Growing on a Curve

As a species that is beginning to try to reach out further into space, growing plants during the long travel times to reach new worlds will be critical. Just getting to mars with our current methods will take a 7 month journey and this just gets exponentially longer as we go further out. While we may be good at growing food on Earth, space offers major challenges of size due to the cost of transporting large objects into space. Currently it costs 10,000$ to transport just 1lb into space.

Even if an object is light due to being mostly empty space such as a grow box prior to being filled with plants that still requires a larger vessel to transport it into space increasing weight and cost. This leads to a need to maximize the useable space of anything brought to orbit. Empty space in a growing chamber is wasted effort and resources.

Currently our grow chambers brought to space have operated much like growing on earth. Soil on the bottom of a container with lights on top. This leaves a ton of empty space and could be greatly improved with a new design taking into account the benefits and challenges of microgravity.

A new grow chamber design should incorporate 3 goals:

The it should have the highest density of useful space possible with the highest output of usable plant material.

It should be lightweight

It should be compact.

(8) 450mm 8020 20mm T-slot extruded aluminum bars.

(3) 24"x36" 3/8" thick marker board

(1) 12v dc hobby motor with long shaft

- https://www.amazon.com/gp/product/B07L11HD3V/ref=p...

(1) Piezo aeration pump

- https://www.amazon.com/Cambani-Aquarium-Oxygen-Sup...

(28) 450mm long LED grow light strips Red and Blue spectrum focus

- https://www.amazon.com/gp/product/B01NH5227Q/ref=p...

(1) Stepper motor

- https://www.amazon.com/ELEGOO-28BYJ-48-ULN2003-Ste...

(4) Reversible computer case fans

- https://www.amazon.com/SilverStone-Technology-Reve...

(1) 5mm thick silicone disc

(6) 5v activated 12v relays

(1) esp8266-12e or other microcontroller with at least 8 digital out pins

(1) 12"x24"x2" soft foam sheet

- https://hydrobuilder.com/hydroponics/hydroponic-gr...

(1) 2' 3/4" PVC tube

(1) 2" 10" PVC tube or thin walled tube

(lil bit of) Vaseline or other non setting hydropobic grease

(1) low profile 90 degree barrel connector

(1) 12v 20a power supply

3d Printer with several rolls of PLA filament, we do not want to use ABS as it will be toxic to plants.

Currently grow chambers for the ISS are 50cm cubes. With lights on the top and grow medium on the bottom, the plants grow up through the box towards the lights. Linear growing of plants leaves large portions of dead space above the plants when they are young, and when the plants grow larger, they block all light from reaching any plants below them. This eliminates the possibility of starting off seedlings or growing microgreens under the main plants, but also will stunt the growth of leaves (and in the case of a lettuce plant, the leaves are the food) under the main canopy because little to no light will reach them.

Plants grown linearly will also need to be planted as seedlings as far apart as they will need to be once fully grown. If plants are planted too close they will stunt their growth in multiple ways. First they will directly compete for light resources leading to stunted growth for one or both plants simply due to competition for resources. Second, plants do not like their leaves to touch other plants around them. this causes signals to be sent not only throughout the plant itself but also to all surrounding plants through the roots. This tells the plants they are at risk of being shut out from light resources and can cause 2 different effects both of which are disadvantageous to growing food. Either this stunts their growth as they conserve their resources to wait for more optimal growing conditions. Or this could cause plants to "bolt", they quickly try to grow tall and rapidly bloom to reproduce before they get closed out of light resources. For lettuce bolting ends its productive lifespan as they will produce a waxy substance that will ruin the plant for food consumption.

These effects however to not happen when roots contact roots of a different plant. roots will happily grow in and among the roots of other plants without changing plant behavior as long as they receive sufficient water, nutrients, and oxygen.

Being in microgravity offers unique advantages for growing geometry. We no longer need to think about plants in this linear fashion as in these conditions they will always grow towards the light and away from the grow medium. So we need to find a geometry that will maximize room for leaves while not concerning root spacing.

If we instead use a cylinder, with leaves pointed outward and roots pointed inward, centered in a chamber lined with lights we offer an advantage in that as plants grow larger the leaves will grow away from each other gaining more room to grow as the plants grow taller. This will prevent the plants from forming a cohesive canopy that blocks all light below them, and minimizes the effects of plants touching each other limiting plant stressors. This allows plants to be planted much closer to each other at the base then as they grow in somewhat of a cone shape the taller the plant grows the more room it has available to grow without competing with its neighbors. A second benefit this offers is we will have lights surrounding the cylinder at all angles, this allows more light to hit each plant as they will receive light not only from above but also from their side. By using wide angle lights like florescent or LED lights plants will receive light not only from above, but from all sides as well, greatly decreasing the competition for light for each plant. Reducing the stunting of growth due to excessive crowding allows for a much larger crop to be in each box.

A cylinder also allows double the height for roots as they can grow past the center of the cylinder meaning the full diameter of the tube is available for the plant roots rather than only the radius.

Another benefit we achieve by growing on a cylinder surrounded by lights on all sides is that we can get more light underneath the main plants. While the main plants will absorb most of the light at the center of an LED aimed at the center of a cylinder, using wide angle LEDs will allow some light to reach under the more disperse canopy compared to a linear grow operation. This offers the possibility of growing seedlings, a mat of microgreens which are naturally less light hungry, under the main plants, further increasing the density of usable plant material that can be produced by one cube.

Growing inward on a cylinder exacerbates this canopy effect and quickly causes plant leaves to touch each other as the plants will grow toward each other increasing the interference each plant experiences and likely stunting their growth due to the massive competition for light resources. This will lead to lower yield for every crop without decreasing the grow cycle time.

We can further improve upon this cylinder in regard to wasted space above plants by having the cylinder rotate at a very slow rate. We can start new seedlings or freshly harvested plants in the area that has minimal distance between the cylinder and the cube faces, then as it slowly rotates, the plants grow up into the corners of the cube where there is more room between the cylinder and faces. Once the plants reach their max height in the corner ot just past the corner the row that is at this max height will be harvested and depending on the plant age, replaced with a young plant or allowed to start its next grow cycle. This staged harvesting and planting allows the max space in the box to be filled with plants at all times.

Additionally, having this staged growth keeps plant heights different around the whole cube further increasing the amount of light that can penetrate the canopy of the main plants. This then increases the efficiency of growing any plants beneath the main canopy like microgreens or seedlings.

If this is to be used beyond research use - for example to supply food to a crew on a mission through space where resupply would be infrequent or nonexistent - food waste and food supply consistency would be of critical concern.

When plants in a linear grow chamber reach maturity, since most plants will not stay ripe indefinitely, there is a possibility that a portion of the harvest cannot be utilized before it spoils. Then, once your crop is harvested, there is a lag time of 3 weeks or more for most plants grown for food before the next batch is available for that cube. Most fresh vegetable material will spoil in much less time so without cooking and preserving at least some of the food obtained that cube it will likely not last until the next batch is ready. While this could be improved in a linear grow by having several grow chambers all started out of phase with each other, that requires a significant amount of extra space due to the additional grow chambers, and would require leaving multiple chambers empty for a period to allow the first chamber to grow out of phase.

Using a rotating cylinder allows for periodic harvesting of a single cube. You do not need to harvest the entire crop of the cube at once. You can select only mature plants at the corners of the cube while leaving the next row to mature. The next crop will take substantially less time than waiting an entire grow cycle which would cut the difference between harvesting crops from 3+ weeks down to a week or less depending on the spacing of the rows. This assures an even and consistent rate of ripe fresh food, limiting the chances of spoiled food, and never leaves a full grow cycle to wait for the next harvest.

Incomplete harvesting in a linear grow container would not allow the same benefit as all plants grow at roughly the same rate, or if staged planting is used, earliest planted sprouts would block out all light from the later planted sprouts as plants will typically expand to fill the room available to a certain point.

Now that we know the optimized geometry for maximizing the useful space in the cube we need to determine how we will grow them in this cylinder.

Because of the enormous cost to transport material to space we will want this to be the lightest possible option.

