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The architecture and capabilities of the newly developed third stage for the Angara-5V was expected to be similar to previously proposed hydrogen boosters, such as such the second stage of the cancelled Rus-M launch vehicle or the UKVB stage that was previously proposed as the third stage for the Proton-M2 and for the Angara-5 rockets. According to an unofficial estimate made by a veteran of the rocket industry Dmitry Vorontsov, the new booster was to have a total propellant mass of around 48 tons and sport a propulsion system producing 40 tons of thrust with a specific impulse of 462 seconds.
Under pressure to give the Angara-5V its hard-to-achieve payload of 37 tons, designers aimed to keep the dry mass (structure without propellant) of the URM-2V stage to an absolute minimum of six tons. However, by the end of April 2015, some conservative estimates pointed toward a dry mass up to two times higher.
Choosing the propulsion system
Still, the biggest challenge to the initial design of the URM-2V stage was the selection of a propulsion system. In March 2015, Yuri Koptev said that the booster would be propelled by the yet-to-be-built RD-0150 engine.
The development of a new-generation propulsion system was apparently deemed more practical than any attempts to revive the Soviet-era hydrogen engines such as 11D57M (RD-57M) and KVD1 or later experimental RD-0122, KVD-1M3 and RD-0146 engines, even though the latter propulsion system was to be used on the second stage of the Rus-M rocket.
The developer of the RD-0150 engine -- KBKhA design bureau in Voronezh, promised to increase the projected thrust of a single RD-0150 engine from 40 to 77 tons, however, this capability would have to be confirmed in the course of the early work on the Angara-5V project. (744) According to Vorontsov, such a propulsion system could theoretically produce as much as 80 tons of thrust.
Other industry sources also said that although a one-engine propulsion system served as a baseline for the design of the stage, it presented its own technical challenges. The engine was expected to feature a special nozzle extension, which allowed to keep the length of the vehicle to a minimum. After the separation of the lower stage in flight, the extension would slide into operational position to provide a better performance for the engine. However, engineers would have to make sure that the rocket remains stable during the "cold" separation, when the lower URM-1 central booster had already dropped, but the upper URM-2V booster was yet to fire, leaving the entire vehicle in the unpowered free flight.
As an alternative to the one-engine design, the four-engine cluster also remained on the table in the first half of 2015. Engineers at KBKhA bureau were also re-examining the legacy from the RD-0122 engine for its possible use in the project. However by the end of April 2015, engineers reportedly favored a two-engine propulsion system, with each engine producing 45 tons of thrust.
In March 2016, First Deputy Director at GKNPTs Khrunichev Aleksandr Medvedev confirmed that it had been decided to equip the URM-2V with a pair of RD-0150 engines. In June 2017, Deputy Prime Minister Dmitry Rogozin promised to initiate the development of the RD-0150 engine, but he did not specify any timeframe for the project. At the time, the same engine was also expected to be used on the third stage of the future super-heavy launcher.