Phase 0–Terrestrial Analog ISRU Prototyping: This is where we’re at now. As far as I know there has been almost no experimental development of the sort that some of our commenter have suggested which would use simulated Venusian atmosphere to attempt various approaches for extracting the different constituents for further processing. Obviously that which hasn’t even been tried in the lab is nowhere near ready to try in situ. This stage will likely be characterized by small, non-flight like, breadboard/brassboard-level prototype processes.
Phase 1–Venus In-Situ Demonstration: The first real Venus ISRU development phase will likely be in the form of small experiments mounted on robotic atmospheric balloons. We’re likely talking about experimental apparatus of less than 200kg, which are not so much focused on producing large masses of extracted materials, but just demonstrating and validating basic extraction processes. These steps will likely be focused on the concepts we’ve talked about so far of condensing out and separating condenseable species, and processing the atmospheric species to remove key hazardous materials, to demonstrate the ability to extract safe feedstocks for future larger-scale processes.
Phase 2–In-Situ Propellant Production and ECLSS Revitalization: This is the point at which the first steps beyond what we’ve already discussed will be taken. This phase may start with unmanned systems, demonstrating the ability to refuel rocket stages for transportation back into Venus orbit, and to provide fuel for Venus orbit propellant depots. But this phase will likely also include demonstration of the ability to revitalize the breathing air and drinking water for manned missions. This may also include trying to create enough lighter-than-CO2 gas to provide buoyancy for the robotic and manned systems. This stage isn’t necessarily about creating voluminous open habitat spaces and floating cities. Depending on the rocket approach taken for transportation between the cloud level and orbit, this could involve processing hundreds of tonnes of atmosphere into propellants, and tens of tons into lifting gasses and life support elements. At this point most chemical processing will be limited to that necessary to create propellants. Depending on what propulsion style makes the most sense, this could be LOX/LH2, LOX/Methane, or LH2 or Ammonia for nuclear thermal, solar/microwave thermal, or solar/laser thermal propulsion systems. These can mostly be created by simple one or two step processes from the basic atmospheric constituents previously discussed.
Phase 3–Small Settlements: At this phase, permanent settlements are first being attempted. So in addition to processing the atmosphere to create propellants for flights in and out of the Venusian atmosphere, and creating lifting gasses for supporting those smaller facilities, we’ll now be talking about creating large amounts of breathable air and water for filling these colonies. Also this phase will likely include the creation of simple construction materials to try and reduce the amount of material that needs to be shipped from earth. This will likely start requiring taking the initial chemical feedstocks and performing several processes to create materials such as carbon fiber, simple polymers, and sulfurcrete. These materials would be used for the structure of the settlements, and possibly even the atmospheric barrier film. This phase will be focused on the low-hanging fruit of materials that would require the most shipping mass from earth, but that are easiest to produce on Venus. Peter Kokh had some clever acronym for this for lunar ISRU, but I’m forgetting it at the moment. But basically, the more processing steps necessary to get to an object, the more likely it would be best to still import this from earth.
Phase 4–Advanced Settlements: At this phase large-scale permanent settlements will exist, and even some limited mining of the surface of Venus will likely have started. As this phase progresses, more and more materials of increasing complexity will be sourced locally, including some simpler metals, and more advanced plastics and composites. As this phase continues imported materials will focus on high-value hard-to-manufacture items like advanced electronics, complex machinery, etc. I think this Phase while interesting is probably beyond the scope of this series. If we get to this phase, we’ve “already won”.
The first place to start with ISRU processing of the Venusian atmosphere is to try and remove all five of the easily condenseable atmospheric constituents: Sulfuric Acid, Water, Sulfur Dioxide, Hydrogen Chloride (in the form of Hydrochloric Acid), and Hydrogen Fluoride (either directly or in the form of aqueous Hydrofluoric Acid).
At this point in the process, the remaining constituents (assuming you’ve properly removed the previous-listed five condenseables) can be lumped into two categories:
* Semi-reactive gasses like Carbon Dioxide, Carbonyl Sulfide, and Carbon Monoxide
* Neutralish gasses like Nitrogen, Argon, Helium, and Neon