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The Viking rocket engines were members of a series of bipropellant engines for the first and second stages of the Ariane 1 through Ariane 4 commercial launch vehicles, using storable, hypergolic propellants, N2O4/UH 25 (mixture 75% UDMH and 25% Hydrazine.)
The earliest versions, developed in 1973, had a thrust of about 390 kN. The version used on the Ariane 4 first stage, which clustered four together, had 667 kN thrust each. The second stage of Ariane used a single Viking. Over 1000 were built, and achieved a high level of reliability from early in the program.
The 144 Ariane 1 to 4 used a total of 958 Viking engines. Only 2 engines have led to a failure. The first failure was due to a crack in the combustion chamber, the second is of human origin: a rag had been left in a pipe during installation. Initially, all the engines were tested before being integrated on a launcher. Beginning in 1998, engineers, confident of the reliability of the engine, authorized the use of untested engines on launchers. Some engines, randomly taken in the workshops of assembly, were tested from time to time. This confidence is very rare in the world of space engines.