Why Russian rocket engines are so popular today

Credit to Roskosmos https://www.youtube.com/user/tvroscosmos
Translation courtesy of Mparovios3000 https://www.youtube.com/channel/UCYHtuA_ayTicJQ873GUopVQ
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Russian and American rocket engines using liquid oxygen as oxidizer and kerosene or RP-1 (some kind of kerosene) as fuel. It is clear that the specific impulse (Isp) for the Russian engines is much higher, offering a significant reduction in propellant mass.

The performance difference is explained from that the Russians use a much higher chamber pressure, which allows for a higher (geometric) expansion ratio and hence a higher exhaust velocity. The higher chamber pressure is possible, because the Russians apply a closed cycle system to drive the pumps, whereas the US designs are based on a gas-generator cycle. The latter essentially is an open cycle meaning that the turbine drive gases of the pumps are dumbed separately, without using it to provide thrust (at least not in an optimal way). In contrast, in a closed cycle system the (cool) turbine drive gases are fed to the combustion chamber thereby allowing these gases to be used again, but this time to produce thrust.

The closed cycle system was first implemented by the Russians around the 1960-s. The reason that the Russians were able to use a closed cycle design, is because at that time they had developed the technology, i.e. the materials, necessary to use oxygen-rich combustion gases from the pre-combustor to drive the turbopump(s), whereas in the USA NASA for long held the view that the use of oxygen-rich turbine drive gases induces corrosion problems. Lately, this view is shifting since there are a significant number of materials available today that are fully capable of being used as turbine components in hot oxygen-rich gases for service lifetimes of many hours.

Notice that the reason to use oxygen-rich turbine drive gases in a closed cycle LOX-kerosene system is because the kerosene propellant is also used as coolant, with maximum coolant temperatures being limited to about 490 K to prevent varnish deposits.
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