Soil is a very well-known option. The benefits of soil are that well selected soil will contain all nutrients needed to grow a plant. But soil is heavy, very heavy. Plus, much of soil is not even nutrients for a plant; a large portion is composed of essentially inert filler. This is not ideal when every pound of dirt to grow a plant must be carried to orbit. A 50cm box with a bottom filled with soil 6in deep for very shallow rooting plants will contain 1.35ft^3 of material. One cubic foot of soil is ~74lbs assuming it was not watered before transport. The dirt alone required for a single traditional grow will weigh ~100lbs and will cost about 1million dollars to transport to orbit.

Furthermore, as was learned with Earth farming practices, soil will get depleted of nutrients if the same plants are grown in them over and over again. While this can be fixed by tilling in new nutrients into the soil that adds to the weight needed to carry to orbit. Additionally, soil is inherently granular and tilling further disturbs and kicks up these small granules. This is something to be avoided when in micro-gravity as any uncontrolled granular material will spread throughout the entire vessel and can be much harder to clean, because it will never settle on a surface, compared to earth bound debris which would eventually settle on a surface where it could be easily wiped up with a cloth. Additionally, because it does not settle it will stay at eye level and can be injure astronauts. Imagine floating into a room and running into an eyeful of dirt. This problem of small granules is reason that tortillas replace all bread on the ISS (https://www.fi.edu/blog/5-foods-astronauts-cant-eat-in-space).

For these reasons, I have chosen to pursue a hydroponic setup. Using hydroponics, we only need a very small amount of inert grow medium to hold the base of the plants in place, so this greatly limits the wasted weight compared to soil. Typically, this consists of a sponge like medium making the small amount that we use even lighter, 1 cubic foot of foams weighs only 1-3lbs and the design of hydroponics will limit this quantity to small plugs no more the 2in diameter and 2in deep or 0.004ft3 per plant, we could plant over 100 plants, ~ 2 grow cubes worth, with 1 cubic foot of material and assuming the high end of weight it would weigh 33 times less than the soil needed for a single traditional box.

We will still need to bring nutrients to add to the water but these are relatively light because they can be dehydrated and there is no inert filler. The ISS is already equipped with large stores of water for the humans onboard so that will not need to be a unique item to bring to the orbit. We could use a unique specially engineered formulation, but earthbound hydroponics have let us develop good premixed solutions that can come in powder form and are water soluble. Additionally, many of these premixed solutions contain pH buffers which help stabilize water acidity further reducing the need for additional ingredients. This keeps the weight of the nutrients required very low for transport to orbit. A 2.2lb bag of nutrients will contain enough to treat about 200 gallons of water. The design I will outline below will take less than 6 gallons per grow cube. Water will need to be replenished in the hydroponic setup as the plants respirate. Assuming our grow cube uses up this whole bag in the same rate that a traditional setup would deplete the soil our total weight to orbit of unique medium for growing is only about 5lbs or 1/20th the weight of soil. That said, user experience on a bag like this used in an earth bound setup of similar size shows this will last over a year which would mean 18 grow cycles which would typically deplete a soil based set up which typically can last no more than 7 cycles before needing to be refilled with nutrients via changing to a pasture or tilling in nutrients. This would further increase the benefit of weight for a hydroponic setup.

The main weight of a hydroponic setup is the water used for growing in. We are already bringing large quantities of water to the ISS and have developed a system to reclaim much of that water so it would not be something to bring unique to the hydroponic setup. Both soil grown or hydroponically grown will need this water. Despite the seemingly large quantity of water sitting in a hydroponic setup, earth bound studies have shown that hydroponics do not use more water overall compared to soil and in fact can use as much as 10 times less depending on the setup as the only water loss from the system is what is respirated by the plants. In soil growing, much simply drains and disperses through the soil. Even if it is collected at the bottom of the pot that puddle will typically be unavailable to the plant and eventually will evaporate. Closed system hydroponics will assure the maximum efficiency of water use.

To address the issues with soil tilling or plant removal for nutrient addition we will inject a premixed nutrient solution dissolved in water from the on board supply. Currently, NASA provides astronauts dehydrated beverages in self-sealing beverage bags which are rehydrated using the onboard water filling station. These use a septum valve that will allow a straw to puncture the valve and will close itself when the straw is removed. Because water in microgravity will not have weight pushing it out of the bag the primary force acting on it is surface tension which makes for a valve needing low closure force. A similar prefilled bag with dehydrated plant nutrients could similarly be filled by the water filling station then mixed by shaking and squeezing the bag similar to how the currently used dehydrate beverages are consumed. To inject this nutrient solution into the root chamber we can make use of a similar septum valve on the chamber where a straw is forced into the filled and mixed nutrient bag then the opposite end of the straw can be forced into the valve on the root chamber.

For the reasons outlined above, I have chosen to pursue a hydroponic setup.

Using hydroponics, we only need a very small amount of inert grow medium to hold the base of the plants in place, which greatly limits the wasted weight compared to soil. Typically, this consists of a sponge like medium making the small amount of filler that we use even lighter. 1 cubic foot of foams weighs only 1-3lbs and the design of hydroponics will limit this quantity to small plugs no more the 2in diameter and 2in deep or 0.004ft3 per plant, we could plant over 100 plants, at least 2 grow cubes worth, with 1 cubic foot of material and assuming the high end of weight, it would weigh 33 times less than the soil needed for a single traditional box.

We will still need to bring nutrients to add to the water but these are relatively light because they can be dehydrated and there is no inert filler. The ISS is already equipped with large stores of water for the humans onboard so that will not need to be a unique item to bring to the orbit. We could use a unique specially engineered formulation, but earthbound hydroponics have let us develop good premixed solutions that can come in powder form and are water soluble. Additionally, many of these premixed solutions contain pH buffers which help stabilize water acidity further reducing the need for additional ingredients. This keeps the weight of the nutrients required very low for transport to orbit.

2.2lb bag of nutrients will contain enough to treat about 200 gallons of water. The design I will outline below will take less than 6 gallons per grow cube. Water will need to be replenished in the hydroponic setup as the plants respirate. Assuming our grow cube uses up this whole bag in the same rate that a traditional setup would deplete the soil our total weight to orbit of unique medium for growing is only about 5lbs or 1/20th the weight of soil. That said, user experience on a bag like this used in an earth bound setup of similar size shows this will last over a year which would mean about 18 grow cycles which would typically deplete a soil based set up which typically can last no more than 7 cycles before needing to be refilled with nutrients via changing to a pasture or tilling in nutrients. This would further increase the benefit of weight for a hydroponic setup.

The main weight of a hydroponic setup is the water used for growing in. We are already bringing large quantities of water to the ISS and have developed a system to reclaim much of that water so it would not be something to bring unique to the hydroponic setup. While both soil grown or hydroponically grown will need water, despite the seemingly large amounts of water in hydroponics, earth bound studies have shown that hydroponics do not use more water overall compared to soil and in fact they can use as much as 10 times less depending on the setup. The only water loss from a hydroponic system is what is respirated by the plants. In soil growing, much of the water simply drains and disperses through the soil. Even if it is collected at the bottom of the pot that puddle will typically be unavailable to the plant and eventually will evaporate. Closed system hydroponics will assure the maximum efficiency of water use.

https://cals.arizona.edu/swes/environmental_writing/stories/2011/merrill.html

To address the issues with soil tilling or plant removal for nutrient addition we will inject a premixed nutrient solution dissolved in water from the on board supply directly into the water reservoir. Currently, NASA provides astronauts dehydrated beverages in self-sealing beverage bags which are rehydrated using the onboard water filling station. These use a septum valve that will allow a straw to puncture the valve, but will close itself when the straw is removed. Because water in microgravity will not have weight pushing it out of the bag the primary force acting on it is surface tension, which makes for a valve needing only low closure force. A similar prefilled bag with dehydrated plant nutrients could similarly be filled by the water filling station, then mixed by shaking and squeezing the bag similar to how the currently used dehydrate beverages are consumed. To inject this nutrient solution into the root chamber we can make use of a similar septum valve on the chamber where a straw is forced into the filled and mixed nutrient bag then the opposite end of the straw can be forced into the valve on the root chamber.

