3D Printed PLA Hybrid Rocket With 3D Printed PLA Fuel

This may be the world's first rocket made of the 3D printed plastic! NASA, SpaceX, and others have been building 3D printed rocket nozzles and other parts for a few years now, generally using metal via the direct metal laser sintering (DMLS) process. I thought I might as well try to do the same kind of thing using a hobby plastic printer (FFF/FDM™). I also wanted to make as much of a whole rocket as possible out of 3D printed PLA. For my rocket, this currently includes the nozzle and fuel, and in the future could include the oxidizer tank as well. This instructable will relate how I went about this project, and should provide enough guidance for you to replicate my experiment.

Credits: Chris B, a member of Protospace, made suggestions and helped with the PLA burn tests as well as the rocket firing tests. Aleks R, Matt F, Nick B, and Steve T helped with the rocket firing tests and video recording of the same. waterside on Thingiverse designed the rocket nozzle.

Project venue: I worked on this project at Protospace, a makerspace located in Calgary, Alberta, Canada. I used our only constantly present and usually working 3D printer—others keep coming and going, or aren't running—an ORD Bot Hadron.

Prize: If I win a 3D printer in the 2016 3D Printing Contest, I will make it available long-term at Protospace (via either donation or long-term loan with agreement) for use by the members. I think the most suitable contest prize printers to enhance Protospace's 3D printing capabilities would be any of the Ultimakers, the DeltaWASP, the Witbox 2, the Rostock Max v2, the CEL Robox, and the Lulzbot Mini.

Following: This project also has a Hackaday Projects project page here, where you can follow this project to see any future progress I might make, if you would like.

Step 1: Decide on the General Design

When I came up with the idea of a fully 3D printed rocket, 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.

Step 2: Sanity Check

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 just performed a static thrust test, 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.)

Step 3: Obtain Supplies and Equipment

To replicate my experiment, you will need:

Equipment

• FFF (filament-based) 3D printer capable of printing with PLA filament
• Woodworking tools (or other tools as necessary if you use a different material for your thrust stand)
• Air compressor, air hose, and air gun
• Electronic weighing scale
• Face shield, or at least safety goggles
• Smartphones or other cameras that can record video (optional)

Supplies

• PLA filament
• BBQ igniter, electric flyswatter with wires attached, or other ignition source that you can somehow get inside the motor
  • Wire for the above
• Wood or other construction material for your thrust stand
  • A hinge
• Aluminium tape (the stuff that's actually used for ducts, which duct tape isn't)
• Sawdust
• Butane/naphtha/other volatile fuel

Step 4: Design and Print 3D Printed Parts

I used OpenSCAD to design my 3D printed parts. These include the air gun adapter, the fuel grain, and the rocket nozzle. The rocket nozzle is a customized version of Customizable Rocket Nozzle by waterside on Thingiverse, used under the 2-clause BSD License. I modeled the other two items myself; they are also parametric.

The fuel grain is just a single cylinder in OpenSCAD; the grain pattern is just the honeycomb infill. I used the following settings in Slic3r for the fuel grain:

• layer height: 0.35 mm (same as nozzle, for fast printing)
• top and bottom solid layers: 0 (to let air/starting fuel/combustion products flow through)
• infill pattern: honeycomb (to give a nice porous pattern for flow)
• infill density: 0.3 (guessing at what might work well)
• perimeters: 1 (might need more if it burns vigorously)
• random start points: yes (to avoid stress concentration)
• minimum layer time: 30 s
• raft: 2 layers, rectilinear, 2.5 mm then 0 mm spacing
• brim: no
• skirt: as usual

I used the following settings for the nozzle and air gun adapter:

• layer height: 0.35 mm (same as nozzle, for fast printing)
• top and bottom solid layers: 1 (just for a bit of strength)
• solid infill pattern: concentric (my usual for solid infill, even though it leaves a ring-shaped gap often)
• infill pattern: rectilinear (strongest and fastest according to tests)
• infill density: 0.1 (suggested to increase for strength and durability against dropping)
• perimeters: 3 (for withstanding hot gases)
• random start points: yes (to avoid stress concentration)
• minimum layer time: 30 s
• raft: no
• brim: 5 mm (for the upside-down one in particular, so it doesn't fall over while printing)
• skirt: as usual

waterside suggested printing two rocket nozzles at once, to give each layer time to cool, with one nozzle upright and one upside-down. I did that, with lots of separation between the nozzles, but the nozzle that I printed upside-down (i.e. bell facing up) didn't turn out well because of poor bridging performance. The overhanging parts on the insides of each didn't print well, but on the upright-printed nozzle, this was inside the bell, not inside the combustion chamber, so I think it will be okay. (I also coated that area with epoxy when I repaired that nozzle after it fell on the floor and broke in two during an early air test.)

