Why doesn't the ISS have more oxygen generators?

Question

According to this article, the O2 reclamation on the ISS is capable of producing about 2kg of O2 per day. However, crews need slightly more than this. What is preventing NASA/Roscosmos/anyone else from just installing more and completely closing the loop?

Answer 1

The question is too complicated, because I will quote a lot from the article. The main air source is not generators. Generators are an emergency. The air source is a regenerator.

Сегодня на МКС частично замкнутая система регенерации воды Where does water and oxygen come from on the ISS?

• СОГС — система обеспечения газового состава,
  
• СВО — система водообеспечения,
  
• ССГО — система санитарно-гигиенического обеспечения,
  
• СОП — система обеспечения питанием,
  
• СОТР — система обеспечения теплового режима.
  

The cosmonaut's life support system includes the following systems:

• SOGS - gas composition support system,
  
• SVO - water supply system,
  
• SSGO - system of sanitary-hygienic provision,
  
• SOP - food supply system
  
• COTR - thermal management system.
  

Системы жизнеобеспечения советских космических кораблей типа «Восток», «Восход», «Союз», а также американских «Меркурий», «Джемини», «Аполлон» и транспортного корабля многоразового использования «Шаттл» были основаны полностью на запасах расходуемых материалов: кислорода, воды, пищи, средств удаления СО2 и вредных микропримесей.

The life support systems of the Soviet Vostok, Voskhod, Soyuz spacecraft, as well as the American Mercury, Gemini, Apollo and the Shuttle reusable transport ship were based entirely on supplies of consumables: oxygen, water, food, CO2 removers and harmful micro-impurities.

Впервые в мировой практике пилотируемых полётов на космической станции «Салют-4» функционировала регенерационная система «СРВ-К»-система получения питьевой воды из конденсата атмосферой влаги. Экипаж в составе А.А.Губарева и Г.М.Гречко использовал воду, регенерированную в системе «СРВ-К», для питья и приготовления пищи и напитков. Система работала в течение всего пилотируемого полёта станции. Аналогичные системы типа «СРВ-К» работали на станциях «Салют-6», «Салют-7», «МИР».

For the first time in world practice of manned flights at the Salyut-4 space station, the SRV-K regeneration system, a system for producing drinking water from condensate with a moisture atmosphere, functioned. The crew of A. A. Gubarev and G. M. Grechko used the water regenerated in the SRV-K system for drinking and preparing food and drinks. The system worked during the entire manned flight of the station. Similar systems of the SRV-K type operated at the Salyut-6, Salyut-7, and MIR stations.

Эффективность использования регенерационных систем подтверждена опытом многолетней эксплуатации например орбитальной станции «МИР», на борту которого успешно функционировали такие подсистемы СЖО, как:

• «СРВ-К» — система регенерации воды из конденсата атмосферной влаги,
  
• «СРВ-У» — система регенерации воды из мочи (урины),
  
• «СПК-У» — система приема и консервации мочи (урины),
  
• «Электрон» — система генерирования кислорода на основе процесса электролиза воды,
  
• «Воздух» — система удаления диоксида углерода,
  
• «БМП» — блок удаления вредных микропримесей и др.
  Аналогичные регенерационные системы (за исключением «СРВ-У») успешно функционируют в настоящее время на борту Международной космической станции (МКС).

The efficiency of using regeneration systems is confirmed by the experience of many years of operation, for example, the MIR orbital station, on board of which such subsystems of the life support system successfully functioned as:

• "SRV-K" - a system for the regeneration of water from atmospheric moisture condensate,
  
• "SRV-U" - a system for the recovery of water from urine (urine),
  
• "SPK-U" - a system for receiving and preserving urine (urine),
  
• "Electron" - an oxygen generation system based on the process of electrolysis of water,
  
• "Vozdukh" is a carbon dioxide removal system,
  
• "BMP" - block removal of harmful microimpurities, etc.
  