Currently the most popular earth bound method of hydroponics is Nutrient Film Technique (NTF) or drip systems. NTF consists of a water basin where some aeration and all nutrient addition occurs. This mixture is then pumped up to the high side of a sloped plant tube, causing a slow flow downward along the pipe due to gravity. The pump is sized to be slow enough to not fill the plant tube, allowing roots to breath in the space above the water, limiting the need for aeration and helping prevent root decay. These systems can, however, develop pooling in the water film due to inconsistent pipe slopes or other blockages which, due to the weak aeration of the water, can cause plants to die. Drip systems are similar but are typically done in completely vertical tubes, thus eliminating water pooling issues, but requiring a stronger pump due to increased height and need to be always on to prevent roots from drying out. Both rely on incomplete filling of the water chamber to allow roots to breath and oxygenate and both techniques require gravity to work in order to allow the water to flow. Additionally both require a separate water reservoir which would occupy space in a grow cube limiting space for plants.

Because our setup will be in microgravity, we cannot rely on gravity to provide the constant flow required in these systems; but a bigger issue is water’s behavior in microgravity. In microgravity there is no bias force drawing the water in any particular direction; the primary force acting on water will be surface tension. This means that any open air space in the system will not be consistent. Furthermore, because surface tension is the primary force acting on the water in the system, water will favor clinging to surfaces with the most surface area. Roots in a hydroponic system will form a root mat of tightly grouped roots, thus providing a very large surface area drawing in any water that contacts it and holding that water among the roots even if the rest of the system is drained. See (https://www.youtube.com/watch?time_continue=144&v=o8TssbmY-GM&feature=emb) title for a good demonstration on how water will cling to surfaces especially porous surfaces.

This further rules out possibilities like aeroponics (spraying water on roots that drains away), top fed deep water culture (partially filled root chamber with flow over top part of roots), flood and drain (filling the root chamber for a period then draining it away and repeating that cycle indefinitely), or even Kratky method (still water that as the plant absorbs and respirates the water level drops allowing a breathing window), as all rely on consistent air gaps facilitated by the bias force of gravity. Of these, Kratky would have the best chance of working but again due to surface tension in the root mat the water would need to be completely used up before the densest parts of the mat can get oxygenated, causing either decay if not enough time is given, or plant death from drying out if too much time elapses. A balance could possibly be reached but it would likely be inconsistent. A quickly rotating cylinder with roots toward the outside could be used to create a bias force but that creates a more hazardous grow system and will need more power to spin a large assembly quickly.

This leads us to methods that do not rely on consistent air spaces meaning we need to use fully submerged root systems which is far from uncommon in hydroponics but less popular than other methods as it requires manual aeration of the water. The most common is Deep Water Culture (DWC) where plants are submerged in the water nutrient solution with an aeration device such as an aquarium air pump providing oxygenation to the water. This again runs into problems in microgravity where air bubbles produced by the aerator will typically stick to the bubbler and create a foam or one big bubble at the aerator diffuser as there is no bias forces that would draw them away and mix them evenly into the rest of the water. To avoid this issue, the method I chose for this design will involve a pump that will push water over the roots in a closed system and will have a bubbler positioned at the intake of the pump such that it will draw away bubbles as soon they are produced before they clump up around the bubble. Then, as the bubbles pass through the pump they will be further incorporated into the water and pushed throughout the system evenly dispersing them among the roots.

This lack of bias forces and domination by surface tension leads to a unique benefit in microgravity in regard to aeration of hydroponic cultures that leads to much different selection of aeration method. Typical DWC setups will have an aeration pump capable of providing 1-2 liters be minute for every gallon of water in the system. This leads to very high rate bubblers for relatively small quantities of water. One main reason this high rate is required is because each bubble produced will only have a very short time in contact with the water, giving it a very low efficiency of air pumped to oxygen dissolved. In microgravity those bubble will no longer exit the water, so they will have a very long time to dissolve into the water and oxygenate the roots. For this reason, we can size our air pump much smaller than those needed in earthbound DWC systems.

Additionally, in microgravity, air will occupy a large amount of space among the water as it does will not be compressed by the weight of the water and will not bubble out of the water surface, but will instead begin to displace the water around it. The traditional estimate of 1lpm per gallon would fill an entire 6gal grow chamber in under 4 minutes, forcing large portions of water out of the system, which would be a problem on an ISS system. This is another reason why a smaller pump will actually be beneficial, to avoid pushing the water clear out of the system.

If you have not seen videos of these behaviors, there are many demonstration videos I would recommend checking out. (Effervescent tablet in water: https://www.youtube.com/watch?time_continue=2&v=Vx0kvxqgC1c&feature=emb_logo, At 1:05 bubbles staying in water indefinitely: https://www.youtube.com/watch?v=cXsvy2tBJlU, Bubble “foam” in a bag: https://www.youtube.com/watch?v=tLCeAD6Z6FI, Air displacement of water: https://www.youtube.com/watch?v=J7eccvsVsTc )

The same connection used to add nutrient solution to the system can also be used to add air into the system to oxygenate the water. One important experiment that would need to be carried out in space will be to determine how much more efficient aeration is when properly mixed in microgravity. Because the bubbles will remain in the water for an indefinite period, this will give more surface area and much more time for these bubbles to provide oxygen to the water. This will tell us if we even need a constant supply of air to properly oxygenate the roots.

Possibly, proper oxygenation could be provided by simply blowing up an empty beverage bag with air, connecting to the same nutrient injection septum valve, then squeezing the air into the water each day or a few times a day. If that proves to be too little oxygenation, then a relatively small aquarium air pump could create a constant stream into the system. Either method would require the circulating water pump to draw the bubbles away from the injection site and to incorporate and distribute them throughout the system. Another possibility would be if an air stone could supply enough force behind the bubbles to give them enough momentum to prevent them from simply accumulating on the air stone then the pump could be eliminated and an airstone would run the length of the tube and replace the inner recirculation tube. However that would limit the mixing and distribution of new nutrient solution. This testing will need to be done in orbit to determine the increase in oxygenation as there is no good way to test the difference in efficiency on earth.

Another benefit we get from being in microgravity is that water will no longer create large forces on its containers when stored due to its weight. Each gallon of water weighs about 8.34 lbs on earth, or 50lbs for this 6 gallon setup. This would normally require strong thick PVC pipe to withstand the weight of the water alone, but in microgravity the pressure on the cylinder and end caps will be very minimal. This allow us to use a much lighter and thinner material for the main water container compared to what would be necessary to support that same quantity of water on earth. The same applies to all aspects of the grow box. Additionally, we do not need to worry about water dripping or spilling out of an upside-down container, the water surface tension will keep it within its container assuming no large open holes are present. For most hydroponic setups the small amount of filler to hold the plants will completely surround the holes where plants enter the water, the sponge like materials typically used will provide enough resistance that water will naturally stay within the water filled root chamber and not try to flow out.

One further consideration for a growing chamber for the ISS is the room it would take up on a transport. While most of the frame and assembly could be made very light weight prior to starting the production of plants in the chamber, a 50cm cube is far from compact. If the frame is not collapsible for transport to the ISS, there will be a large portion of empty space, which will not be economical when dealing with the costs of a rocket flight to orbit. This wasted space leaves less room available on the transport and offers no benefits to the people aboard the ISS; it is wasted space that could be taken up by valuable supplies like water or food. For cost reasons, a transport company would like to fit as many pounds of material into the smallest craft possible. A rocket carrying x pounds of light-density cargo would necessarily have to be larger, and therefore more expensive, than a rocket carrying the same pounds, but of a higher-density cargo. Therefore, the grow box assembly should be collapsible - without compromising on the ease of setup once onboard the orbiting craft.

One key aspect for testing this design will be plant selection. Plants with shallow roots would be preferred as this would limit the size of the tube needed to contain the roots allowing more room for edible parts of the plants. Second, we would like the majority of the plant to be edible material to maximize used space. Third, plants with shorter maturity times are preferred as this will allow for more frequent harvesting of food and shorter wait times in between harvesting. Lastly, plants that need to grow very tall are less desirable as we are limited by the 50cm cube and placing the tube in the middle further limits this height. For these reasons I chose to grow lettuce as the main plant, while not the most nutrient dense food it could be replaced with other similar plants like broccoli, spinach, arugula, cabbage, etc. I’m just most familiar with growing lettuce and it is very forgiving in growing. Some of the key aspects are short root depth (~6-10in),very good hydroponic performance (very commonly grown and so easy to start you can resprout cut heads of lettuce in just a cup of water) and a max height that nicely fills the distance between the corner and tube in the given sizes(~8in).

