Inside the Harrier’s Heart: A Closer Look at the Pegasus Turbofan

The Harrier Jump Jet, famed for its ability to take off like a rocket and hover like a helicopter, owes its unique capabilities to one revolutionary engine: the Rolls‑Royce Pegasus. This vectored‑thrust turbofan powers the Harrier family—RAF Harriers, USMC AV‑8s, and Sea Harriers—enabling vertical/short takeoff and landing (V/STOL) performance previously deemed impossible. But what exactly makes the Pegasus so special? Let’s explore its engineering marvels and why it remains a legend in aviation.

AV-8 Harrier V/STOL jet fighter, produced for the U.S. Marines, 1983. Developed by Hawker Siddeley Aviation (later part of BAE Systems), the original model first flew in 1966.
Image from Britannica

A Brief History of Innovation

Pegasus traces its origins to the late 1950s when Bristol Siddeley began experimenting with vectored thrust turbofans to enable aircraft to hover. First flown in tethered hover in 1960 on the P.1127 prototype, it soon evolved through early variants like Pegasus 2, 3, and 5, delivering around 15,000 lbf of thrust. By 1969, the Pegasus was operational with the RAF, powering the Harrier GR.1 and, later, US Harriers under designation F402.

Type: Turbofan, vectored thrust VTOL engine, 4 swivel exhausts Thrust: 68,944 N (15,500 lb) at take-off Compressor: 2-stage front fan, bypass; 7-stage axial Combustor: Cannular Turbine: 3-stage axial Weight: 975 kg (2,150 lb)
Image from National Air and Space Museum 

Core Design: Two‑Shaft, Three‑Fan Architecture

At its core, the Pegasus is a two‑shaft turbofan featuring:


  • Three-stage fan and eight-stage high-pressure compressor

  • Contra‑rotating LP and HP spools, reducing gyroscopic effects and helping low‑speed hover control. 

  • Titanium blades, built for strength and heat resistance. 


It was the first turbofan to position the fan ahead of the shaft bearing, eliminating the ice‑hazard posed by inlet struts. 

Turbofan of Bristol Siddeley "Pegasus" in 1963
Image from Grace's Guide

Vectored Nozzles: The Secret to Hovering

Perhaps the most iconic feature is the Pegasus’s four swiveling nozzles:


  • Front nozzles channel cooler fan air, while rear nozzles handle the hot turbine exhaust.

  • Nozzle orientation is synchronized via air‑motor‑driven chains, rotating through approximately 98.5°, splitting thrust roughly 60/40 between front and rear.


By redirecting thrust downward, the engine produces both lift and forward motion in hover, transition, and conventional flight—defining V/STOL capability. 

Water Injection: Boosting Thrust When It Counts

During hot conditions, turbine temperature limits maximum thrust. To push thrust further during critical VTOL maneuvers, Pegasus injects distilled water into the combustion chambers to cool turbine blades and increase airflow. 


With a 500 lb reservoir, water injection offers peak thrust for up to 90 seconds—enough for vertical lift or heavy takeoff runs. 

A Hawker Siddeley P.1127 in flight at the NASA Langley Research Center, Virginia (USA). The P.1127 research aircraft was tested extensively at Langley to evaluate V/STOL handling qualities and the use of thrust vectoring as an aid in enhancing aircraft maneuverability.
Image from Wikipedia

Variants and Performance Upgrades

Pegasus has evolved significantly:


Variant

Thrust (lbf)

Notes

Pegasus 5

15,000

Kestrel evaluation

Pegasus 11

21,000

GR.3, Sea Harrier, AV‑8A

Pegasus 11‑21/106

21,500–21,750

Mk107 naval variants

Pegasus 11‑61 (Mk 107)

23,800

AV‑8B+, Harrier II+


These upgrades boosted thrust by more than 50% over the original, improving payload, range, and hot-weather performance. 

Underside view of the first prototype P.1127. The rotating jet nozzles were a key design element of its VTOL capability
Image from Wikipedia

Why Pegasus Still Matters

The Rolls‑Royce Pegasus was a game‑changer, ushering in V/STOL aviation and inspiring modern designs like the F‑35B. Its engineering elegance—vectored thrust, dual shafts, water injection—proved jet fighters could be compact and flexible without sacrificing combat effectiveness. 

Rolls-Royce Pegasus: A Jet Engine Ahead of Its Time

The Pegasus turbofan remains one of aviation’s most ingenious engines. It transformed the Harrier from a theoretical novelty into a combat‑proven aircraft, capable of unparalleled flexibility and strategic deployment. With an engineering legacy spanning decades, Pegasus reminds us that true breakthroughs often come not from quantum leaps, but from bold, integrated innovations.


Even in the age of advanced fifth‑generation fighters, the Harrier’s heart—the Pegasus—continues to capture imaginations and earn respect as a masterpiece of aero engineering.