Similar regeneration systems (with the exception of SRV-U) are currently operating successfully aboard the International Space Station (ISS).

В состав системы обеспечения жизнедеятельности (СОЖ) МКС входит подсистема обеспечения газового состава (СОГС). Состав: средства контроля и регулирования атмосферного давления, средства выравнивания давления, аппаратуру разгерметизации и наддува ПхО, газоаналитическую аппаратуру, систему удаления вредных примесей БМП, систему удаления углекислого газа из атмосферы «Воздух», средства очистки атмосферы. Составной частью СОГС являются средства кислородообеспечения, включающие твердотопливные источники кислорода (ТИК) и систему получения кислорода из воды «Электрон-ВМ». При стартовом запуске на борту СМ имелось всего лишь 120 кг воздуха и два твердотопливных генератора кислорода ТГК.

The ISS includes a subsystem for ensuring the gas composition (SOGS). Composition: means for monitoring and regulating atmospheric pressure, means for balancing pressure, equipment for depressurization and pressurization of PXO, gas analytical equipment, a system for removing harmful impurities from the BMP, a system for removing carbon dioxide from the atmosphere, “Vozdukh”, and means for purifying the atmosphere. An integral part of SOGS are oxygen supply facilities, including solid fuel oxygen sources (TEC) and the Electron-VM water oxygen production system. At the start-up, there was only 120 kg of air and two solid fuel oxygen generators of THC onboard the SM.

На МКС цеолитовые поглотители системы «Воздух» захватывают углекислый газ (CO2) и высвобождают его в забортное пространство. Теряемый в составе CO2 кислород восполняется за счет электролиза воды (разложения ее на водород и кислород). Этим на МКС занимается система «Электрон», расходующая 1 кг воды на человека в сутки. Водород сейчас стравливают за борт, но в перспективе он поможет превращать CO2 в ценную воду и выбрасываемый метан (CH4). И конечно, на всякий случай на борту есть кислородные шашки и баллоны.

On the ISS, zeolite absorbers of the Vozdukh system capture carbon dioxide (CO2) and release it into the outboard space. The oxygen lost in the composition of CO2 is replenished due to the electrolysis of water (its decomposition into hydrogen and oxygen). This is done on the ISS by the Electron system, which consumes 1 kg of water per person per day. Hydrogen is now pushed overboard, but in the long run it will help turn CO2 into valuable water and methane (CH4) emitted. And of course, just in case, there are oxygen checkers and cylinders on board.

К сожалению полного круговорота веществ на орбитальных станциях пока не достигнуто. На данном уровне технологий с помощью физико-химических методов не удается осуществить синтез белков, жиров, углеводов и других биологически активных веществ. Поэтому диоксид углерода, водород, влагосодержащие и плотные отходы жизнедеятельности космонавтов удаляются в вакуум космического пространства.

Unfortunately, a complete cycle of substances at orbital stations has not yet been achieved. At this level of technology, using physicochemical methods, it is not possible to synthesize proteins, fats, carbohydrates and other biologically active substances. Therefore, carbon dioxide, hydrogen, moisture-containing and dense waste products of cosmoonauts are removed into the vacuum of outer space.

Where does water and oxygen come from on the ISS?

P.S. March 6, 2019 NASA: Piloting a New Procurement Paradigm

In the $65 million contract that NASA established with private contractor UTC Hamilton Sundstrand Space, Land & Sea, UTC would engineer the Sabatier system with the stipulation that 100% of NASA’s investment would be refunded if the system did not perform upon in-orbit activation. Throughout the development time frame, NASA provided UTC with milestone payments to meet UTC’s need for development cash flow. Importantly, the agreement also removed more than 70% of NASA’s standard requirements, and verification of the remaining requirements was left as flexible as possible.

