How to Build a 2-Liter Water Bottle Rocket with Foam Fins

This tutorial shows how to build a 2-liter water bottle rocket. It has a foam football nose cone and three foam core fins, and you check its balance with a string before you launch it. It flies from the launcher in How to Make a Water Bottle Rocket Launcher from PVC, so build that first. This page follows video 3, the last video in the set.

Normally a rocket is built to be as light as possible. This one has a heavy nose cone on purpose, because the extra weight in front moves the center of gravity forward and keeps the rocket flying straight.

Safety first#

Tools you need#

• A scrap piece of wood
• 3/8" wood screw
• Electric drill and Phillips driver bit
• Carpenter's square
• Permanent marker and pencil
• Box cutter or other razor knife
• Hot glue gun
• Duct tape
• Bike pump
• A length of string for the balance test

Step 1: Build the assembly stand#

The stand holds the bottle upright and keeps it from turning while you mark and glue.

1. Take the cap off the 2-liter bottle and set the bottle aside for now.
2. Use the drill and screw to attach the cap to the scrap piece of wood.

• Bottle cap screwed to the wood

Step 2: Mark the fin lines on the wood#

You need three lines on the wood, 120 degrees apart. They show where the fins go.

1. Unscrew the cap from the wood. The screw hole is the center point for your lines.
2. Press the carpenter's square against the edge of the wood and draw a straight line through the hole with the pencil.
3. Draw a second line through the hole at 90 degrees to the first.
4. Put the square's pivot point on the hole. Turn the square until its 30 degree mark lines up with the 90 degree line, and draw a line there. That line is 90 plus 30, or 120 degrees from the first line.
5. Flip the square over and repeat on the other side to get the second 120 degree line.
6. Screw the cap back onto the wood at the hole, and screw the bottle on.

The 90 degree line is only a guide. The three fin lines are the first line and the two new ones, 120 degrees apart.

• Three lines marked, 120 degrees apart
• Cap screwed back on and bottle on the stand

Step 3: Make the nose cone#

1. Use the carpenter's square to draw a straight line all the way around the foam football. Check that the lines meet on the other side.
2. Cut along the line with the box cutter. It does not have to be perfect. Clean up any rough edges.
3. A full half of the football is much too large. Trim it down to a smaller, more appropriate size.
4. Tape the nose cone to the end of the bottle that points up, with vertical strips of duct tape. Vertical strips keep it streamlined.

• Football cut in half and trimmed
• Nose cone taped on with vertical strips

Step 4: Transfer the fin lines to the rocket#

Do not turn the rocket on the stand for this step. If it moves, the lines will not match.

1. Line the carpenter's square up with one of the lines on the wood.
2. Hold the rocket still and draw a matching straight line up the side of the bottle.
3. Do the same for the other two lines.

• Three lines on the bottle, 120 degrees apart

Step 5: Make and glue the fins#

Fins keep the rocket stable. They move the center of pressure toward the tail. For a rocket to be stable, its center of gravity needs to be closer to the nose than its center of pressure.

The video uses a simple shape called a clipped delta.

1. Decide how high up the bottle the fins go. Approximate it, and draw a line around the bottle at that height so all three fins sit at the same height.
2. Draw one fin on the foam core with the carpenter's square. Mark a 4" point and a 3" line, draw a reference line, then draw a 45 degree line from the 4" mark down to the 3" line. That is the clipped delta shape.
3. Draw two more fins the same way. Leave some foam between them so cutting one does not damage the next.
4. Cut out the three fins with the box cutter.
5. Put hot glue along the edge of one fin that touches the bottle, and press it gently onto one of the lines. Hold the carpenter's square against it so it stands straight up and down. Wait for it to cool.
6. Rotate the rocket on the stand and glue on the next fin the same way.

• Three matching fins cut out
• Fins glued on at the same height, standing straight

Step 6: Check the balance#

Once the rocket is fully assembled, find its center of gravity.

1. Tie a string around the rocket. Slide it along the body until the rocket hangs close to level without you touching it. That point is the center of gravity. A few millimeters of error is fine.
2. The fins put area at the tail, so the center of pressure sits toward the tail. The center of gravity should be ahead of it, closer to the nose.
3. Do a preflight swing test. Make sure nobody is within reach. With the string attached, spin the rocket around your head quickly. A stable rocket points the way it is moving, like an arrow.

In the video, everything checked out stable and the rocket went to the launcher.

• Balance point found with a string
• Swing test passed

Step 7: Launch#

1. Fill the rocket half way with water, and set it on the launcher from the launcher tutorial.
2. Pump it up with the bike pump. Do not go over 60 psi.
3. Stand well back to the side and pull the string.
4. After it lands, check the rocket for damage before the next flight. The rocket in the video took some damage on landing.

Troubleshooting#

The rocket wobbles or tumbles. The center of gravity may be too far back. Add weight to the nose, or check that the fins are straight, even, and all at the same height.

The rocket flies too far for your space. Launch from a bigger open area, or use less pressure. Stay under 60 psi.

A fin comes off on landing. Foam core and hot glue are easy to repair. Re-glue the fin along its line and let it cool. You can also try adding a thin strip of duct tape connecting the fin to the rocket bottle.

The fins are not 120 degrees apart. Go back to the stand. The lines on the wood set the angles, so make sure the rocket was not turned when you transferred them.