There's a company called "Relativity Space" that's supposedly 3D printing rocket engines. https://www.relativityspace.com/

I am highly skeptical. A number of things come to mind: porosity, stress risers and crystallization. Like battery powered aircraft, I feel like this is a way of depriving ignorant investors of their money or giving them a write-off, and that the technology will fail for its marketed goal, but the R&D that goes into it will be useful for other things.

Cost-is-no-object 3D printing is quite amazing.  Aerospace companies usually do it to save time, rarely is it to only save money.  The material cost is similar - metal powder is much more $ per pound, but you waste very little of it.  The printing machines are very expensive, but a printed part needs much less machining to finish it.

Turbine engine manufacturers have been using it to prototype parts for many years.  You can print a part overnight instead of waiting weeks/months for the same part made by forging/casting.  To the best of my knowledge there are no engines with rotating or hot section printed parts, but what I know is at least 6+ years old.

Their website makes them seem like a jobs program that is a wanna-be space x.  The CEO is 7 years out of college and worked at blue origin for 2 years.  Seems their only claim to fame is copious use of 3D printing.  3D printing is hyper popular in the space industry because dreamers say you just ship metal powder to mars and then print whatever part you need.  Big dreamers with big $ backing.  It's not tax money, so I say good for them!

You are behind the times.

3D printing jet engines for airplanes
The 3D printing of both jet engine prototypes and end-use parts is already having a significant impact on development and production.

The GE90 series of high-bypass turbofan aircraft engines first entered service in 1995. Now, the next-generation GE9X from GE Aviation will use 19 3D-printed fuel nozzles to help power the next generation of wide-body aircraft like the Boeing 777X. The GE9X is the largest and most powerful jet engine ever manufactured — the front fan alone measures 11 feet in diameter. While it is approximately 10 percent more fuel-efficient than its predecessor, it still generates 100,000 pounds of thrust at takeoff.

In April 2016, Airbus took delivery of the first two LEAP-1A engines for its next-generation A320 passenger jet. GE Aviation and France’s Safran are equal partners in the engine’s manufacturer, CFM International. CFM has received more than 12,000 orders for the LEAP engine and its 3D-printed fuel nozzles. A manufacturing facility in Alabama will produce the thousands of nozzles required to fulfill the orders.

Functional prototype testing is often cheaper, simpler and quicker when AM processes are used. In late 2017, in collaboration with the United States Army, GE Aviation completed prototype testing of the Future Affordable Turbine Engine (FATE) engine. Additive manufacturing was key to the design and production of the prototype’s turbine rig. The FATE engine performs at heretofore unknown levels of efficiency and performance. Compared to current engines in the field, it reduces fuel consumption by 35 percent while cutting production and maintenance expenses 45 percent.

The ITEP engine is intended as a drop-in replacement for Apache and Black Hawk helicopters. The engine is to offer 25 percent fuel savings and a 35 percent reduction in production costs. The T901 prototype of the proposed production model makes use of functional prototypes of 3D-printed parts. The use of AM processes in the FATE and ITEP projects builds upon insights acquired in the previous development of the commercial LEAP engine.

When used to produce select lightweight, higher-durability jet engine parts, AM processes deliver a perfect trifecta of simplified design, reduced cost and shortened lead times.

3D printing airplane parts
In April 2015, GE Aviation received its first FAA clearance to use a 3D-printed part in a commercial jet engine. The sensor housing for a compressor inlet temperature sensor is produced by a selective laser sintering (SLS) system. GE contracted with Boeing to use the part in more than 400 GE90-94B engines to be used on Boeing 777 jets.

Jet engine combustors are notoriously difficult to manufacture using traditional means. Using an AM process with cobalt-chrome alloy powder as the build material, high-quality components are fabricated that protect the highly sensitive electronics in the temperature sensor from surging airflows and significant icing inside the engine. AM processes are also being used to fabricate other aircraft parts, including those used in ducting systems.

Additive manufacturing also produces sophisticated tools for the aerospace industry. In 2016, researchers at Oak Ridge National Laboratory (ORNL) fabricated a trim-and-drill tool that will help Boeing manufacture aircraft wings for its 777X aircraft.

The special tool is 17.5 feet long, 5.5 feet wide and 1.5 feet tall. The 1,650-lb device took approximately 30 hours to print from carbon fiber and composite plastics. The Guinness Book of World Records subsequently certified it as the largest solid object ever created on a 3D printer.

In one aviation and aerospace application after another, 3D-printed, lightweight products lead to energy savings, reduced carbon emissions and enhanced return on investment (ROI).

3D printing in space
In general, manufacturing for space applications requires a very high degree of precision. AM processes like EBM and DMLM excel at producing parts to close tolerances. A high degree of dimensional accuracy is attainable when layers are as thin as 20 or 40 microns, substantially less than the width of a human hair.

