he "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".