Advanced Composite Habitats
Extraterrestrial habitats are required for sustained human exploration of space. The trade-off between mass, size, and safety is a concern for all proposed systems. The goal is to provide a low mass system that is easy to transport, while providing maximum space to astronauts and protection from the harsh environment. In addition to large temperature swings, structures need to survive multiple g’s during transportation, radiation, and micrometeorite impacts.
In order to survive on the Moon or Mars, the initial wave of astronauts will use structures built on Earth and transported to location. These structures offer a couple of advantages, including being able to utilize sophisticated building techniques and advanced materials. However, the key drawback of Earth built structures is the need to launch and transport them to their operating location. This means that habitat size and mass must fit within available launch vehicles.
One approach for reducing habitat mass is to use advanced composite structures that maximize strength while reducing mass. Engineers at Advanced Space Manufacturing Systems (ASMS) presented such a design for a cylindrical vessel made from a combination of Kevlar, carbon fiber, and titanium.
This composite design is in contrast to systems that use full metallic structures. While being cheaper and simpler to build, full metallic structures have almost twice the mass of composite structures. Costs are important, but current launch costs per kilogram generally exceed construction costs.
The composite design by ASMS is based on a three layer construction. The outer surface is a Kevlar layer, providing impact resistance from space particles. The middle layer is a carbon fiber honeycomb structure, providing rigidity and pockets for insulation. Lastly, the inner layer is a titanium shell with titanium ribs, which acts as the pressure vessel. In addition to high strength and low mass, titanium has a low thermal expansion rate that is important when using it as a layer within a composite system.
The thickness of each layer is a trade-off between strength and mass. For instance, the thicker the Kevlar layer, the greater the protection from micrometeorites yet the more mass. Tests performed by ASMS indicate that a Kevlar layer 76.2 mm thick can survive the impact of a particle 2 cm in size at an impact velocity of 7.8 km/s. Despite this positive outcome, a layer of Kevlar 76.2 mm (3 in) thick is likely to have a substantial mass and construction cost. A trade-off in Kevlar thickness will be required to balance strength vs mass.
Rigid prebuilt structures are just one design for extraterrestrial habitats. Inflatable structures, such as those designed by Bigelow Aerospace, also hold promise as they concentrate mass in a small, transportable package that is inflated once delivered.
Ultimately, a more sustainable and scalable approach is to building structures using local materials and ISRU principles. Proposed systems include structures that are 3D printed and structures that are buried under regolith. Using local materials allows the design to be less efficient with material usage, yet still provide equivalent protection, especially in regards to radiation and micrometeorites. These structures still require more testing, so pre-built structures are important for short term human exploration.
Gutti Shashidhar Gowda, Aishwarya, Fred Barez, and Peter Humphries. "Structural Analysis of the Advanced Space Manufacturing System's Space Utility Module." AIAA Scitech 2019 Forum. 2019.