Proposal: The Checkerboard

This is a submission for the "Growing Beyond Earth" challenge in the professional category.

Design by Draper engineers: Carly Buchanan, Martin Sinclair, Isaac Whipple, Max Turnquist, Kevin Myers, Lexi Neese, Rob de Saint Phalle, and Jeffrey Burrell.

Our proposal is a two-sided checkerboard of root modules, suspended in the center of the shell, with lights, electronics and reservoirs on the “upper” and “lower” outer sides. The driving factor behind this design is to take advantage of microgravity and have plants growing “up” and “down” in relation to each other. This gives each plant space to grow while more effectively using the volume of the center plane. Keeping the root volumes within the same plane instead of stacked on top of each other means we can fit light sources for both sides and keep them at a close, but plausible distance away from the leaf canopy of fully matured plants. The entire checkerboard is held on slides so that it can be pulled out of the shell like a drawer for easy access to the plants.

We are not experts in space agriculture, so we did our best to research the constraints that affect the growth of the lettuce. Our design is based on the assumptions we made from this research, which is summarized below.

Root Volume & Growth Medium

For the lettuce to receive enough nutrients and hydration, a minimum root volume of 250 cubic centimeters is required[1]. We used sphagnum moss for the growth medium due to its natural capillary abilities, which helps draw water away from the source. We did also experiment with a foam growth medium which nests neatly into the root modules and may be less prone to mold or fungus, but does make it harder to run the water plumbing.

The modules could be filled with any growth medium, although granular growth mediums may need some sort of netting to keep them contained within the module.

Lighting

The type of lighting used and the distance from the light source to the lettuce have a major impact on the health of the plant and the shape that it grows in. To fit as many plants into the volume as possible, it was important to minimize that distance. We found commercial off-the-shelf hydroponic lighting units that claim they can be mounted as close as 4” from the canopy[2], so we chose to base our design around that number. However, professional hydroponic LED setups are carefully designed to create a consistent field of photosynthetic light across the growing area. We did not have the expertise or time to design a custom LED module, and COTS modules are designed for much larger growing areas than our small volume, so we used spools of flexible LED tape in our prototype. This means our prototype will not perform as well as it would with bespoke LEDs for our specifications. However, we do believe that the fundamental distances within the design are plausible.

Since in space there is no gravity to tell plants which way is “up”, they grow straight toward the light. To try to encourage the lettuce plants to grow outward as well, we added up-lighting at the base of the plants. These LEDs will be dimmer than the traditional above-canopy lights since they are much closer to the leaves. We did find studies that had successfully used up-lighting for terrestrial growth operations to help plants receive more light and grow faster.[3],[4] To maximize lighting efficiency, we made the majority of flat surfaces inside the shells white to reflect light around the inside of the shell.

Growth Modeling

Staggering the growth stages of the plants allows for a more continuous harvest of lettuce and a more efficient use of space. The growth cycle of lettuce generally follows a Gompertz function[5], characterized by slow early growth, a mid-stage growth spurt and a final stage of slow growth until the lettuce is harvestable. The growth period is estimated to be 35 days (5 weeks) from a seed, with the fastest growth between days 10 and 20.

For our modeling purposes, the lettuce plants were idealized as paraboloids opening upward, as we conjectured that lettuce growing in microgravity would be more voluminous at the top of the plant, closer to the lights. The maximum size assumed for mature lettuce plants is 10 cm in height and 15 cm in diameter at the widest point. Dry leaf masses were converted to paraboloid volumes to model the lettuce plants at different stages of growth. Based on this model, our design assumes two plantings staggered two weeks apart. When the first planting is harvested, this allows more space for the second planting to reach full volume at maturity, and then the second planting is harvested and the relationship is reversed.

Watering Method & Water Reclamation

We don’t have any experience in microgravity fluidics, so we did not design a new watering system. In our prototype, we use a garden soaker hose that weeps water to simulate the porous tube in the Advanced Plant Habitat (APH). We couldn’t find details of how PONDS works, since it contains Tupperware intellectual property, so we don’t know if the root modules could be reconfigured slightly to support a passive watering system.

As there is no nutritional value in the growth material itself, fertilizer must be added to the water or the growth medium. We chose a liquid fertilizer in the water, but the design could support either method. As the International Space Station has a water reclamation system, our design takes advantage of it with one side of the shell open to the ISS environment to allow moisture transfer from the plants to the ISS's water reclamation system, similar to the method that VEGGIE uses. Fans are mounted throughout the shell for proper air circulation.

[1] Massa, G. D., Newsham, G., Hummerick, M. E., Morrow, R. C., & Wheeler, R. M. (2017). Plant Pillow Preparation for the Veggie Plant Growth System on the International Space Station. Gravitational and Space Research, 5(1), 24–34. Retrieved from https://pdfs.semanticscholar.org/c5a4/a623c0fe8de...

