RD-253

The RD-253 (Ракетный Двигатель-253 (РД-253)) and its later variants, the RD-275 and RD-275M, are liquid-propellant rocket engines developed in the Soviet Union by Energomash. The engines are used on the first stage of the Proton launch vehicle and use an oxidizer-rich staged combustion cycle to power the turbopumps. The engine burns UDMH/N2O4, which are highly toxic but hypergolic and storable at room temperature, simplifying the engine's design.

History
Development of the RD-253 started in 1961. Preliminary investigations and development of the engine and its further production was performed under the guidance of Valentin Glushko and finished in 1963. The RD-253 uses an oxidiser-rich staged combustion cycle. It was used for the first time in July 1965, when six engines powered the first stage of the Proton rocket. Development and production of RD-253 was a qualitative leap forward for rocketry of that time by achieving high levels of thrust, specific impulse and pressure in the combustion chamber. This engine is one of the most reliable engines in the USSR and modern Russia.

As the first stage of the Proton-K rocket used six RD-253 engines, the system played a pivotal role in Russian space missions when this rocket was chosen as a carrier, including the following programs: "Luna", "Venera", "Mars probe", crewed orbital stations "Salyut", "Mir", and it launched several principal modules for ISS. It is was widely used for the launches of heavy satellites. The last RD-253 rocket engine was used on a Proton-K rocket and launched on March 30, 2012.

Since the original development of the engine, several modifications were designed that were not flown. One of them was the RD-256 engine, for which development stopped on experimental models. It was not used in flights and was designed for a cancelled vehicle. The RD-254 variant was supplied with an extended nozzle for use in vacuum.

Current state and development
All rights to sell and employ the RD-253 for Proton rocket are held by Energomash which produces it in Perm, Russia.

The modification RD-275 (14D14) appeared as the result of development in 1987–1993, with the purpose to achieve a more powerful version of the engine. Its 7.7% higher thrust was reached by raising pressure in the combustion chamber and enabled an increase of payload mass to geostationary orbit (GEO) up to more than 6000 kg. The successful maiden flight of a Proton rocket with the new engine was completed in 1995.

Energomash started the development of next more powerful version of engine in 2001. It has 5.2% higher thrust and has the designation 14D14M (RD-275M). It was designed to allow the rocket to deliver 150 kg more payload to GEO.

In the period from 2002 to 2003 years some experimental work was completed with this version of the engine. It included four test firings of three experimental RD-275M with a total time of 735s. In the middle of 2005, this engine went into production by government commission. The first launch of a Proton-M with 14D14M engines was launched on July 7, 2007.

The final version RD-275M is sometimes designated as RD-276, but through 2009 the name RD-275M (14D14M) was more common. Some sources points out the cost of production per engine as much as 1.5 million USD and sometimes calls some lower figures around 1 million USD per unit.

Versions
During the years there have been many versions of this engine:
 * RD-220: Initial proposal for the N-1 first stage.
 * RD-221: Initial proposal for the N-1 second stage.
 * RD-222 (GRAU Index 11D41): Development program for the N-1 first stage.
 * RD-223 (GRAU Index 11D42): Development program for the N-1 second stage.
 * RD-253 (GRAU Index 11D43): Serial production version for the Proton (8K62) first stage. Was proposed for the N-1 first stage.
 * RD-253F (GRAU Index 11D43F): Project for R-36M (15А14) first stage.
 * RD-254 (GRAU Index 11D44): Project for UR-700 third stage and for Proton and N-1 second stages, it was a RD-253 high-altitude version.
 * RD-275 (GRAU Index 14D14): Serial production for Proton-M first stage. RD-253 with increased thrust by 8%.
 * RD-275М AKA RD-276 (GRAU Index 14D14М): Serial production for Proton-M first stage. Improved RD-275.