Editing Saab 37 Viggen (section)

=== Propulsion ===
[[File:Viggen cockpit.jpg|thumb|Closeup view of the cockpit and air intake of a JA 37 Viggen]]
The Viggen was powered by a single [[Volvo RM8]] turbofan. This was essentially a heavily modified [[Licensed production|licence-built]] variant of the [[Pratt & Whitney JT8D]] engine that powered commercial airliners of the 1960s, with an afterburner added for the Viggen. The airframe also incorporated a thrust reverser to use during landings and land manoeuvres, which, combined with the aircraft having flight capabilities approaching a limited STOL-like performance, enabled operations from 500&nbsp;m airstrips with minimal support.<ref name = "bomber 244" /> The thrust reverser could be pre-selected in the air to engage when the nose-wheel strut was compressed after touchdown via a [[Pneumatics|pneumatic]] trigger.<ref name = "warwick 1261">Warwick 1980, p. 1261.</ref><ref>Ahren, B. "Viggen Thrust Reverser", ''AIAA Journal of Aircraft'', vol. 18, no. 5, 1981.</ref>

The requirements from the Swedish Air Force dictated Mach 2 capability at high altitude and Mach 1 at low altitude. At the same time, short-field take-off and landing performance was also required. Since the Viggen was developed initially as an attack aircraft instead of an interceptor (the [[Saab 35 Draken]] fulfilled this role), some emphasis was given to low fuel consumption at high subsonic speeds at low level for good range. With turbofan engines just emerging and indicating better fuel economy for cruise than [[turbojet]] engines, the former was favoured, since the latter were mainly limited by metallurgy development resulting from limitations in [[Turbojet#Cycle improvements|turbine temperature]].{{Citation needed|date= January 2013}} Mechanical simplicity was also favoured, so the air intakes were simple D-section types with boundary layer [[Splitter plate (aeronautics)|splitter plates]], while the fixed inlet had no [[Inlet cone#Alternative shapes|adjustable geometry]] for improved pressure recovery. The disadvantage was that the required engine would be very large.<ref name = "bomber 244" /> In fact, at the time of introduction, it was the second-largest fighter engine, with a length of 6.1&nbsp;m and 1.35&nbsp;m diameter; only the [[Tumansky R-15]] was bigger.{{Citation needed|date = January 2013}}

[[File:Goteborg Volvo Museum 09 RM8.jpg|thumb|left|Volvo RM8 on display, 2014]]
Saab had originally wanted the [[Rolls-Royce Medway]] as the Viggen's powerplant.<ref name = "bomber 244" /> Owing to the cancellation of the Medway, the JT8D was instead chosen as the basis for modification. The RM8 became the second operational afterburning turbofan in the world, and also the first equipped with a thrust reverser. According to aviation author Christopher Chant, the RM8 has the distinction of being the first engine to be fitted with both an afterburner and a thrust reverser.<ref name = "chant 458">Chant 2014, p. 458.</ref> It had a bypass ratio of around 1.07:1 in the RM8A, which reduced to 0.97:1 in the RM8B.<ref name="Nativi">Nativi 1993</ref>{{Page needed | date = January 2014}} The RM8A was the most powerful fighter engine in the late 1960s.<ref name = "warwick 1260" />

The AJ, SF, SH and SK 37 models of the Viggen had the first version of the RM8A engine with uprated internal components from the JT8D that it was based on. Thrust was 65.6&nbsp;kN dry and 115.6&nbsp;kN with afterburner.<ref name = "Janes88-89 p 702" /> For the JA 37, the RM8A was developed into the RM8B, achieved by adding a third low-pressure compressor stage over the preceding model, increasing the turbine inlet temperature and fuel diffusion within the combustion chamber.<ref name="auto">Warwick 1980, p. 1264.</ref> Thrust is 72.1&nbsp;kN dry and 125.0&nbsp;kN with afterburner.<ref name="Janes88-89 p 702">Taylor 1988, p. 702.</ref> Owing to the increased length and weight of the RM8B engine over its predecessor, the airframe of the JA 37 was stretched in order to accommodate it.<ref name="auto" /> Onboard electrical power was provided by a 60 [[Volt-ampere|kVA]] generator. In the event of an in-flight engine failure, emergency power was provided by an automatically deploying [[ram air turbine]] (RAT), capable of generating 6 kVA.<ref name = "bomber 245" />