RD-870 could become Ukraine's first booster engine
More than quarter of a century after the end of the USSR, rocket engineers in the former Soviet republic of Ukraine are embarking on the development of a large rocket engine for the first stage of an indigenously built space vehicle. The RD-870 engine, intended to propel the first stage of the Tsyklon-4M (Cyclone-4M) rocket will be based on a Soviet-era second-stage engine but redesigned to lift the rocket off the launch pad, instead of firing in the stratosphere.
The RD-870 engine at a glance:
Burning non-toxic kerosene and cryogenic liquid oxygen, RD-870 is a closed-cycle engine, which means that the oxidizer gas created in the gas generator first performs its primary job of driving the engine's turbo-pump, which sucks propellant into the combustion chamber, and then it flows into the combustion chamber to burn with the rest of the propellant generating thrust. This complex system gives the engine some extra performance, compared to simpler designs where generator gas is dumped overboard after it has driven the pump.
The RD-870 engine will fire for 200 seconds beginning at liftoff and it can not be restarted in flight.
History of the project
In 2016, KB Yuzhnoe in the city of Dnepr, Ukraine, began work on the Tsyklon-4M (Cyclone-4M) rocket, which would launch from Canada. The new commercial vehicle would be closely based on the Zenit and Antares space rockets but relying on Ukrainian-built engines, instead of the Russian RD-171 on Zenit and RD-181 on the Antares.
With its roots deep in Soviet rocketry, KB Yuzhnoe accumulated extensive know-how in rocket propulsion. After the end of the Cold War, the company was able to retain its cadre of propulsion engineers, as well as production and testing facilities for rocket engines, thanks to its cooperation with China, Russia and South Korea. The ability to build rocket engines would be crucial if Ukraine wanted to build a self-reliant rocket industry after its very close ties to Russia were interrupted in 2014.
The Tsyklon-4M program would be the first significant step in the process of building a fully Ukrainian space launcher, resembling the Russian effort in the 1990s to develop the Soyuz-2 launch vehicle.
The best candidate for the first-stage propulsion system on Tsyklon-4M would be a cluster of four RD-120 engines with a thrust of 85 tons borrowed from Zenit's second stage. Although RD-120 was developed at NPO Energomash in Moscow, it was serially produced in Ukraine, with the exception of its combustion chamber supplied by a manufacturer in Samara, Russia.
Early on, there was a proposal to build the RD-120K variant with a new nozzle optimized to operate at sea level, instead of nearly 60 kilometers above the Earth's surface, where Zenit's second stage would normally ignite its RD-120. From the manufacturing standpoint, the easiest way to optimize RD-120 for work at sea level would be to order the combustion chamber with a new nozzle from NPO Energomash, but politics made such a deal impossible.
To make the engine fully indigenous, KB Yuzhnoe proposed to use 50 already available combustion chambers for the Russian RD-263 engine. Four RD-263 engines used to propel the first stage of the Soviet-era R-36M ICBM, burning hypergolic propellant.
The combustion chamber used in the RD-263 engine is similar to that used on the RD-120, but it has a nozzle optimized to work at sea level. However, because these chambers use a fuel injector that would not work with the kerosene and oxygen combination, the new engine would receive a fuel injector borrowed from the RD-120. The resulting hybrid was designated RD-870.
Early attempts to develop RD-870
The RD-870 engine was first proposed for the US Antares rocket to replace the troubled NK-33 engine on its first stage. However, because the Antares needed a replacement as soon as possible with a minimum of technical risk, its developer never seriously considered RD-870 and opted for the Russian RD-181 instead.
Another variant of the RD-870 engine, customized for ignition at high altitude and designated RD-872, was considered for the Mayak-S3.9A launcher, which never left the drawing board. Now, the engine's best hope rests with Tsyklon-4M.
Gimbal suspension system
Unlike its fixed position on the second stage of Zenit, two of four main engines on Tsyklon-4M would be attached to the first stage via a special gimbal mechanism. The gimbal will enable a pair of RD-870s to tilt up to six degrees from its vertical axis along the two planes perpendicular to each other in order to steer the rocket in flight. The gimbal will be located just above the injection system and connected to the combustion chamber via an adapter cone.
Other modifications to RD-120 to build RD-870
The combustion chambers for the RD-120 and RD-263 both feature a cooling system using fuel as a coolant, but the design of the cooling loop on the two engines is different, affecting the location of the fuel valve. On the RD-870, the fuel valve, borrowed from the RD-120, will be installed directly at the entrance into the cooling loop of the combustion chamber.
Several other modifications of the pneumatic and hydraulic systems of the original RD-120 were also introduced to increase the reliability of the new engine.
To reduce the time necessary for the engine to transition from ignition mode to full-thrust mode, a special hydraulic switch will be replaced with an electric valve. Its design will be based on a similar device developed at KB Yuzhnoe for the KRD-120 engine under a commercial agreement with China. Under the name YF-100, that Ukrainian engine successfully propelled a Chinese launch vehicle for the first time in 2015.
The RD-870 will also have modified hardware for filling the tanks of its pneumatic system. The pneumatic valves which are used to fill or drain pressurized gas, had previously shown some reliability problems, therefore they will be replaced with valves borrowed from the RD-8 (11D513) engine developed at the KB-4 division of KB Yuzhnoe for the second stage of the Zenit.
Also, the tank with a volume of 20 cubic decimeters, which was previously supplied by RKK Energia in Russia, will be replaced with a 25-cubic decimeter tanks also borrowed from the RD-8 engine.
Because the transition from the RD-120 to the RD-870 means a move from the second to the first stage, which lifts the rocket off the ground, the new engine will be upgraded with a fuel drainage valve, in case the liftoff has to be aborted on the launch pad.
Also, when RD-120 was ignited above the atmosphere, its fuel line would be vacuumized to expel any air and ensure its complete filling with fuel. For launching from the ground, RD-870 will receive a special ejector system to flush air out of the pipeline before filling it with fuel.
All these changes will require the development of a new design documentation for the RD-870 engine.
Switching to serial production
According to preliminary evaluations, KB Yuzhnoe could begin the manufacturing, assembling and testing the RD-870 engine with relatively minor upgrades to its existing production base, as long as the project relies on 50 already manufactured combustion chambers of the RD-263 engine. However, in order to resume the production of the combustion chambers, the existing factory equipment would have to be modernized. From eight to 10 chambers would be required for bench testing of the engine, leaving enough hardware for around 10 missions of the Tsyklon-4M rocket.
The RD-870 technical heritage:
A concept of the RD-870 engine, as of 2017. Credit: KB Yuzhnoe
A historic RD-120 engine displayed in demo room at NPO Energomash in Moscow. Copyright © 2002 Anatoly Zak
The Ukrainian-built version of the RD-120 (11D123) engine will be a basis for the new-generation RD-870 engine. Copyright © 2001 Anatoly Zak
Available combustion chambers from RD-263 (15D117) engine could be used for building RD-870. Credit: KB Yuzhnoe