Now that we know what we are planning on planting we can start to layout the size of our grow tube based on the max height of lettuce ~8in max. A standard size 10” tube will leave a 8.5in gap to the corner of a 50cm cube. That said, due to the height of led strips and frame a more ideal solution would be a 9” thin walled tube, but for cost and availability issues of such a tube and the fact I am building this on earth I will simply use a more common 10in tube.

To create the closed path for water to circulate we can use a tube within a tube and a pump with a 360 degree outlet. One reason we would not simply put a divider down the center of the tube and have water circulate through either side of the divider in because we are looking to create a continuous circle of plants around the circumference that all get even treatment in the water flow. Even if multiple dividers are used to make a more consistent flow this creates dead spots in the tube where the divider meets the tube wall, and would limit the total size the roots could grow as parallel partitions could not share a root mat. A tube inside a tube creates dead spots only at the very ends of the cylinder where we already cannot grow as it meets the walls of the box. Based on a 10” pipe estimation the total system will be approximately 6.5 gallons of water.

Basing the flow rate on other similarly sized hydroponic systems we will need only about 2gph over the root section. That said, we are not concerned with how quickly the water moves through the return tube however we do not want to cause cavitation or unnecessarily high speeds in the return tube. We can confirm this by checking the rated flow rates of PVC pipe. Checking a table for this we find that a ½” pipe would still be good for 420gph, so far greater than what we will be subjecting it to so we can determine that inner tube flow rate will be negligible in our decision making. A ½” pipe in the center would make for a very small prop that can fit in the tube to hide it from roots. This will limit the efficiency of the circulation pump because blades length will be significantly reduced just from the shaft of the prop. A ¾ in tube gives us a more comfortable 20mm prop diameter which is comparable to many RC boat propellers.

For this design we need a pump to gently push the water through the system and pull bubbles from the aerator. This will help keep water from stagnating and will help mix in and evenly distribute additions to the water such as air or nutrients.

We need a pump that will create a slow flow across the outer tube while still having enough head pressure to push through the bed of roots that will grow into the outer tube. One additional concern would be how we create an even distribution. If we simply draw water from one side and output it on the opposite side then one end of the tube will have an uneven distribution of air bubbles. If we are able to push water in either direction forward or backward we can more evenly distribute oxygenated water to more of the plants.

A centrifugal pump with a 360 degree exit similar to many shop vac pumps is one option for use. Centrifugal pumps are very capable of dealing with higher pressures and will evenly disperse the pumped water across the circumference of the outer tube at their exit, but centrifugal pumps do not allow for reversing the flow as the impeller on these pumps only creates pressure when driven in one direction; reversing the direction of the motor does not reverse the direction of the flow, it simply makes the pump inoperable. Additionally, centrifugal pumps require very different impellers and volutes to move water compared to air due to the major differences in density. Because bubbles will be readily present in the water and will not leave the water volume, we would prefer something that is capable of moving either water or air or most likely a mixture of the two.

For these reasons a traditional boat style propeller would be a better option. Props can be run in forward and reverse with little to no difference in efficiency depending on the blade geometry. Additionally, a prop will also be capable of pushing air if bubble density increases which will prevent stalling movement of the fluid mixture. To protect the roots from getting chewed up by the prop we can hide this prop in the inner tube with a long shaft leading to the motor. To protect the motor from water we will use a very simple seal design that prevents the need for gaskets that could wear out or an engineered sealed bearing. Most hobby RC boats are able to seal their entire motor and electronics chamber from the hole the shaft exits by means of a channel surrounding the shaft that is filled with grease or another hydrophobic paste that will not harden or solidify. This allows the shaft to spin and will repel low pressure water due to being much more viscous. As shown before water in space exerts very little pressure on its surrounding container and prefers to cling to itself due to surface tension. The main pump body will be built into the end cap of the 2 tubes and will provide channels from one tube to the other after the prop. We will drive this pump with a simple hobby motor, again looking to RC boats for inspiration. These motors will operate from 3v to 12v and can be reversed by simply reversing the current path. When ducted through a tube they can achieve flow sufficient to disperse bubbles throughout our system. We should not need to worry about over current on this motor as it will not be among the roots where it could tangle and halt rotation of the rotor.

The air pump on this will take some experimentation in microgravity to determine the efficiency gains from the unique behavior of bubbles in space. Potentially a pump would not be needed if simply blowing bags of air into the system could create enough dissolved oxygen, but that will require testing. For my purposes I chose a pump that can provide closer to 1 liter per hour for every gallon in the system. This may prove to be too much and could push water out of the system, but could be cycled on and off to reach a proper balance. The pump I am using is a small aquarium air pump that provides ~5lph. This uses a diffusion stone to minimize the size of the air bubbles to better disperse and the prevent air gaps in the pump. It will fit into the same septum valve used to inject nutrients. The nutrient mixture will be able to pass through the pores of the stone and when the air pump is connected back to the inlet it will push any remaining fluid trapped in the stone.

To rotate the cylinder at a rate that will give ¼ rotation over the course of a grow cycle this will need a very slow rotation. For lettuce that takes 3 weeks to grow this will need the cylinder to make one full rotation every 12weeks. This equates to about 0.000008 RPM. This motor will not need a lot of power due to the slow speed, low friction when in microgravity, and a significant gear reduction.

If we look at a typical hobby motor like the one we will use for the circulation pump motor these typically spin at about 2000rpm and come with gears with 10 teeth - less than that makes it difficult to fill an entire circle. If we calculate the necessary gear reduction for such a decrease in RPM we will need about a 2.4 billion tooth gear. If we swap the standard gear for a worm drive, we will still need a 240 million tooth gear to get this kind of reduction in speed. Even if we break it up into 3 sets we would still need (3) 1:1000 gear reductions. All of these options are preposterous for the space restraints; additionally, this will only work for one length of plant cycle, a 4-week cycle massively increases these numbers.

Because of this I will use a stepper motor to allow us to worry much less on the gear teeth ratio as we can tell the stepper to move even as little as a single step per day. This also allows us to change the length of the cycle by only changing the programming rather than getting new gears manufactured. For this I will use a small 28BYJ-48 stepper motor. This will allow very fine steps because it is internally geared down 1:64 speed of the actual rotor. This gives us 4096 steps per rotation. Using a 23 tooth gear on the drive and a 300 tooth gear on the cylinder we will need about 26.5 steps per hour to achieve our proper rotation speed.

The frame will be very simple extruded aluminum T slot with fittings to connect them into a cube. Aluminum is light weight and the T slot profile aids in simply connecting the cube and holding the faces, additionally it allows cubes to be rigidly linked together in a farm like scenario, and easily allow instrumentation to be attached to the frame using common methods.

The faces will be made of lightweight semi rigid flexible plastic sheets, to allow for rolling up into the main tube for transport, that are either clear or white. Clear would give more visibility of the plants from all angles and would help in viewing issues with plants and aiming your hand when harvesting or planting, but would offers no benefit in light saturation. If we use white colored walls all light that hits them will be more efficiently reflected back at the plants further amplifying the light that can be used by all plants in the system. Because I am not in microgravity and need my walls to support themselves and the attached lights against gravity for my prototype I will be using more rigid pieces of marker board which are white and semi water proof on the inside. Most walls can simply slot into the openings of the extruded aluminum rods as it is assembled however the wall at the opposite side of the pump we will want to be removable so it will be rigidly fixed to a piece of the T stock which can slide in and out of the remaining 3 walls, this allows access to the inside of the cube for harvesting or planting.

