Working on a 3d printed water rocket launcher.

Right. A moisture sensitive filament that has been out for almost a year, and advertises great tensile stress characteristics. This won’t help layer bonding much, or use around water. At the end of the day, fdm printing just won’t hold pressure on its own, because the layer adhesion is the biggest strength limitation.

I’d suggest getting a two part urethane resin @Cameron_Spiller and trying a technique similar to roto-molding, where you pour a small amount of the liquid into your chamber and continuously rotate it through all axis to coat and make a shell. The print would then be able to provide more mechanical strength, without having to withstand a uniform, 100 pounds-per-square inch (!)

Good luck!

@Anthony_White apparently you need to refresh yourself on the meanings of “like” and “similar”.

How we got to PET-G, ongoing research.

For now @Cameron_Spiller try your PET-G with a slightly higher than usual print temperature (245-255) and a much slower print speed limit. Like 25-30 mm/s, and no cooling fan.
Also experiment with different infill patterns and densities. They are certainly going to be a factor in the strength.
If using Cura, I’d try Concentric 3D at 80% infill. Enable “Extensive Stitching” to catch any hoes in the mesh, and go to a thicker Z layer height to minimize the number of layers to worry about. At least above .2mm Maybe .26 ish. Definitely use a multiple of your printer’s “magic number” or one from it’s magic number list if anyone has pre-calculated one for that printer (likely).

Oh, and definitely bake your filament before you use it to drive out moisture that can lead to steam pocket holes during printing, greatly weakening the structural integrity.
60°C for a few hours.

O-ring not sealing in the PLA nozzle🤔

@Cameron_Spiller Don’t feel too bad…better rocket scientists than you have had trouble with their o-rings. ;’)

@Christopher_Gaul sadly very true.

o-ring seals usually spec a surface roughness outside anything any fdm printer can achieve. You may be able to sand or smooth the groove?
@Christopher_Gaul no need to be rude over our clear disagreement - I’m open to being proven wrong. (and like and similar are synonyms, go figure.)
Just relating my own extensive experience trying solutions to the same problem (sealing pressure) with printed parts. While 910 wasn’t out, I used several other Taulman materials with similar speed and over-extrusion settings, all to know avail.

@Cameron_Spiller I reread some of this thread and I really want to point you to @Christopher_Leger 's suggestion that you look into annealing your PLA. I think this might get you what you’re looking for. Or at least end up being a key part of a multi-part solution.
Also there’s the option of gentle heat gun application to finished parts.

Oh, and check out this Hackaday post and the comments.

Print with 100% infill, higher heat, and try again. It should work. I saw you say “5 top and bottom layers and 4 perimeters” and I was all like “OHHH Now it all makes sense!” Seriously dude, try it again. Report back your findings. Godspeed.

Seriously, I see some guy saying to print at 80%. Don’t do that. You need 100% for this to work. Do it.

@Adam_Mizban You don’t just go straight to 100% infill. This is not like using the fill tool in a paint program. It’s 3 dimensional and algorithmic and infills higher than 80% can cause more problems than they solve depending on a bunch of other factors. Also, infill doesn’t affect the walls, which are always 100% filled.
In most cases 80% is equivalent to what you think 100% infill means.

The problem is the print time and the amount of filament that high density would require, kind of makes the concept unattractive.

I’m sure I could get this to work, even just with coating with epoxy… It is strong enough, it’s just the natural porosity leaks at high pressure.

What I’m looking for is a design that prints quick and easy and works well and safely with no extra process. Particularly wet processes.
Something I can share on thingiverse, and anyone with a reasonable printer can print and launch on the same day with no messing around.

The problem I have is that there is a very small duct required to connect from the air fitting to the launch tube. This duct is what needs to hold air, not the body itself. The body only became pressurised because the duct leaked into the body.

My temporary solution was to drill the duct out and line it with a short section of pressure hose.

This works perfectly and the launcher has taken at least 20 launches with no failure so far.

Id love to prove that a 3d print can take the pressure, and perhaps that is the next project, however I believe the issues I am having can and should be addressed in the design.

The o-ring issue I have addressed by going back to the normal method of sealing on the bottle, not on an extended printed nozzle.


I really don’t think you need 100% infill. If anything I think it’s more about exploring infill patterns to find one that gives the structural strength you need with the least amount of infill, and weight, which is also an issue for rockets. Almost everything in the aerospace industry uses honeycomb “infill”.

If I were you, I’d create a smaller test object which is essentially a sphere shaped bottle (basically something that looks like a cartoon bomb about the size of a tennis ball) with a connector you can use to pressure test it with different print settings using a pressure gauge. Even a quality bicycle pump with an built in gauge.
They have brass adapters that go from a garden hose type threaded fitting to a schrader valve. This would be perfect since you can readily get o-rings for both the valves and the hose fittings.
Then just keep printing these with different print settings and document what settings reach what pressure, and hold it for how long.

Basically this is your test object, but with a garden hose thread on the top.

Then you can experiment with walls, wall thickness, infill % and type, etc.

@Christopher_Gaul like the cubic infill in slic3r. It’s incredibly strong for very little material.

It’s also very strong in all 3 dimensions, unlike honeycomb.

Cubic isn’t used in industry because you can’t mass produce it.

The part that started my discussion is easily withstanding 100psi, other than the angled “terraced” surfaces leaking.

I think, and now I’m intereted, that a shape with vertical sides and horizontal top, has more chance of success in holding pressure without bleeding because it has complet layers.

A spherical shape will be worse due to the maximum number of incomplete, terraced layers… If that makes sense.

In terms of pressure testing, we do a heck of a lot of that in my industry. It’s very hard to see a fine bleed with a gauge. A slow leak can take a very long time to move a pressure gauge needle… Best way is to set the pressure with an accurate regulator, so the pressure is constant regardless of bleed, and then drop it in water. Even the finest, slightest bleed will either show as a stream of bubbles, or a growing bubble.

Creating a pressure vesle by 3d printing was not the original plan for this project… But now I want to know. I might do some tests and start a new thread.

To clarify one point, I have never seen cubic used in industry (non 3d printed) because it is more difficult to make. But honeycomb is very common because it is easy to manufacture.