The result was that a 249-kilogram (550-pound) stainless steel cube the size of a small refrigerator arrived via Space Shuttle Discovery on April 7, 2010, and was operational by October of that year. The system performed for more than 6 years in orbit, and produced more than 1,080 liters (1,043 kilograms [2,300 pounds]) of clean, safe water for crew members. This represented significant and immediate cost savings in the operation of the space station, and provided a way to produce water rather than transport it all from Earth, thereby increasing the goal of self-sufficiency and broadening the path for extended human survival in LEO and beyond.

Aug. 16, 2019 NASA: Recycling Water and Air

Host: Interesting. So ten years, so there was a starting point obviously, we had to start somewhere. Where did the International Space Station start and then how did it evolve to get to that regenerative system, to ten years?

Laura Shaw: ...So we added those to the ISS to make things comfortable for crew where they were living, and then we evolved to the water processor, the urine processor, the oxygen generation system that creates oxygen from water. And then we had some additional, we'll talk about the Sabatier, which takes in CO2 and hydrogen and creates water out of it. So that's kind of how we've evolved.

Host: Okay. Now on top of this, let's go into Sabatier. Now this is just pulling the existing water, any source of currently existing water, urine, and humidity, and sweat, and all this stuff, bringing it back and recycling it and getting it ready for it to drink again. Sabatier, though, is a system of actually creating water from different sources. And how does that work?

Laura Shaw: Okay. So we start with, back to CO2 removal -- .

Host: Yes.

Laura Shaw: Briefly. We, instead of venting that CO2 to space, we use it, we compress, we pull on it with a compressor and we put it into a tank, so we save it. And then Sabatier takes that CO2, it also takes hydrogen that's generated in the oxygen generator. And we'll talk about that.

Host: Yeah. We skipped that one.

Laura Shaw: Yeah. It’s okay

Host: But, yeah, let's do Sabatier.

Laura Shaw: There's a product of the oxygen generator that's hydrogen. So we take hydrogen and CO2 and there's a catalyst inside the Sabatier that combines those two together and the product is methane and water. So you're literally taking C, you know, carbon, oxygen, and hydrogen, mixing them up and you get, you know, CH4 which is methane.

Answer 2

Not to sound dismissive, but here's text from another part of the article:

It's for this reason that the ISS has two other methods of receiving oxygen. It is provided from Earth whenever the ISS receives a supply shuttle and pumped into pressurised tanks mounted outside the airlock; and by a backup solid-fuel oxygen generator called Vika, or SFOG, developed by the Russian Federal Space Agency for the Mir space station.
The Vika system isn't ideal for several reasons. The first is that they use canisters that need to be shipped from Earth. These canisters contain a mixture of powdered sodium chlorate and powdered iron. This is ignited, which heats the iron to a temperature of 600 degrees Celsius (1112 degrees Fahrenheit), which creates the energy required to break down the sodium chlorate into sodium chloride and oxygen gas. These temperatures can be hazardous. In 1997, a Vika canister aboard Mir malfunctioned and caught fire, which melted the canister and launched globules of fire on to the bulkhead. Additionally, 1 kilogram of material produces only about 6.5 man-hours of oxygen. With an estimated cost of tens of thousands of dollars per kilogram of cargo shipped to the ISS, this is no small matter. That's why Vika is a backup system, rather than primary source of oxygen. The astronauts' day-to-day oxygen needs are adequately filled by electrolysis oxygen generation and supplies from Earth.

Basically the existing reclamation system is supplemented by CRS water resupplies and pressure vessel volumes. If the astronauts are losing 0.84 kg of air per day, the ISS pressured volume is 915 m^3, meaning they have well over 1000 kg of air. They could run without resupply for three years with their current regeneration system and losses and then begin to have issues. The longest they have ever gone without a resupply is 128 days. See: https://space.stackexchange.com/a/2079/33077 They of course have the backup generators from MIR and the ability to leave via Soyuz any time, so they aren't concerned.

I guess the best answer would be that they don't have more because they don't need more. The current system has worked for well over a decade, and if it ain't broke, don't fix it.