49th International Conference on Environmental Systems ICES-2019-195 7-11 July 2019, Boston, Massachusetts Astro GardenTM Aeroponic Plant Growth System Design Evolution Sam A. Moffatti, Robert C. Morrowii, and John P. Wetzeliii Sierra Nevada Corporation, Madison, WI, 53717, USA By providing dietary nutrients and unburdening physiochemical life support equipment, Hybrid Life Support Systems (HLSS) can be an attractive option for longer duration space exploration such as Mars transit missions. State of the art microgravity plant nutrient delivery utilizes a physical media in which the plant roots grow (e.g., clay-based arcillite). This added media increases the mass to orbit necessary for growing plants in space – limiting the appeal of large-scale plant growth systems in terms of Equivalent System Mass (ESM) when considering long duration mission architectures. Plants grown hydroponically or aeroponically represent a logistically favorable alternative for space-based plant growth systems, as nutrients are delivered without the added mass of a soil-like media. However, by removing the media material, controlling water delivery and recovery in a microgravity environment presents a challenge. Astro Garden is developed to provide a large- scale, space-based aeroponic plant growth system, leveraging modifications to current designs meant to operate within a gravitational environment. This paper details the design evolution of Astro Garden from the Phase 1 prototype through the upcoming ISS technology demonstration and future flight systems. Nomenclature CEA = Controlled Environment Agriculture CFM = Cubic Feet per Minute DAQ = Data Acquisition System ECLS = Environmental Control and Life Support ESM = Equivalent System Mass HLSS = Hybrid Life Support Systems ISS = International Space Station LED = Light Emitting Diode LEO = Low-Earth Orbit NFT = Nutrient Film Technique NI = National Instruments PCB = Printed Circuit Board PPM = Parts Per Million SNC = Sierra Nevada Corporation TRL = Technology Readiness Level WCD = Water Capture Device WPM = Water Processing Module Zero-G = Zero Gravity Corporation I. Introduction lants have been grown in space for over 40 years1. Research payloads such as Astroculture, Advanced P Astroculture, Biomass Production System, Veggie and Advanced Plant Habitat have provided valuable tools for plant physiology research in a microgravity environment. These systems have almost exclusively relied on nutrient delivery through a soil-analog medium such as arcillite. Scaling these research systems towards food production systems becomes problematic due to increased mass requirements2. To achieve plant growth on the scale necessary for significant astronaut dietary supplementation, as well as Environmental Control and Life Support (ECLS) system i Mechanical Engineer, Sierra Nevada Corporation, and 1212 Fourier Dr. Madison, WI, 53717, USA ii Principal Scientist, Sierra Nevada Corporation, and 1212 Fourier Dr. Madison, WI, 53717, USA iii Program Manager, Sierra Nevada Corporation, and 1212 Fourier Dr. Madison, WI, 53717, USA © 2019 Sierra Nevada Corporation hybridization, alternate methods for nutrient delivery and recovery are highly desirable. Systems that eliminate the need for a rooting media interface present opportunities for significant mass reduction of large-scale plant production systems. The Astro Garden system is being designed in a phased approach. The Phase 2 Astro Garden prototype builds upon the Phase 1 design and parabolic flight testing data to create a large-scale plant growth system capable of the level of production necessary to meet the NASA salad-diet architecture3. This level of plant production also has the added benefit of increased ECLS system hybridization (supplementing physiochemical processes with biological processes). The Astro Garden prototype relies on conditioned inlet gas (controlled humidity & temperature as well as enriched CO2 levels) to provide optimal conditions for peak plant growth. In terms of ECLS hybridization, the system provides a means for additional oxygen production to supplement the traditional physiochemical systems and can provide a near potable water source through the recovery of transpired water. The modular, scalable nature of the Astro Garden design allows for plant growth in a variety of environments and potential mission scenarios, enabling future low-Earth orbit (LEO), cislunar, lunar surface, Mars transit or martian surface plant food production. II. Aeroponics in Space Aeroponics is a plant nutrient delivery mechanism that is characterized by spraying a nutrient solution directly on to plant roots, typically through use of pressurized nozzles. Aeroponics is a form of hydroponics where the roots are exposed directly to air (thus the