Volvo RM8

The Volvo RM8 is a low-bypass afterburning turbofan jet engine developed for the Saab 37 Viggen 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 engine, as used for airliners such as the Boeing 727, 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, and was produced by Svenska Flygmotor (later known as Volvo Aero).[1]

RM8
An RM8 on display at the Swedish Air Force Museum
Type Turbofan
National origin United States/Sweden
Manufacturer Volvo Flygmotor/Pratt & Whitney
First run 1964
Major applications Saab 37 Viggen
Developed from Pratt & Whitney JT8D

Variants and applications

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.[2] 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.[3] The Viggen was designed with a maximum speed at sea level of Mach 1.2.[4] 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.[5] RM8 thrust boosting of 76% at take-off[6] was much higher than possible with a turbojet. In comparison the Saab 35 Draken with a Rolls-Royce Avon turbojet engine and Swedish-developed afterburner had a thrust boost at take-off of 38%.[7] 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.[8] In comparison the braking force with the reverser design used on the JT8D engine installation on the McDonnell Douglas DC-9 airliner was 40/45% of take-off thrust.[9]

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.[10][11]

Specifications (RM8B)

Data from Flight International.[12]

General characteristics
  • Type: Afterburning turbofan
  • Length: 6.23 m (245 in)
  • Diameter: 1.03 m (40.55 in)
  • Dry weight: 2,350 kg (5,180 lb)

Components
  • Compressor: Axial flow, 3-stage fan, 3-stage LP, 7-stage HP
  • Combustors: 9 chambers in can-annular arrangement, four injectors per chamber

Performance

See also

Related development

Related lists

References
Notes
  1. Gunston, p. 236
  2. Thunder at Trollhattan - The Volvo Flygmotor RM8, Flight International 14 December 1972,p.871
  3. Powerplants For The Concorde Supersonic Civil Airliner,S.G.Hooker,Proc Instn Mech Engrs 1963-64 Paper presented at Summer Meeting 1964, Fig.15 Concorde ram intake total pressure,Fig.16 Concorde ram intake total temperature
  4. Jane's All The World's Aircraft1982-83,ISBN 978 0710607485, p.183
  5. Thunder at Trollhattan - The Volvo Flygmotor RM8, Flight International 14 December 1972,p.871
  6. Jane's All The World's Aircraft 1982-83,ISBN 978 0710607485, p.748
  7. Jane's All The World's Aircraft 1972-73,ISBN 978 007 0321625, p.178
  8. Combat Biplane Saab's unique Viggen canard explained,Flight International 20 April 1967, p.637
  9. Systems For The Deflection Of Jet Streams Of Turbo-Jet Engines,NASA Technical Translation NASA TT F-603,March 1970, p.38
  10. Thunder at Trollhattan - The Volvo Flygmotor RM8, Flight International 14 December 1972,p.874
  11. JAS 39 Gripen, p.32&35, Gunnar Lindqvist / Bo Widfeldt, Air Historic Research AB, 2003, ISBN 91-973892-5-0
  12. Thunder at Trollhatten - The Volvo Flygmotor RM8,Flight International 14 Dec 1972, p.874
Bibliography
  • Gunston, Bill (1999). The Development of Piston Aero Engines, 2nd Edition. Sparkford, Somerset, England, UK: Patrick Stephens, Haynes Publishing. ISBN 0-7509-4478-1.
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