The Zenit is a medium lift booster and its first stage was used for the Energiya booster stap-on's. It was developed at the Yujnoye Scientific Production Association. Flight testing began in 1985, and since becoming operational it has been used exclusively from Tyuratam for launch of ELINT satellites. In the mid-1970s, development got under way of a unique, new-generation launcher. Ecologically clean, capable at a launch weight of 459 tons of putting a satellite weighing 16 tons into orbit. Zenit-2, 2 stage booster could launch up to 15,000 kg. or smaller payload up to 2000 km. altitude (±3.5 km., 2.5 sec., 2.5 minutes inclination). Zenit-2 weighs 460,000 kg, can launch 12,000 kg. to 200 km. circular 90° orbit. Zenit-3 a Zenit-2 with the Block-D, can launch up to (4500 kg. to GTO) 1100 kg to geostationary orbit. Possible launches from a Australian facility and using 2 solid kick motors could raise payload to 3,000 kg. to GEO (±.01 Hour, ±60-160 sec., ±6 min inclination). Weight on launch is 467,000 kg. Zenit-SL is a Zenit-3 which gains additional payload capacity by launching near the equator. The Zenit first stage had a single four nozzle RD-171 engine that used LOX-Kerosene as propellant. 804,000 kg. thrust for first stage, 93,000 kg. thrust second stage. The RD-171 engines four nozzles were supplied by a single 250,000 horse power turbo pump, in a design much like the RD-107 on the Soyuz booster. The engine is a variant of the RD-170 designed for the "Energia". Glushko guided the design of the engine and over a period of 15 years it was developed with a staff of hundreds of workers.The engine produced about 740,000 kg. to 806,000 kg. vacuum thrust each. Glushko said that the biggest problem in developing the RD-170 was eliminating high frequency pressure oscillations in the engine which demanded most of the time and money allocated. 900 test firings of the entire engine were carried out during its development program.
Adding a newly designed kerosene/LOX second stage with an RD-120 engine made the booster into the Zenit-2. The Zenit-3 version added the Block-D third stage to the booster with 8500 kg. thrust. In 1990, the Zenit-3 was scheduled for testing for 1993 but it never came about due to low demand until the Sea Launch project adopted it. The Zenit is carried to the launch pad on a rail car/erector combination similar to the Proton. The booster was designed to be assembled in 10 working days, to be storable in that configuration for up to a year, and payload integration took 10 days for the booster and 4.5 days for a standard payload. After 24 hours of preparation on the pad the booster is ready to launch. Booster is 3.9 m diameter, 57 m long, inside payload shroud diameter is 3.4 meters, length 8.37 or 7.3 m depending on upper stage. The Zenit booster was tested in at least four sub-orbital tests in 1985 (one on April 13). The first orbital launch of a Zenit was on June 21, 1985, and was the unintentional result of a sub-orbital test in which the upper stage exploded with some resulting pieces pushed into a 64.4° orbit, a characteristic of a launch from Baykonur. The Soviets were unaware of this and the pieces became the first unannounced launch in many years. The second orbital test was Kosmos 1697, the third was Kosmos 1714. Kosmos 1767 in July 30, 1986, and Kosmos 1786 were the fourth and fifth orbital test flights. The next tests were Kosmos 1820 and Kosmos 1833. Kosmos 1697 and Kosmos 1714 were probably ELINT satellites (but 1714 was stranded in a low orbit after the upper stage failed to restart and 1767 showed no activity. Kosmos 1786 was put into a highly elliptical orbit and probably failed to restart to circularize at 2560 km. All the missions seem to leave four objects in orbit before separating the upper stage from the payload. The Zenit program was downscaled by 1990 eliminating some payloads and delaying others mainly due to the slowdown in Energiya operations. On Oct. 4, 1990 a Zenit launch failed shortly after launch due to an explosion in the first stage. The Zenit also introduced a revolution in launch pad systems with highly automated system allowing for 10 times faster than comparable Proton preparations.
Because Zenit is one of the most modern of the Soviet launch systems, its design shows many improvements compared with older Soviet vehicles. The Zenit uses only two stages, each fueled with kerosene and oxygen propellants that are safer and more environmentally friendly than the storable propellants used on launch systems such as Proton and Tsiklon. The propulsion system is much more advanced than the previous LOX/kerosene engines, such as those used on the Soyuz, for example. The RD-171 engine uses a high-pressure staged combustion cycle that results in 25% higher sea level specific impulse than the older RD-107 used on Soyuz. Zenit also demonstrates improved technologies in its highly automated launch infrastructure. Zenit can launch within 90 min of reaching the launch pad.
