Question: Which chemical archaically called diamidogen was used in the Viking rocket engines?

Context paragraphs:
NK-33: The NK-33 and NK-43 are rocket engines designed and built in the late 1960s and early 1970s by the Kuznetsov Design Bureau.  The NK designation is derived from the initials of chief designer Nikolay Kuznetsov.  The NK-33 was among the most powerful LOX/RP-1 rocket engines when it was built, with a high specific impulse and low structural mass.  They were intended for the ill-fated Soviet N-1 rocket moon shot.  The NK-33A rocket engine is now used on the first stage of the Soyuz-2-1v launch vehicle.

Airbreathing jet engine: An airbreathing jet engine (or "ducted jet engine") is a jet engine propelled by a jet of hot exhaust gases formed from heated and expanded air that is drawn into the engine via a compressor, typically a centrifugal or axial type.  They are typically gas turbine engines.  The opposite of airbreathing jet engines are non-airbreathing jet engines, such as rocket engines, for example, which are propelled by a jet of hot gases created by the chemical reaction of two or more compounds internally.  While the majority of the mass flow of an airbreathing jet engine is provided by air taken from outside of the engine and heated internally, using energy stored in the form of fuel, a rocket engine's fuel provides both the energy and the mass flow to create thrust.

Val Cleaver: Arthur Valentine Cleaver OBE FRAeS (14 February 1917 – 16 September 1977) was a distinguished British rocket engineer.  He co-authored a paper which discussed the possibilities and problems of nuclear rocket engines in 1948.  After the Second World War he developed de Havilland's Sprite and Spectre rocket engines.  He moved to Rolls Royce in 1957 and in 1960 he became general manager and chief engineer of the Rolls Royce's rocket departments, where he was responsible for the engines which powered the Blue Streak missile and Black Arrow launch vehicle.  While the ELDO vehicle was ultimately unsuccessful and abandoned, the Blue Streak vehicle and its engines worked perfectly on every launch, and Cleaver was awarded the OBE for his part in developing them.

Monomethylhydrazine: Monomethylhydrazine (MMH) is a volatile hydrazine chemical with the chemical formula CH(NH)NH.  It is used as a rocket propellant in bipropellant rocket engines because it is hypergolic with various oxidizers such as nitrogen tetroxide (NO) and nitric acid (HNO).  As a propellant, it is described in specification MIL-PRF-27404.

Hydrazine: Hydrazine is an inorganic compound with the chemical formula N2H4 (also written H2NNH2 ), archaically called diamidogen.  A simple pnictogen hydride, it is a colorless flammable liquid with an ammonia-like odour.  Hydrazine is highly toxic and dangerously unstable unless handled in solution.  , approximately 120,000 tons of hydrazine hydrate (corresponding to a 64% solution of hydrazine in water by weight) were manufactured worldwide per year.  Hydrazine is mainly used as a foaming agent in preparing polymer foams, but significant applications also include its uses as a precursor to polymerization catalysts and pharmaceuticals. Additionally, hydrazine is used in various rocket fuels and to prepare the gas precursors used in air bags.  Hydrazine is used within both nuclear and conventional electrical power plant steam cycles as an oxygen scavenger to control concentrations of dissolved oxygen in an effort to reduce corrosion.  Anhydrous hydrazine is corrosive towards glass, in a manner similar to hydrofluoric acid.

Reaction Motors: Reaction Motors, Inc. (RMI) was an early American maker of liquid-fueled rocket engines, located in New Jersey.  RMI engines with 6000 lbf thrust powered the Bell X-1 rocket aircraft that first broke the sound barrier in 1947, and later successors including the X-1A, X1E, and the Douglas D-558-2 Skyrocket.  A 20000 lbf thrust RMI engine also powered the Viking research rocket, the first large liquid-fueled US high-altitude rocket.  RMI was merged with Thiokol in 1958, where it produced the XLR-99 engine that powered the X-15 rocket aircraft.

Simple chemical reacting system: The simple chemical reacting system (SCRS) is one of the combustion models for computational fluid dynamics.  This model helps us to determine the process of combustion which is a vital phenomenon used in many engineering applications like aircraft engines, internal combustion engines, rocket engines, industrial furnaces, and power station combustors.  The simple chemical reacting system (SCRS) refers the global nature of the combustion process considering only the final species concentrations.  The detailed kinetics of the process is generally neglected and it postulates that combustion does proceed via a global one-step without intermediates.  Infinitely fast chemical reaction is assumed with oxidants reacting in stoichiometric proportions to form products.  SCRS considers the reaction to be irreversible i.e. rate of reverse reaction is presumed to be very low.

Viking (rocket engine): 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.)

LEROS: LEROS is a family of chemical rocket engines manufactured by Moog-ISP at Westcott, Buckinghamshire, United Kingdom.  LEROS engines have been used as primary apogee engines for telecommunications satellites such as the Lockheed Martin A2100 as well as deep space missions such as Juno.

Rocket engine: A rocket engine is a type of jet engine that uses only stored rocket propellant mass for forming its high-speed propulsive jet.  Rocket engines are reaction engines, obtaining thrust in accordance with Newton's third law.  Most rocket engines are internal combustion engines, although non-combusting forms (such as cold gas thrusters) also exist.  Vehicles propelled by rocket engines are commonly called rockets.  Since they need no external material to form their jet, rocket engines can perform in a vacuum and thus can be used to propel spacecraft and ballistic missiles.

Supporting paragraphs: Viking (rocket engine) Hydrazine
Answer: Hydrazine