In 2014 I won a UK Space Agency Grant for Exploratory Ideas to explore a concept for creating large structures in space using additive manufacturing.
The following is taken from the grant application.
Project summary: Building massive structures in orbit.
When engineers and science fiction writers imagine creating large habitats in space they often conceive of their construction in a manner analogous to terrestrial buildings, where crews of space suited builders and sophisticated robots construct large structures from precisely engineered components ferried to orbit from the ground.
We imagine a different approach, one part inspired by natures own builders, and part by the recent explosion of additive manufacturing technology. Our exploratory idea will investigate how mechanically simple and reliable machines can automatically form very large habitable structures in space, paving the way for large scale human occupation.
Project Description: Rotational forming of large habitable structures in orbit.
Current conventional habitat construction for space involves the terrestrial fabrication of complete modules that are then shipped to orbit and assembled into a larger structure in a manner loosely analogous to a conventional construction site. This mode of construction requires considerable technical capability in orbit including the use of robotic systems, and human EVA activity.
We propose an approach whereby a single, simple and low cost machine is shipped to orbit. Once deployed it proceeds to form a large diameter shell in orbit using processes derived from additive manufacturing. The resulting shell encloses a space in a pressure safe, thermally insulating and impact resistant composite material. Once complete the primary task of the machine is over and the space it has created can then be pressurized, allowing humans to enter it and work in relative safety. In addition to the novel machine and construction process, we are also proposing an approach to hull construction based on a low density layered composite material that can be supplemented or ‘bulked out’ with raw material obtained from non terrestrial sources such as asteroids.
The rotational forming process
The method of construction and machine design is based around a revolute additive manufacturing system whereby a pair of rotating arms, each supporting extrusion heads, lay down material onto a pre-made axial core. With each rotation the material is built up in successive layers and the arms pivot round to follow the rising form. This process will, in its simplest incarnation, form a spherical shell around the axial core, completely enclosing the machine, and the machine can then be used to form some internal features if required. The pre-manufactured core also serves as the entry point to the structure and a possible axis of rotation. In this way the structure can work as a centrifuge, generating a degree of gravity around the internal equator for the inhabitants.
The shell produced by the machine would consist of many layers. The primary blended layering of materials would happen when the shell is initially created. These layers are designed to complement each other with properties that combine to provide thermal and radiation insulation, structural rigidity and an excellent ability to resist penetration by high velocity projectiles. Once the primary shell is complete additional layers can be formed on the interior of the structure.
Part of the project involved building a very crude demonstrator, a device that would build a plastic shell around its self using a conventional FDM printing process. Unfortunatly it was a bit of a disaster. The build arm and the mechanism was driven by a stepper motor, and the arm was fairly long. As the build process started to work the stepper motor had to adjust the arms speed to compensate for the increasing diameter of the shell, and the stepper very quickly hit the resonant frequency of the arm, and the whole thing started to oscillate. This is quite a pertinent problem for a machine like this, but I was naive for not realising that this would be a problem when designing the machine.
In 2018 I presented a paper about the concept at the British Interplanetary Society Reinventing Space Conference in London with some fresh thinking on the concept and a slightly more refined approach. The paper is available here: Building massive structures in orbit.
There is still a lot wrong with the concept, for example when writing the RISpace paper I tried to address the issue of precession and stability of the rotating elements, but hadn’t understood the intermediate axis theorem properly. Now I think that the whole process needs to be thought through better because its potentially possible for the build process to transition the rotating part to something that is rotating about the intermediate axis, and is therefore unstable.