where there is little to no ambient air pressure. This gives it a higher thrust and specific impulse, but makes it longer and heavier. It has a thrust-to-weight ratio of about 120:1. The predecessors of NK-33 and NK-43 are the earlier NK-15 and NK-15V engines respectively. The oxygen-rich technology lives on in the RD-170/-171 engines, their RD-180, and recently developed RD-191 derivatives, but these engines have no direct connection to the NK-33 except for the oxygen-rich staged combustion cycle technology, the kerosene/RP-1 fuel, and in case of the RD-191 and its variants like the RD-193 and the RD-181, the single combustion chamber instead of the multiple chambers in previous Russian rocket engines. History N-1 The N-1 launcher originally used NK-15 engines for its first stage and a high-altitude modification (NK-15V) in its second stage. After four consecutive launch failures and no successes, the project was cancelled. While other aspects of the vehicle were being modified or redesigned, Kuznetsov improved his contributions into the NK-33 and NK-43 respectively. The 2nd-generation vehicle was to be called the N-1F. By this point the Moon race was long lost, and the Soviet space program was looking to the Energia as its heavy launcher. No N-1F ever reached the launch pad. When the N-1 program was shut down, all work on the project was ordered destroyed. A bureaucrat instead took the engines, worth millions of dollars each, and stored them in a warehouse. Word of the engines eventually spread to the US. Nearly 30 years after they were built, rocket engineers were led to the warehouse. One of the engines was later taken to the US, and the precise specification of the engine was demonstrated on a test stand. Combustion-chamber design The NK-33 closed-cycle technology works by sending the auxiliary engines' exhaust into the main combustion chamber. This made the engine design unique. This technology was believed to be impossible by Western rocket engineers. The fully heated liquid O2 flows through the pre-burner and into the main chamber in this design. The extremely hot oxygen-rich mixture made the engine dangerous: it was known to melt thick castings "like candle wax. One of the controversies in the Kremlin over supplying the engine to the US was that the design of the engine was similar to Russian ICBM engine design. The NK-33's design was used in the later RD-180 engine, which had twice the size of the