Most locations off the Earth do not have automatic recycling of waste products, and a space project will have a finite amount of materials that have been processed beyond the raw state. We define Recycling as the application of energy and processes to convert materials from a non-useful state back to a useful state. It shares some technology with production from raw materials, except using materials that have already been processed previously and are not in their raw state. In fact some recycling methods will involve feeding waste products back into a production process.
A goal for recycling is to achieve Closure, where all waste products are converted back to useful states and the only external input is energy. We expect this to be a theoretical limit similar to conversion efficiency in electrical transformers and motors. Those devices can reach 98% efficiency but not 100%. Similarly, practical recycling is expected to reach a high percentage, but not 100%. To the extent your percent closure CL% approaches 100, the amount of new raw materials required is reduced to 100-CL% of an open system with no recycling. So a high percent closure can have a dramatic effect on the need for new raw material processing or replacement items brought from elsewhere.
Some items by their nature are not amenable to recycling. A prominent example is reaction mass expelled outside a gravity well by a propulsion method. You are deliberately throwing that mass away in order to get thrust, and outside a gravity well there is no practical way to recover it to use it again. Within a gravity well, such as launching from Earth using a chemical rocket, all the reaction mass is sub-orbital - ranging from about -1/2 orbital velocity to +1/2 orbital velocity depending on the vehicle velocity. Therefore it all ends up back on Earth and can be used again. While some reaction mass cannot be recovered, you can deliberately choose high efficiency methods that lose less mass this way, and use methods such as gravity assist that do not lose propellant mass.
Closed Loop Life SupportEdit
Humans are biological organisms with certain requirements to continue living, including clean air, water, and sufficient food. They also produce waste products from the human standpoint, such as CO2, urine, and feces. On Earth those wastes from the human standpoint are necessary inputs to plants and other organisms, and solar powered evaporation produces clean water in the form of rain. The natural cycles form closed loops. For space projects we have to replicate the function of those closed loops with artificial systems, or supply air, water, and food from external sources. The more time and the higher the number of humans the greater the outside supply mass becomes, and so the more desirable a closed loop system becomes.
Besides inputs such as air, food, and water, humans additionally need controlled lighting, temperature, pressure, radiation levels, and acceleration. Prepared food and feces are complex from a chemical standpoint, there are secondary volatiles and shed skin produced by humans, and body and clothing cleaning are desirable. Therefore a full life support system for humans is complex. We apply the Systems Engineering approach of dividing it into simpler sub-systems that each perform part of the total functions required, and then optimize the full system as a whole. In addition to the direct functions that provide life support, such as growing plants producing food, there are also indirect functions caused by meeting the requirements of the plants, such as water, illumination, gravity, and C02 concentration. The methods listed below include meeting both direct and indirect functions.
Humans and plants are evolved in a 1 g environment. Without gravity, human bones deteriorate and some plants may not grow properly. With gravity, water circulation and dust settling operate by familiar methods. If local gravity from a large satellite or planet is not sufficient, artificial acceleration can be generated by rotation.
Photosynthetic plants need sufficient illumination of the right wavelengths to grow, and humans need sufficient light to see and navigate. Having both evolved on Earth with the Sun as the main source of illumination, both use the wavelength band that the Sun emits the most power at, known as Visible Light. There are slight differences in intensity, day cycle, and wavelengths for various plants, and for what humans need to see properly. Natural sunlight is easily obtained at many locations in space, so it certainly should be considered if available. Artificial lighting is available for circumstances where natural sunlight is not sufficient, but that usually requires a power source to operate. For a large area of plants that amounts to a lot of power, so natural sources are usually preferred, even if it needs to be concentrated.
Plants on Earth often grow in soil, which is broken down rock plus microorganisms, water, nutrients, and organic matter. Hydroponics dispenses with the inert rock component, which can save mass for space projects. If radiation shielding is required, though, the inert rock component can serve a dual function. While fully developed soils are only known from Earth, there are many sources in space of small rock particles, water, and carbon which make up a large part of soil mass.
Human food requirements can be measured in terms of total energy, commonly measured in Food Calories which are equivalent to 4184 Joules of available energy. The requirements are also measured by a wide variety of specific nutrients in specific amounts. It may not be efficient to supply all the low mass nutrients in a given situation, while supplying the main ones by mass. Whatever nutrients are not obtained from the closed loop system would have to be supplemented from outside, or simply done without for short missions. Human requirements can be translated to specific growing areas per person using known data on agricultural productivity. This data can be modified by designs of space systems. For example, if a plant needs 12 hours of sunlight per day, you can grow twice as much in the same area by using trays that are swapped every 12 hours, and storing the other tray under the illuminated one.