The combination of water, compressed air, and a plastic soda bottle serves as a great source of propulsion for a backyard rocket (assuming you have a big back yard). Typical versions of these rockets involve taping or gluing on the nose cone and fins which can be challenging to keep attached and often end up fairly crooked and wobbly. Why not try a different approach where you build a rocket frame around the soda bottle and don't rely on direct attachment to the bottle?
I made this at TechShop www.techshop.ws
Step 1: Materials & Equipment
Vector drawing software (I used CorelDraw)
Laser cutter (my TechShop has a Trotec Speedy 300)
1.25 liter soda bottle
Cardboard box (taller than your rocket, 16" in this case)
Hot glue gun
This instructable assumes some basic familiarity with CorelDraw and a laser cutter.
Step 2: Measure Bottle for Key Dimensions
First you need to understand the dimensions of your plastic bottle. A set of large calipers makes this much easier, but a ruler will work just fine with some patience. If you have to err one way or the other, tend towards the larger side as the bottle will expand some when pressurized and you don't want the frame to be pushed or bowed out.
I converted these measurements into a series of reference rectangles. The height of each rectangle matches the key vertical dimensions in the hand drawn diagram. I added a rectangle to the top where the nose cone will go. I also added a rectangle to the side of the bottom where the fin will go. I decided that I wanted the long part of the frame (that runs most of the length of the bottle) to be 0.5" wide, and that became the basis for the width of the other rectangles. If you squint you can see that these rectangles represent a very coarse outline for one side of the rocket.
Step 3: Design Vertical Side Pieces
Now you can use the various intersections of the reference rectangles to start adding curves. For most of the curves I used the simple 3-Point Curve tool. For the nose cone the shape wasn't looking quite right, so I used the B-Spline tool which lets you add more control points to the curve. This led to a smoother transition from the long vertical section into the nose cone, as well as into the tip of the nose cone.
Once you have the desired shape, use the Virtual Segment Delete tool to remove the unnecessary reference lines. If you haven't already, change all of the lines into red lines with a hairline width to indicate that these are cut lines and not engraving lines. The final step is to add a slot at the top and bottom areas where the support discs will go. This is where the vertical and horizontal pieces will interlock to hold the assembly together. I measured my cardboard and it was 0.15" thick so I made my slots 0.16" wide so that there wouldn't be too tight of a fit.
Step 4: Design Top Support
The top horizontal piece consists of a round disc with four slots to interlock with the four vertical side pieces. Referring back to the design of the sides, you can see that this disc intersects after the shoulder of the nose cone (where it is already narrowing). The radius of the nose cone at that point is about 2", so you should create a disc that is a little smaller to make sure it doesn't protrude after assembly (due to tolerances). This disc is 3.8" in diameter, and you should add two perpendicular rectangles to represent the slots. These should be 0.16" wide to allow the 0.15" thick cardboard sides to slip in reasonably. I also obtained the measurement for where the slot starts on the sides from the other design file. To make it easier to draw I added a reference rectangle centered on the circle. This makes it easy to use the Virtual Segment Delete tool to remove all of the extra line segments.
Step 5: Design Bottom Support
The design of the bottom support ring starts out similarly to the top disc, but with a hole in the middle through which the neck of the bottle will pass. I decided that I didn't want a complete ring at the bottom for two reasons. It adds aerodynamic drag, and it looks a little funny. So I added three reference rectangles that let me uniformly scoop out a section of the ring between each set of fins. The scoop is created with the 3-Point Curve tool and the appropriate nodes and intersections on the reference rectangles. Finally use the Virtual Segment Delete tool to get rid of all of the unnecessary line segments.
Step 6: Play with the Laser
Off to the laser for some very fast cutting (cardboard doesn't take much to cut). Here I set the laser to 100% power and 1% speed. It turns out that was a little slower than I would have liked and left some smoky residue on the back side of the pieces. Had I not run out of cardboard I would have tried a faster setting for a nicer finish.
Step 7: Assemble and Hot Glue
Since all of the pieces are appropriately slotted, they go together quickly and hold themselves roughly in place. Assemble them around the bottle (which is about to become a permanent part of this rocket). Using the hot glue gun, apply a bead of glue to all of the seams that you can reasonably reach. If you want a less permanent solution, and perhaps an easier way to make repairs, you could tape all of the joints. Hot gluing is faster and I plan to just cut out another cardboard frame as necessary.
BE VERY CAREFUL NOT TO HIT THE BOTTLE WITH THE HOT GLUE OR THE GUN ITSELF!!!!
The heat will weaken the bottle (even if it does not put an actual hole in the bottle) which can lead to a frightening rupture on the launch pad.
Step 8: Go Launch Your Rocket!
The basic rocket is now ready for launch! Building a launcher is outside the scope of this instructable, but there are lots of online references for that. For even better performance you can improve the rocket with an actual aerodynamic nose cone. The simple way to do this is wrap the nose cone area with tape (making the three dimensional nose cone shape). If you want to get fancy you can first fill the cavities with expanding foam, shape them with a knife, then wrap the nose cone. As a final note many school physics classes have rocket building challenges often involving adding a raw egg payload that has to survive the flight. This same design can be adapted to a detachable nose cone with an area to hold an egg capsule and a parachute, but I'll leave that up to the students to experiment with. :-)