The reason I asked was basically because the amount of plants, as well as plant needs would dictate the best type of system. But overall for a continued supply of lettuce heads, 3 separate grow stages in rotation is a good plan (seed propagation, seedlings, then maturing plants). However the NFT/spray system will become complicated and more expensive on a large (especially commercial) scale. As well as the spraying will become unnecessary with the 3 growing stages because the roots will grow long enough in the second stage. The two most common methods to grow lettuce commercially are either NFT, or Water Culture systems. Weather the system you use for the maturing lettuce plants (any variety) is an NFT, or Water Culture, I would build the the propagation and seedling setups the same.
The proprigation
Propagation trays you can get at most hydroponic shops and cost only a couple of dollars. They generally hold about
72 sprouts (cells) in each (10x20) tray. Two twin 2 foot florescent
lights (T5, or T8), would be sufficient for up to two 72 cell propagation trays for lighting (less light would probably result in elongation). Or two 4 foot twin lights for up to five 72 cell trays. Depending on location and time of year, you may need to use artificial heating or cooling to keep the air temperatures in range.
The seedling setup
Each sprout would now be transferred/transplanted into the basket/cup they will mature in to keep labor to a minimum, as well as allow the roots to grow through the baskets/cups (making it imposable to transplant them again without root damage). Smaller heads like butter lettuce, or grand rapids leaf can probably do fine in 2 inch cups, while large heads like romaine lettuce will probably take 4 inch cups.
The easiest, simplest, cheapest, as well as most space productive seedling setup I can think of is a water culture system. How large depends on how many plants you will be growing. For a commercial setup, a custom built system would be the cheapest as well as most productive. But for a home system, modifying short plastic storage totes with a lid would be the cheapest, as well as simplest (about $8 ea at wal-mart). Using storage totes like the ones I've used in the past. (picture attached). Light proofing the base by painting it black, then white to reflect light. Then cutting holes in the lid spaced closely together (like the attached pictures). They wont be in the system long enough to become crowded.
The main system for maturing plants
The easiest main system could be either a Water Culture, or NFT system. Depending on how many plants will be growing in it either would be very productive. Fallowing the same design setup as the seedling setup, and expanding it to multiple growing chambers as needed is relatively simple. Just spacing out the holes for the lettuce plants farther apart to accommodate the full grown plant. Like to 8 spots per container like I did with my lettuce. However if a large number of plants is planed, or large verity like romaine lettuce are planed. A costume built system would be more cost effective, as well as productive. But again NFT systems are very productive for growing lettuce, although the 4 inch square fence post tubing is very expensive for such a setup. But again large varieties like romaine would benefit from the oversized tubing because of their size. Though small varieties could just as easily use rain gutter downspout tubing instead.
Plant Rotation
Deciding on how many plants you want to harvest weekly will tell you how large of a system/s you'll want/need to rotate production. As an example if you wanted to harvest 1 lettuce head every day. That would be 7 a week. Generally lettuce plants (most smaller varieties) take about 6 weeks to go from seedling to harvest. So 7x6=42, then add another week for the
seeds to germinate (7 more plants)=49. So in that case you would want a minimum of 49 total plants going at any given time to ensure the harvest quota of 1 lettuce head harvested every day. Also each week starting a new batch of at least 7 new sprouts (probably a few extra in case some don't germinate).
So depending on your growing conditions (a large factor in how fast they grow). Sprouts would be ready to put into the 'seedling system" about a week after germination, where they would remain for about 3-4 weeks. From there they would be large enough to go into the main maturing system for another 2 to 3 weeks. So I would expect it to go something like this;
14 to 28 germinating sprouts at any given time (first 2-3 weeks)
21 to 28 plants in the seedling system at any given time (next 3 to 4 weeks)
14 to 21 plants in the main plant maturing system at any given time (last 2 to 3 weeks)
Total growing time "once seeds germinate" between 6 and 9 weeks. Then taking these numbers I would design and build the system/s to accommodate at least that amount of plants. Then I would also know what I could realistically expect in the way of harvest, as well as cost to build, space needed etc.. Even energy consumption once I knew the amount of lights I needed, air
pump/s, water pump (if the maturing system was a NFT). By gathering the total wattage for each electrical device, and figuring the amount of hours each will be used. Then imputing that data, as well as what I pay per kilowatt hr (found on your electric bill) into a
Electricity Cost Calculator.