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Comment by Animats | Hacker News Reader

Space-X makes the Dragon engine block by 3D printing.

It makes sense. Rocket engines are mostly a big single piece with lots of internal voids. The fuel is used as a coolant, so there are channels inside the engine bell and some other sections. It's mostly plumbing.

That's the kind of part where 3D printing is useful. Making a big object with internal plumbing out of multiple parts is a fabrication headache. NASA's engines have vast amounts of welding work in them. High-pressure joints are always a problem. Making something with few or no joints is just better. It still has to be inspected, but that's what industrial-sized CAT scanners are for.

(Most of the real-world problems in both rockets and nuclear power involve plumbing and welding.)

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Sure, but on the other hand in 3D printing everything is a joint/weld.

Am I correct to say that means you only need to get it right once?

Once or millions of times depending how you count.

Yeah, but a good weld is stronger than the metal around it. Win!

"It makes sense" in a way yes

In other ways it doesn't (depending on the techniques). Things like metallurgical resistance, defect creation and tolerances

Elon recently noted that Raptor (unlike SuperDraco) won't be fully 3D-printed. It will have 3D-printed parts, but most of the engine will be machined. I guess for a larger engine the things you mentioned have to be more tightly controlled to be able to print the whole engine. Might also be a cost issue as size increases, I guess.

Also, build size and volume is still a constraint in metal 3d printing these days.

wouldn't it be possible to use a cnc machine, with maybe a robot arm to mill the inside too?

It's definitely possible to use a CNC machine to mill the shape of a rocket engine. You can do some pretty crazy internal milling with a 5-axis machine:

https://youtu.be/RnIvhlKT7SY?t=14s

But the problem isn't the shape of the bell at all. It's that the wall of the bell is not solid, but hollow, filled with plumbing.

http://heroicrelics.org/info/f-1/f-1-thrust-chamber.html

The fuel actually flows through the pipes and voids in the red-hot engine, where it absorbs the heat that would otherwise melt the engine. It then flows back up to be burned.

You'd need an endoscope, not a robot arm, to mill that out!

And neither an endoscope (obviously) nor a robot arm would likely be rigid enough to mill in these materials. Robot arms are typically only used when milling wood, plastic, or soft modeling materials. A CNC mill in metals like the exotic Nobium alloys used in these engines needs incredible rigidity which robot arms just don't have. Consider the hardened tool-steel shafts of the end mills used in the first video - and those are easily broken, while just cutting aluminum!

EBM would work if the tool end could be made sufficiently tiny.

I think you mean EDM, electrical discharge machining. Typical wire EDM usually needs a hole all the way through the material to start, but sinker EDM would work. Interesting idea, wonder if anyone's tried it.

There are edm milling setups. I'm not sure what the major constraints are but I'm assuming the usual: accuracy, cost, repeatbility.

yeah, i was thinking more of an endoscopic robot arm, it would probably need quite a few entry points, and the those holes would need filling again, you could tap those and put a screw it afterwards.

but maybe it's the coming towards end of subtractive manufacture.

I don't think that would be practical. I assume the volume to be milled out is pretty convoluted if you want yo do it mono-bloc. If you break it down, you're back to welding issues.

That said, I'm not a rocket scientist.

I just had an image of a bunch of insect-like robots crawling over and through a giant metal block, chipping away at internal spaces with precision cutting tools to produce the desired shape, carrying away waste material, like an robotic ant colony digging a smooth steel nest.

they wouldn't have to carry anything, you can blow air into the cavity to get the material out. they could have wheels and drive up and down the cramped cavity with a drill on the end. in the end I think additive has won.

You can print voids then flow mill it? https://en.wikipedia.org/wiki/Abrasive_flow_machining

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