Previously, sodium halide lights were the most common grow lights due to producing high intensity light in a spectrum that is favorable to plant growth, but these lights are large, hot, and very energy hungry. The next step form sodium lamps was florescent lamps, which run much cooler, can be smaller, and are more energy efficient. However, since the advent of blue LEDs and the rise of the semiconductor industry we can now make very cheap LED strips that are much smaller than either florescent or sodium lamps, generate very little heat and take a fraction of the energy florescent lamps require. Further more LEDs can be tuned to the most absorbed wavelengths for plants maximizing their efficiency. Strips wide angle LEDS with a mixture of blue and red wavelength are commonly produced for indoor grow setups. Strips can be mounted to the faces of the grow cube with stand offs that assure they are directed toward the center of the tube maximizing light intensity reaching the plants. Wide angle LEDs will also assure the maximum amount of side angle light can penetrate past the taller plants and enable the undergrowth that this circular design enables.

The fan selection for this is minimal. Plants grow stronger if they have a slight breeze compared to still air, and a fan helps push respirated air away and draw fresh air in, increasing the rate of growth of the plants. I will use low profile computer fans to keep a steady air flow with minimal energy usage in a compact package. One special consideration for these fans will be to have fans that can reverse direction. This will allow us to put 4 fans around the back of the enclosure only and have the fans flip flop between pushing or pulling the air. This lets us have multiple wind directions to avoid biasing the plants in a certain direction - without needing to put fans on the front removable face. One key reason we chose to put them only on the back is because this will allow all sides with LEDs to be the reflective white material and the front face is removable. This also allows introduction of new air into the system even if an arbitrarily large number are stacked side to side because the fans would not be blocked if 2 cubes are put next to each other.

This setup will need a control board not only for the stepper motor but also to control cycles of lights fans and pump and aeration. For this I will be using an esp8266, this is a microcontroller similar to an Arduino but it is cheaper, smaller and offers the ability to generate an access point which will allow us to remotely monitor all aspects of the design such as pump cycle time, light status, grow cycle state, and any other aspect we include, for example; if we add a temperature, PH, or water level sensor we will be able to get notifications if any of these fall out of range. Additionally, this will enable us to make on the fly changes via a convenient interface to adjust any parameter of the build like cylinder rotation rate, aeration level, pump speed, etc. These boards are commonly used to create IOT devices an are very reliable and are capable of deep sleep modes to conserve energy.

This microcontroller will need to connect to all aspects of our design and allow controls of each. This micro will be connected to a PCB that contains 6 relays, one for on off of the aeration. One will be used for on off of the LEDs, while the LEDs have 3 connections, we will only need all lights on or all lights off so we can combine the ground connection for both the blue and red LEDs and control the on off via a single relay. 2 will be used for the circulation pump, this will allow us to reverse the direction of the pump forward and reverse. And 2 for the fans, again to allow forward and reverse wind directions. Additional control is capable using this microcontroller such as PH monitoring and control, or water level monitoring and control, it will simply take a PCB with more connections and likely more relays, however this will be the most I will add for this prototype. That said, the design of this cube could allow the PCB to be as large as a 450mm square which could accommodate far more relays and other components that would be needed to monitor every possible aspect of each cube.

I will be using Catia to model the design outlined in the steps above and I will try my best to walk a user familiar in 3d design through the general steps that should be available in most 3d software programs. The frame will be largely modeling the t slot beams based on the dimensions given by 8020 t slot supplier 8020.net. I will be using the 20-2020 which is a single set of 20mm wide t slot profiles dimensioned as below. This is mostly for being able to make earthbound version of this but a frame for space use could potentially save some small amounts of space by using a smaller profile.

One key step that I use to facilitate modeling is using a master sketch for important parts of the design. This sketch will not directly drive geometry but many sketches we use later will reference this sketch by projecting lines that are in the master sketch. Two main things we will layout on the master sketch are the frame outside dimensions to assure we never exceed the necessary 50cm cube and the 2 pipe sizes and their location. Shown earlier in the introduction section is the majority of this sketch. This can be a convenient single point to adjust major decisions like tube size, frame size, etc. One important note of how I have positioned this sketch is that I have centered it on the axis of the part file, this will let us very conveniently pattern other elements of the cube without creating extra axis’s.

Next we need to create a single bar that matches the outer dimensions of the 8020 bars. In this sketch we will use projections of our master sketch to define a single corner frame element.

Extrude this sketch to the total length of the t slot beam you are using. I chose to make the t slot length 450mm leaving me 25mm to create a fitting to connect the multiple t slot beams. Again, I chose to make this centered on the axis system to aid in patterning the rest of these beams to create the total cube.

Create an axis through the center of the bar, a quick way to do this is to create 2 planes that run through the corners of the beam and define an axis on the intersection of those 2 planes. This will be used to create the patterns around this one bar.

Create a sketch that matches the dimensions of the slot based on the 8020 provided dimensions shown in the figure above. Again, we can utilize our previous sketches and planes to grab the outer dimensions of the first 8020 piece and the center of the beam we defined earlier, this lets us easily center our sketch on the first. We will only need to define one slot as we will be able to pattern it around the bar.

Create a pocket from this sketch that run the length of the bar.

Pattern the slot around the bar using the axis we created defining a 360degree total angle with 4 instances, and voila we have our first 8020 section.

Next, pattern this bar around the axis parallel to the bar length on the main axis system that we centered the cube master sketch on, and we will have 1/3 of our frame done.

This lets us now pattern this pattern to create the other axis of bars by patterning this assembly in 90 degree increments about the other axis’s of the main axis system to create an entire cube.

We are now finished with the body or part (depending on your modeling software) of the 8020 used for the frame.

We will now move on to the connecting corner elements of the 12 bars.

All but 2 will be identical, those 2 will be the ones that provide a slot for the removable face to be drawn up and out of the frame to allow access to the plants. and extra support points for the rotation axis holders on that same face. These 2 unique corners will simply have a slot cut through the main body and will be missing the t slot elements on the side the slot runs through.

We will start with the corner elements that we are able to pattern due to their similarities. Pick a face on the main axis system that will be perpendicular to the axis of the main grow tube, we will start in a corner opposite of what will be the front removable face of the grow cube. Start with a sketch, again centered on the main axis that projects the sketch of the t slot we defined earlier. Then, offset that sketch by about 0.1mm to 1mm depending the tolerance of the manufacturing method you are using for making the corners. I typically use 0.5mm offset for my 3d printer and get reasonably easy to assemble results but higher grade printers or skilled machinists can usually achieve better and lower grade printers or hand cut parts may need more. The difference in this just leads to more slop in the assembly but this can be mitigated when the assembly is screwed together.

The extrusion for this sketch will need to be offset from the main axis system to allow us to continue to use the that axis to pattern the parts to the other 7 corners. 10mm should be sufficient to align the part in the 8020 then the offset of the total length of 8020 bar from the total 500mm allows us to create a space for a block to connect the alignment features of each bar. That block will also hold the bolts or screws that secure to the 8020 to hold the frame together.

To make the rest of the corner clips we will start by patterning this offset slot profile around the first bar then around each subsequent bar in the corner it is based in. We will then create a block that joins these. Lastly, we will pattern this clamp in a similar way to how we patterned the bars around the whole cube. The axis's necessary for these patterns can be achieved with methods similar to the first axis by drawing 2 planes between corners and defining an axis as the intersection of those axis’s.

Next we will need to add holes for screws that will drive into the center of the 8020 bars. This will provide most of the strength of the cube. We cannot however simply have a face mounted screw for 2 reasons. First, it will exceed the outside dimensions and prevent the cubes from mating up flush. Second, the screws will intersect through the center of the cube because holes are centered on the 8020 bars. To get around this we can over counter sink these screws such that the face of the screw when fully driven is inside from the center line of the other screws and clears the screw head diameter.

Start by making a hole centered on one the axis’s of the 8020 bars. The hole in the center of the 20mm 8020 is about 5mm diameter. To allow clearance for the screw head of a screw that will fit the 5mm hole we will over size the countersink hole to 12mm diameter.

Repeat this for all 3 outside faces of the cube to make all 4 clearance holes.

Now create a hole matching in size of the 5mm hole in the 8020 bar in the center of each counter sink hole.

Do this for all 3 faces again.

Now we can pattern this part around the central axis system similar to how we did for the bars to fill out the structure of the cube.

