Ceramics were used to create art and various kinds of dishes for thousands of years. Today, we can use them to craft ultra sharp durable knives that can make their high tech steel counterparts appear like ancient relics. From life experience, we also know that ceramics do not deal well with sudden strong forces acting upon them. For example, a floor hitting them at a few meters per second.
The inherent brittleness of ceramics is the main reason why they are unsuitable as structural material for most applications. However, ceramics do stand out in many highly specific aspects, especially when it comes to high temperatures and chemical stability. In this article, we explore the crucial roles of ceramics in spacecraft and how they save the day where metals falter.
Designing the most efficient thermal protection system
One of the more common use cases for ceramics in spacecraft is as part of the thermal protection system. To understand why a ceramic material is the ideal candidate for this application, it is important to look closely at the different heat dissipation mechanisms.
When a spacecraft enters any kind of atmosphere at orbital speeds, it experiences significant surface heating through atmospheric drag. This holds even true for the relatively thin Martian atmosphere, which has only 1% of Earth’s atmospheric density. The heat absorbed by the spacecraft can then take two ways: it can be radiated into the environment or conducted into the interior of the spacecraft, as indicated in the figure 1.
Radiation would be a favourable way for the spacecraft designer to get rid of the absorbed heat, since the environment is hardly affected by the radiated heat, while the spacecraft could disintegrate and/or melt if too much heat is accumulated during the entry phase.
However, the efficiency of radiation is tied to the fourth power of the surface temperature. This means that it plays hardly any role for surface temperatures most materials can comfortably handle but becomes the dominant heat transfer/cooling mechanism at temperatures above ~1000 K. You may be familiar with this temperature range as virtually all solid materials start to visibly glow red around here .
Specialised coatings are the key
The conduction of the heat into the spacecraft is the less favourable way to handle the surface heating because of the temperature limitations of all used materials within the spacecraft. There is only so much heat the spacecraft can absorb before the material limits are exceeded and catastrophic failures may occur.
Engineers came up with a smart solution that utilises both heat transfer mechanisms. For example, the heated surface of the space shuttle orbiter is covered with a good heat-insulating material, namely silica (silicon dioxide). In addition, a black borosilicate coating is applied to this material in order to maximise the radiation emission properties of the surface. This way, up to 95% of the encountered heat is shed away immediately, leaving only 5% of the heat to be absorbed by the interior of the tiles.
The entire lower surface of the space shuttle orbiters is covered with these black tiles, consisting out of a silica fibre system with a volume content of only 6%. The remaining volume is filled with air. Each tile is marked with an identification number to ensure the correct maintenance and assembly in its unique position. The tiles are bonded to the underlying aluminium structure with a silicon rubber “glue”.
This afternoon, @SuperclusterHQ photographer @erikkuna returned to Pad 39A with #SpaceX to collect our remote cameras.— Supercluster (@SuperclusterHQ) June 25, 2019
What you're seeing is the fire and fury of the Falcon Heavy's 27 Merlin engines igniting to generate 5 million pounds of thrust. pic.twitter.com/CBVBzJSUek
Other thermal protection systems, such as abrasive systems, also use insulating materials, which are deliberately being eroded by the excessive heat. By design, the abrasive systems can only be used once before they require a complete replacement. In contrast, the silica tiles are reusable, despite their impressive peak service temperature of about 1900 K during the reentry phase.
For especially strongly heated areas, like leading edges of aerodynamic structures, thermal insulation can be insufficient, requiring active cooling. In this case, there is no insulating ceramics layer but a relatively thin, thermally conductive material in place.
This principle is comparable to the cooling of the main combustion chamber in the space shuttle main engine, which was described in detail in our previous article, Metals in Space: How superalloys changed the rocket landscape. Moreover, Elon Musk is planning on using active cooling of stainless steel on the entire windward-facing surface of the newly designed spacecraft Starship.
