1/8 inch) diameter and one sharp end. The sharp end is originally used to slide the meat/marshmallows on the skewer.

	Fig 1: Skewer

	Note: If such skewers cannot be found in the stores near you any round wooden stick will do, you can make the sharp end with a knife or a pencil sharpener. Just make sure it can be connected to the fin as described in section 3.3.

2.3	Polycarbonate Structured Sheets (Polygal): Ploygal is a commercial name for Polycarbonate Structured Sheets that are hollowed out in one direction. Typical "normal" applications are large notebook covers, portfolios, boxes for office use, etc. Here it is used for making the stabilizer fin; I usually use a sheet with a 2.2 mm thickness. All that is required is that the skewer can be forced into the holes as described in section 3.3. Commercial polygal site. Other sites that use this material: Nick's Water Rockets under the commercial name: "corex". water rocket workshop under the commercial name: 'Flyweight' cellular corrugated plastic. Kevin's H2O Rockets under the commercial name: "Coroplast".

	Fig 2: polygal (with cut-out for illustration)

2.4	Adhesive tape: any type, even low grade will do.

2.5	Rubber bands.

2.6	Scissors or sharp knife, thin tip permanent marker, pencil, cardboard, and ruler.

3.	Construction
3.1	Create a pattern for the fins: Use a piece of cardboard to make a pattern for the stabilizer fin. Hold the pattern next to the bottle and make sure it fits the bottle's contour at the neck as shown in fig. 3. No exact fit is required, just one contact point is enough. Any shape that makes sense is ok, but 2 conditions should be met: Leave space for the launcher cable ties and lock tube (thick red line in fig 3) Leave space for hand with launcher (area between green lines in fig 3) Note: most pictures on this page show old fins that do not meet this condition. When the result looks ok, draw a straight line on the pattern that is in line with the bottle contour.

	Fig 3: bottle and fin pattern (see picture)

3.2	Cut fins: Place the fin pattern on the Polygal sheet such that the line drawn on the pattern is aligned with one hole in the Polygal. Hold the pattern in place and mark the fin on the Polygal with a permanent marker. Cut shape of fin with scissors or knife. Mark the hole in the Polygal that was aligned with the line on the pattern. Repeat this to create 3 fins.

	Fig 4: fin (see picture of fin and pattern)

3.3	Create stabilizer set (fin and skewer): Insert a skewer in the marked hole of the fin from sec 3.2 using the sharp end starting at the side of the hole in the flight direction. The skewer should be forced by hand into the fin at least 5 cm (2 inch) so that the friction holds the two parts firmly together. You will notice that the skewer creates a bulge in the Polygal. Repeat two more times to create 3 stabilizer sets.

	Fig 5: stabilizer set (fin and skewer) (see picture)

3.4	Positioning the stabilizer sets: Mark the location on the bottle - three equally spaced marks on the circumference. Place a few rubber bands on the bottle. Insert the skewers under the rubber bands at the marked locations. The structure at this point is strong enough so the rocket can be placed on the fins in vertical position. Make sure each fin is touching the bottle's neck at least at one point.

3.5	Connecting the stabilizer sets: Wrap adhesive tape around the bottle at two locations: next to fin and front of rocket. This is a good job for two people: one rotating the rocket horizontally while the second guides the adhesive tape, making sure the tape is following the bottle near the skewer. At this point the stabilizer set is fixed in the longitudinal direction but can still rotate around the skewer. Next, cut 6 pieces from a spare bottle and bend them to L shapes. Then place 2 L shapes between each fin and the bottle and use adhesive tape over each leg of the L to connect the fin to the bottle.

	Fig 6: finished rocket with detail of L shape (see picture)

	THIS IS IT !!!

	You are ready to launch. If launched "as is" (without parachute), the stabilizer set will probably need some adjustment after landing since the "Bang" may disconnect the adhesive tape that was holding the L shapes (but never the tape wrapped around the bottle). Make sure the fins are attached with L shapes before each launch, If the fin is not fixed this way it will rotate in flight with considerable decrease in the rocket's performance.

3.6	Additional description:

	Fig 7: Exploded view (without adhesive tape)

	3D view (in new window)

4.	Possible improvements
4.1	Use adhesive tape to fix a tennis ball at the top of the rocket. This will help in two ways: first, it provides a softer landing. Second, it contributes to move the center of gravity forward.
4.2	Connect the stabilizer set with a piece of adhesive tape along the skewer, not wrapped around the bottle. This is smoother and causes less drag.

Water Rocket - Skewer Design

1.

Introduction

2.

Materials and equipment

3.

Construction

4.

Possible improvements

Last modified July 7, 2006