Once Coral proves itself capable of reliably and repeatedly producing usable products such as equipment replacement parts or construction elements on the Moon, it will be ready for business.
The development curve for these capabilities will take off from simple but robust elements to more sophisticated and complex elements. This progressive development embodies improving performance and sophistication both in the Coral printer and the products it makes. This article explains the development strategy to bring Coral to operational maturity through six evolutionary steps.
Step 1: The First Brick
Bricks are one of the world’s oldest and simplest construction elements. On its own the robust, primitive nature already lends itself to an endeavor that favors simplicity over sophistication. This isn’t to say that beginning with lunar bricks would constrain the conceptual and physical architecture of future space missions that use them. Diverse masonry structures spanning millennia of history and culture underscore the flexibility of this seemingly rudimentary technology.
In a more fundamental sense, manufacturing a brick is broadly applicable to a wide range of engineering applications beyond masonry construction. The first brick will not only be a brick, but also a representative test sample from which we can measure and derive a variety of key engineering properties — including compressive strength and stiffness — that must be known in order to design any structure.
This technology demonstration mission, therefore, will advance two distinct yet interrelated capabilities required for lunar construction. First, Coral will prove that lunar-derived masonry is feasible. Additionally, Coral will provide the first ground truth to quantitatively define what kinds of structures can be built in subsequent projects on the Moon. At the successful close of the first step, in fact, we will turn our efforts to subsequent projects.
Step 2: Landing Pads and Roadways
Before the first lunar landings, scientists were worried that the lunar surface was in fact covered by a sea of dust that would swallow whole any lander. The truth was less strange but even more sinister: Apollo astronauts and Earth-based scientists found that lunar dust posed a significant hazard to both equipment and human health. Unmitigated, our future Moon towns and factories would suffer. This problem is one of the first that Coral can address.
One of the popular concepts for constructing landing pads and roadways that do not generate lofted dust is to sinter the surface in-place. Germany’s RegoLight project and JPL researchers independently explored this concept.
The problem with sintering regolith in this way is that we can never know the bearing strength very well since we do not know what is under it. In the absence of reinforced spanning elements, the surface crushing resistance of the sintered material will be close to its bearing strength, which may not suffice to carry vehicles and may inadvertently create the Moon’s first pothole-filled roads.
Coral can provide a superior alternative to sintering the lunar surface material in place to because it makes the complete structural element, instead of just melting and fusing the top layer. The Pacific International Space Center for Exploration Systems (PISCES) shared a tantalizing glimpse of this construction method in late 2015.
Coral offers the ability to produce structural slabs that sit on the surface and span across any defects in the regolith. The Coral slab bearing strength comes from more than its resistance to surface crushing. The slab acts as a beam that spans any surface weaknesses or imperfections, allowing a spacecraft to land and a heavy pressurized rover to travel over it without causing damage.
Step 3: Elementary Support Pedestals for a Far-Side Radio Telescope or Interferometer
The Astronomy from the Moon Community constitutes a family of ready-made potential customers for Coral. The scientists and researchers in this group rightly note that the far side of the Moon is among the best locations in the solar system for astronomy unblemished by Earth’s atmosphere or transmissions.
In this application, Coral could be used to construct the vertical support structures to shape a cable mesh into a parabolic curve suitable for use as a radio telescope or interferometer. Such a task is a useful intermediate step between roadway construction and more complex freestanding structures with regards to qualitative difficulty and quantitative requirements like precision and size.
The design and fabrication of these structures would require a precision at a scale of 20 to 30mm. The pedestals would consist of simple structures up to about 2 m square and up to 10 m tall. The Coral system would excavate regolith on the site of the pedestal emplacement, and manufacture it right there. This excavation can provide “on the spot” extraction and sourcing of the regolith to provide the 3D printing feed. In fact, robot-rovers could excavate the regolith from the same hole where the pedestal will be implanted.
Coral system robots will perform all the construction work: surveying, excavation, collecting local material, processing, and printing the pedestals. Running the cables through the eyelets or other securing devices mounted on the pedestal will comprise a separate phase of robotic operations. It might be preferable to lay out the cables first and then lift them and move them laterally to attach to hardware on the side or top of the pedestal.
Step 4: Infrastructure for a critical payload
As Coral capabilities grow, the scale and importance of the projects it can be applied to will grow in proportion. This evolutionary step for Coral enables a critical infrastructure emplacement, such as a foundation and shield wall for the fission reactor that makes it possible for humans to stay through the 336-hour (14 Earth Day) lunar night. The design of this foundation would require greater precision, perhaps on the order of 5 to 10 mm.
This step may require the introduction of reinforcing fibers into the feedstock mix to carry tension forces generated by dead and live load compression, and possibly impact in the case of the shield wall. In this step, may it may be possible to repeat the “extraction on the spot” of previous steps.
Step 5: Protective structures for a human or robot base
Vault or dome structures can provide a measure of protection against radiation and micrometeoroids over pre-integrated Class 1 or deployed Class 2 habitat modules (terms defined by Kriss J. Kennedy of NASA’s Johnson Space Center).
These structures can also provide thermal screening for habitats during the 140° C heat of the lunar day. The main challenge of building these structures would be to produce the continuous arch, vault or dome that rise at least 5 m above the surface, spanning probably up to 10 m. The precision required would be similar to the previous step. This step will require longer and heavier fiber tensile reinforcement to support the long-span arches, vaults, or domes.
In this step, it is unlikely that it will be viable to repeat “on the spot” extraction because the production will require a much larger quantity of regolith and digging such large holes in close proximity to an operational area could create a serious hazard.
Step 6: Pressure Vessels for Habitats
The holy grail of in situ construction will be to produce a pressure vessel (PV) capable of safely and reliably providing a pressurized living environment. The atmospheric pressure that this PV contains exerts a powerful outward force on its walls, hatches, and windows. The precision required will be on the order of 0.5 to 1 mm. This step in the evolution of Coral structures will require tensile strength in the 3D printed walls that can take the complete hoop stresses around the cylindrical, spherical, or other geometric form of the PV. For this step, not only will it not be possible to use “on the spot” extraction, but it may be necessary to import regolith of special composition from sites some distance removed from the habitat or base.
These six steps represent at a generic level the scale and scope of the technology development trajectory for Coral. Establishing these levels of capability and identifying stakeholders who stand to benefit from each phase of Coral’s development is key to ensuring that we build a tool that is beyond an intellectual exercise or all-to-brief public curiosity.
Instead, Coral will be a useful tool to the myriad future participants in whatever sort of lunar economy will emerge. Providing an essential service to these stakeholders will, in turn, align market forces to more strongly motivate the necessary continued research and development of Coral’s foundational technology.
If you feel that Coral may be of use to your endeavors or want to help bring it into existence, then please feel free to reach out to us by joining the public chat on Riot or signing up with this form.
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Space Decentral is a decentralized autonomous space agency that leverages blockchain technology to reinvigorate the push for space exploration with global citizens in control. Space Decentral promotes collaborative design of space missions, sharing research for peer review, crowdsourcing science, and crowdfunding worthy projects that accelerate human progress.