We still need to modify 2 of the cubes which will be unique for the face that is removable. This face will be the face that provides the water filing port and must be perpendicular to the axis of the main growing tube. The main change to these 2 corners is to remove the prongs that reach into the slots and create a pocket that will allow the face plate to slide up and out from the slots in the 8020.

Now we can make the faces of the cube. All faces will be identical for now, but we will later add holes for parts that get mounted to the faces like the LEDs, the air pump, the grow tube mounting points, the fans and the rotation motor.

Make one face then pattern around the cube like before. I’m only showing the faces as clear for purposes of being able to see the whole model. As stated in the planning section it would be beneficial for light intensity and distribution if the faces are made of opaque white material to reflect light back on to the plants.

One last step before we move on to other components will be to remove the lip of the face above the removable front face. Without doing this the portion that slots into the frame piece of this face will lock in the removable face. Create a sketch on the face above the 2 modified corner clips. Then create a rectangle that is just outside of the frame piece and overhangs all other edges of the face we will modify. Lastly create a pocket that goes completely through the face in question but does not affect the other faces.

This will conclude what we will be designing on the faces for now. We will revisit the faces after we have created more parts and especially so once we create the bought parts as these will all interact closely with the faces.

Next we need to start on the grow tube.

Using the sizes we defined in the master layout sketch we can simply project the circle that define the tubes onto a sketch that we will extrude to create the grow tube. We chose a 10in tube for the outer tube and we will be using a ¾” tube for the inner tube. Even though we proved we will not have flow rate concerns in a ½” tube we will get more purchase on any air gaps caused by the bubbles by going with a slightly wider prop which is better suited for moving air and therefore a wider tube. A ¾” tube will allow us to use a 20mm prop for the pump propeller which should be plenty to create a breeze in any air gaps that will draw water towards it to avoid stalling in a bubble as could happen in a centrifugal pump or a pump tuned just for water. We need to offset the face of the tube from the faces of the cube to allow room for the end caps with rotating mounts which we will design next.

The inner tube will need to be slightly shorter on both ends to accommodate the flow paths between the inner and outer tube so we will use a thicken operation to reduce each side to allow channels to pass the flow between the 2 tubes. This will limit the need for extra sketches or hassles like that. We will need space for an air stone on one side and space for the pump on the opposite side. The motor is about 12.5mm tall, the shaft is 32mm long with about 12mm of propeller. The air stone is about 25mm long. For this I will reduce each end of the internal tube by 30mm, this will ensure our prop still remains in the tube (much more efficient flow is achieved by keeping the walls of the pipe close to the blade of the prop) and the sealing area for the shaft and motor as well as the air stone on the opposite side will not block the tube.

Next we need to add holes for the plants. Lettuce root bases are on average around 3in diameter or smaller. For this we will create an array of 3in holes around the tube. The holes would ideally be around 6in apart on the length to allow room to grow. However due to the length of the tube it would be better to more closely space and fit 3 plants over 2. These will be made from one sketch patterned around the tube 8 times.

Lastly we need to add some smaller holes for plants that will live under the main plants such as young versions of the large plants or other smaller growth plants. This could be extended further to even smaller holes in between the large and medium holes to saturate a mat of microgreens that would fill the entirely of the unused space below the main plants however I am not going to include this in the model.

Now that we have all the plant entrance holes we will add some plugs into each hole to represent the foam plugs that will hold the plants in place and prevent water from leaving the cylinder. For this we can use the same sketch we used to create the holes but will create a pad using this sketch instead of a pocket. We will pattern this pad same as we did for the holes, then repeat this with the smaller holes.

We can now start on the end caps. These will be a single piece that connects both tubes while also providing ducted channels for the water path and all the mounting and assembly points for motor or the airstone. Additionally, the side with the motor will have teeth around the perimeter of the end cap, we will use these teeth to interface with our stepper motor to control the rotation rate of the tube. Because the main portion of the end cap will be rotationally symmetric, we can utilize one sketch and a revolution command to create the majority of the end caps. Due to slight differences in the 2 end caps we will not mirror the revolved parts and will instead make 2 unique ones. There are 6 main features we need to include in the motor side:

· Fitting for outer tube

· Fitting for inner tube

· Flow path for water

· Chamber for hydrophobic paste

· Motor mounting slot

· Gap for cover plate to seal paste chamber

Revolve this sketch around the central axis of the pipes and we will have one end cap mostly complete.

You may however notice that we have created 2 parts that are not connected. To resolve this we will create some ribs through the flow channel that will connect the 2 parts and evenly duct flow around the pipe. We do not want to obstruct the outlet near the pump so we will make the ribs start just outside the outer diameter of the pipe and stop just short of the hooked shape in the channel. The fins should be a very smooth profile and preferably thin with pointed tips and tails to avoid creating turbulent flow. We will pattern this rib profile around the center axis of the part.

Next we need to create some mounting points for one more part that will mate with this piece that will cover the motor from the outside and provide a shaft for the assembly to rotate on, in the center of that shaft will be a barrel connector for the motor, using a barrel connector will allow power to be provided to the motor without twisting wires despite how slowly that will happen with a 12week rotation time. It also provides and easy way to unplug power from the motor for disassembly or moving. We will need to make one more adjustment to the main plate to aid in assembly of this, a groove that runs around the width of the cover plate, and 8 holes for screws to hold the plate to the motor cover. The groove can be added to the main revolution sketch, the holes will be patterned around the center axis. We can place these features in the thick area caused by the back hook in the flow channel to keep the part robust.

Last thing for the pump side cap we need to create is a large gear around the perimeter of the part that will be used in conjunction with the stepper motor to control the rotation of the part. This will sit off the current back face of the part to get this gear closer to the face that the rotation motor will be mounted on. First we need to define a sketch for the backing rib to support the gear teeth. We want the outside of these ribs to sit a tooth length inside from the outer edge of the main part to keep the teeth well supported and the entire assembly compact.

Next we need to create a sketch for one tooth of the gear which we will then pattern around the part. We want this to come approximately to the edge of the main circle and we want to make sure we make it large enough to be robust but small enough to fit many on the gear. Because we will be using a stepper motor we do not need an extremely large number of teeth but more is better. Using a 2.5mm root and 2.5mm height we will have a comfortably robust tooth and can fit 300 on the main gear. Using these numbers, we can map out 3 of these teeth to better show how it will mesh with our drive gear. Using the same tooth profile we will map out the drive gear, doing this we can fine a happy medium of a 23-tooth gear with a. 8.8mm radius. Because this will move so slowly we do not need helical gears or well rounded teeth to prevent gear chatter as it will move very slowly, we just want to assure this will not bind.

Pattern this around the gear body with a complete crown with 300 teeth.

This completes this part and it is now ready to print. I will be using printed parts for both pump end caps for convenience, but this could prove to be a very good method for parts to be sent to space as they can very easily made hollow yet strong and can create internal areas such as the complex shape inside the pump exit without the need to make it 2 parts.

Next, we will skip the motor plate in favor of creating the opposite end plate because we can leverage the design of the first plate by copying the sketch then creating small modifications. Pasting the pump side sketch not as a result or a curve but as specified in part document lets us create a sketch that has all the same features as the first sketch. We will then modify this sketch to suit the need of the injection side end cap. This side will need:

· Same flow path as pump side (to keep even flow speed independent of pump direction)

· Space for airstone without blocking pipe entrance

· Channel to fit air stone inlet

· Gap for soft silicone septum valve

· Channel for alignment of seal plate

· Lips for outer tube

· Lip for inner tube

Again we will simply revolve this around 360 to create the body.

Again we will need to create fins to connect the 2 parts of the volute, for this because we copied the sketch of the previous end cap we can simply reuse the fin sketch and pattern it just like last time.

Lastly we will create the screw holes for the cap on this side that will seal the septum valve in its gap and like the other cap will provide a shaft that will control rotation of the cylinder. This will again be patterned around the central axis, however due to the difference in geometry the best thick spot is closer to the central axis so we will only use 6 for this side.

Now you may realize that we just created a part on top of another part because we simply copied the sketch. Because we made the inner tube offset the same for both side and we constructed this centrally around the main axis we can simply mirror this over the central axis to move it to its correct location to finish this part.