aero-prefix) with the nutrient solution applied as a spray or mist. In more traditional hydroponic plant growth systems, the roots remain submerged in nutrient solution. Aeroponics has been used to grow and research plants on the ground dating back to the 1920s2. Commercialization and subsequent large-scale plant production through aeroponics took hold in the mid-1980s, beginning with systems such as GTI’s Genesis Machine4. Aeroponic plant growth offers a number of advantages over soil-based plant growth. From a research standpoint, aeroponics provides easy access to the plant roots, negating the need to remove soil for examination. Because of this access, roots are less likely to be damaged during the inspection process, allowing for quick, relatively undisturbed data collection throughout the growth cycle5. Aeroponics can also inherently isolate one plant from another, preventing the spread of disease through the soil medium. If disease does occur, the plant in question can be easily removed from the growth volume preventing further spread in the system. Aeroponically grown plants can also be grown faster. Because the roots are exposed directly to the nutrient solution, their absorption rate is typically higher than through a soil medium. When done in a recirculating mode, aeroponics is very efficient at regulating water delivery to the plants, decreasing the overall water needs for growth. Planting density can be increased through vertically stacked aeroponic nutrient delivery, requiring less overall volume. Because of the volume, water and nutrient absorption efficiencies, aeroponics is a highly desirable platform for urban plant growth, where space and water consumption are at a premium. Coupled with the reduction in mass through removing the soil medium, aeroponics presents an ideal solution for space- based agriculture. In addition to the benefits of using aeroponics on Earth, a number of space-focused attributes exist as well. Aeroponics can function the same in 1-g as it does in microgravity for nutrient delivery; because of the relative droplet size (~50 microns) and the velocity at which the droplet is discharged, the inherent momentum of the particle overcomes the effect of gravity acting on the particle mass. This has been demonstrated through numerous parabolic flight tests for various nozzle geometries and inlet pressures as noted in Section V. The nutrients are also typically delivered via a preset timed spray cycle. By determining the duration and frequency that a nozzle is effectively “on” and “off” a highly controllable quantity of nutrients can be delivered to plants roots without the use of additional sensors (such as pressure sensors used to control the level of negative pressure in porous media based systems). Nozzles can be easily replaced with the use of quick-disconnects, allowing fast on-orbit replacement when necessary. There are two primary concerns for using aeroponics in the absence of gravity: how to recover spray that does not impinge upon the roots, and how to prevent nutrient depleted water from adhering to the roots for extended periods. Removing depleted water from the roots is important to prevent blocking fresh nutrients from absorption sites and preventing oxygen uptake, resulting in a hypoxic condition. Without gravity acting upon the droplets, the solution will float as globules until coming into contact with a surface in the root zone. Depending on the wetting characteristics for that surface, and the capillary angle that the surfaces make with one another, the solution will adhere more or less strongly to that feature. Highly wetting materials and sharp angles will create regions that tend to collect more of the solution. Sealing becomes especially important as small, tight features can drive the solution to other regions over time through capillary action. Many of the challenges identified with microgravity aeroponics can be addressed through parabolic flight testing with Zero-G Corporation, but other challenges will need to be addressed through extended duration testing on the ISS. 2 International Conference on Environmental Systems III. Phase 1 Astro Garden Design and Testing The Phase 1 Astro Garden program included design of a modular, open loop plant growth system prototype intended to demonstrate future flight system packaging. The module design relied on readily available parts and materials in an effort to reduce complexity, cost, and development-time. The system received inlet air from a controlled environment room via internal fans and rejected plant chamber air back to the room. Nutrients were delivered aeroponically to the roots through an internal pump and accumulator system used to control nozzle delivery pressure. Gravity was the primary means to drain the