The Rolls-Royce WR-21 is an advanced marine gas turbine engine, designed with a view to powering the latest naval surface combatants of the partner nations, and currently fitted to the Type 45 destroyer of the Royal Navy.
Developed with government funding input from the UK, France and the United States, the WR-21 was designed and manufactured by an international consortium led by Northrop Grumman as prime contractor. The turbine itself was designed primarily by Rolls-Royce with significant marine engineering and test facility input from DCN, with Northrop Grumman responsible for the intercooler, the recuperator and system integration.
The original design and development of the WR-21 was carried out by Westinghouse Electric Corporation (later Northrop Grumman Marine Systems) under a U.S. Navy contract placed in December 1991. Later the Royal Navy and the French Navy became interested in the WR-21, leading to Rolls-Royce and DCN involvement.
The WR-21 is the propulsion system of Royal Navy Type 45 destroyers.
The WR-21 is the first aeroderivative gas turbine to incorporate Gas compressor intercooler and Exhaust heat recovery system (ICR) technologies that deliver low specific fuel consumption across the engine's operating range. It offers a reduction in fuel burn of 30% across the typical ship operating profile.
- The intercooler cools air entering the high pressure compressor, reducing the amount of energy required to compress the air.
- The recuperator preheats the combustion air by recovering waste heat from the exhaust, improving cycle efficiency and reducing fuel consumption.
It is a candidate power plant for propulsion of cruise ships and other large commercial vessels, where fuel efficiency and its small size mean improved operational flexibility and significant lifetime cost reductions to operators.
- Rated power: 25.2 MW
- Specific fuel consumption: ~190 g/kWh
- Main module wet weight: 45974 kg
- Twin-spool design + free rotating power Turbine
- Six-stage LP compressor
- Six-stage HP compressor
- Exhaust heat recuperator
- Nine radial combustors
- Single-stage HP turbine: 8,100 rpm (135 Hz)
- Single-stage LP turbine 6,200 rpm (103.33 Hz)
- Five-stage free power turbine 3,600 rpm (60 Hz)
The Northrop Grumman intercooler as fitted in the WR-21, on Type 45 destroyers, were not designed to operate in water temperatures beyond 30c. Rolls-Royce said that the NG intercooler had been built as specified, but that conditions in the Middle East were not "in line with these specs". The House of Commons defence committee had in 2009 warned that "persistent overoptimism and underestimation of the technical challenges combined with inappropriate commercial arrangements" would lead to increased costs. The Northrop Grumman intercooler of the first Type 45 destroyer, HMS Daring, failed in the mid-Atlantic in 2010 and had to be repaired in Canada, with further repairs for intercooler failure in 2012 in Bahrain. The Type 45's pioneering Integrated Electric Propulsion (IEP) system uses two WR-21s and two Wartsila 2MW diesel generators to power everything on board, including weapons systems in addition to propulsion and other functions, leaving the ships vulnerable to "total electric failure". The Ministry of Defence said: "The Type 45 was designed for world-wide operations, from sub-Arctic to extreme tropical environments, and continues to operate effectively in the Gulf and the South Atlantic all year round."
The Rolls Royce turbines themselves are extremely well-designed, but the Northrop Grumman intercooler has a design flaw causing the turbines to fail occasionally. 
IEP has many advantages in fuel efficiency and flexibility in engine position, and is supposed to reduce maintenance and manning requirement; the Type 45 is otherwise successful. However, the WR-21 engines will have to be supplemented by one or two additional diesel generators, fitted by cutting open the hull in dry dock.
Current First Sea Lord Admiral Philip Jones clarified that "WR-21 gas turbines were designed in extreme hot weather conditions to what we call “gracefully degrade” in their performance, until you get to the point where it goes beyond the temperature at which they would operate... we found that the resilience of the diesel generators and the WR-21 in the ship at the moment was not degrading gracefully; it was degrading catastrophically, so that is what we have had to address." The Admiral still argued that despite the problems, the Royal Navy has been able to deploy Type 45 destroyers in nine-month cycles to the Gulf region where temperatures are high with little fault. The Royal Navy has also been able to maintain at least two Type 45s at operational readiness.
- "Northrop Grumman/Rolls-Royce Team Selected as Preferred Supplier". defense-aerospace.com. 1 November 2000. Retrieved 30 January 2016.
- "WR-21 gamble pays off". Jane's Navy International. 2000. Retrieved 2009-11-16.
- "WR-21 Marine gas turbine". Rolls-Royce plc. Retrieved 2009-11-16.
- Carl L. Weiler, John Chiprich (October 1997). WR-21 Intercooled Recuperated Gas Turbine System Overview and Update. ASME 1997 Turbo Asia Conference. American Society of Mechanical Engineers. ISBN 978-0-7918-7867-5. Retrieved 30 January 2016.
- "WR-21 engine selected for Royal Navy type 45 destroyers". Retrieved 2009-11-16.
- Richard Norton-Taylor (7 June 2016). "Destroyers will break down if sent to Middle East, admits Royal Navy". The Guardian. Retrieved 8 June 2016.
- "Putting the Type 45 propulsion problems in perspective". Savetheroyalnavy.org. 3 February 2016. Retrieved 11 June 2016.
- "Oral evidence: Naval Procurement: Type 26 and Type 45 HC 221". UK House of Commons Defence Select Committee. 21 July 2016. Retrieved 21 July 2016.
- Rolls-Royce WR-21 Marine gas turbine page (archived 2013-11-08)
- Commander Colin R English (November 2003). The WR-21 Intercooled Recuperated Gas Turbine Engine - Integration Into Future Warships (PDF). Proceedings of the International Gas Turbine Congress 2003 Tokyo.