Launcher rocket made with 3D printing

Materials World magazine
,
30 May 2019

3D printing a copper alloy has helped boost the performance of a liquid rocket engine.

Copper is commonly used to make the chamber liner of high-performance rocket engines. Its high thermal conductivity makes it a great material for increasing the performance of the combustion cycle, but machining the part from a copper billet can be incredibly time-consuming, generating large volumes of waste material.

3T Additive manufacturing Ltd, UK, (3T-AM) had been trialling 3D copper printing, and recently collaborated with US liquid rocket engine startup Launcher to test the material and the process.

Regenerative cooling

Regenerative cooling is often used in rocket engines where there is a radial array of coolant channels surrounding the engine’s combustion chamber. Fuel is flowed into a manifold near the lower end of the chamber, then passed up and around it through the channels, where it acts as a coolant to the product itself. It is then flowed into the injector and shot into the product to combust with a chosen oxidiser. However, 3T-AM Applications Engineer – Metals, Sam Rogers, explained that a chamber material with higher thermal conductivity will allow higher heat transfer from the hot chamber wall to the coolant.

Working with copper

According to Rogers, copper is such a highly used material for combustion chamber liners due to its high thermal conductivity. ‘For applications such as heat exchangers – which is essentially what a rocket engine is – the high thermal conductivity of copper allows greater heat transfer through the exchanger than alternative viable alloys such as Inconel,’ he said. ‘Most rocket engines use copper as their chamber liner. That is so more thermal energy can be soaked from the inner wall of the chamber, maintaining it below its maximum operating temperature whilst the combustion gases inside the object are at a higher temperature. You will get increased transfer from the chamber inner wall to the coolant itself.’

A copper chrome zirconium (CuCrZr) alloy had been developed by 3T-AM. The company had experimented with it on a variety of different applications. Heat exchangers, inductors, and demo rocket engine nozzles were built to learn how the surface roughness on the copper performed at low build angles, the surface finish of the material on vertical walls, and to optimise the mechanical properties. The company is still developing the copper alloy and it is currently classified as a beta material.

Printing copper engines

Rogers said using 3D printing to make rocket engines is becoming increasingly popular. He explained regeneratively cooled rocket engines have an array of intricate cooling channels around the chamber. To make these, all of these channels would be machined from a copper billet. Then the channels might be filled with wax, and then a nickel alloy outer liner would be braised over the top to close off the channels. The wax would then be melted out. Manifolds or volutes would then be welded onto the liner, where required. This is a time-consuming process.

‘If you design with a good knowledge of the AM process, you can optimise out the majority of issues that you might encounter during build. So, you can design the chamber optimally for the direct metal laser sintering printing process such that of all the cooling channels, volutes and manifolds form as desired. As the part is formed, a laser will scan the cross sectional layer of the part itself in a layer of metal powder. The machine will then lay down another layer of metal powder and the laser will scan that area again. The cross sectional area will gradually change as you move up through the layers, slowly agglomerating the part itself. You can return a few days later and the part is complete – you just need to remove the excess powder. You can build a very intricate part with complex internal channels in one part.’

Rogers said there is far less waste material with AM because any powder in internal channels or not entered into the final part is removed from the powder bed and all excess can be sieved and reused. ‘If you were to machine the same chamber from a billet, then you would start with a cylinder of copper and machine away all of the material you don’t require’ he said.

‘If you think of a rocket engine, it is essentially a vase shape, so all you want left is the wall of that vase. Everything else internally and externally is machined away from a diameter of copper cylinder that is slightly larger than the vase’s maximum diameter. So you’re wasting a lot of material when you’re subtractively manufacturing it.’

Collaborating with Launcher

Launcher, founded by Max Haot in 2017, is working to build high-performance AM liquid rocket engines to help fly satellites into space. Test models are being built and the company is aiming for commercial takeoff in 2026. In test models, Launcher made the chamber with Inconel, but was not able to produce the desired performance. Following 3T-AM’s success with printing copper, Launcher was keen to set up a collaboration with them. By 3D printing a specific copper alloy, the joint team was able to attempt building an engine that operated at a higher internal temperature.

Testing in the US in April 2019 showed the process had produced a large performance increase in Launcher’s E-1 test rocket engine. The copper part was proven to be 20 times more conductive than a comparable Inconel part, resulting in a coolant temperature of 280°C compared with 153°C for Inconel.

‘Because of copper’s significantly higher thermal conductivity, it allows you to operate the engine at a higher mix ratio of oxidiser and fuel, closer to the optimum,’ Rogers said. ‘The closer you can get that ratio, the hotter the chamber temperature is going to be, causing higher pressure and a more efficient engine. The higher the temperature gets, the better your chamber material needs to be at transferring heat into that coolant. That’s where the better thermal conductivity comes in,’ Rogers said. The learnings from this engine will go into models.

Future prospects

In addition to proving copper as an ideal material for heat exchange and in turn, rocket performance, the technology used to make the part drives efficiencies in materials and labour. As copper 3D printing increases in popularity, the costs are sure to decrease.