Now that we have both endcaps designed, we will create the caps starting with the motor side cap. This will again be made using a revolve but will need much fewer features, just a rib for the alignment slot, a gap for the barrel connector and a shaft to rotate on. Because this will be used in microgravity, the friction will be low and the rotation rate will be so slow, we will not need a bearing, simply a shaft and a hole to capture it. Because this will sit so close to the face, and will need to exit that face, and that face is not removable, we can simply have a hole in the face this cap fits into to stabilize it.

Next we will add the holes for the screws using the same sketch we used to make them in the main piece to assure they are perfectly aligned and will stay that way if we change the position of the holes.

Now we will make the cover for the aeration end cap. This cover will have a bump similar to the bump on the aeration end cap to compress the silicone septum valve and keep it from moving. Additionally, it will mate with the alignment groove we made in the end cap.

One important note on the rotation shaft for this side. Because we want to make the face on this end removable we cannot simply use a hole in the face to support it. Because we do not have a gear on this end, we have room for a thin piece of material to run from edge to edge mounted to the frame to support this end. That said we do not have much extra room on this side so we will draw up the outer edge of the cover plate to act as the rotation shaft, this allows twice the shaft length compared to using a shaft off the face of the screws rest on. This will also make it so the screw heads are recessed from the cube face

The thin support piece for this side of the tube assembly will be a very simple piece of the face material that is connected to the frame just inside the removable front face. The main considerations of this piece is it must be wide enough to contain the shaft with width for strength, but it should not be so wide you cannot access the plants for planting and trimming. Lastly it must be tied into the frame pieces perpendicular to the removable frame end such that this piece can remain in place when the front face is removed to access the plants.

Now we will create the drive gear for the stepper motor used for rotation. We defined what the size and tooth number would be in the previous design of the main gear on the pump body. For this we will simply copy the geometry defined as construction geometry in the big gear sketch.

After we extrude this we will pattern it to make the 23 tooth drive gear.

We still need a hole for the shaft of the motor, this will be a semi square hole through the entire part based on the shaft dimensions of the 28-BYJ48 stepper motor.

Next we will make a mount for the stepper motor we will use. We will be using a 28-BYJ48 stepper motor which will be too large to sit outside the cube because to maximize the root chamber we need this drive gear to sit directly against the face of the cube. Because of this we will make a stepper motor mount to let the motor enter the gear in the opposite direction. This will be a simple sketch that will define 2 screw holes for the motor and 4 to mount to the face. It will be just tall enough to let the motor clear the gear.

Now we will make some standoffs for the LED strips. Because we will have our plants growing out of a cantrally located tube we will want out LEDs to have the most of their viewing angle directed at the center of the tube. Because the walls of this are flat we will need some risers that will angle the led’s toward the center of the tube. These will simply be thin strips lued to the side walls that are angled such that the center of the LED viewing angle is pointed at the center of the tube. Like with other rotationally symmetric features we will only create a quarter of the standoffs then pattern it around the cube.

Next, we need to create a multi directional propeller that will fit on the shaft of our motor.

This must fit within the inner tube of the system with very little gap between the propeller blades and the walls for maximum efficiency. We will want relatively wide props to increase the head pressure of the pump to better allow it to push through the root mat but keeping some open space allows it to more efficiently push air that might get stuck around the prop due to bubbles.

We will start by creating the shaft that will mount to the motor. This will be a sketch that defines a 2mm diameter hole for the motor shaft with a 2mm wall surrounding it to hold the blades. Extrude this 12mm to form the shaft.

Next, create a sketch perpendicular to the shaft which will be used to define the max width of the blades and will be used to define the angle of the blades. We want the max available width to be at least as wide as our tube interior or 10mm radius. For this prop we want it to be suitable for both forward and reverse flow so we do not want to bias the blade in any way, so we will use a straight profile centered on the midpoint of the body. We will make this pad much longer than the blade as we will trim the outer edge of the prop in a later step, but what this allows us to do is create the offset angle of each prop with a chamfer for a convenient place to adjust the angle of overlap.

A chamfer angle of 45 will align each blade edge directly inline with the next prop, values above this leaves more open space, values less creates overlap between blades, if we didn’t over size the first blade pad then we would not be able to make an angle more than 45deg. For this design we want just a small amount of free space between the props. This will allow for better flow in air gaps while minimally affecting head pressure of the pump. For this I will use a 70deg blade angle.

We will then define a sketch that will trim the blades down to their final diameter which will be just less than 20mm tube diameter.

Add filets to all edges of the blade to decrease turbulence around the prop, then pattern the blade around the shaft to complete the prop.

The septum valve is simply a thick silicone pad with a cut down the center that will hold itself shut until something is forced into the slit like a straw or a syringe.

Instead of showing this just as a flat sheet, I will add a very narrow gap to show where the cut would be in the pad. This material would not actually be removed when making the part, it would simply be cut through with a sharp knife.

This slit will be shown with 2 very large circles spaced just barely closer together than their diameter. I will use 100mm circles spaced 99.9mm apart to show a thin slit. This will then be extruded to fill the space we created in the aeration end cap. We will not make a groove to correspond to the ribs we created as we want those ribs to pinch the septum which will be soft enough to simply squish around those ribs

The motor shaft seal cap is a very simple sketch, just 2 concentric circles one matching the shaft diameter the other matching the width of the space we created to hold it. Extrude this to fill the space we created in the end cap.

The pump motor is another simple revolve but in this same revolve I will include the barrel connector to allow the motor electrical connection to spin freely. The motor is a small 5-12v dc motor and should be sufficient to create a steady flow through the circuit. The barrel connector is a low-profile right angle 10mm barrel connector with 2 contacts. The barrel connection could potentially be swapped with one with more connections in order to add water sensing such as PH or water levels to add more control to the system.

The stepper motor is a main cylinder with an offset shaft and 2 flanges for mounting points.

We will start with the shaft to be sure our model aligns correctly with our gear, then we will create the mounting points to be sure they align with our mount, then we will create the main cylinder based on the dimensions of the motor.

Create a sketch on the face of our motor mount and align a circle with the slot we created on the drive gear, add 2 lines corresponding with the flats of the drive gear hole then trim the extra. We designed the motor mount to be in the proper position to put the shaft end at the far face of the gear, so simply extrude up to the face of the gear.

The mounting holes will be a sketch with each hole centered on the mounting holes we defined in the stepper mount so create 2 concentric circles around each mounting hole and size them to the dimensions of the data sheet. These are connected to the main body with by a straight strip of metal. To make this easily in the sketch we can just make 2 tangent lines connecting each then trim the unneeded part of the outside holes. Extrude this 1.5mm.

Create the main body cylinder with another sketch based on the mounting bracket face, this is centered on the 2 mounting holes so we will create a construction line between the 2 centers where we will define the main circle at the midpoint of this line. Extrude this 19mm.

Lastly we will show the port the wires leave from to aid in routing wires. This will be centered about the main body and will have a face 17mm offset form the center, and 14mm wide. Extrude this with an offset from the face with the shaft. This completes the stepper.

The air pump, because we will need less aeration compared to an earth bound DWC, will simply be a small round piezo driven pump, I will use a Cambani Aquarium Air Pump, this significantly reduces the size and weight of the pump without compromising on the amount of dissolved oxygen due to the effects of bubbles in microgravity. This will create a steady stream of small bubbles.

For the pump I will show it as a circle the height of the housing with a cylinder for the wire exit and a cylinder for the air exit. This pump will live in a cutout on the end with the removable face and aeration port to limit the hose length.

The pump body is 52mm diameter and 17.8mm tall with a 7.5mm aeration port tangent to the top end of one side of the cylinder with a round electric connection opposite the aeration port and centered on the body.

To limit room lost inside the grow chamber we will make this pump sit in line with the frame and we will cut out a notch in the removable face, the pump will need to be at the bottom corner of this face to allow removal without a large slot in the housing.

Lastly there is a small depression around the perimeter of the bottom of the pump.

After completing the air pump we will make a simple clip to hold the air pump in place. This will slot into the 8020 bars we used to make the frame similar to how the corner clips fit in.

