Engine Encyclopedia

How It Works

The turbojet is the earliest form of jet engine. Air is drawn in and compressed through multiple compressor stages (up to 10–25× intake pressure), then mixed with fuel and ignited. The hot, high-pressure exhaust gases drive the turbine (which powers the compressor), then exit through the nozzle at high speed to produce thrust.

Advantages: Relatively simple design, excellent high-speed performance, high thrust-to-weight ratio

Disadvantages: High fuel consumption, poor efficiency at low speeds, extremely loud

Supersonic Aircraft Using This Engine

How It Works

The turbofan adds a large fan in front of the turbojet core. Supersonic fighters use a low bypass ratio that varies by engine turbofans, where most air flows through the core and a smaller portion through the bypass duct. This design balances supersonic performance with subsonic fuel efficiency.

When equipped with an afterburner (reheat), fuel is injected directly into the exhaust stream for secondary combustion, increasing thrust by an amount that depends on the engine and operating condition — at the cost of massively increased fuel consumption. Many modern supersonic fighters use low-bypass turbofans with afterburners.

Supersonic Aircraft Using This Engine

How It Works

The SR-71 Blackbird used the J58 afterburning turbojet as part of an integrated inlet, engine and nozzle system. At high Mach number, bypass valves bled some air from the fourth compressor stage and routed it around the rear compressor stages, combustor and turbine toward the afterburner. The turbojet core continued operating; the engine did not simply switch off its turbine and become a pure ramjet.

The inlet, J58 and nozzle were designed together for sustained flight above Mach 3. Their efficiency depended on the operating point. It is misleading to infer that the aircraft always used less fuel simply because it flew faster.

Aircraft Using This Engine

How It Works

A basic ramjet does not need a rotating compressor, turbine or fan. It compresses incoming air through the vehicle’s forward motion and inlet geometry. A practical installation may still have moving components in its fuel system, valves or variable geometry.

When flying supersonically, air is “rammed” into the intake. Through carefully designed geometry, shockwaves slow the supersonic airflow to subsonic speed while dramatically compressing it. The compressed air mixes with fuel, ignites, and exits at high speed to produce thrust.

Main limitation: a ramjet cannot provide useful static takeoff thrust. Another system must first accelerate it to a suitable operating speed, which depends on the design. Missiles commonly use a rocket booster for this initial acceleration.

Aircraft & Missiles Using This Engine

How It Works

The scramjet — supersonic combustion ramjet — is the evolved version of the ramjet and represents the a demanding branch of air-breathing propulsion technology. Unlike a regular ramjet, the airflow inside the combustion chamber remains supersonic at its intended operating condition.

How hard is that? Imagine igniting and maintaining stable combustion in airflow moving at thousands of km/h — like lighting a match in a hurricane and keeping it lit. Air passes through the combustor in just a few milliseconds, and fuel must mix, ignite, and burn completely in that incredibly short time.

NASA’s X-43A reached Mach 9.6 in 2004, and the X-51A maintained Mach 5+ flight for over 3 minutes in 2013 — both landmark achievements for scramjet technology.

Aircraft Using This Engine

How It Works

The fundamental difference between rocket engines and all jet engines: rockets carry their own oxidizer and don’t need to breathe air. This means they work outside the atmosphere and produce thrust at any speed — including from a standstill.

The X-15 used the XLR99 rocket engine in its later research flights. The program reached Mach 6.7 in October 1967 and about 108 km altitude in August 1963 on separate flights. The earlier Bell X-1 used the XLR11 for the 1947 controlled level-flight breakthrough beyond the speed of sound.

Limitation: propellant is consumed rapidly. NASA describes typical X-15 powered flight as about 80–120 seconds; external tanks on the X-15A-2 allowed a longer burn. Specific-impulse comparisons with air-breathing engines also need to account for the rocket carrying its own oxidizer.

Aircraft Using This Engine