Original article written for QM Magazine by John Challen
Magna Parva were contracted by ESA to develop a additive manufactured (3D printed) material for a crushable thermal protection system for the Earth re-entry capsule, the capsule will hold a 500g Martian soil sample, and was subjected a number of tests and analysed at various facilities.
In order to transport 500 grammes of Mars soil safely back to earth, the Mars Sample Return capsule must be:
- Strong enough to withstand freefall impact on re-entry
- Crushable enough to reduce impact shock for its contents
- Heat proof to keep the sample below a specified temperature
Part of the project involved investigations into additive manufacturing, as a suitable crushable material will very likely be based on cellular material and additive manufacturing is a means to tailor crush properties to be used within the capsule’s structure. Magna Parva used the Manufacturing Technology Centre in Ansty (MTC) for their additive manufacturing expertise to assist with the manufacturing process.
Having put in a bid for the project in 2012, Magna Parva started working on the thermal protection system in January 2013. The development of a crushable material for the system had a number of objectives. The material needed to be attached to an ablative material forming the heat shield that burns during re-entry to the Earth’s atmosphere. It also needed to be attached to the much cooler structure of the sample container, while limiting its acceleration level on landing. Finally, the material needed to provide thermal insulation between the hot ablative material and the cold structure.
To these mechanical challenges to the design must be added that of protecting against ‘bioburden’, or the bacteria that may be present in an unsterilized sample. “The difficult thing is that you are bringing a sample from one planetary body into another and this causes issues with bioburden control,” explains Magna Parva design engineer Zeshan Ali. “Whatever we send to Mars has to be sterilized. you have to take a sample that is clean, get it into a sealed biocontainer, and then get it back to Earth.” The challenge here, for Magna Parva, was ensuring that the soil sample stays at the permitted temperature, and that the bio-container is not crushed when it lands. “You have to protect it very well, and this means the dual challenge of thermally protecting it and impact-protecting it,” confirms Ali.
Within the Earth re-entry capsule, the volume in between an ablative material and the bio-secure container – where the Mars sample soil will be stored – is where Magna Parva is developing a crushable material as part of the thermal protection system. The outside of the capsule is covered in the ablative material, which burns off during re-entry. “For that material, you have to work out the exact thickness you will need, considering its job is to protect the internals from high temperatures,” explains Ali. ” It can reach above 2000 Celsius on the outside, dependant on the re-entry capsule design, but the objective is to also protect the inside. As the material burns off, it is also transmitting heat towards the bio-container that we are trying to protect. so, thermally, we are trying to protect the bio-container with the crushable material because if the sample gets too hot, it might burn off imported chemistry. Crucially, we need to protect the integrity of the container by not allowing any ingress or egress,” says Ali.
Although they form part of the same structure, the design of the crushable Earth re-entry capsule and the selection of the material itself were two different parts to the project. On the materials side, says Ali, work started by looking at available materials that were used for energy absorption, such as crushable damping systems for cars, and trains. “We looked at a number of different materials, such as RVC (Reticulated Vitreous Carbon) foams, hollow titanium spheres, and aluminium sandwich panels. We concentrated on a lot of foam systems – such as copper, silicon carbide, corecell and aluminium – but didn’t look into polymers,” he adds. “A target requirement of the material was a service temperature of up to 600 Celsius, which ruled them out.”
The decision was taken to go with a titanium lattice that was constructed using additive manufacturing ” because it ticked all the boxes of our criteria, and it also provides design flexibility,” comments Ali. “You can develop something that is electron beam-melted, or selective laser-melted. Material trade-offs were performed for each material, using criteria such as energy absorption; maximum component dimensions; yield strength; thermal conductivity; relative density and oxidation resistance. The ability to tailor properties such as yield strength and thermal conductivity made the titanium lattice material high scoring in the trade-off,” confirms Ali.
The MTC then looked at what additive manufacturing methods were available. This extensive list included Selective Laser Melting (SLM), Electron Beam Melting (EBM), Digital Part Materialisation (DPM), Laser Metal Deposition (LMD), Plasma Transfer Arc (PTA) and Ultrasonic Consolidation (UC). “We eventually chose SLM and EBM as the best routes forward,” says Ali.
Integration and joinability of the new material
On the grounds of availability and cost, component and powder removal, heat treatment, and parent material properties, as well as the availability of a machine at the MTC, EBM was selected for material development. “We then designed the crushable cell geometry required, and worked out how we would bond the material to itself, because currently the machines aren’t large enough to build a whole structure,” explains Ali. “The earth re-entry capsule is roughly 1.5 m in diameter, so we would need to build it in sections, and then join them together. The protective material also had to be joined to a cold structure – representing the biocontainer – and the ablative material. This configuration meant reviewing and trading-off several methods of joining the three interfaces – crushable material to the bio-container; crushable material to itself, and crushable material to the ablative material.”
After designing test samples to characterise the material, the next challenge was to manufacture the samples. “We need to conduct various tests on the manufactured samples in order to characterise the crushable material allowing correlation of the empirical data and design analysis,” explains Ali. “The MTC was able to provide knowledge of the additive manufacturing tooling capabilities and detail exactly what happened during the manufacture phase, so we could record and review the process as well as the design methodology for further development. It is this advancement that is helping the technological process, and giving us the manufacturing input we need.”
“A crushable thermal protection system is seen as a more reliable solution than parachutes, and offers a mass-saving opportunity.” says Ali. “We calculated an impact energy based on a general earth re-entry capsule design, entry trajectories, ballistic and drag coefficients to work out our material strength requirement – it can’t be too weak, or else it won’t damp the impact energy enough, but can’t be too strong because there will be too much g-force transmitted into the container.”
The proposed – or earliest possible, depending on how it is viewed – date for launch for the Mars sample return is 2025. This is a long way to go in terms of time, but also in the amount of development work that is required. In the meantime, Magna Parva will look at how it can exploit the technology elsewhere. “One of the key advantages of additive manufacture exploited in this project is the ability to incorporate features into the crushable material design which allow the integration of different components. This enables the attachment of ablative and cold structure components, as well as additional sections of crushable material, onto a single processed part,” concludes Ali. “It is a small part of a big mission, but it is an essential part.”