PowerLabs Bottle Rocket Car

 Introduction:

 Photo Description:

1-      All the materials and tools required for building the pop bottle car:

Materials:

�� balsa wood sheet (4x48�)

1/8� balsa wood sheet (4x48�)

1/2:� Balsa wood square (48�)

4 lightweight model airplane foam wheels (1� dia)

8 wheel collars

steel wire for the wheel axles

Tools:
X-acto knife (the picture shows a knife set. All you need is one sharp knife)

Marker

Steel ruler

Not shown: Superglue and epoxy (I used 3 minute, 2 part epoxy).

2-      Step1; remove the plastic ring around the bottle neck. It�ll get in the way otherwise.

3-      Step2: Mark the size of the car on the �� balsa. You can see the front bumper, the nose, and the bottle compartment. Remember to give yourself at least �� between the back of the bottle and the bottle stop otherwise you won�t be able to slide the bottle in and out. I usually cut a �� thick Styrofoam sheet and use that to lock the bottle in place afterwards. This wasn�t necessary on this car because the bottle cap holds the bottle in place.

5-      Cut the �� balsa to size. Remember to use a very sharp knife and take several cuts. Don�t score the wood and snap it; that will make it splinter.

6-      Cut two 4� pieces from the 1/4� balsa; one will be the back and the other will be the front of the car.

9-      Mark the hole that the bottle mouth will fit through.

10-  Drill the hole for the bottle mouth. Start with a small drill and work your way up. I finished mine off with a dremel cylindrical sanding bit. If you have a dremel tool, it will make everything a lot easier J

12-  Glue the back of the car in place with superglue (Cyanocrylate adhesive). Make sure it is straight.

13-  Cut a 4� length piece from the � balsa square and glue it behind the back of the car. This will provide added support to it, since all the thrust of the bottle is transferred there.

14-  Glue the front piece in place. Make sure the bottle can slide in and out freely.

17- Cut two 1� tall, 12� long pieces of the 1/8 balsa and glue them to the sides of the car, between the back and front bottle supports. These will strengthen the structure and prevent buckling.

19 / 20 � Assemble the axles. Make sure the wheel collars sit flush with the car body and that the wheels are lose enough inside the axles that they can turn.

21 � Mark and cut arches on the back and forward bottle supports. The edges do not add strength but they do add weight.

25- Cut a 4� piece of 1/8� thick balsa and glue it between the nose (right behind the front bumper) and the front bottle support. This will triangulate the front and give it strength. It also helps with the aerodynamics.

26- I closed the gap between the nose and the body with tape. Tape is light and looks OK once painted.

27/28- Test fit the wheels, make sure they turn freely

29- Cut a slot on the body where the wheel axle can fit.

30- Put epoxy on the axle and hold it down with tape. SUPERGLUE WILL NOT GLUE STEEL TO WOOD. As the epoxy begins to dry, roll the car back and forth carefully; adjust the axle if the car doesn�t track straight.

34- Paint the car. It is ideal to sand the balsa lightly with some 200 grit sandpaper, apply pore filler to it, sand again, then paint. I just sprayed it with automotive primer and used 2 half empty cans of different car paints because it was all I had an was in a hurry ;) Oh, the last layer on the body is automotive clear coat. Still looks like crap, but at least if you spill something on it won�t be ruined. I recommend varnishing the wood regardless; it will add strength.

 Last: Drill a hole on the bottle top for the nozzle. Make sure it is centered. It helps to make a guide dimple with a nail and work your way up gradually with increasingly larger drill bits.

 Tips:

1-      These are just guidelines; you can make the car whatever way you want it as long as it holds a bottle and rolls on wheels. Actually, it doesn�t even have to roll; on a smooth enough surface you could make a sled out of this.

2-      Make everything as light as possible.

3-      The nozzle size will depend on the kind of fuel you are using. � is a good start.

4-      It is very hard to get the air/fuel ratio right. If it doesn�t work, try venting it a little bit (give the bottle a couple squeezes for it to �breathe� and then try again).

 The science:

 The bottle is working as a rocket engine; when a small amount of fuel is put inside it, it mixes with the air to form a flammable mixture. This mixture, when ignited, will burn rapidly producing a change in pressure. As the excess pressure jets out of the nozzle, it creates thrust (action and reaction). The fuel will only burn for a second or so, but it takes a little bit longer for all the gases to exit. Once the excess pressure is relieved the remaining gases cool down, creating a partial vacuum; this causes the bottle to collapse. The bottle kinks because it collapses warm, but it will still work for several other tries.

 If the nozzle size is too small the gases inside the bottle will begin to cool down before all the excess pressure is released, and energy will be lost. This results in low efficiency.

 If the nozzle size is too big the exit velocity of the gases will be too low resulting in little thrust.

 The nozzle size has to be matched to how quickly the fuel burns.

 A few fuels that work very well:

 Alcohol; It is an oxygenated fuel so you can burn more of it in the same space. It also has a wide flammability range making it easy to ignite.

 Hydrogen: Very wide flammability range in air, and very fast burning. One of the very best.

 Butane: Works well but its tricky to get the air fuel ratio right.

 Hair spray: Essentially alcohol.

 Carburattor starter: Ether. Works too well; can be dangerous. It burns very fast and over a very wide range of air/fuel ratios.

 Whatever you do don�t try acetylene. A lot of hobbyists will have an oxy acetylene torch at home and filling the bottle off that will make a bomb.

 More to come; I have to get to class! Enjoy the video!

 The Car: