By Michael Fiske, Jennifer Edmunson, Ph.D., John Fikes, Mallory Johnston, and Michael Case
The informal motto of Marshall Space Flight Center’s In-Space Manufacturing Group is “Make It, Don’t Take It.” The National Aeronautics & Space Administration’s (NASA) planetary habitat manufacturing and assembly technologies that incorporate in situ resources provide options for autonomous, affordable, pre-positioned habitats with radiation shielding features and protection from micrometeorite and exhaust plume debris caused by launch and landing. Without a significant atmosphere, surface dust can be accelerated and cause substantial damage to adjacent structures unless precautionary measures like a wall or berm are implemented.
Not surprisingly, the U.S. military can realize many of these same benefits in the development of expeditionary structures. The ability to use in situ material to construct structures either autonomously or with minimum personnel will allow payloads that would otherwise make long-term Moon and Mars habitation cost-prohibitive. Equally, the ability to use local materials to construct terrestrial military expeditionary structures autonomously or with minimum personnel can reduce transport of materials in-theater and reduce waste. Additive construction provides better overall ballistic protection and thermal conditioning performance than many standard military construction techniques.
On Earth, in-situ materials refer to local sources of the components of Ordinary Portland Cement-based concrete: sand, gravel, Portland cement, water, and both wet and dry additives. Using local materials minimizes the need to transport these materials in-theater. On the Moon and Mars however, in-situ materials refer to the materials on the planetary surface.
This inorganic material (identified as regolith to distinguish it from organic Earth soil) will be the basis for the aggregate in any planetary cementitious material, with the goal to extract a binder from the same material. Any available water will likely be used for propulsion or life support, so a waterless binder is being pursued for these applications. Recently, NASA’s Marshall Space Flight Center and Jacobs Space Exploration Group delivered a fullscale additive construction system to the U.S. Army Corps of Engineers (USACE) Construction Engineering Research Laboratory in Champaign, Ill.
The system—known as Automated Construction of Expeditionary Structures (ACES-3)—was based on prior work performed at Marshall Space Flight Center from 2004 to 2007, and again from 2013-2017, during the Additive Construction with Mobile Emplacement (ACME) project. Funded by USACE and NASA’s Game Changing Development Program, ACME’s mission was to develop a fundamental understanding additive construction, evolving from batch processing to a continuous-feed system. Work also included development of cementitious materials based on planetary (Moon and Mars) resources as well as understanding and controlling the rheological (viscosity) properties of these cementitious materials.
Contour crafting is an additive construction technology developed by Dr. Behrokh Khoshnevis of the University of Southern California and Contour Crafting Corp. Contour crafting shapes a continuously flowing bead of construction material, providing structural consistency and a more appealing aesthetic. This process has been used to build structures of gypsum, Ordinary Portland Cement-based concrete, sulfur concrete and ceramic slurries.
Contour crafting has influenced aspects of almost every additive construction technology that has been developed. USACE is interested in the technology because of its ability to support development of on-demand structures in a variety of settings using local materials. Such structures can include standard, culverts, anti-tank obstacles, and Army B-Huts (troop housing). In addition, it is expected to take less time to build a B-Hut (16-ft x 32-ft x 8-ft) than using traditional construction methods (one day compared to five days). Additive construction may need fewer construction personnel and fewer security personnel to protect construction workers. The amount of material brought into the field is reduced (a 50 percent reduction is anticipated) and can reduce waste from about 1-T to less than 500-lb/B-Hut. Another advantage is that if the Army has an agreement with a foreign government resulting in that government keeping the structures when the Army leaves, those structures can be designed and printed to reflect local architectures. Forward personnel would all benefit from these improvements.
The NASA/Jacobs team worked with USACE to define requirements for the ACES-3 system that reflected the additive construction performance goals.
• Break the system down for transportation into no more than three 8-ft x 8-ft x 20-ft volumes, consistent with a standard Army Palletized Loading System.
• Complete system setup and alignment in under 11 hours.
• Print in the X and Y axes at up to 500-in/min concrete deposition speed with a volumetric flow rate of up to 800-in³/min.
• Maintain absolute nozzle positional accuracy of +/- 1/8-in in all three axes during printing.
• Operate entire system with no more than six personnel (long-term goal of three).
• Print concrete with aggregate up to 3/8-in diameter.
• Include an automated dry goods storage and feed system for up to seven dry materials and an automated liquids storage and feed system for up to five liquid materials.
In 2004, the University of Southern California delivered a contour crafting system to NASA that supported batch processing of Ordinary Portland Cement-based mixtures. The system known as ACME-1 allowed fabrication of relatively long slender walls. Typical structures fabricated with ACME-1 include a dome structure with interior walls.
Between 2004 and 2007, the NASA/Jacobs team experimented with different nozzle and trowel configurations, initial characterization of optimum rheological (“soft solids”) properties, and understanding the differences between commercial off-the-shelf concrete materials and Portland cement, stucco and additives. Significant effort also was spent programming and printing various geometries and experimenting with concrete deposition speed versus concrete cure time and strength to optimize the process.
The project was inactive from 2007 until 2013 (due to funding constraints) at which point USACE contacted Marshall Space Flight Center to express interest in further development of the ACME-1, converting it from a batch process to a continuous-feed process (ACME-2). During batch placement, layers deposited on different days (wet on dry) did not adhere well. But same-day runs (wet on wet) yielded excellent bond strength. This continuous-feed system produced larger structures (though still sub-scale); eliminated poor layer-to-layer bonding; and eliminated discontinuities between the end of one batch and the beginning of the next.
As the technology advanced, preparations for the design of the full-scale system began. The evolution from ACME-2 focused on the transition from sub-scale to full-scale. Many areas requiring further development and detailed evaluation were identified, including selection of an optimized nozzle mobility system (gantry versus truck/boom arm versus robotic arm versus other); selection of components for the larger system (pump, motors, drive system) to meet print speed and volumetric flow rate requirements; hose management (minimizing vertical pumping and curves); positional accuracy; system mobility; cleaning; and assembly/disassembly.
TOWARD NEW HEIGHTS
ACES-3 represents a unique asset for the U.S. military. The ability to transport, deploy, and use a large-scale concrete 3D printer opens the door for rapid development of numerous infrastructure elements that do not have to be transported in-theater.
It is expected this technology will continue to generate new applications and resources for the military engineer…and beyond.