[2] Valoya BX and C Series. https://www.valoya.com/wp-content/uploads/2019/06... https://www.valoya.com/wp-content/uploads/2019/06...

[3] Zhang, Geng, et al. “Supplemental Upward Lighting from Underneath to Obtain Higher Marketable Lettuce (Lactuca Sativa) Leaf Fresh Weight by Retarding Senescence of Outer Leaves.” Frontiers in Plant Science, vol. 6, 2015, doi:10.3389/fpls.2015.01110.

[4] Takagaki, Michiko. “Chapter 27 - Challenges for the Next-Generation PFAL.” Plant Factory: an Indoor Vertical Farming System for Efficient Quality Food Production, by Toyoki Kozai and Genhua Niu, Academic Press, 2016, pp. 387–393.

[5] Barker, David J. et al. 2010. Analysis of Herbage Mass and Herbage Accumulation Rate Using Gompertz Equations. Agronomy Journal 102:849-857.

Supplies

Materials

1 box shell
4 slide rails
17 straight root volumes
4 left turn A root volumes
4 left turn B root volumes
24 LED strips (20 in)
2 frame slide panels
2 frame face panels
Cable chain
4 fans with spacers
1 microcontroller
1 electronics breakout
1 power supply
1 User Interface breakout
Wire & connectors
1 reservoir
¼” ID soaker (porous) hose
¼” ID nonporous hose
90-degree connectors for ¼” ID hose
Threaded inserts and bolts
Cable holders

CAD

Our CAD is located here

Construction Notes

Since the purpose of this write-up is to specify the “form, fit, and function” of the major components, we do not include detailed construction methodology in this write-up. In our prototype, we sometimes used screws for ease, but we assume that all fasteners can be replaced with bolts paired with threaded inserts as captive nuts.

Step 1: Design & Planning

Based on the research that LED lights should be at least 4 inches (~10 centimeters) away from the leaf canopy to avoid tip burn (necrotic tissue on the lettuce leaves), we did not think there was room within the 50 centimeter cube to fit two trays of growth medium. However, we realized that by interweaving the two trays into a single bi-directional tray, we could fit two layers of lettuce plants with two light sources within the given space (see diagram). This interweaving also meant we could densely nest the modules that house the roots, while still giving the lettuce leaves room to stretch out and grow without too much collision. The attached video above shows the modeling for lettuce growth from planting to first maturity. The green circles represent lettuce growing on the "up" side and the blue circles represent lettuce growing on the "down" side.

Animated_CheckerBoard_5x5.avi
Download

Step 2: Prepare the Shell

Steps

Construct the shell walls. We routed the walls out of a plastic composite and 3D printed the corner joints.
Install threaded inserts into the walls.
Assemble the shell.
Install slides.
Cut the LEDs to length, add connectors and install them on the walls. Our LED strips have an adhesive backing.

Notes

The shell is designed to fold and flat-pack. See the renderfor how constructing the shell with hinges would allow the shell to collapse. However, due to our limited machining capabilities and time, our prototype uses a rigid shell with plastic walls joined together by 3D-printed corners. To make assembly in orbit as easy as possible, we designed the shell so that some features are installed on the ground, including the slides and the LEDs onto the walls.

Drawer slides are bulky for the tight space constraints. We used a simple square extrusion that slides within a dado in the sides of the growth tray. It is important that the tray and the slide are low-friction so that they slide smoothly and easily.

Step 3: Install Life Support

Steps

Attach the power supply, microcontroller, electronics breakout, reservoir and pump to the interior back wall of the shell where they won’t interfere with the tray.
Connect the microcontroller to the electronics breakout, and connect both to the power supply.
Connect the pump’s power connection to the electronics breakout.
Connect the pump to the output line of the reservoir.

Notes

In orbit, the rest of the plumbing and electronics support (other than the growth tray) can be added to the shell. The power supply, microcontroller, electronics breakout, reservoir and pump are attached to the shell with hook-and-loop tape so we could easily remove them for modification during prototyping. This ability could be a benefit on the ISS as well for maintenance, but the hook-and-loop should be replaced with a longer lifespan material.

The diagram above gives a general idea of how things are electrically connected. Essentially: using drivers such as Darlington arrays and/or MOSFETs to to allow the microcontroller to power the fans, pump, and lights on and off as determined by parameters that can be set by a user interface. The up-lighting and down-lighting should be wired separately so that the up-lighting can be run dimmer than the down-lighting since it is supplementary and closer to the leaves.

