Blok D (Russian: Блок Д meaning Block D) is an upper stage used on Soviet and later Russian expendable launch systems, including the N1, Proton-K and Zenit. There were plans to use it for some other rockets as well (project Air Launch).
The stage (and its derivatives) has been included in more than 250 launched rockets as of 2005. By 2002 its modification Blok DM had a 97% success rate in 218 flights since 1974, and 43 successful missions in 1997-2002.
The stage was developed in 1960s as the fifth stage ('Д' is the fifth letter in the Cyrillic alphabet) for the Soviet Moonshot N1 rocket. The stage first flew in March 1967 while testing Zond of the moonshot program system. During manned lunar flight Blok D would be used for mid-course corrections on the flight to the Moon, then to place the lunar orbiter and lander into a lunar orbit, and decelerate moon-lander out onto its landing trajectory.
Blok D was also included as fourth stage of Proton rocket and as such flew on unmanned Soviet missions to Moon, Mars and Venus. It is still used in Proton K configuration of the rocket (the newer Proton M uses Briz-M as fourth stage).
Blok DM also flies as the third stage for Zenit-3SL rocket, which is used by the Sea Launch project to launch geostationary satellites. In 2002 the failure of a Blok DM3—used in the attempted launch of Astra 1K -- caused considerable concern.
The stage uses liquid oxygen (LOX) and kerosene as propellants, and has one single-chamber RD-58 main engine. The LOX tank has a spherical shape; the kerosene tank is toroidal, inclined to 15 degrees for better fuel extraction, with the engine mounted in the center of torus. Tanks include the first pump stage for the engine; the main pump is mounted on the engine.
Blok D weighs 3.5 tons during liftoff, but some parts are jettisoned and the dry mass in space is 2.5 tons. It has 5.70 meters length and generates 83.300 kN thrust for 600 seconds burn time. Blok D was modified as Blok DM in 1974, with 11D-58S engine. The unit cost is $4 million.
RKK Energia, the company that created Blok D, used it as a platform for many modifications over many years for different purposes; for example, the main propulsion unit on Buran started as a modification of the Blok D.
- Blok DM (1974) - Blok D's modification for Earth-based orbits
- Blok DM-2 (1982) - Used (as recently as 2009) with Proton M for GLONASS launches 
- Blok DM-2M
- Blok DM-3
- Blok DM-5
- Blok DM1 - commercial Blok DM-2
- Blok DM2 - commercial Blok DM-5
- Blok DM3, DM4 (1996) - commercial Blok DM-2M
- Blok DM-SL (1999, derivative of DM3) - used in Sea Launch missions as upper stage of Zenit 3SL rocket, as of January 2006
- Russian Space Web
- Boeing stands by Block DM's reliability
- "Rockets: Launchers: N1: Block D". RussianSpaceWeb.com. Retrieved 2007-09-20.
- "Sea Launch Stands Behind the Reliability of the Block DM". Boeing (press release). Retrieved 2007-09-20.
- "11D-58 - Summary". Andrews Space & Technology. Retrieved 2007-09-20.
- "Block DM 11S86". Retrieved 2008-09-26.