How Jet Engines Work

The Basic Principle: Newton's Third Law

Every jet engine operates on a deceptively simple idea: accelerate a mass of air rearward, and an equal and opposite force pushes the aircraft forward. This is Newton's third law applied at enormous scale. A typical turbofan engine on an Airbus A320 ingests roughly 400 kg (880 lb) of air every second during takeoff — equivalent to emptying a large bathtub in under a second — and expels it at higher velocity. That difference in momentum is thrust.

What distinguishes jet engines from piston engines is continuous combustion. Rather than discrete power strokes, a jet engine burns fuel in a constant, steady flame, allowing far higher power-to-weight ratios and enabling flight at altitudes and speeds impossible with propeller-driven aircraft.

The Compressor Stage

Air enters the engine through the inlet and first encounters the compressor, which can be a fan, a low-pressure compressor (LPC), and a high-pressure compressor (HPC) in sequence. The compressor's job is to squeeze incoming air to much higher pressure before it reaches the combustion chamber.

Modern high-bypass turbofans use axial-flow compressors: rows of spinning rotor blades alternate with stationary stator vanes, each stage adding a small pressure rise. The CFM LEAP-1A on the Airbus A320neo achieves an overall pressure ratio of around 45:1 — meaning air entering the core at atmospheric pressure (roughly 14.7 psi) exits the compressor at over 660 psi. Higher pressure means more energy available for combustion, directly translating to greater efficiency.

Compressor blades must handle enormous centrifugal forces while operating at temperatures that soften ordinary steel. High-pressure compressor blades are machined from titanium alloys or nickel superalloys and may spin at 15,000–25,000 RPM at the core shaft.

Combustion

Compressed air flows into the combustion chamber (combustor), where fuel — typically Jet-A kerosene — is sprayed and ignited. The combustor is designed to burn fuel as completely as possible while keeping temperature distribution even enough not to destroy the downstream turbine blades.

Modern annular combustors surround the engine core in a ring, replacing older can or can-annular designs. Combustion temperatures reach 1,500–2,000°C — far above the melting point of the nickel alloys used in the surrounding structure. The combustor liner is protected by a film of cooler compressor bypass air that flows along its inner surface and by a thermal barrier ceramic coating only a fraction of a millimeter thick.

The Turbine

Hot, high-pressure combustion gases expand rearward through the turbine section, which consists of high-pressure turbine (HPT) and low-pressure turbine (LPT) stages. The turbine extracts energy from the gas stream to drive the compressor — up to 80 percent of the energy produced by combustion goes back to running the compressor. Only the remaining energy creates thrust.

HPT blades operate in gas temperatures exceeding their own melting point, surviving only because of sophisticated internal cooling channels that circulate cooler air through the blade and release it as a protective film. The blades in modern engines like the GE9X are single-crystal nickel superalloy castings — the entire blade is one grain of metal, eliminating grain boundaries where fatigue cracks start.

Bypass Air and the Fan

A turbofan engine adds a large fan at the front that moves a much larger volume of air around the engine core, bypassing combustion entirely. The ratio of bypass air to core air is the bypass ratio (BPR). The CFM56 engines powering older 737s have a BPR of about 5:1. The CFM LEAP-1B on the 737 MAX raises this to 9:1. The GE9X on the 777X achieves 10:1.

Higher bypass ratio means more thrust from slower-moving air, which is far more efficient than a small amount of very fast air. It also dramatically reduces engine noise, since jet noise scales with approximately the eighth power of exhaust velocity.

Types of Jet Engines

• Turbojet: The original jet engine type — all thrust comes from the core exhaust. Extremely loud and fuel-hungry at subsonic speeds. Still used in some supersonic military aircraft.
• Turbofan (high-bypass): Dominant in commercial aviation. The fan provides most of the thrust; the core provides power to drive the fan. Examples: CFM LEAP, Rolls-Royce Trent XWB, GE9X, Pratt & Whitney GTF.
• Turboprop: Uses a turbine to drive a conventional propeller through a gearbox. More efficient below about 450 mph; used on regional aircraft like the ATR 72 and Q400.
• Turboshaft: Extracts nearly all energy to drive a shaft — used in helicopters and APUs rather than for direct jet thrust.

The thrust-to-weight ratio of modern turbofans can exceed 7:1, meaning the engine produces more than seven times its own weight in thrust — a level of performance unimaginable to the pioneers who watched Frank Whittle's first jet engine run in 1937.