The atmospheric entry phase is not the only operational phase where a spacecraft is subjected to considerable surface heating. Simply being exposed to the sunlight in space can raise the surface temperature quickly up to about 500 K.
Against this, the space shuttle orbiters were protected by the same silica tiles using a white coating, consisting of a mixture of silica compounds and aluminium oxide , to maximise the surface reflectivity and absorb only a miniscule fraction of the incident solar energy flux.
The downsides of highly reflective spacecraft
In some cases, reflectivity of a spacecraft can be problematic. Just recently, SpaceX received serious complaints about their satellites interfering with observations from astronomers .
The optical reflectivity was not considered for the overall design of the Starlink satellites. However, SpaceX acknowledged this flaw and is actively working towards a solution by putting a coating on the Earth facing side of the satellites .
This coating strongly affects the thermal properties of the satellites as the light emitted and reflected by the Earth can also act as a substantial source of heating that needs to be taken into account in the overall system. Hence, this is not a straightforward change but one that needs to be carefully designed and validated via trial and error.
The research on electromagnetic-wave-absorbing materials dates back to World War II, when the Germans faced the highly successful first radar systems of the Allies. This research resulted in a ferrite-based paint, which can be considered as the first artificially created radar absorbing material .
Nowadays, the radar signatures of satellites need to be suppressed in some cases for strategic reasons, cloaking them from enemy detection systems. However, satellite operators that choose to apply radar absorbing technology need to pay extra attention to ensure that their satellites do not contribute to the strongly increasing issue of space debris in Earth’s orbit after the end of service, as they are even harder to find and remove.
Why multifunctional materials/composites are a must
From the considerations above you can see that spacecraft such as satellites are highly complex systems embedded in one of the most demanding environments we know. The strongest deterioration that exterior satellite structures experience is typically related to surface erosion originating from the UV irradiation in space and from bombardment with atomic oxygen , in addition to the severe thermal cycles, depending on their orbital characteristics.
The outermost layer of a satellite is the defining surface for all thermal interactions with the environment. If it is optimised for only one purpose, for example to minimise electromagnetic reflectivity, other features required for the nominal functionality like a certain surface emissivity for cooling or impact protection from micrometeorites and debris of the satellite might be missing. Therefore, the outermost layer has to fulfil a multitude of functions and requirements.
Multilayered carbon-based ceramics have proven to be an effective material to achieve a multifunctional, lightweight and robust spacecraft skin. Figure 3 shows how carbon fibre reinforced carbon (Carbon/Carbon or C/C) provides a high thermal stability over a large temperature range. The C/C components can be manufactured with a chemical vapour infiltration process.
A real cloak of invisibility
It might seem trivial to hide an object in space by simply colouring it the same way as the background: black. However, even objects that absorb all visible light can be perfect reflectors of electromagnetic irradiation at other wavelengths, for example microwaves.
The superior thermal protection that C/C provides can be combined with the electromagnetic-wave-absorbing characteristics of an epoxy matrix with added multi-wall carbon nanotubes. Not only do carbon nanotubes have the potential to increase the absorbance of electromagnetic waves, but they can also be used to fabricate ultra strong nanomaterials, as described in this article by Wade Lanning.
The outer layers of the shielding multilayer shown in figure 4, with a carbon nanotube content of up to 1.5%, provide excellent microwave absorbing characteristics, acting as a cloak of invisibility for the satellite. The thickness of the individual layers as well as their composition are optimised using a machine learning approach, following a recent trend in materials science.
As you can see, cutting-edge materials and the application of multifunctional composites are required to withstand the demanding environment of space.
No matter how well a spacecraft is designed and tested down here on Earth, some surprises in its functionality and unintended effects can still be encountered in orbit, as SpaceX and their Starlink satellite constellation have shown. Ceramic materials, composites, and coatings offer highly desirable characteristics like long term stability and thermal protection, enabling a new era of advanced space exploration.
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