Our prototype does include a power supply. However, on the ISS power would be supplied by the Express Rack. The control electronics also could be packaged much more compactly on a custom PCB. This would open more space for a larger reservoir. Our reservoir is currently large enough to hold 2.5L of water, or approximately 1 day of water for 25 fully mature plants, which would require the reservoir to be refilled daily. We constructed our reservoir using a boxed beverage bladder contained within a rigid box. For a space-ready design, we propose a rigid-wall tank and a quick connect junction between the reservoir and the pump. This quick connect could be used to temporarily connect a water supply to refill the reservoir.

Step 4: Prepare the Growth Tray

Steps

Attach the sides to the back frame.
Place the bars that are used to support the root modules into their slots along the back. We lightly pressed these bars into place by hand.
Add the 90-degree hose connector to the outside of output module against the back face before adding all the modules.
Slide the root modules along the bars into place. See the CAD for more details. They should form an alternating up/down checkerboard pattern, with holes connecting to provide a single zig-zag path through all the modules for the plumbing. We propose alternative methods of routing the hose in the photos above, but this would require different variations in the CAD of the root modules.
Add the front face so its slots capture the ends of the bars that hold the root modules and another 90-degree connector to guide the plumbing out of the tray. When the front frame attaches to the rest of the frame, the root modules are held securely in place. We designed the root modules to be 3D printed, since 3D printing can be done in a food-safe manner, and the ISS does have 3D printing capabilities. We assumed launching filament would be more compact and weight efficient than sending up printed modules. If the modules are printed prior to launch, the tray could be assembled on the ground.
Fill the root modules with growth medium.

Notes

A .STEP file of our CAD assembly is available here: https://github.com/CarlyBuchanan/Checkerboard-AstroGarden

Currently our root modules mate flatly, face to face, to each other, which does create a leak zone. In our prototype, we ran a bead of hot glue around the hole where the hose traveled between modules and pressed the modules together to create a watertight seal. For a real version that would be launched into space, we recommend adding a flange around the hole or creating a rubber gasket that can be used to overlap the root modules and prevent leaks. Similarly, the lids of the root module would benefit from an O-ring or some other type of rubber gasket to help them mate securely to the root module body.

We used sphagnum moss as a growth medium because it was affordable, lightweight, acted as a capillary system to wick the water away from the porous tube source, and didn’t fall out of the hole where the lettuce sprouted. We also experimented with foam growth mediums, which nest neatly into the root module boxes. The modules could be filled with any growth medium, although granular growth mediums may need some sort of netting to keep them contained within the module.

Step 5: Final Assembly

Steps

Connect a hose to the output of the pump and the input of the tray.
Connect another hose to the output of the tray and the other end into return into the reservoir.
Add LED strips on each side of the tray and connect them to the wires that run to the control electronics.
Mount the fans to the walls, using spacers to keep clearance between the fan and the wall for airflow. Wire the fans to the control electronics. Adhesive cable holders are useful for routing the wires neatly.

Notes

For prototyping purposes, we ran long strips of LEDs along the seams between rows of root modules. However, we probably would change this in a future iteration and use round LED rings either around the base of each plant or on the solid faces of the modules facing the other way. That setup would involve more connections, but we think it would provide a better distribution of light and would allow these LEDs to be installed on the root modules prior to assembling the tray, which might simplify the assembly process.

We also used coiled tubing between the tray and the reservoir to allow the tray to be pulled out of the shell for easier access. However, we would change this design to use a cable chain along the side of the tray to keep both plumbing and wires tidy without limiting the tray's motion.

A user interface module is intended to allow a user to interact with the control electronics to change the photoperiod, tune the LED wavelength, adjust the watering schedule and otherwise program the parameters that control the environment within. It is intended to mount onto the front face of the shell, but is currently mounted onto our control electronics on the back wall as we ran out of time to build a cable connection for it. Since the purpose of this contest was the geometry and layout of the design, we have not programmed the adjustable functionality into our prototype.

Step 6: Program, Prime, Plant

Steps

Program the microcontroller to control the photoperiod of the lights and the frequency of watering.
Prime the pump if it needs to be primed.
Plant lettuce seeds into the growth medium.
Monitor for seedlings!

Notes

As we mentioned in the Growth Modeling section, we designed the spacing of the root modules based on 2 plantings, staggered 2 weeks apart. The diagram above shows our proposed planting schedule. This staggering allows smaller plants to nestle with large plants, packing them into the volume more compactly. A cut-and-come-again harvest schedule should preserve this staggered timing.

Step 7: Thanks & Acknowledgements

We’d like to thank Lauren Kessler, John West, Kim Slater, Seamus Tuohy, Chris Yu, John West, Kim Slater, and Bryan Dolan for their support of this project; Abigail Bonnice, Erik Weir, Steven Popkes, and Manwei Chan, and Allison Looney for their contributions; Bill Wells and Dave Reed for their advice; the rest of Draper Strategic Communication including Sarah Von Oldenburg and Allison Looney for their assistance; and Jay Schwartz for his carpentry skills.