Zenit was developed in the mid 1970s and is the only large rocket developed by the former Soviet Union since the ill-fated N-1 lunar rocket of the late 1960s. Therefore, its general design and many of its systems are more modern than other Soviet and Russian launchers. Zenit uses two or three in-line stages, each fueled by LOX/kerosene. The first stage was developed in parallel with the Energia strap-on boosters, and it shares the same basic structure and propulsion systems. The first-stage RD-171 engine is the most powerful liquid engine ever flown, and its high-pressure staged-combustion turbopumps provide higher sea-level specific impulse than any other LOX/kerosene engine. The primary difference between Zenit's RD-171 and the RD-170 used on Energia is that the Energia RD-170 thrust chambers gimbaled in two axes, while the Zenit RD-171 thrust chambers can gimbal in only one axis. The engine was qualified for up to 10 restarts as part of the Energia program. The upper LOX tank fits in a concave depression at the top of the kerosene tank, and the LOX feed line runs through the middle of the lower tank. Separation is achieved with four solid retro-rockets located at the base of the stage. The Zenit-3SL first stage is very similar to the Zenit-2, with strengthened structures for sea-based operations, and additional propellant loading lines for the Block DM-SL upper stage. The second stage is connected to the first by an open truss structure. It is propelled by a single RD-120 engine, with steering provided by an RD-8 vernier engine with four thrust chambers spaced around the outside of the RD-120. A toroidal kerosene tank surrounds the main engine, and oxygen is stored in a standard cylindrical tank. The second stage is topped by an instrumentation compartment containing the avionics. The third stage for the Zenit-3SL is a derivative of the Block DM stage used on Proton.
The Block DM-SL includes a spherical LOX tank, a toroidal kerosene tank around the single 11D58M main engine, and a pressurized instrumentation compartment. These elements are connected with a truss structure. When integrated on the launch vehicle, the Block DM-SL is contained inside a cylindrical skin-stringer interstage that is jettisoned before Block DM-SL ignition. The interstage is divided into two sections, referred to as the lower adapter and middle adapter. The mass of these two components is 526 kg and 715 kg (1160 lbm and 1575 lbm), respectively. The stage can function for 24 h or more on orbit, and is capable of five to seven restarts, allowing complex orbital maneuvers. The Block DM-SL is capped by an aluminum interface skirt. This flared structure serves as the transition from the 3.7-m (12.2-ft) diam stage to the 4.15-m (13.6-ft) fairing, and also carries the conical payload support structure.
Other modifications from the Proton version include updated avionics, modified interfaces to the lower stages and the payload section, and modified prelaunch support interfaces. For example, on Proton the fueling and umbilical connections are provided by an umbilical tower. On Zenit, these connections are routed up through the lower stages. The instrument compartment of the Block DM-SL is also mounted lower than in previous versions of the Block DM in order to provide clearance for the payload adapter.The four thrust chambers of the RD-171 engine can be individually gimbaled ±5 deg in one axis to provide three-axis control. One of the few differences between Zenit's RD-171 and the RD-170 used in the Energia strap-on booster is that the RD-170 engine supported two-axis gimbaling of each thrust chamber. Attitude control during second-stage flight is provided by the RD-8 vernier engine, which uses one turbopump to feed four thrust chambers, each of which can be gimbaled. The main engine is fixed and does not contribute to attitude control. The RD-8 burns the same LOX/kerosene propellant as the main engine and operates from 10 s before second-stage ignition to 75 s after main-engine shutdown. During third-stage burns, the main- engine gimbal provides pitch and yaw control, and turbopump bleed gas is used for roll control. The Block DM-SL upper stage also has two sets of independent ACS thruster units for three-axis attitude control during coast phases and payload deployment, and for propellant settling before main-engine ignition. These are fueled by small tanks of N2O4/UDMH.
The Zenit-2 vehicle uses a digital flight control system. The navigation system is aligned using an optical update unit that is jettisoned on the pad immediately before launch. The Biser-2 flight computer has three redundant channels, with majority voting to detect a failure in any channel. During Flight #31, two channels malfunctioned and were interpreted as failures by the remaining channel. As programmed, the remaining channel commanded flight termination rather than continue on a single channel, since there was no way to confirm that the remaining channel was not also faulty. Future vehicles will use the upgraded Biser-3 computer, which has been modified to prevent this problem. As is typical with Soviet launch vehicles, flight termination is performed by automatic shutdown of the engines, which brings the vehicle down largely intact, rather than by triggering explosive systems to destroy the vehicle as is typically done in the West. However, the Zenit-2 has a much more extensive flight safeguard system than most vehicles, probably stemming from early man-rating requirements for proposed spaceplane programs. The flight safeguard system can transmit a launch vehicle emergency signal to the spacecraft resulting from either an onboard computer failure or a loss of stability.
