It's clear that additive manufacturing (3D printing) is the future of short-run fabrication. However, it's far from clear whether the mechanical properties of typical 3D printed parts are sufficient for high-power rocket construction. This article documents my experiments get some numbers by testing three hypotheses:
- 3D printed parts are weaker than those made of traditional materials.
- Traditional materials produce parts that are stronger than they need to be.
- By optimizing the material, design and printing techniques, we can make printed parts strong enough.
Clearly these hypotheses are somewhat contingent on each other, but I'm testing the first two in parallel because I think both are interesting in themselves. Of course, proving the third is the ultimate goal.
The first round of tests includes four different structures to test the absolute strength of ¼" (6mm) plywood centering rings (CRs) and bulkheads and the strength of a typical epoxied assemblies using them. I chose the most common HPR size: 98mm (3.9") body tubes and the CRs are for 54mm MMTs.
|Test 01||Test 02|
Break CR by pushing down on center.Test the absolute strength of a CR by pushing a lip on the inside against a lip on the outside.
Break epoxy bond by pushing on center.Test the weakest component of a bonded CR by pushing a lip on the inside of the CR.
|Test 03||Test 04|
Break bulkhead by pulling up on center.Test the absolute strength of a bulkhead by pulling a ¼" bolt/fender washer through a hole in the center against a lip on the outside.
Break epoxy bond by pulling on center.Test the weakest component of a bonded bulkhead by pulling a ¼" bolt/fender washer through a hole in the center.
Tests 01 and 02 simulate motor thrust being transferred to the body tube through the CRs. Tests 03 and 04 simulate parachute opening force where it's tethered to a bulkhead. Note that there are two series for Test 04: without and with and reinforcing coupler ring above the bulkhead.
The baseline parts are ¼" 5-ply "birch" plywood and the body tubes are Giant Leap flexible phenolic. All parts are for standard 3.9" (98mm) ID tubing. Epoxy bonds were AeroPoxy ES6209 with "gloved finger" fillets.
The baseline material was ¼" 5-ply "birch" plywood, and the 3D printed samples were various materials and printers. Note that the filament was purchased from the printer manufacturer and the prints were all run with default printer settings.
|Onyx + CF||Markforged MarkTwo||FFF||11|
|Tough PLA||Ultimaker S5||FFF||23|
|Tough 2000||FormLabs Form3||resin||40|
|PA12||HP Multi-Jet Fusion||FDM||50|
Note that all of these are FFF printers except for the Form3 which is resin and the HP MJF which is FDM. The HP MJF parts were purchased through Shapeways; the other parts were from printers I own.
Three samples of each series were tested in order to guage the variance in material and fabrication. (PLA was not used for CRs because of its low heat tolerance.) Each sample is identified with three numbers, for example "01-20-03" is:
- test 01 (CR absolute strength)
- series 20 (ABS on Ultimaker S5)
- sample 03 (3 / 3)
Here are some of the samples for tests 01 and 03 and the aluminum fixtures used for testing them.