How Jet Engines Work: From Turbojets to Scramjets

Every supersonic aircraft — from the Bell X-1 to the F-35 Lightning II — relies on one fundamental principle: Newton’s Third Law. For every action, there is an equal and opposite reaction. A jet engine accelerates gas backward; the aircraft moves forward. Simple in concept, extraordinarily complex in execution.

The Four Types of Jet Engine

1. Turbojet

The original jet engine. Air enters the intake, gets compressed by spinning blades, mixed with fuel and ignited in the combustion chamber, then blasted out the back through a turbine (which drives the compressor) and the exhaust nozzle.

Pros: Simple, excellent at high speeds
Cons: Very fuel-hungry, extremely loud
Used on: Early fighters (MiG-21, F-104), Concorde (Olympus 593)

2. Turbofan

The modern standard. A turbofan adds a large fan in front that moves a huge volume of air around the core engine (the “bypass” air). This bypass air provides most of the thrust while being much quieter and more fuel-efficient.

Low-bypass turbofans (bypass ratio ~0.3–1.0) power military fighters — they’re a compromise between efficiency and high-speed performance. Examples: Pratt & Whitney F119 (F-22), General Electric F414 (Super Hornet).

High-bypass turbofans (bypass ratio 5–12+) power airliners — maximum fuel efficiency at subsonic speeds. Examples: CFM LEAP (737 MAX), Rolls-Royce Trent (787).

3. Ramjet

At high speeds, incoming air is compressed by the aircraft’s own velocity — no spinning compressor needed. A ramjet has no moving parts: air rams in, fuel is added, combustion occurs, exhaust shoots out.

Catch: Ramjets don’t work from a standstill — they need to be accelerated to around Mach 2+ before they can function. This is why ramjet-powered missiles are launched from aircraft or boosted by rockets first.

Used on: SR-71 Blackbird (J58 — technically a turboramjet hybrid), BrahMos missile

4. Scramjet (Supersonic Combustion Ramjet)

The frontier of air-breathing propulsion. In a regular ramjet, incoming air is slowed to subsonic speed before combustion. In a scramjet, the air flows through the engine at supersonic speed the entire time.

This is incredibly difficult — it’s been compared to “lighting a match in a hurricane.” But scramjets can theoretically operate at Mach 5 to Mach 15+, far beyond what any other air-breathing engine can achieve.

Used on: NASA X-43A (Mach 9.68 — fastest air-breathing vehicle ever), Boeing X-51 Waverider (Mach 5.1)

Afterburners: Raw Power

An afterburner injects additional fuel directly into the exhaust stream after the turbine. The fuel ignites in the hot exhaust gases, producing a dramatic thrust increase (40-70%) at the cost of massive fuel consumption. The iconic blue-orange flame cone behind a fighter in full afterburner is this extra combustion in action.

What’s Next?

The future of jet propulsion includes:

• Adaptive cycle engines: The GE XA100 and P&W XA101 can change their bypass ratio in flight — efficient like a turbofan at cruise, powerful like a turbojet at sprint
• Rotating detonation engines: Use continuous detonation waves instead of steady combustion for dramatically higher efficiency
• Combined cycle engines: Turbojet + ramjet + scramjet in one package, operating from standstill to Mach 5+ (like the proposed SABRE engine for Reaction Engines’ Skylon spaceplane)

Explore engine types in our Supersonic Engine Encyclopedia.

References

1. Fry, R.S. (2004). A Century of Ramjet Propulsion Technology Evolution. Journal of Propulsion and Power, 20(1), 27-58. DOI: 10.2514/1.9178
2. Curran, E.T. (2001). Scramjet Engines: The First Forty Years. Journal of Propulsion and Power, 17(6), 1138-1148. DOI: 10.2514/2.5875
3. Mattingly, J.D. (2006). Elements of Propulsion: Gas Turbines and Rockets. AIAA Education Series. DOI: 10.2514/4.861789

Frequently Asked Questions

What are the four main types of jet engine?

Turbojets (simple, used in early fighters and Concorde), turbofans (modern fighters and airliners — the dominant type), ramjets (no moving parts, only function at Mach 2+), and scramjets (supersonic combustion, capable of Mach 5 to 15+). Afterburners are not a separate engine type but an add-on that boosts thrust from any of the first two.

What is the difference between a turbojet and a turbofan?

A turbojet sends all incoming air through the core combustor. A turbofan adds a large fan in front that bypasses much of its airflow around the core, providing most of the thrust more efficiently and quietly. Low-bypass turbofans power fighters; high-bypass turbofans power airliners.

Why don’t ramjets work from a standstill?

A ramjet relies on the aircraft’s own forward speed to compress incoming air — there is no spinning compressor to do the job. Below roughly Mach 2, the inlet pressure rise is insufficient for stable combustion. That is why ramjet missiles are launched from aircraft or first boosted by rockets.

What is a scramjet and why is it so hard?

A scramjet is a supersonic combustion ramjet — air flows through the engine at supersonic speed the entire way, even during combustion. This has been compared to lighting a match in a hurricane. NASA’s X-43A scramjet vehicle reached Mach 9.68, and the Boeing X-51 reached Mach 5.1, but practical scramjet engines remain at the frontier of propulsion research.

What does an afterburner do?

An afterburner injects raw fuel into the hot exhaust stream after the turbine. The fuel ignites in the oxygen-rich exhaust, producing 40–70% extra thrust at the cost of dramatically higher fuel consumption. The blue-orange flame cone behind a fighter in full afterburner is this extra combustion.