The journey of open rotor engine technology spans several decades, tracing back to its initial development phases in the 1970s by United Technologies Hamilton Standard Division, in collaboration with NASA. Known by various names over the years—from Propfan and Unducted Fan (UDF) to Advanced Turboprop, Contra-Rotating Open Rotor (CROR), Unducted Single Fan (USF), and simply Open Rotor (OR)—this technology has been a testament to aviation’s relentless pursuit of efficiency and performance.
A Glimpse into the Past
In the 1980s, an unducted fan engine prototype mounted on an MD-80 aircraft marked one of the early attempts to bring the open rotor concept to the skies. This was followed by significant developments, such as the CROR concept in a pusher configuration developed under the CS1 program, and more recently, the USF in a puller configuration proposed by CFM, analogous to Avio Aero’s concept for the CS2 IRON project.
Left: Unducted Fan Engine (open rotor) from the 1980s, mounted on an MD-80 aircraft Middle: Contra Rotating Open Rotor (CROR) concept in pusher configuration developed in CS1. Right: Unducted Single Fan in puller configuration proposed by CFM (Analogous to the concept proposed by Avio Aero in the CS2 IRON project)
In the 1980s, an unducted fan engine prototype mounted on an MD-80 aircraft marked one of the early attempts to bring the open rotor concept to the skies. This was followed by significant developments, such as the CROR concept in a pusher configuration developed under the CS1 program, and more recently, the USF in a puller configuration proposed by CFM, analogous to Avio Aero’s concept for the CS2 IRON project.
The principle of efficiency
At its core, the open rotor engine seeks to maximize the concept of propulsion efficiency inherent to jet engines. Unlike modern turbofans, where a significant portion of air bypasses the engine core, open rotor designs aim to optimize the mix of bypassed and core-flow air to produce thrust more efficiently. This approach promises a considerable reduction in fuel consumption—by 25-35% compared to the latest turbofan engines and even offers a 10-20% improvement over the newest turbofans on the market.
The quest for higher bypass ratios has led to the development of engines like the Pratt & Whitney geared fan engines, boasting ratios of 12:1, and plans by Rolls-Royce to introduce new generations of engines with even higher ratios. However, the limitations of increasing fan diameters—namely, added nacelle weight, increased aerodynamic drag, and duct losses—necessitate a reevaluation of propulsion strategies. Herein lies the open rotor’s advantage, capable of achieving effective bypass ratios exceeding 30:1 without the encumbrance of a fan duct, thus significantly enhancing propulsion efficiency.
Addressing the Challenge
Despite their efficiency, open rotor engines face challenges, particularly at higher cruise Mach numbers (Ma), where losses due to shock formations in tip regions become pronounced. Consequently, open rotor engines are typically designed to operate optimally at about Ma=0.8, minimizing component losses and maximizing overall performance.
The journey of open rotor technology from its inception to the present day reflects the aviation industry’s ongoing efforts to find a balance between performance and fuel economy. As we continue to explore and refine this promising technology, the open rotor stands as a symbol of potential—a beacon guiding the way toward a more efficient, sustainable future in air travel.