The entire rocket and the fuel are all made of 3D printed PLA.
Credit: waterside on Thingiverse (http://www.thingiverse.com/thing:280483). Used under 2-clause BSD License (https://opensource.org/licenses/bsd-license.php).
scad - 10.56 kB - 06/28/2016 at 05:00
Just a cylinder. Print with honeycomb infill.
scad - 35.00 bytes - 06/28/2016 at 05:00
scad - 1.12 kB - 06/28/2016 at 05:00
Here's what the parts look like after the testing we did a few hours ago.
(The hot glue is because we drilled a hole to try putting the electrodes there, and later decided not to use it.)
Nozzle (damaged by Chris's torch only):
Inside the back of the combustion chamber, pretty clean:
Inside the front of the combustion chamber (after having to separate the parts with a vise and pliers), showing a bit of burnt sawdust and a bit of melting on the front of the fuel grain:
You can see there's not much damage—far less than would be expected from holding a blowtorch to a piece of relatively-low-melting-point plastic, while filling it with fuel and attempting to set it on fire. The damage to the nozzle was only from Chris's torch (which you can see him aiming into it in the videos). The front of the fuel grain shows a small melted area, which is the only evidence that it was close to igniting. However, given the blowtorch-like behaviour in the second burn test, and how well the PLA stood up to the thermal abuse we gave it, I think that the only thing standing in the way of this rocket's success is Ignition. I/we may try again soon (or not so soon) to ignite it in different ways. Suggestions are welcome :)
With the help of Protospace members Aleks R, Chris B, Matt F, Nick B the Shorter, and Steve T, I tested my rocket tonight around midnight. We tried all sorts of things including different starter fuels (both butane and naphtha, along with some sawdust), different igniter positions (in the back, in the front, and in holes drilled in the sides), and a different igniter (electric flyswatter instead of BBQ igniter). The last test resulted in some flame inside the motor, but it didn't ignite the PLA fuel grain.
I ended testing around 12:45 am (MDT) so I could submit to the Instructables 3D Printing Contest (deadline 11:59 pm PDT). I submitted about one second before the deadline, and it confirmed my entry, but the contest entry list doesn't include my entry, so I'm going to inquire about this. Hopefully it ends up getting accepted; Protospace really needs another good, useful, and reliable 3D printer.
Without further ado, here are the four test videos I uploaded. We did lots more tests, but nothing really interesting happened.
Last night, Protospace member Chris and I performed a test of how well PLA burns.
Before testing the rocket, I wanted to first test how well PLA would burn when its surface area-to-volume ratio (SA:V) was high. There have been videos showing how well PLA filament burns (drippingly), and MakerBot and others have warned against making 3D printed candle holders, but I had not seen any that showed it burning furiously as would be desired in a rocket.
So, I took one of the two rocket nozzles I'd printed (the one that didn't print well) and put some PLA bits (chopped-up skirt from printing the nozzles) into it, topped with some sawdust and butane to start it. Chris and I went out in the parking lot to test it.
The first few attempts didn't work—the igniter spark gap was too far from the sawdust.
The BBQ igniter ignited it successfully after several attempts, with Chris squirting butane as I squeezed the igniter repeatedly. It burned pretty well, but the sawdust and butane burned out before it got down the PLA.
We tried again, this time with forced air from below using the air gun (not actually the same air gun as used later in this experiment, not that it really matters). This resulted in the side of the nozzle catching fire, and it burned very well, with a blue flame like a Bunsen burner or blowtorch.
The actual PLA bits that I had intended to test didn't really catch fire much, but that's OK—the nozzle itself demonstrated that PLA burns strongly with forced air. This is quite promising for using PLA as rocket propellant.
I first came up with the idea of making a 3D printed rocket for the Instructables 3D Printing Contest. I wanted to make as much of the rocket out of 3D printed plastic as possible, to see how well it would work.
When I first came up with the idea, I was planning to make a solid-fuel rocket, which is the easiest to construct. However, I soon realized that this would require an oxidizer to be mixed in with the plastic, which would have been difficult. (I would have had to either pre-blend the oxidizer into the PLA filament, or blend it inside the extruder, both of which would be difficult and might damage the printer, which isn't mine.)
Therefore, I decided to switch the design to a hybrid rocket. A hybrid rocket has a solid fuel and liquid or gaseous oxidizer (or, rarely, a solid oxidizer and liquid/gaseous fuel). The advantages of hybrid rockets in this case are that the fuel and oxidiser do not have to be pre-mixed, and the oxidizer can be turned off to shut down the rocket, which is not possible with a solid rocket.
Initially, I had been planning to refill a spent CO2 cartridge with oxygen for the oxidizer, and to build the rocket into a 3D printed airplane. I found that it is possible to add a valve to a CO2 cartridge and refill it, though it is somewhat difficult. However, a stoichiometric calculation and an ideal gas law calculation showed that I could not fit anywhere near enough oxygen into the cartridge at any sane pressure. As well, I didn't have enough time before the contest deadline to design and print an airplane.
So, I am now planning to just perform a static thrust test for now, using our shop air compressor for the oxidizer source. (I have looked into 3D printed pressure vessels, and, quite surprisingly, they can be made pretty easily. So I might still make the rocket plane in the future, maybe for next year's contest.)