[Note: This post was featured on Hackaday on 3/8/2016]

I believe they said it best in Anchorman:

“They’ve done studies, you know. 60% of the time, it works every time.”

There was a great post on reddit today entitled “What happens to PLA that has been sitting in water for 3.5 years? – An accidental experiment” making the case that PLA does *not* break down in water.

However, my experience is that PLA really does break down under some conditions.  So, what’s the deal?  Turns out, I have an accidental science experiment of my own!

1) The case *for* biodegradability:

In March 2015, I 3D printed a filter basket from PLA for use with my fish tank filter. I wasn’t happy with the first one, so I replaced it a week later with a slightly shorter one.  I never threw out the first one, so that set the stage for an interesting science experiment.

Here they are, side-by-side, a year later.

PLA after 1 year in fish tank (left) vs 1 week in fish tank (right)

The one on the right is just dirty – no breakdown at all. The one on the left, though, has become faded, porous and brittle, and parts of the honeycomb walls are starting to crumble.

closeup of degraded PLA

That being said, this is just about the most conducive environment possible for PLA bio-degradation.  After all, a fish filter is *designed* for optimal biological breakdown — lots of fresh oxygen-rich water circulation, high levels of nitrifying bacteria, and a constant temperature of 80F/26C — so this degradation should come as no surprise.

2) The case *against* biodegradability:

Lest you think the case is closed in favor of biodegradability, I have some conflicting evidence.  At about the same time last year, I printed and installed this end cap for a filter spray bar. As you can see, it is dirty but undamaged after a year in the fish tank:

PLA dirty but undamaged after a year underwater

So, what’s the difference?  In terms of materials, this one is a different brand of PLA, and obviously uses a different pigment.  All of these prints are from cheap generic PLA from ebay so I can’t say anything about their purity, chemical composition, or additives.

There are also environmental differences between the two results.  This cap spent its year in the tank near the surface of the water, exposed to light, with lower levels of bacteria and a generally lower water flow rate.

Conclusion: PLA sometimes breaks down in water

Based on my experience, it’s hard to make any definitive statements about whether a particular 3d printed part will break down with exposure to water, and if so, how quickly.  Clearly, it *can* break down, but at least *sometimes* it does not.

Further investigation is needed to say how environment factors (temperature, light, biological activity) and/or chemical makeup of the filament (brand, pigments, processing additives) influence these different results.

Further Reading

That sums up my experience with PLA in a bacteria-friendly aqueous environment.  You might want to check out this post on Lazy Gecko Blog to see a different experience in a very different environment.

In Part 1 of this post, I presented the math behind a rocket nozzle.

Here, in part 2, I’ll show you how I turned that math into a 3d model!

First, the fun stuff… I posted my code to Thingiverse at http://www.thingiverse.com/thing:280483 where you can play with it live via the Thingiverse customizer.

How does it work? Well, first, I built an OpenSCAD script, using the math from Part 1 to build a 2D shape like this:

full2dNozzle(Rinlet = Rinlet, Ain = Ain, Rthroat = Rthroat,
             Rexit = Rexit, Aout = Aout, pct = pct);

This shape is actually two half-nozzles, one a conic nozzle and one a parabolic nozzle. Here, the inlet arc is 1.5 * Throat Radius, and outlet arc is .382 * Throat Radius, as shown in Part 1.

Once I have defined a function ‘full2dNozzle’ to draw the 2d shape, it is a one-liner in OpenSCAD to twist and extrude it into 3d:

rotate_extrude() rotate([0, 0, -90])
         full2dNozzle(Rinlet = Rinlet, Ain = Ain, Rthroat = Rthroat,
                 Rexit = Rexit, Aout = Aout, pct = pct);

This is part 1 of a multi-part post about rocket nozzles.

I have been kicking around some rocket nozzle ideas for a while now, and now that I have a 3d printer, I actually have fabrication capabilities at my disposal. Granted, plastic filament is not a very practical material for rocket nozzles, however I have never practicality get in the way of good fun.

I was going to wait a while before posting my work, but I see people collaborating online at github/thingiverse/reprap forums working on a similar project, so I figured I’d draw a line in the sand and share what I’ve got so far.

In part 1, I will lay out the sketches of the nozzle that I set out to build — a de Laval converging-diverging nozzle using G.V. Rao’s geometry — and develop the equations that describe such a nozzle.

The idea behind a converging-diverging nozzle is that exhaust gases will accelerate through the converging cone, reaching mach 1 — the speed of sound — as they pass through the throat. Once past the constriction, the gases will expand and further accelerate and become supersonic. The final velocity reached depends on gas thermodynamics and the exact geometry of the nozzle, but today’s rockets routinely achieve exhaust velocities of several times the speed of sound.

The geometry underlying the nozzle is fairly simple — a cone, two arcs, and a parabola, as described by G.V. Rao in 1958. See section 2.4.1 and 2.4.2 for some excellent bedtime reading about Dr. Rao’s results: http://www.scribd.com/doc/58809639/13/Bell%C2%A0nozzle

In order to model this nozzle in OpenSCAD, I first needed to determine up a few key points mathematically, so I threw together a more detailed sketch suitable for reasoning about the trig:

Here it is again, with the details worked out for the cone-to-throat transition:

And here are the details for the inlet cone itself:

The arc transitioning out of the throat is set up the same as the arc in, just with a different radius.

The arc out of the throat needs to join smoothly with the parabolic expansion section. Mathematically, that means that both the arc and parabola are tangent to the same line at the transition point. I admit, I was stuck here until I saw github user buback note that a quadratic bezier curve forms a nice parabola. I adapted the algorithm he found to use a trapezoidal approximation of the curve.

The last bit of math is to find point P1…

And that’s it! If you followed this far, you paid attention in High School Trigonometry! Wish you paid more attention? Khan Academy has great free tutorials!

In Part 2 I will use all of this math to design an OpenSCAD script to build 2d and 3d models of the nozzle!

One of many flavors of the popular Mini DV 1280. I got mine on Amazon for about $10

I just got a new “Mini DV 1280” video camera for use with various flying machines.

The camera is PERFECT for onboard video on a Syma X1 Quadcopter — tiny size, decent video quality, amazing price — but the instructions are a bit lacking.

After spending an hour deciphering the cryptic instructions, I decided to clean up my notes and post them as an infographic. So, here is my Visual Instruction Guide to the Mini DV 1280 Video Camera, published with hope that someone else will find it useful.

Visual Instruction Guide for the Mini DV 1280 Video Camera

1) Turn the camera on by hold “on/off” for 3 seconds

2) Cycle through the three main modes by pressing ‘Mode’. You can identify the current mode by looking at the blue and red LEDs. For example, a solid Red light indicates “Standby” mode.

3) The “on/off” button takes a different action depending on the mode you are in. For example, in Standby mode (indicated by a solid red LED), the “on/off” button will start recording (indicated by flashing red LED). See the table and/or infographic above for details.