How Jet Engines Generate Thrust

Seventy-Seven Thousand Pounds of Thrust from a Tube of Compressed Fire

A GE Aviation GE9X engine—the powerplant on Boeing's 777X—generates up to 105,000 pounds (467 kilonewtons) of thrust while burning roughly 2.8 liters of jet fuel per second at takeoff. Its fan diameter is 3.4 meters—large enough to walk through upright. The engine weighs approximately 9,400 kilograms. Yet despite this scale, the fundamental operating principle is straightforward: continuously suck in air, compress it, burn fuel in it, expand the hot gases, and eject them rearward faster than they arrived. Newton's third law does the rest. The force that pushes the exhaust backward pushes the engine—and the aircraft—forward.

The Brayton Cycle: Jet Engine Thermodynamics

All jet engines operate on the Brayton cycle (also called the Joule cycle), a thermodynamic cycle consisting of four processes:

1. Isentropic compression: Air is compressed without heat loss, raising its pressure and temperature.
2. Constant-pressure heat addition: Fuel burns at essentially constant pressure in the combustion chamber, dramatically increasing temperature (to 1,400–1,700°C in modern engines) while pressure remains roughly constant.
3. Isentropic expansion: Hot gases expand through the turbine and nozzle, extracting work and accelerating.
4. Constant-pressure heat rejection: Exhaust gases leave the engine at atmospheric pressure, carrying waste heat with them.

The thermal efficiency of the Brayton cycle improves with higher pressure ratios (more compression before combustion) and higher turbine inlet temperatures. The GE9X achieves an overall pressure ratio of approximately 60:1, meaning air at the combustion chamber entrance is 60 times denser than outside air. Achieving this requires materials that can withstand temperatures exceeding the melting point of nickel alloys—solved through ceramic thermal barrier coatings, single-crystal turbine blades (eliminating grain boundaries that fail under stress), and internal cooling channels that route cooler air through the blade's interior.

Engine Architecture: From Turbojet to Turbofan

The first practical jet engines, developed independently by Hans von Ohain in Germany (flew September 1939, He 178) and Frank Whittle in Britain (flew May 1941, Gloster E.28/39), were pure turbojets: all thrust came from the jet exhaust. Turbojets are fast and compact but inefficient at subsonic speeds—they accelerate a small mass of air to very high velocity, which wastes energy.

Modern commercial aviation uses turbofan engines, which add a large fan stage at the front that drives far more air around the engine core (the bypass flow) than through it. The bypass ratio is the key parameter:

High bypass ratios improve specific fuel consumption (SFC) dramatically because moving a large mass of air slowly is more efficient than moving a small mass very fast—a consequence of the thrust equation. A turbofan with a 12:1 bypass ratio uses roughly 15–20% less fuel than an equivalent turbojet for the same thrust at cruise.

The Thrust Equation

Newton's second law applied to propulsion gives the thrust equation:

F = ṁ × (V_exit - V_intake)

Where F is thrust in newtons, ṁ is the mass flow rate of air through the engine in kg/s, V_exit is the exit velocity of the gases, and V_intake is the intake velocity (equal to aircraft speed). At cruise altitude (~35,000 feet), a large turbofan may process 1,000–1,500 kg of air per second through the fan. The GE9X moves approximately 1,200 kg/s. Increasing thrust means increasing either mass flow or exit velocity—high-bypass engines maximize mass flow.

The Three Major Engine Families

Commercial aviation is dominated by engines from three manufacturers:

• CFM International (GE/Safran JV): The CFM56 has been the world's best-selling commercial engine family with over 33,000 sold. Its successor, the LEAP engine (on Boeing 737 MAX and Airbus A320neo), uses 3D-woven carbon fiber composite fan blades and ceramic matrix composite (CMC) turbine shrouds, improving fuel efficiency 15% over the CFM56.
• Rolls-Royce: The Trent XWB (powering the Airbus A350) was named the world's most fuel-efficient large civil aircraft engine upon its 2013 service entry. Rolls-Royce's UltraFan demonstrator engine, tested in 2023, demonstrated a 25% fuel efficiency improvement over the original Trent using a power gearbox to allow the fan to spin slower than the core turbine.
• Pratt & Whitney (RTX): The PW1000G (GTF—Geared TurboFan) engine uses a gear system between the fan and low-pressure compressor/turbine, allowing bypass ratios of 12:1 while keeping both fan and turbine at optimal speeds. It powers the A220, A320neo family, and Embraer E-Jets E2, with certified fuel burn improvements of 16–20% over previous-generation engines.

Materials at the Limits of Physics

Modern turbine blades operate at temperatures 200–300°C above the melting point of the nickel superalloys from which they are made. This is possible through:

• Single-crystal casting: Turbine blades are cast as single crystals with no grain boundaries—the failure mechanism for polycrystalline metals at high temperatures. Developed by Pratt & Whitney in the 1960s.
• Thermal barrier coatings (TBCs): Ceramic coatings (typically yttria-stabilized zirconia, YSZ) of 100–300 microns applied to blade surfaces, insulating the metal from gas temperatures by up to 300°C.
• Cooling channels: Intricate internal passages machined or cast into blades route cooler compressor air through the blade and out through film-cooling holes on the surface, creating an insulating air film.
• Ceramic matrix composites (CMCs): GE Aviation introduced CMC components (silicon carbide fibers in a silicon carbide matrix) into the hot section of the LEAP engine—the first such use in a production civil engine. CMCs are one-third the weight of nickel alloys and can withstand higher temperatures, enabling future efficiency gains.

The jet engine is among humanity's most refined engineering achievements—a machine operating at extremes of temperature, pressure, speed, and stress, turning 40 years of incremental material science into aircraft that cross continents on less fuel than they did a decade ago.