Therefore, I would recommend printing two or more rocket nozzles at once, all upright (bell down, combustion chamber up), and spaced out on the bed.

Step 5: Modify BBQ Igniter

I bought a BBQ igniter at Home Depot. After testing it out—yay sparks!—I mounted it in a little piece of wood to make it easier to hold and press.

For the PLA burn tests, I used the electrodes it came with (which conveniently attach to opposite terminals on it) held by a helping-hands. (This particular one is from Protospace's electronics room. I cobbled it together from a couple of broken ones a few months ago. It has three hands and a magnifier now.)

For the actual rocket tests, I cut off the electrodes, extended the wires, and attached a spark gap made of magnet wire to the end. This needed to be narrow because I inserted it into the rocket via the nozzle, whose hole is only 1.5 mm in diameter.

Step 6: Test How Well PLA Burns

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 using 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. Protospace member Chris and I went out in the parking lot to test it. 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 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. (I'm pretty sure that this is the PLA burning, not the butane, because the tiny amount of butane Chris sprayed would have evaporated many seconds earlier.) 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.

Step 7: Assemble Rocket

The rocket motor is assembled by wrapping the fuel grain in aluminium tape, and inserting it into the air gun adapter and then the nozzle. The adapter and the forward half of the nozzle together make up the combustion chamber. The photos should make it pretty clear how it goes together.

It's a good idea to sand the inside of both outer pieces before assembling. This will make it much easier, and avoid scratching the aluminium tape.

Step 8: Build a Thrust Stand

The thrust stand is a bit outside the scope of this instructable, and also, the one I built was extremely crude/expedient—the hinge is the door of a toaster oven that some members cannibalized to fix theirs the other day, because that was the only hinge I could find here*—so it would not make for a very good tutorial. You can find a few tutorials on building thrust stands by searching for 'thrust stand' or 'measure thrust' on Instructables or various radio control aircraft forums.

However, as part of your stand, build a mount for your air source. This is to ensure that you can't accidentally influence the thrust readings by pushing or pulling on the air gun. Also, make sure your hose is as straight as possible between the air gun and the rocket, to avoid the Feynman sprinkler effect, which could also influence your readings.

My thrust stand mounts the rocket by clamping the front and back halves between V-grooved blocks of wood, which are then clamped together, and then the whole thing gets clamped to the the upper arm.

I inserted the igniter into the rocket through the nozzle, and I poked a tiny hole in the PVC tube to admit the butane starter fuel. (Later, we tried inserting the igniter into the front of the rocket motor, putting the fuel directly in before attaching the tube, and other things.)

*I should have printed this hinge (also a 3D Printing Contest entry). Too bad I didn't see it in time.

Step 9: Perform Air-only Test

To determine if your rocket produces thrust, you need to first determine how much thrust is produced by the air alone. So, do a thrust test with just the air running, without igniting your rocket. Also, it might help to put some kind of block on the air gun to set how far you can squeeze it, so that the air flow is consistent between the two tests. On mine, this purpose is served by the hose clamp that holds the air gun to the stand.

Step 10: Perform Actual Rocket Firing Test

Hopefully yours succeeds. We tried all sorts of things including different starter fuels (butane and naphtha), different igniter positions (in the back, in the front, in holes drilled in the sides), and a different igniter (electric flyswatter). The last test resulted in some flame inside the motor, but it didn't ignite the fuel. That's it for now—we might try again later, but the time is about up for this contest. Good luck!

Step 11: Epilogue: Aftermath of Testing

Here are some photos of the parts after the tests were concluded. 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 :)