Ukraine's success with Tsyklon led to its selection as the lead for the Zenit medium capacity launch vehicle in the second half of the 1970's. From its inception, the Zenit program was assigned a dual purpose: (1) develop a new low-cost, 15-metric-ton capacity launch vehicle and (2) design its first stage as a strap on booster for the Energiya heavy-lift launch vehicle. Although employing the more environmentally friendly liquid oxygen and kerosene as propellants, the Zenit adopted many of the attractive traits of the Tsyklon space transportation system. Zenit is highly automated and can be launched within hours of being erected on its launch pad.
Ukraine chose two Russian power plants to lift Zenit: the 11D521 (RD-171) engine for the first stage and the 11D123 (RD-120) engine for the second stage. Both were developed by the Energomash Scientific Production Association near Moscow. The Zenit was intended to replace the Soyuz-U launch vehicle for many programs, but economic and political forces combined to prevent Zenit from becoming a major part of the Russian space transportation infrastructure. Since its introduction in 1985, the Zenit has primarily been used to support a single Russian military space program. However, Zenit picked up two new programs in 1994 and is slated to be one of the principal logistics links for the International Space Station (References 445-455). In addition, Zenit launch facilities at the Plesetsk Cosmodrome should be completed for launches beginning in 1997.
After a moderately successful initial flight period during 1985-1990 (three failures in 12 missions), Zenit encountered three successive failures, one of which destroyed its launch pad at Baikonur. The 4 October 1990 failure just seconds after launch was so severe that Complex 45 right had still not been repaired by the end of 1994. The launch vehicle successfully returned to flight in November 1992 and had not lost another payload through the end of 1994. A total of six missions were flown during 1993-1994. A small problem did arise following the 25 December 1992 and the 26 March 1993 flights. A modification to the second stage engine, which had caused launch failures in 1991 and 1992, led to an explosion of the stage within two days of launch. A subsequent fix resolved this problem (Reference 456).
Commercialization of the Zenit, once considered highly marketable, has been met with numerous setbacks. An early concept to launch Zenits from the proposed Cape York, Australia, spaceport faded with the demise of that project. In 1993 a US firm, Commercial Space Management, signed an agreement witha Russian consortium for exclusive marketing rights for Zenit, but this deal, too, fell through when the US company went bankrupt (References 448, 457-480).
Later in 1994 a new marketing and operations arrangement was concluded by Yuzhnoye, the US Boeing Commercial Space Company, and the Norwegian ship-building firm of Kvaerner Group. The new venture, under the name of Sea Launch, envisioned launching Zenit vehicles from floating platforms in the Pacific Ocean, perhaps as early as 1997. The Sea Launch global partnership includes Boeing Commercial Space Company, Seattle, Washington, (provides spacecraft integration and the payload fairings); Kvaerner Maritime a.s., of Oslo, Norway (the vessel builder); RSC Energia of Moscow, Russia (provides the Block-DM upper stage and its integration with the launch vehicle); and KBYuzhnoye/PO Yuzhmash of Ukraine (provides the first two stages of the launch vehicle and launch support operations). Meanwhile, Space Systems/Loral was considering the Zenit launch vehicle to deploy its Globalstar spacecraft. Those LEO missions would originate from the Baikonur Cosmodrome (References 461-463).
The Sea Launch concept provides commercial satellite customers such as DIRECTV, with a direct route to geostationary transfer orbit without requiring a change in flight inclination. Launching from the equator also affords operational benefits including increased performance, high launch availability and reduced launch infrastructure costs. From the ocean-based launch site, the Sea Launch Zenit-3SL rocket can lift a heavy spacecraft mass or place a payload into geostationary transfer orbit, helping satellite operators attain a longer satellite service capability.
The ultimate success of the Zenit remained dependent upon both technology and politics. For GEO missions a third stage must be added to Zenit. Already several years behind schedule, this effort vacillated between accepting a modified Proton Block DM stage and developing a new Ukrainian stage. As early as 1994, the former was assessed as the more likely outcome. Politically, disagreements (1) between Ukraine and the Russian Federation concerning the "nationality" of Zenit, (2) about the utilization of the Baikonur Cosmodrome, and (3) concerning Government approval for US spacecraft to be launched on Zenits must be settled (References 464-470).
On 14 December 1995, the United States and Ukraine signed an agreement for up to 20 geostationary-orbit launches of U.S.-built payloads on Ukrainian launchers through 2001, following similar agreements signed in previous years with China and Russia. Five of the launches are unrestricted; another 11 are set aside for joint U.S.–Ukrainian ventures such as Sea Launch, and four more will be added should market growth warrant them.
In the first year of operation, 1999, SLLP intended to conduct three launches (one demonstration payload and two satellites); six launches are proposed for each subsequent year. Sea Launch, a multinational, ocean-based launch services company, officially began commercial operations on October 9, 1999 with the launch of the new DIRECTV 1-R direct broadcast satellite. From the equatorial launch site at 154 degrees West longitude, the Sea Launch Zenit-3SL vehicle lifted off from the Odyssey launch platform. In March 2000 Sea Launch executed its first launch successfully - that of a demonstration payload which confirmed the design and operation of the complete Sea Launch system.
The launch vehicle that Sea Launch uses consists of the Zenit rocket, the Block DM-SL upper stage, and a payload adapter and fairing. The adapter, which accommodates the satellite payload on the rocket's Block DM-SL upper stage, and the nose cone fairing (a protective shroud for the satellite) are manufactured in Seattle, Washington. Following manufacture of the LP, the ACS, and the first payload adapter and fairing, a full-system with the two-stage Zenit rocket and Block-DM upper stage is deployed from the Home Port. The three dry rocket segments, the payload fairing, and the payload adapter are transported to the Home Port in Long Beach harbor, California. Satellite payloads are transported to the Home Port by the launch customers, most of whom are located in the Southern California area. The rocket segments, fairing, adapter, and payload are processed and integrated at the Home Port and prepared for ocean transport. Propellants and hazardous materials are loaded onboard the LP at the Home Port. The ILV, personnel, and supplies (including kerosene and liquid oxygen as primary propellants of the launch vehicle) are transported onboard the LP and ACS to the launch location at 154 W on the equator. During the seven to ten day sailing to the launch location, ILV electrical systems are checked and charged, and launch command processes and contingency measures are rehearsed.
In the hours prior to launch, the LP is lowered to a more stable, semi-submerged position. The ILV would be erected to a vertical position on the deck of the LP and then mated to remotely operated systems for fueling and launch ignition. Prior to fueling, all personnel on the LP transfer to the ACS, which is positioned five km from the LP. The commands for fueling and launch are initiated remotely from the ACS. Any system failure prior to Stage 1 engine ignition would be detected remotely from the ACS, prompting commands to remotely defuel and stabilize the ILV. A few seconds prior to ignition of the launch vehicle’s Stage 1 engines, launch controls from the ACS are relinquished and an automated (computer controlled) launch sequence is initiated. After ignition, hold-down clamps are released when adequate thrust is achieved. Onboard computers automatically monitor rocket performance, azimuth, and system deviations. In the event of uncorrectable deviations from the flight plan, the computer would initiate thrust termination.
An air-launched version of Zenit, named Svitiaz, has also been proposed to circumvent the existing launch site problems, albeit at significant penalty in payload capacity. Using the An-225 aircraft as a launch platform, modified Zenit could deliver up to nine metric-tons into LEO or one metric ton into GE0. A maiden flight of this configuration may be possible by 1998 (References 452 and 471).