This is a classic toy from the 1950s (given away free in cereal boxes) that you don't see around much anymore.

I remember my dad getting me one when I was a boy in the 1980s, but haven't really seen them around since.

After doing some research, here is a really good comprehensive website dedicated to them:

http://torgo.org/bpsubs/

On this website it states that there is no 'clear answer' as to how to make your own baking soda submarine. Well, now with 3d printing technology, the answer is simple - model and print it!

Date Made: June 2013
Approx Cost: (Cost of printing?)
Approx Time: 3 hours modelling (plus printing time)
Difficulty: Easy/Medium

(Note: image taken from http://jamesrising.com/2011/04/23/what-wine-goes-with-fruit-loops/ under fair usage)

Step 1: How to Operate

First fill a container with water (about 6 or 7 inches).

Next You fill the small compartment of the submarine with baking powder and put the lid back on. These toys use baking powder not baking soda. (I presume it is the same terminology in the USA?)

Then launch your submarine - place it in the water and give it a quick shake, ensuring that the lid doesnt come off. The submarine will sink. After a short amount of time it will float and then it will repeatedly sink and float.

Here is a video showing you one working:

http://youtu.be/XS4HSPCjC28

(Note: image taken from http://www.inlieuofpreschool.com/submarine-science-for-kids/ under fair usage)

Step 2: How It Works

The sub should naturally sink in water.

Baking powder is a mixture of cornstarch plus powdered acids and bases. The baking powder mixes with the water and creates carbon dioxide gas, by way of a chemical reaction.

This gas builds up until there is enough to make the submarine more buoyant and hence causing the submarine to float.

The submarine will get to the top where the gas will be released and so it sinks again.

And then the process it repeated.

(Note: image taken from http://makezine.com/26/toyinventor/ under fair usage)

Step 3: The Model

I have built a 3d model of a submarine with the correct holes, compartments and a lid, hopefully ready for 3d printing.

The model is approx 130mm x 20mm x 30mm

This was done using tinkercad and can be found on their website by searching for "baking powder diving submarine" or here:

Please feel free to copy and tinker with it yourself.

I guess really this should be classed as a prototype, as I am unable to print, test and optimize.

Obviously the material used and the size of it (whole submarine and bubble chamber) will affect the buoyancy and hence how long it takes to float/sink etc.

Step 4: Optimization

As I said, I do not have access to a 3d printer (hence why I would like to win one), so am unable to try it out and to optimize the design.

If this model does not work very well, the next step is to analyse, evaluate and adjust the model.

Depending on whether the submarine is not floating or not sinking, then a number of alterations can be made to alter the buoyancy:

1. Make the bubble chamber smaller/bigger (by making partition thinner or thicker).
2. Make the submarine wall thinner/thicker.
3. Make the triangular 'base' smaller/bigger (wider/longer).
4. Make the holes in the body smaller/bigger (top and bottom).
5. Move triangular base and turret along sub length (to balance sub).
6. Alter ratio of powder area to bubble chamber (by moving partition up or down).

The aesthetics could be changed, but I would do this before optimizing, as it may change the buoyancy etc as well.