How Ramjets Function

NASA engineer Laura O'Connor examines a supersonic ramjet (scramjet) engine prototype at Langley Research Center, located in Hampton, Virginia. © Corbis

Key Insights

Anyone who has belly-flopped off a high dive knows the feeling: when you hit a fluid without giving it a chance to move out of the way, it pushes back hard. Divers cheat physics by plunging with a more streamlined shape, and faster cars and aircraft do the same with more aerodynamic designs. However, near the sound barrier, streamlining isn't enough — a speed where the air that keeps your plane in flight begins to overwhelm you with drag, turbulent forces, and intense shock waves. Many thought the sound barrier was insurmountable until Chuck Yeager’s rocket-powered Bell X-1 shattered that belief on October 14, 1947.

Imagine if you could turn all that trapped air to your advantage. Instead of forcing it through with propellers or blasting it away with rockets, you could trap it in a specially designed tube, ignite it with an explosion, and release it through a nozzle at speeds beyond sound, all without any major moving parts. What you'd have is a unique type of jet engine, a 'flying stovepipe' capable of slicing through the air at thousands of miles per hour. This is the essence of a ramjet.

The simplicity of a ramjet is somewhat misleading; it demands cutting-edge aeronautical design, modern materials, and precise manufacturing to build one successfully. This is partly why an idea almost as old as powered flight was repeatedly discarded until it saw some success during the Cold War.

In contrast to its chief rival, the rocket, which burns fuel with onboard oxidizers like ammonium nitrate, potassium chlorate, or ammonium chlorate, a ramjet uses air. While rockets can operate in the vacuum of space, ramjets depend on the atmosphere. To work, they must travel at extremely high speeds—around Mach 2.5-3.0, or three times the speed of sound—because ramjets function by harnessing 'ram pressure', the natural compression that occurs as an aircraft speeds through the air. Essentially, ramjets use the shock waves and compression forces that once hindered high-speed flight to their benefit.

Ramjets are more fuel-efficient over long distances compared to rockets, but they come with a major drawback: they're ineffective at low speeds. As a result, they need booster rockets or other systems to accelerate to the required velocity. Standalone ramjet-powered planes typically rely on hybrid engines.

If all of this went over your head at supersonic speeds, it's probably because we skipped over some fascinating details. Let’s take a closer look at how jet engines have evolved to produce this modern technological wonder.

Detonations and Arrivals

A cameraman captures the intense flame from the thrust augmentor of a ramjet I-40 engine at the Lewis Flight Propulsion Laboratory in Cleveland. (This facility would later become the John Glenn Research Center.) © Corbis

Jet engines rely on continuous explosions. While that may sound unusual, car engines work on a similar principle: air is drawn in, compressed, mixed with fuel, ignited, and boom! The piston moves. However, unlike the cyclical bursts of gasoline or diesel engines, jet engines operate with continuous combustion, where fuel and air are constantly mixed and burned. In both cases, more power means more fuel and more oxygen. High-performance cars use superchargers to achieve this, but in jet engines, the process is more intricate [source: Encyclopaedia Britannica].

The first operational jet aircraft entered combat towards the end of World War II, powered by turbojet engines—an ingenious yet simple design following the Brayton (or Joule) Cycle. As the plane flies, air is drawn through an intake and into a diffuser, a chamber that slows the airflow and reduces shock waves. It then moves through a set of spinning rotors and stationary stators, which together act as a compressor, raising the pressure inside the combustion chambers. Here, fuel mixes with the high-pressure air and ignites, creating temperatures ranging from 1800-2800°F (980-1540°C) or higher [sources: Encyclopaedia Britannica; Krueger; Spakovszky].

As pressure increases with temperature, this ignition generates powerful force that seeks a rapid escape. As the exhaust passes through the rear nozzle, it creates thrust that propels the aircraft forward. Along its path, the exhaust also flows through a turbine connected to the rotors via a torque shaft. As the turbine spins, it transfers energy to the compressor blades, completing the cycle.

In aircraft with turboprops or helicopters powered by turboshaft engines, turbines transmit power to the propeller or rotor of the helicopter through a series of gears.

Turbojets generate immense power but face difficulties at lower speeds. As a result, during the 1960s and 1970s, low-supersonic aircraft began adopting

turbofans, which are still the standard in most private jets and commercial airliners. A turbofan can be thought of as a 'turducken' of engines—essentially a turbojet nestled inside a larger casing, with a big fan mounted at the front. The fan draws in more air, which the engine divides into two streams: one passes through the turbojet, while the other flows around it. The two streams come back together when the redirected cooler air mixes with the turbojet's exhaust, slowing it down and creating a larger, slower thrust stream that works more efficiently at low speeds [sources: Encyclopaedia Britannica; Krueger].