The clip for the air pump will be made by outlining the pump body with a squared profile stopping short at where the tangent of the pump body meets the frame. We will start by outlining the grove on the bottom of the air pump and extruding the clip body into the depth of the groove.

Next create a profile that outlines the outer perimeter of the circular pump body and extrude this the width of the t slot bar.

Next create a cut out in this clip around the air hose of the pump.

Then, create a small rib that will outline part of the top of the pump body keeping the over hang to only about 1mm, this will allow the pump to be pressed into the clip and held in place.

Lastly, we reuse the sketch for the corner clips to add some t slot profiles that will fit into the frame to hold the clip in place on both edges that meet the T slot.

Now we will create the LED strips. I will not be showing the individual LED’s just the over all shape of the strips in their proper locations.

The strips are small domes about 3mm tall and 10mm wide. We already created the LED mounts to be 10mm wide at the angled face. We can project each or these faces then use those to place the arcs to create the LED strip profiles. The only one we do not have a projection to reference for placement is one directly at the closest point between the tube and face. For this strip we can center it on the central axis, and place the bottom of the arc flush with the cube face.

Again we will only define a quarter of the light strips and pattern this around the cube. Simply extrude the profile up to a few mm offset from the edges of the frame.

Now we will add the fans for air circulation. I will not be designing these myself, for these because computer case fans are very standardized we can simply use a model from online for 120mm case fan. Any 120mm case fan will approximately match these.

McMaster-Carr has models of nearly everything they have on their site so we can simply find a case fan on McMaster that matches the size fan we are looking to use, in this case a 120mm by 20mm case fan, we need not find one specified as reversible as this will only mean a small change in the blades of the prop but the overall size will be standard to any fan of this size.

We can download this as a step file then import this into our model as a body. Once we have it imported, we will need to move it into position using translations and rotations as these step models will come into a model centered around the absolute axis which in our case will mean the center of the cube.

We want to move it to where it is in a corner of the pump side face then we pattern it around the cube.

We will now add wire routing representations to all the electrical components.

The LEDs run of a 3 wire connection, one for 12v+ and one for blue range LED ground and one for red range LED ground these will be routed to connect all led strips consolidated to one conection as each are wired in parallel so we can simply connect each wire to its corresponding wire on the next strip. Then from one center strip we will run a 3 wire connection up to the control board.

The fans will be similarly parallel routed to one another then connected to the control board via one connection.

The pump will be routed quite simply directly to the control board via the barrel connector.

The aeration pump comes with jacketed connector that makes it look like one wire, this will be routed along the frame edge until it reaches the same point that the led wire exits to limit the number of holes in the housing.

Finally, the 5 wire connection for the stepper will be routed to the control board leaving the faces immediately from the motor then routing to the control board.

For all of these wires I will use a rib/sweep with a profile of tangent circles defining the number of conductors needed, 3 for leds, 2 for fans and pumps, and 5 for the stepper.

Now we will make the control board. For this I wont show every small component such as resistors, but will show the major components.

The main large components are a barrel connector to provide 12v power to the entire system, 2 relays for the water pump motor to allow forward and reverse operation, 2 relays for the fan assembly again to allow forward and reverse, one relay for creating a light cycle (we do not need 2 relays because both red and blue connect to ground to activate so we can connect both via one relay), one relay for the aeration pump on/off, and one esp8266 microcontroller.

The microcontroller will allow us to control the timing of all aspects of this including all relays and the stepper motor.

These will all be placed on a single PCB mounted to the pump side of the cube where we routed all the wires to.

We need to show the connection of the air pump to the septum valve. This will be created much like we did for the wire connections; a center curve leading from the air exit of the pump to the septum valve.

Finally, now that we have defined the locations of everything that will need to penetrate the faces of the cube we will now add the holes necessary to accommodate the parts that must pass through the cube. These include; a hole to allow access to the septum valve, a cutout for the air pump bracket, a hole for the pump motor standoff and barrel connector, a hole for the aeration pump wire pass through on the water pump end, a hole for the led wire pass through, a hole for the stepper motor wire pass through, and 4 cutouts for the wind fans and perforations on the opposite end of the fan to allow circulation.

Gather the marker board and cut out (6) 18.5" sections. These will be our faces

We then need to create the gaps for the corner clips. For this non functional proto I will simply chop the corners off to create this gap rather than create the exact outline.

Once we have 6 of these we need to modify 3 of them.

One will be the rear sheet which will have a small hole for the barrel connector and motor side mount and (4) 120mm cutouts for the fans.

One will be the front face which will have a cutout for the aeration pump mount which we can simply outline to create this cutout.
The last one we need to modify is the top face which will need a strip removed to allow the removable front face to slide out.

Start with a bottom LED face trapping the face in between the frame pieces. then add the 4 support vertical to this and slide the face pieces in. Then finish by adding the corner clips and sliding in the top face piece.

Don't forget to include the pump motor mount like I did or you will have to disassemble that corner to allow it to slide in past the corner clip.

Then add screws to all 3 holes in every corner clip except the ones for the removable face which will only get 2 each.

This completes the frame.

For this non functional prototype I will simply be using a concrete form tube to show the grow tube, the working prototype will use a PVC tube rated for potable water use.

Cut the form tube to the total length which for this design happens to match the length of the 450mm 8020 bars so we can use one to measure the cut line.

Next add the grease to the motor shaft seal. For this i will be using a grease that i will fill into a syringe to easier add it to the chamber.

Next add the grease cover, then add the motor and press the prop onto the shaft. Run a quick test to be sure everything spins freely.

Then add the motor shaft cover with the motor wires coming through the barrel connector hole. Solder the barrel connector to the motor wires and press fit it into its hole.

Add the air stone to the hole we made in the aeration cover. Then make a quick septum valve by cutting a circle of silicone and push an exacto knife through it to create the slit.

put this in its hole and add the cover. Run a quick test to be sure a syringe can pass through and can push water through the air stone.

Next mount the inner tube to one of the covers, then add the outer tube to the same cover. Then press the other cover onto both tubes.

Now we need to make plant holes. This will be done with a 3" hole saw. Cut 3 holes spaces 140mm apart and create 8 of these patterns around the outer tube.

This completes the grow tube for this non functional prototype, we can now add it to the main cube with a strip of the housing material to trap the aeration end and with the other end poking through the hole in the rear face.

The aeration pump simply snaps into the the bracket we made for it., then measure the air hose length and trim it. Then add a tip to penetrate the septum valve. When you insert this into the septum valve it should be able to spin without dragging the tube.

The LED strips attache to the faces with adhesive, i did not make LED standoffs for this nonfunctional prototype as this is just to show the general concept.

Add the drive gear to the stepper, then mount it to its mounting bracket. We will then use this assembly to place where it will mount to the cube. It should engage with the main gear on the grow tube you can check this by turning the main tube manually to be sure both turn to assure engagement. Mount the bracket to the rear face.

Lastly we need a foam plug that would hold the plant. For this i just cut the foam with the same hole saw used for the main holes

This finishes all that will be done on the nonfunctional prototype.

For a prototype that can be built and tested on earth I will demonstrate it will need to be able to hold water without the benefits of microgravity. To do this I will demonstrate this with a vertical drip system. This will be able to demonstrate the benefits of this geometry in use of space and can show how a micrgreen mat can grow beneath plants when they are grown on a cylinder surrounded by lights. This will also demonstrate how a setup like this can allow staged growth and partial harvests to further maximize the usage of space while also creating a more steady anc constant rate of food production. is is because I cannot have the plants live upside down on earth as all water in the setup will quickly drain away.

Simultaneously I will assmble a way to prove out the deep water culture aspect of this design, however this also must necessarily make allowances for functioning in gravity. It will have grow holes around only 1/4 of the pipe and instead the cube will rotate instead of the cylinder. Additionally because i do not have the benefits of bubbles in microgravity this will use a full size aeration pump. This too will be able to show the benfits of stages growh and benefits of growing on a cylinder surrounded by lights.

I believe that the combinaiton of these 2 tests should fully prove this design is worth of further pursuit in taking it to microgravity.

Thank you for your time in reading this report.