Baking Powder Submarine

What are Baking Powder Submarines?

While they came in all shapes, colors & sizes, they all worked on the same principle. When placed in water they began to dive & surface multiple times on their own.  The secret?  A pinch of Mom's baking powder (not baking soda) placed in a secret compartment.

After downloading & printing several designs I came across online, I found that none of them worked properly, regardless of the printing material used.  Some would tilt to the side & not orientate themselves upright once placed in water.  Some would not sink or float & some had gas chambers that didn't seal properly.  I therefore decided to take on the challenge & try to design my own in the hopes of uncovering the underlying physics behind a working baking power submarine. 

As part of a past baby boomer icon, the original subs were commonly included as premiums in Kellogg's cereal boxes.  Additionally, there were also other versions out there which were not submarines. They included Navy Frog Men, killer whales & dolphins.  There was also a boat variation (most widely known as the PT Boat), which didn't dive but propelled across the water surface using a similar principal.

Specifications

• Dimensions (LxWxH): 79 x 28 x 44 mm
• Weight: 17 grams (without baking powder)

Some Chemistry

Baking powder is made up of 47% (by weight) mono-ammonium phosphate, 43% sodium bicarbonate (baking soda) & 10% corn starch. When mixed with water the compounds react producing carbon dioxide gas.  IMPORTANT NOTE:  Baking soda & baking powder are not the same!  Baking soda will not react well on its own in water but baking powder will as it already contains the acid needed to produce CO2 gas.

How do Baking Powder Submarines work?

There is a chamber located inside the submarine which holds the baking powder. This area closes up completely, except for a small hole located at the bottom of the chamber. When placed in water, the submarine sinks. After a few seconds the baking powder reacts with the water to produce carbon dioxide gas. As a result, a bubble begins to form underneath the sub. As the bubble grows in size, it eventually provides just enough buoyancy to rise the sub to the water surface. Once the submarine reaches the top of the water level, the submarine tilts to the side resulting in the carbon dioxide bubble to be released and causing the sub to sink again.  As it goes down it continues to produce more carbon dioxide and the process repeats itself. The entire reaction can go on for some time causing the submarine to dive and resurface many times on a full chamber of baking powder.

Watch this video from the North Carolina School of Science and Mathematics to see it in action:  http://youtu.be/XS4HSPCjC28

How was it Designed?

I initially guessed at many of the physical parameters based on available videos, photos, etc. of the original sub.  Also came across several patents (see file section) which helped clarify a few things.  

I found that when designing a baking powder sub, it is difficult to achieve a construction that: 

1. Has just the right amount of buoyancy, allowing it to barely sink once placed in water;
2. Generate just the right amount of gas which allows the sub to rise to the surface, while maintaining it in an upright position;
3. Then upon reaching the surface, releases a bubble of gas sufficiently large so that the sub sinks again to the bottom;
4. and to finally have the cycle repeats itself multiple times over several minutes.  

After multiple iterations, I finally came up with a working model.  Not as aesthetically pleasing perhaps as the original sub but it's functional. I did, however, add some improvements over the original design; like a cone section that screws onto the baking powder chamber, thus forming a perfect seal and preventing the escape of resurfacing gas which is needed by the sub.  Also, the cone & chamber sections were designed to be removeable from the sub body so that someone can experiment with different shapes, weights & materials.

Design Criteria for a Working Submarine:

