‘To boldly go where no man has gone before…’, the alloy that has made space travel possible.

By Karolina Jackiewicz

Since the first Moon landing, we have taken a huge leap in pushing what we consider to be the boundaries of human space exploration.

The introduction of private companies such as SpaceX or Blue Origin in the last 20 years has propelled the development of space-travel technology. Thanks to cooperation with NASA it became feasible to decrease the cost of sending each crew member beyond Earth’s orbit by introducing various cost efficiencies previously deemed impossible, e.g. by sending people to space on a reusable rocket, a big selling point for SpaceX.

This opens doors to more people being able to travel into space and to travel there safely, already now, and in years to come.

Since the science of space exploration really took-off in the 1950s, several different materials and components have been tested and used to build a reliable rocket and all the related technology and equipment that goes with it. From solar panels, acting as the energy source for satellites, to built-in antennas, allowing communication with the Earth, among many more.

To enable a space rocket to lift-off, which weighs over 13 empty Airbus A320 planes or more, including its own payload, and to steer it through outer space, a vast amount of energy and an adequate propulsion system is required. Due to the complexity of space travel - a number of phases and different forces are involved at each step of the journey - a typical space rocket consists of several stages involving different materials, usually at least 2.

Each of the rocket’s stages engages a different propulsion system – one to apply enough power to lift it off the Earth’s surface, one to enable travel beyond the low Earth orbit in a vacuum environment and one to finally return it home, to Earth.

One of the most important and reliable materials which has been used in these demanding space applications, since travel beyond low-Earth orbit was made possible, is the C-103 TM alloy commonly used in the engine expansion nozzle – the enormous engine bell used in propulsion from the second stage onwards, perfect for use in the vacuum of space. C-103 is therefore not an alloy that makes the rocket reach outer space and disappear from our view, but the one that allows it to cruise through it, in the absence of Oxygen.

Figure 1. Apollo Service Propulsion System Engine with Exhaust Nozzle. Source: Frontiers of Flight Museum.

C-103 is an alloy developed and trademarked by ATI Specialty Metals in the US and consists of approximately 90% Niobium, 10% of Hafnium and a little bit of Titanium and Zirconium, offering exceptional properties. It was designed to replace weaker, incumbent Niobium alloys, such as the previously used Nb-1Zr alloy; due to its superior tensile strength, yield strength and creep stress resistance at higher temperatures, while maintaining excellent formability and weldability properties at the same time.

Figure 2. Stress to produce 1% creep after 1000 hours. Source: Thermomechanical Processing and Properties of Niobium Alloys by Wah Chang
Figure 3. Tensile yield strength vs. temperature for common Niobium alloys. Source: Thermomechanical Processing and Properties of Niobium Alloys by Wah Chang

All Niobium alloys can be considered very ductile at room temperature, however C-103 maintains its high strength at sustained temperatures up to 1482oC and is resistant to high frequency vibrations in cryogenic conditions, due to its low ductile-to-brittle transition temperature (a property which made the steel of Titanic’s hull so brittle in the icy waters of the Atlantic and which contributed to its famous sinking upon collision with a floating iceberg).

These two properties however make the C-103 alloy an ideal candidate for rocket chambers and nozzles applications. According to ATI’s research, thermomechanical properties of the alloy can be further improved by applying a protective thermal coating, e.g. silicide, enhancing its thermal stability and emissivity (a characteristic important for materials cooled radiatively in space), which is based on the structure emitting most of the heat energy before it can absorb and affect the structure.

Figure 4. Apollo Service Propulsion Module Engine.

Most famously, C-103 made it possible for humans to travel to the Moon in July 1969, as it was historically used in the Apollo program, the first ever program to send people beyond low-Earth orbit, and specifically on Apollo 11 Service Propulsion Module engine nozzle of the Saturn V rocket, which landed humans on the Moon.

The alloy and its behaviour in space is so significant that even now it is widely used in the SpaceX Merlin Vacuum engine range for use on its Falcon 1, Falcon 9 and Falcon Heavy rockets, in the form of a fixed non-deploying expansion nozzle.

Figure 5. SpaceX’s Merlin Vacuum engine and expansion nozzle.

While the constant work on new materials continues, C-103 is still the material of choice for the nozzle application. Perhaps one day it will be the alloy making it possible for us to make our first journey to Mars and beyond.