Similarly, it can receive a spacecraft emergency signal from the spacecraft. If an emergency occurs after a predetermined time late enough in flight that the spacecraft can still complete its mission or perform an abort, an emergency main-engine shutdown is commanded, the payload fairing is jettisoned, and the payload is separated. If a launch vehicle emergency occurs in the first 20 s of flight, instead of commanding engine shutdown, the safety system commands a 6-deg/s pitchover maneuver to move the vehicle downrange as rapidly as possible so that the launch complex will not be destroyed. When used in the three-stage configuration, the two lower stages of Zenit are still controlled by their own avionics, with separate systems on the Block DM-SL upper stage. The avionics for the lower stages are similar to Zenit-2, but with a new, more-redundant guidance computer that has been flight proven on other Soviet launch systems. Block DM-SL avionics are installed in a toroidal pressurized compartment at the top of the stage. The Block DM-SL has an updated flight computer, and a new inertial navigation unit that is better able to operate from a floating launch platform. The Zenit-3SL telemetry system can transmit to either the tracking antennas on the assembly and command ship, or to TDRSS communications relay satellites. Zenit was intended to become one of the primary Soviet launch systems once it became operational, with a launch rate of 20-40 flights per year (this total may include production of the Energia strap-on boosters). However, this did not occur. Some planned programs that would have used Zenit never materialized, and government funding for the three-stage Zenit-3 version, which could deliver payloads to GTO, never became available. Following the collapse of the Soviet Union, Zenit is in the awkward position of being a primarily Ukrainian launch vehicle, while its primary user has been the Russian government. Russia now prefers to launch most of its spacecraft on domestically built vehicles such as Soyuz and Proton. As a result, Zenit's flight rate has remained low, and its use for launching Russian spacecraft to the International Space Station has apparently been canceled. The planned Zenit launch complex at the Plesetsk cosmodrome was never completed. Zenit continues to carry occasional government payloads, but its survival depends now on its commercial viability.
Zenit 2 is launched from Baikonur only now. Rocket stages arriving from the Ukraine on railroad cars can be put in storage, or delivered directly to the Zenit Assembly and Test Building. Three rail cars are used, one each for the first stage, second stage, and fairing. All vehicle processing takes place in a horizontal orientation. Using overhead bridge cranes, the elements of the launch vehicle are removed from the rail shipping cars and transferred to special mating trolleys that are capable of translating or rotating the stages to ensure proper alignment for mating. Any additional required flight hardware is integrated onto the stages, the stages are mated together, and the vehicle undergoes electrical and other tests. Only 76 work hours are needed to prepare and integrate the arriving stages to the point where the spacecraft can be integrated. Following vehicle preparation, the spacecraft is integrated onto the vehicle, electrical connections are checked, and then the spacecraft is covered by the payload fairing. The integrated launch vehicle can then be transferred to the transporter / erector rail cars for transport to the launch pad. Spacecraft integration onto the launch vehicle requires 21 work hours (not counting separate spacecraft preparation activities), for a total of 91 h from arrival of the vehicle stages at the cosmodrome to the completion of vehicle integration. Spacecraft processing may be performed in Area 31, Area 254, or at other facilities used for commercial launch services of other launch vehicles. The facilities include an Assembly and Test Building (ATB) for checkout operations, vacuum testing, and antenna testing. Spacecraft fueling operations are conducted in a separate building.
Zenit-2 launches are conducted from Launch Complex 45, which includes two launch pads. However, Pad 45 Right suffered heavy damage when Zenit Flight 15 exploded at liftoff. It has not been repaired since, so all subsequent launches have used Pad 45 Left. Each pad is a large, flat, concrete structure including an exhaust duct with a water noise suppression system. The launch mount is a metal structure with four hold-down clamps, and automatic fittings for automatic propellant filling and, if necessary, draining. The launch complex area includes storage facilities for LOX and kerosene, as well as pressurized helium, nitrogen, and air. Tall lightning towers protect the erected vehicle from lightning strikes.
The launch complex does not include a traditional service tower or umbilical tower. Instead, the Zenit arrives in a horizontal position on a transporter / erector system mounted on two railcars. The transporter / erector automatically erects the vehicle into a vertical position using large hydraulic pistons. The vehicle is supported by a simple erector structure during this process. During the erection process the electrical, pneumatic, hydraulic, cooling, and propellant loading connections automatically mate with the appropriate interfaces at the base of the launch vehicle and at the top of the first stage. Once the vehicle is fully integrated, the erector is retracted and moved away from the pad, leaving only a small, simple tubular cable mast that carries the electrical and pneumatic connections between the vehicle and the launch mount. There is also a movable service tower that is capable of enclosing the upper portion of the launch vehicle. However, this was built to support planned manned missions, and while it is available for conventional spacecraft, it is not generally needed.
