Adaptive Cycle Engines

Variable cycles, the third stream, thrust and efficiency modes, mission fuel, and the third stream as a heat sink

Two personalities, 2020

In December 2020 an engine unlike any in service first ran at General Electric: the XA100, built to fit the F-35 (the XA100). Pratt & Whitney's rival, the XA101, ran in 2021 (the XA101). Both have three streams instead of two: an adaptive fan can send air into an outer third stream for fuel efficiency and cooling, or through the fan and core streams for thrust. In 2023 the US Air Force chose an upgraded F135 for the F-35 instead, citing cost and the risk of integrating a new engine. The idea lives on — the XA100 is the technological foundation of GE's engine for the next-generation fighter. This lesson is about why one engine might want two personalities.

Every engine in this course is designed for one point and lives with the compromise everywhere else (Unit 8). A fighter needs high specific thrust for take-off, climb and supersonic dash — a low bypass ratio — and low fuel consumption for cruise and loiter — a high one. Variable-cycle engines change their bypass ratio in flight. The idea is old: GE's YF120 was a variable-cycle engine for the US Air Force's advanced tactical fighter competition, but the Air Force chose the conventional Pratt & Whitney F119 for what became the F-22 (variable cycle engine).

\frac{F}{\dot m_0} = \sum_i x_i\,(1 + f_i)\,u_{e,i} - u_0, \qquad \eta_{p,i} = \frac{2}{1 + u_{e,i}/u_0}

Specific thrust of a multi-stream engine (x_i = share of the inlet air in stream i, f_i its fuel–air ratio, zero for bypass streams) and each stream's Froude propulsive efficiency (Lesson 1-2).

Derivation

Why more air at lower speed saves fuel

A stream of mass flow

\dot m

accelerated from

u_0

u_e

gives thrust

F = \dot m\,(u_e - u_0)

and gains kinetic energy at the rate

\tfrac12\,\dot m\,(u_e^2 - u_0^2) = F\,\frac{u_e + u_0}{2}.

For the same thrust, a lower jet speed — more air, accelerated less — wastes less energy in the wake, so it needs less fuel. But thrust per unit of inlet air is

u_e - u_0

: the same inlet then gives less thrust, and at high flight speed a slow stream barely beats the incoming air at all. Specific thrust and fuel consumption pull in opposite directions.

💡The third stream

An adaptive engine keeps one core and gives its fan a choice. Efficiency mode: open the outer third stream, a lightly compressed flow of extra air — a higher bypass ratio and lower jet speeds. Thrust mode: close it and push that inlet air through the fan and core streams instead. The third stream is also a large, cool flow of air that can absorb heat and feed power systems — on the XA100, a heat sink for cooling as well as a source of efficiency.

📐One core, two modes

A core of overall pressure ratio 36 at

T_{t4} = 1900

K with an inner fan stream (0.4 kg per kg of core air, pressure ratio 4.5) and a third stream (1.2 kg, pressure ratio 2.0), at 11 km (Lab 1):

Cruise, Mach 0.9: thrust mode 820 N·s per kg of inlet air at a TSFC of 30.3 mg/(N·s); efficiency mode 489 N·s/kg at 27.4 — 9.6 % less fuel per newton.
Dash, Mach 1.6: thrust mode gives 73 % more thrust from the same inlet.

📐A mission

Take-off and climb, cruise out, a supersonic dash, loiter and cruise back — 3 hours in all — with the adaptive engine in thrust mode only where it must be (Lab 3): 6.1 % less fuel than a fixed thrust-mode engine, or 19 more minutes on station with the same fuel.

💡Thermal management

Modern combat aircraft are full of electronics that turn power into heat, and the traditional heat sink is the fuel on its way to the combustor. But fuel flow at cruise is small, and fuel cannot be heated far before it starts to break down. The third stream is a large flow of cool air passing right by the engine's hot core.

📐Dumping 300 kW

At cruise (Lab 2):

The engine burns 0.685 kg/s of fuel; warmed by 120 K it can absorb only 173 kW.
The third stream carries 60 kg/s of air: 300 kW warms it by just 5 K — and adds 0.3 % of thrust, because the heat is not wasted.

Adapt or perish, now as ever, is Nature's inexorable imperative.

– H. G. Wells, The Mind at the End of its Tether (1945) (Wikiquote)

💡Supersonic airliners

The same tension rules supersonic transports: high specific thrust to cruise supersonically, low jet speeds for quiet take-offs and efficient subsonic legs. Some form of variable-cycle engine may be what the next generation needs (variable cycle engine).

💡Where you will use this

Multi-stream cycle analysis, mission fuel accounting and thermal management are now part of every combat-engine study, and of hybrid-electric airliner concepts. Lesson 10-2 turns to airliners, whose problem is carbon.

📚Further reading

Jack D. Mattingly, William H. Heiser and David T. Pratt, Aircraft Engine Design (2nd ed., AIAA, 2002).
The XA100 and variable cycle engines on Wikipedia.

↓But there is a problem

A fighter's engine adapts to its mission. An airliner's engine faces a harder adversary: every kilogram of kerosene it burns makes 3.16 kg of carbon dioxide (Lesson 7-4). In 2020 and 2021, two of the industry's giants placed two very different bets on the same deadline.