• The bubble chamber needs to centrally positioned to the length and width of the submarines main body & aligned with the upper conning tower.  In other words, the weight distribution ahead & behind the chamber needs to be the same for it to rise in a level fashion;
• The sub needs to tilt to the side once it surfaces to release the CO2 bubble from the chamber & re-enable it to sink;  
• The body of the sub should be hollow with an open bottom for weight reduction;
• The top deck requires equally spaced openings to allow any trapped air bubbles to
  escape from the sub bottom which could cause the sub not to sink. The openings also help in weight reduction.  
  • In order for the bubbles to escape the deck, the holes must not be less than 1.6mm;
  • Example: For a deck area of 484 mm2, the total hole area should not be less than 32.3 mm2; i.e. 7% of total deck area.
• The material used and the size of the submarine will affect buoyancy and how long it takes to sink & resurface.  With a density of approximately 1.04 g/cm3 for ABS and 1.34 for PLA, a sub built entirely of ABS may not sink and a sub built entirely of PLA might not rise, depending on infill, the size & shape of the sub body & the size & shape of the bubble chamber.  The sub body should be made of a plastic whose density is close or slightly less than that of water and the lower ballast should have a density which is more than that of water:
  • Ideal material for sub body would be polyethylene @ 0.96g/cm3
  • Alternatives:
    1. Polystyrene has a density slightly more than water @ 1.06 g/cm3;
    2. ABS is slightly better @ 1.04 g/cm3;
    3. Others:
       1. PLA @ 1.24 g/cm3           Too high
       2. PETG @ 1.27 g/cm3        Too high
       3. ASA @ 1.07 g/cm3          OK but ABS is better
  • The average density of the sub, including the sealed air chamber & lower ballast should be greater than that of water to allow the sub to barely sink;
• To permit the water to enter the lower chamber & yet not permit loss of too much baking powder, the hole size in the cone section needs to be optimized.  I found that 3mm was a good choice;
• The insertion of the chamber cap or cone should cause the compression of the baking powder to approximately ¾ of its original volume so to render the powder more susceptible to uniform wetting & a steadier generation of gas;
• The volume of the bubble chamber should be large enough to produce a suitable bubble & make the sub rise.  For example, a sub weighing 10 grams of ABS, the capacity of the lower chamber should be approximately 1356 mm3 (11x11x11 mm);
• The total sub weight, weight distribution, sub size & shape, design & location of chamber, holes, aesthetics, etc. all affected buoyancy & therefore all critical in designing a working sub.  Buoyancy is controlled in 3 ways & their combination forms a design which allows the sub to float, sink & remain in the upright position when placed in water:
  1. A sealed air chamber ballast above the sub deck which is fixed in volume;
  2. A CO2 gas bubble chamber located beneath the sub which allows the sub to rise in the water when the bubble is formed & then sinks once the bubble is released from the chamber;
  3. A lower ballast whose density is greater than water located beneath the sub & below the bubble chamber.  This could be incorporated within the bubble chamber end cap or cone.
• The upper air chamber & lower gas bubble chamber should be centered along a vertical line through the center of gravity of the submarine body;
• Optional:  Additional ballast could be placed below the sub which has a density greater than that of water.  This could be in the form of a rod printed in PLA.  The ballast positioning should be adjustable in the longitudinal direction to allow balancing the weight distribution along the body of the sub.  It also should be adjustable in the off-center position relative to the longitudinal axis of the sub body to allow the sub to tilt to the side once it breaks the water surface so that it can release the gas bubble.

Print Settings

• Printer brand:  Prusa
• Model:  MK4S
• Supports:  Yes
• Resolution:  0.2mm 
• Infill:  100% (no air pockets!)
• Brim: Yes - 15mm
• Filament brand:  Doesn't matter
• Filament material: Important!  Do not substitute! 
  • ABS for main sub body, bubble chamber & disc
  • PLA for Cone section only, 
  • FLEX for washer;
• Filament color:  Doesn't matter
• IMPORTANT NOTES:  
  • Print in an enclosure for best results; 
  • Keep infill at 100% otherwise your submarine may not sink or float properly!
  • Follow filament material recommendations for each part listed above in order to get a working submarine!

Assembly

• Slip the FLEX washer into the top portion of the Cone;

• Place the Cone Cover over to of the Bubble Chamber making sure the notches are aligned;

• Slide the Bubble Chamber with Cone Cover through the bottom of the Sub Body, making sure again that the notches are aligned;

• Now screw the Cone Section into the Bubble Chamber at the bottom of the sub.

How to Use

1. With the cone section removed, place a small amount of baking powder (not baking soda) into the bubble chamber, filling it half-way with a loose charge of powder.  Use a bread knife as a spatula to facilitate filling & reduce spillage.  The chamber will hold approximately 1 gram of baking powder;
2. With the chamber half-full, wipe off any excess powder that may have fallen outside of the chamber;
3. Fill the remaining half of the chamber with water then seal the chamber by screwing in the cone section.  Ensure the cone is screwed in snugly against the washer to provide a good seal;
4. Place the submarine in water with a depth of at least 200mm.  Tilting it sideways, shake it several times in the water then let it go;
5. The submarine will initially sink & after several seconds resurface, release a bubble, then sink again.  It will do this multiple times until it runs out of baking powder.

How to Refuel

• Unscrew the cone from the chamber & rinse out any remaining propellant from both parts;
• Shake & blow out any water out of the chamber & cone;
• Note:  Any water left in the chamber or cone before refueling may cause your sub to malfunction!
• Refuel as before. 

Please hit the like button if you think this is a cool Printable.  Thanks for your support!

Note:  All images are provided under fair usage.

References: 

• US Patent 2712710
• US Patent 4052812
• Torgo - Baking Powder Submarines