Changing the game of spaceflight

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Matthew Dale learns how additive manufacturing is optimising the production of rocket components

Hot-fire testing of a GRCop-42 L-PBF chamber and NASA HR-1 LP-DED nozzle with integral channels at the NASA Marshall Space Flight Center. (Image: NASA)

Additive manufacturing (AM) has been a ‘game changer’ for the production of complex components used for propulsion applications in space flight. 

Such was shared by Paul Gradl of NASA’s Marshall Space Flight Center in a recent SPIE webinar hosted in the run up to the Photonics West conference in San Francisco.

‘Liquid rocket engines are inherently complex,’ he began. ‘A lot of challenges exist with traditional manufacturing methods, as none of the alloys that we're using are actually easy to process with traditional techniques. In addition, some of the lead times using these traditional methods, for example forging or casting, might be two months, six months, or in some cases even 12 months.’

NASA has therefore been exploring the field of AM to develop certain components. With it, lead times are being brought down anywhere from 2-10 times, while cost reductions upwards of 50 per cent are also being achieved.

‘It really is a game changer for complex components,’ Gradl confirmed.

Explosive development

To demonstrate his point Gradl shared the example of a combustion chamber – the heart of a rocket engine where temperatures can reach upwards of 3,000°C. 

Made using a copper alloy, with traditional manufacturing methods multiple forging, machining, slotting, joining and assembly operations are required to produce this part, taking approximately 18 months and costing around $310,000.

‘When we started development with AM however, we were able to make this copper alloy combustion chamber in just a couple of pieces using laser powder bed fusion (PBF),’ said Gradl. ‘We cladded on a structural jacket to the outside using electron beam wire directed energy deposition (DED), and were able to reduce the lead time to around eight months and at a cost of $200,000.

 

Bimetallic combustion chamber with GRCop-42 L-PBF liner and NASA HR-1 LP-DED jacket. (Image: NASA)

‘As we continued to evolve AM, supply chains became more accessible and we expanded our knowledge of this, which enabled us to bring these numbers down even further – the lead time is now around three to five months and the cost is $125,000. And you're seeing this across the whole industry right now. Again, it’s very much a game changer for a lot of our component development.’

While NASA has been successful in using machines equipped with infrared lasers to produce the combustion chambers, this has created challenges due to the high reflectance of the copper alloy being used. As a result, Gradl explained that the agency is also looking at certain applications where green and blue lasers can be used to increase efficiency and productivity when printing copper alloys with laser PBF or wire/powder DED processes.

Such laser processes make up the leading AM techniques used by NASA, according to Gradl. Other non-laser AM techniques used by NASA include electron beam PBF, arc wire DED, electron beam wire DED, cold spray, additive friction stir deposition and ultrasonic AM. 

‘There's a lot of different flavors of AM, with each having its own advantages and disadvantages, but they're all very supplementary to each other,’ Gradl remarked. ‘We're going to see each technique evolve over the next several years, and I’m very excited about what this means for aerospace.’

Large rockets, large parts

The DED techniques in particular are facilitating an ongoing ‘exciting’ trend being seen across the entire aerospace industry, according to Gradl: the advancement of large-scale AM.

In space flight, for example, large parts such as a liquid rocket nozzle can be produced using laser powder DED. Here, powder is injected through an inert gas stream using the laser to locally melt that powder and build the parts layer by layer. Gradl showed a nozzle measuring a metre and a half in diameter and almost two metres in height, which despite its size also featured integral channels with wall thicknesses down to around one millimetre.

Progression of 65 per cent scale Integral Channel Wall Nozzle build using LP-DED. (Image: RPMI / NASA)

‘We can vary the different optics used for laser powder DED to control our spot size, which provides not only different deposition rates but also different feature sizes…there’s a lot of active research ongoing in this area,’ he said. ‘This particular part we built in about 90 days. Traditionally, this would have been probably a 12-18 month fabrication process. So again this shows the flexibility of AM to reduce our lead times.’

An even larger demo part measuring almost three metres in height was also shown by Gradl, which he explained was developed for the RS-25 engines set to fly on NASA's upcoming Space Launch System – the most powerful rocket ever built by the agency. ‘So again, you can see the scale flexibility of some of the different laser processes: we can build very small, complex parts, but also these very large parts as well,’ he said.

Full scale RS-25 SLS Nozzle Liner built using LP-DED. (Image: NASA MSFC / DM3D)

The path ahead

Despite the progress being made, AM is still at a very early stage in aerospace, according to Gradl. He remarked that we’re going to see a lot of growth in the field over the next couple of decades, to mature it to the point where it’s more of a traditional manufacturing process. 

For space flight in particular, the path ahead is clear, with NASA having identified emerging development areas for producing liquid rocket engines with AM:

  • The maturing of each AM process and furthering understanding of microstructure, build limitations, and methods for design and post-processing 

  • Continual development of large-scale AM using DED and other processes

  • Continuous hot-fire and component testing to advance the various combustion chambers, injectors, nozzles, ignition systems, turbomachinery, valves, lines, ducts, and in-space thrusters being developed

  • Learning how to combine various AM process for multi-alloy solutions and to increase design opportunities

  • Advancing the commercial supply chain for certain unique alloys, including GRCop-42, NASA HR-1 and JBK-75

  • Developing new alloys (refractory, oxygen resistant, AM-specific [print-friendly] alloys)

  • Producing a comprehensive material database of metal AM properties – tensile, fatigue and thermophysical – to aid conceptual design 

  • Exploring design complexity using lattices and thin-wall structures

  • Development of standards to safely infuse into flight applications

Certain areas stand out more than others however, with Gradl highlighting what he thought were the biggest three challenges in AM for spaceflight:

‘One of the biggest challenges is education: having informed users that understand how to design for the technology, how to properly select different build processes, and how to achieve the best post processing. There's therefore a lot of opportunities in providing education systems and professional training in this area. 

‘One of the other challenges that we're seeing is the availability of alloys that are out there. If you look at traditional manufacturing, there's thousands of alloys that we can select from, whereas with AM there's a couple dozen alloys. While there's a lot of advancements in these alloys, we still need to be able to mature them and truly understand their microstructure, response to heat treatments, and resulting properties. That will be a pretty significant effort to achieve all that.

‘And then of course there’s the qualification side of this too. These are new technologies that we're still learning about, and we have to be able to fly the parts safely. So we have to do this methodically and have the proper standards and qualification tools in place, in addition to people that have a good understanding of all that.’

NASA in particular has been central in the certification of AM and producing documentation outlining how additively manufactured parts can be flown safely. For example, the NASA 6030 standard has recently been released, outlining the qualification of AM parts for human spaceflight, including how the process and materials can be certified, and how to analyse the parts to make sure the microstructure and material properties meet the design intent.

Ready for launch

After years of research, development and qualification efforts, one of NASA’s additively manufactured parts is now finally set to see the stars: ‘We will be flying various laser PBF parts on the Space Launch System as part of our Artemis missions,’ Gradl confirmed. ‘The pogo accumulator and pogo Z-baffle (to dampen propellant oscillations) was traditionally a welded machine component requiring 127 welds, which has now been reduced down to four welds with a significantly reduced part count.’ 

To have such a futuristic process as AM contributing to these missions is quite fitting, as Artemis will facilitate humanity’s next step in space travel. Using innovative technologies, the mission will explore more of the lunar surface than ever before, allowing humanity’s first long-term presence to be established on the Moon. The knowledge and experience gained throughout the mission will then be used to take an even further step in human space travel: sending the first astronauts to Mars.

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