Space Available

Selecting batteries or even solidifying the electronics means figuring out the maximum dimensions of any given rectangular shape that will still fit within the tube.  If only the diameter is limited, then a variety of rectangles may fit.  Time to pull out some geometry…

Geometry
Geometry calculation

Thanks to a particularly good website for batteries, a complete set of dimensions for a variety of batteries is available.  The equation above can show whether a particular battery will fit in the tube or not, based on its width and length (assuming “depth” extends along the tube’s axis). Performing some calculations with the existing central tube, electronics and two batteries, some options emerge:

  • using an alternative 12 V battery, at 3.8 Ah, measuring 7.6″ x. 2.8″ x 1.85″
  • stacking the electronics within a 5″ x 2.25″ area gives 2.68″ of available height for both the electronics and ballast (not much, but usable) 
tube sizing
Tube size calculations

Motor load measurements are still needed to confirm that 3.8 Ah is sufficient for propulsion.  The Arduino Mega and Motor Shield fit within an area 5″ x 2.25″, but the connectors and DHT11 sensor need to be cleverly stacked on top.  And 2.5 lbs of ballast needs to fit under the electronics within about 2.28″ of space, total.  

It’s either this or recalculating displacement using a longer tube, plus actually building it.

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