Examples of using additive manufacturing in space and to get to low-earth orbit and beyond are more prevalent than ever. At the Marshall Space Flight Center (MSFC) in Huntsville, Alabama, NASA researchers designed and printed a functional prototype of a two-piece rocket injector that met the performance parameters of its 163-part predecessor. The highly simplified 3D-printed injector meets strenuous mechanical property and hot fire standards, while reducing manufacturing and assembly times.

In November 2017, the Tubesat-POD (TuPOD) satellite completed its mission. The tube-shaped deployment satellite was the first 3D-printed satellite launched from the ISS.

AM experiments on the International Space Station (ISS) test the feasibility of 3D printing in space. In 2014, astronauts aboard the International Space Station printed their first plastic part on a 3D printer. Objects printed aboard the ISS were returned to earth to determine if microgravity impacted mechanical properties or other essential characteristics in any material way.

Print-on-demand technology could be pivotal in carrying out plans to travel to Mars. The capacity to print tools and replacement parts millions of miles from Earth is vital in the quest to manage risk and control costs. In some anticipated scenarios, some of the build materials for 3D printing would be mined from the Red Planet itself.

A fuel nozzle is very different from a fuel tank meant to hold liquid methane or LOX under pressure. I'm not saying 3D printing of metal parts for high temperature environments is a bad idea, but I'm skeptical of what else is implied: that 3D printing of parts that are under tension is possible.

You are intellicetually lazy.

I said it's common for prototyping due to speed and that I know of no spinning or hot components in production turbines that are printed.  
You googled and cut and pasted examples of exactly what I said.  Line by line above:

Fuel nozzles do not spin and are questionably a hot part.
Prototype.
Prototype.
Housing for an inlet temperature sensor is not hot and doesn't spin.  
Prototype.

Last sentence is exactly what I said.

The exotic printed parts have excellent mechanical properties.  Even metal from a mill can have defects.  A sizeable portion of the high cost of flight critical aerospace parts is due to testing and QC of finished parts to ensure they are OK.  Non destructive testing of every type imaginable gets expensive very quickly.  Just like MIM in gun parts, done correctly it can be more than adequate, done incorrectly it isn't.

The military is looking into 3D printing to make flight critical parts that are out of production.  I have seen 3d printed flight control parts tested.  If you can 3d print a part overnight you have no inventory/storage overhead, which can be a major expense for the number and variety of aircraft the military has.

Get your thinking away from plastic hobby printers that cost a few hundred dollars.  Modern printing can use nearly any exotic metal you can think of, has excellent mechanical qualities, and is an incredible technology.  The financial aspect is continuously getting better and it will only become more common in the future.

Aerojet is working with 3d printing some components for old shuttle RS-25's going in the SLS.

https://www.nasa.gov/exploration/systems/sls/nasa-tests-3-d-printed-rocket-part-to-reduce-future-sls-engine-costs

More about Aerojet 3d stuff here.
https://www.rocket.com/innovation/additive-manufacturing

They also are building a RL-10 engine with a 3d printed core. That would be a version of the engine used in the SLS second stage.

Some of the parts that I mentioned aren't even milled -- they are often spun or formed from cold forming extrusions. Consider the strength of something as common as the aluminum beer can and its strength that comes from the pressure of its contents and the consistency that prevents weak spots. RelativitySpace has test fired rockets that produce 15,000lbs of thrust -- hobbyist levels. I'm more curious to see them do SpaceX style testing, where they force failures of their parts and solve those issues and not just pat themselves on the back for making relatively weak rocket engines fire.

As for rapid production technology, you can always 3D print wax and do investment casting as well, but that requires engineers who are capable of thinking in terms of tool making and casting. The advantages that I perceive with 3D printing metal are for parts that need to withstand heat or can have weight savings through internal structures, especially parts that are in compression more than tension, and parts that would expensive or exceedingly difficult to cast.

All of rocketlab's Rutherford engines are 3d printed.  They just launched another satellite to orbit in the last 48hrs.

That's 10 3d printed rocket engines working perfectly. And they've done over 10 launches.

They are past the point of "developing" 3d printed rocket engines

Not to diminish anybody's work, but get back to us when a human or a $1trillion satellite is orbited by a 3d printed engine managed by An organization other than the government.  There's a reason the commercial space companies haven't flown a human.

Airplane development didn't stop once Orville and Wilbur flew a plane.

Those also only put out around 5,000 lbs of thrust. You could probably have a die made for those nozzles for < $100k and kachunk those out for a few dollars a piece followed by a few minute turning operation. That's not to say that 3D printing engine parts isn't impressive, but for something so small, the variable cost per unit when 3D printed seems like it would quickly eclipse your fixed tooling costs + variable costs, and I don't see the variable costs of 3D printed parts dropping all that much.



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