From space tourism to rocket innovation, man is diving deeper into the great black. But how much is known about the tools that are engineering space exploration? Here, William Durow, global engineering project manager for space, defense and aerospace at Sandvik Coromant, reveals the metal cutting considerations for outer space.
We have seen several giant leaps for mankind in recent years. The European Space Agency’s JUpiter ICy moons Explorer (JUICE) mission launched in April 2023 and is set to arrive in the Jovian System in 2030. It will then take three-and-a-half years to observe Jupiter’s three moons. Roughly 100 launches are also planned by SpaceX this year, and NASA’s OSIRIS-Rex has recently returned to Earth in September 2023. And that’s just a handful of the recent, current and future projects aiming to help us know more about our galaxy.
Ensuring success among the stars takes many considerations. Whether that is thorough mission planning, rigorous simulation, qualified mission managers or effective contingency planning, a successful space trip requires careful planning, preparation and execution. In addition, materials used in space applications must withstand some of the most extreme conditions one could imagine — such as vacuum, radiation, thermal cycling and micrometeoroid impacts.
Tough materials
Building anything destined for space involves a range of material considerations to ensure its safety, performance and functionality in extreme conditions. Structurally, materials must be able to withstand high pressures and stresses experienced during launch and in-flight. Spacecraft will also experience intense heat during re-entry to the Earth’s atmosphere and so external materials must work to stop the vehicle burning up, while other components, such as rocket nozzles, must also be made from heat resistant materials.
Weight is also a consideration, particularly for elements such as rocket propellant tanks where a lighter tank can better withstand structural stresses and can aid with payload capacity. The more the rocket itself weighs, the less payload, including satellites, scientific instruments and crew, it can carry into space. Lighter tanks allow for a larger portion of the rocket's total weight to be allocated to payload, maximising the mission's capabilities.
Popular materials for these applications include heat resistant super alloys (HRSAs). These materials are advantageous for space due to their exceptional ability to withstand harsh conditions. However, their hardiness also brings machining challenges.
HRSAs are designed to withstand extreme temperatures, mechanical stresses and corrosive environments and are primarily used in applications where conventional materials would fail due to their limitations under extreme conditions. Capable of maintaining their mechanical properties and structural integrity at very high temperatures, often exceeding 1000°C (1832°F), and with excellent creep resistance and good thermal stability, HRSAs are used for components including turbine blades, exhaust nozzles and combustion chambers.
But HRSAs do come with their limitations — particularly from a machining standpoint. While the materials are metallurgically composed to retain their properties when exposed to extreme temperatures, this also means the stresses generated when machining these materials are high. The unique capability of these nickel based super alloys to perform close to their melting point also gives them generally poor machinability.
Another key material used for space components is titanium. A lightweight metal with a density roughly half that of steel, titanium helps to reduce the overall weight of spacecraft, which in turn results in greater fuel efficiency and payload capacity. It’s also highly corrosion resistant and has excellent resistance to atomic oxygen, making titanium ideal for applications in low Earth orbit, where its oxide layer can provide protection against this highly reactive form of oxygen.
However, these advantages also make titanium difficult to machine. Cutting tools need to be sharp, maintain their edge line and be incredibly wear resistant to battle the material’s high strength, while its low thermal conductivity compared to metals like steel or stainless steel can lead to heat accumulation during machining, which can result in premature tool wear.
Machining considerations
Machining heat-resistant superalloys requires specialised tools and techniques — so what do space engineers need to consider? First, they’ll want to think about the material of their cutting tools. While carbide is the predominate material of choice, other materials are also available such as ceramic for roughing and cubic boron nitride (CBN) for finishing of HRSAs and polycrystalline diamond (PCD) for finishing on titanium alloys. Tool coatings and geometry are other important considerations.
These materials like to be sheared, so a sharper geometry is typically a better option not to generate heat while machining. Thin tough coatings are preferred. Physical Vapor Deposition (PVD) is generally the first choice for HRSA materials, however, in titanium turning applications an uncoated grade is preferred as a first choice.
HRSAs are typically machined at lower cutting speeds (rpm) compared to conventional materials to prevent excessive heat build-up and notch wear. Adjusting feed rates and depths of cut also play a crucial role in maintaining efficient machining. The right cooling strategy is also crucial due to the amount of heat HRSAs and titanium generate during machining. High pressure coolant is often employed to aid in breaking chips and dissipate excess heat. Manufacturers will also want to prioritise tool wear monitoring to predict tool failure and reduce the chance for insert failure, which can potentially damage an expensive component.
One method Sandvik Coromant recommends for machining space components is high feed side milling. The technique involves a small radial engagement with the workpiece, which allows increased cutting speeds and feed rates and axial cutting depths with decreased heat and radial forces. To support this method, Sandvik Coromant has developed the CoroMill® Plura HFS high feed side milling range. The range features a series of end mills with unique geometries and grades and is made up of two end mill families. One family is optimised for titanium alloys, the other for nickel alloys.
Unique requirements
While titanium and HRSAs are crucial materials in the space race, experts are constantly innovating their own materials too. In a bid to reach new space heights before their competition, most organisations operating in the field also develop their own, unique blend of materials to give themselves an edge.
The contents of those materials are often shrouded in secrecy — it could be titanium alloys, ablative materials, carbon-carbon composites or something totally different. Apart from the spacecraft engineers themselves, the secrets of its material blend will also be revealed to their machine tools supplier.
In Sandvik Coromant’s case, our expertise in space explorations spreads across the globe and includes several dedicated R&D teams tasked with advising on the best tools and techniques for the job. When a customer approaches Sandvik Coromant, the team will work with them to discover the machining solution for their material requirements. This can involve testing in a secure site, consulting on tool selection and advising on machining methodologies.
The stakes are high when developing components destined for outer space. Even the slightest falter in quality can stop a mission from getting off the ground, so careful attention must be paid to every step of the manufacturing process. That includes the materials selected for each component and how they’re machined. To deliver success among the stars, it is important manufacturers consider a balance between tough materials and the machining challenges they bring. Having access to the right machining knowledge and robust tools is key to making the next great leap.
