The Volvo RM8 is a low-bypass afterburning
turbofan
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft engine, aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which ac ...
jet engine
A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, Pump-jet, water jet, and hybrid propulsion, the term ...
developed for the
Saab 37 Viggen
The Saab 37 Viggen (Swedish for ''"the Bolt"'' or ''"the Tufted Duck"'' ( see name)) is a retired Swedish single-seat, single-engine, short-medium range combat aircraft. Development work on the type was initiated at Saab in 1952 and, following ...
fighter. An augmented bypass engine was required to give both better fuel consumption at cruise speeds and higher thrust boosting for its short take-off requirement than would be possible using a turbojet. In 1962, the civil
Pratt & Whitney JT8D
The Pratt & Whitney JT8D is a low-bypass (0.96 to 1) turbofan engine introduced by Pratt & Whitney in February 1963 with the inaugural flight of the Boeing 727. It was a modification of the Pratt & Whitney J52 turbojet engine which powered th ...
engine, as used for airliners such as the
Boeing 727
The Boeing 727 is an American narrow-body airliner that was developed and produced by Boeing Commercial Airplanes.
After the heavy 707 quad-jet was introduced in 1958, Boeing addressed the demand for shorter flight lengths from smaller airpo ...
, was chosen as the only engine available which could be modified to meet the Viggen requirements. The RM8 was a licensed-built version of the JT8D, but extensively modified for supersonic speeds, with a Swedish-designed
afterburner
An afterburner (or reheat in British English) is an additional combustion component used on some jet engines, mostly those on military supersonic aircraft. Its purpose is to increase thrust, usually for supersonic flight, takeoff, and comba ...
, and was produced by Svenska Flygmotor (later known as
Volvo Aero
Volvo Aero was a Swedish aircraft, guided missiles and rocket engine manufacturer. It became ''GKN Aerospace Engine Systems'' following the company's acquisition by British engineering conglomerate GKN during 2012.
It was originally establish ...
).
Variants and applications
* RM8A -
AJ 37 Viggen
Since the original civil engine was designed for subsonic speeds, parts had to be thickened and made from improved materials for the higher pressures and temperatures which occur in a Mach 2 military engine. These are higher than in a Mach 2 civil airliner because supersonic combat aircraft go faster at sea level. Here the ambient temperature and pressure are higher than at altitudes where civil airliners are accelerating through the speed of sound. The Viggen was designed with a maximum speed at sea level of Mach 1.2. As an example of a required material change the diffuser case, where the compressed air is slowed for entry to the flame tubes, was changed from steel to nickel alloy to withstand the higher temperatures leaving the compressor. Air cooling for the HP turbine blades was introduced as the turbine entry temperature was 110 degC higher than on the civil engine. Hamilton Standard had supplied the fuel control for the JT8D but Bendix was chosen for the engine and afterburner fuel control systems due to their experience developing the fuel controls for another augmented turbofan engine, the
Pratt & Whitney TF30
The Pratt & Whitney TF30 (company designation JTF10A) is a military low-bypass turbofan engine originally designed by Pratt & Whitney for the subsonic F6D Missileer fleet defense fighter, but this project was cancelled. It was later adapted with a ...
. RM8 thrust boosting of 76% at take-off was much higher than possible with a turbojet. In comparison the
Saab 35 Draken
The Saab 35 Draken (; 'The Kite' or 'The Dragon') is a Swedish fighter-interceptor developed and manufactured by Svenska Aeroplan Aktiebolaget (SAAB) between 1955 and 1974. Development of the Saab 35 Draken started in 1948 as the Swedish air fo ...
with a
Rolls-Royce Avon
The Rolls-Royce Avon was the first axial flow jet engine designed and produced by Rolls-Royce. Introduced in 1950, the engine went on to become one of their most successful post-World War II engine designs. It was used in a wide variety of ai ...
turbojet engine and Swedish-developed afterburner had a thrust boost at take-off of 38%. For short landings the engine exhaust was deflected partially forwards by an airframe-mounted reverser. The exhaust passed through three slots (which otherwise supply ambient air to the airframe-mounted exhaust ejector) to give a braking force about 60% of the maximum dry take-off thrust. In comparison the braking force with the reverser design used on the JT8D engine installation on the
McDonnell Douglas DC-9
The McDonnell Douglas DC-9 is an American five-abreast single-aisle aircraft designed by the Douglas Aircraft Company. It was initially produced by the developer company as the Douglas DC-9 until August 1967 and then by McDonnell Douglas.
After ...
airliner was 40/45% of take-off thrust.
* RM8B -
JA 37 Viggen
The flight envelope for the fighter-version demanded both more thrust and better stall margins at high altitudes. This led to redesigns for the fan, the low-pressure compressor and the combustion-chambers with four injectors in each for more complete fuel mixing.
[JAS 39 Gripen, p.32&35, Gunnar Lindqvist / Bo Widfeldt, Air Historic Research AB, 2003, ]
Specifications (RM8B)
See also
References
;Notes
;Bibliography
*
External links
RM8 history''in Swedish''
{{Swedish military gas turbine aeroengines
Low-bypass turbofan engines
1960s turbofan engines