Around the time turbofans were gaining traction, research into ramjet-powered aircraft was finally making significant progress. It had been a lengthy journey.

Ramjets, Ahead of Their Time?

Whoever claimed you must crawl before you can walk never met French inventor René Lorin. As early as 1913, while pilots were still flying basic wooden planes, Lorin envisioned the potential of ram-pressure propulsion. Recognizing the design's limitations at subsonic speeds, he shifted his focus to designing a ramjet-assisted flying bomb. The French military rejected his idea. Similarly, Hungarian engineer Albert Fono pursued a similar project in 1915, only to face the same skepticism from the Austro-Hungarian Army [sources: Gyorgy; Heiser and Pratt; Wolko].

Ramjet designs briefly gained popularity between the World Wars. Soviet engineers made early advancements with rocket-based ramjets (discussed further below), but interest waned before 1940. Despite interruptions from the German occupation, French engineer René Leduc's perseverance paid off on April 21, 1949, when his Lorin-inspired 010 model made its first powered flight atop a Languedoc 161 airliner. It flew for 12 minutes, reaching speeds of 450 mph (724 kph) at half power [sources: Siddiqi; Ward; Wolko; Yust et al].

For a while, Leduc's success was all that came from ramjet research. Despite his achievement, lack of funding halted official support for his research in 1957 [sources: Siddiqi; Ward; Wolko; Yust et al.]. The ramjet seemed to have no future. Meanwhile, World War II saw the introduction of the first generation of operational turbojets, including the British Gloster Meteor, the German Messerschmitt Me 262, and the American Lockheed F-80 Shooting Star [sources: Encyclopaedia Britannica; Encyclopaedia Britannica; Encyclopaedia Britannica; National Museum of the USAF; van Pelt].

As the Cold War intensified after World War II, it became clear that turbojets and turbofans offered more viable solutions for subsonic and low-supersonic speeds than ramjets. Consequently, most U.S. and Soviet research into ramjets shifted toward developing intercontinental missiles. In 1950, American engineer William H. Avery and the Johns Hopkins University Applied Physics Laboratory created Talos, the U.S. Navy's first ramjet-powered missile. Future iterations of this technology would refine and streamline the design, introducing hybrid ramrockets capable of reaching high supersonic speeds (Mach 3-5) [sources: Hoffman; Kossiakoff; Ward].

Despite interesting designs like the Hiller XHOE-1 Hornet helicopter, the proposed Republic XF-103 bomber interceptor, and the short-lived Lockheed D-21B unmanned reconnaissance drone, ramjet aircraft remained largely sidelined until the 1964 debut of the Lockheed SR-71 Blackbird. The fastest manned aircraft until its retirement in 1989, the Mach 3+ Blackbird utilized a hybrid engine, sometimes referred to as a turboramjet [sources: National Museum of the USAF; Smithsonian; Ward].

In the following section, we will explore the SR-71 and various ramjet hybrids, along with their different subtypes.

Ramjets: Making a Mockery of Mach

The Lockheed SR-71A Blackbird reconnaissance aircraft gets ready for takeoff. The Blackbird on display at the Steven F. Udvar-Hazy Center previously made a record-breaking flight from Los Angeles to Washington, D.C. in just one hour, four minutes, and 20 seconds. © George Hall/Corbis

If ramjets are so finicky, then what’s the point? Well, at the extreme pressures and temperatures experienced at speeds over Mach 2.5, most jet engines become impractical and, frankly, useless. Even if you could somehow make one function, it would be like trying to operate a windmill in a hurricane or dragging a wave machine to Oahu's North Shore.

Ramjets take the essential concepts of traditional jet engines and push them to their extreme, achieving this without the need for complex moving parts. The air enters the ramjet's diffuser at supersonic speeds, creating shock waves that help increase ram pressure. A diamond-shaped body within the intake further compresses the air, slowing it down to subsonic speeds for better fuel mixing and combustion. Combustion takes place in an open chamber similar to a massive afterburner, where liquid fuel is injected or solid fuel is ablated from the chamber's walls [sources: Ashgriz; Encyclopaedia Britannica; SPG; Ward].

The speed constraints of ramjets eventually led to the development of hybrid engines capable of flying at lower speeds while accelerating to supersonic speeds. The SR-71 Blackbird is the most well-known example of this, featuring a turbojet-ramjet hybrid known as a turboramjet. These engines function like a turbojet with afterburning until surpassing Mach 1, after which ducts bypass the turbojet, directing the ram-compressed airflow into the afterburner, transforming the engine into a ramjet [source: Ward].

Missile designs evolved to eliminate the need for boosters by integrating them within the ramjet itself, leading to the creation of ramrockets or integral rocket ramjets. During rocket acceleration, plugs temporarily seal off the ramjet's intake and fuel injectors. Once the rocket fuel is spent and the ramjet reaches speed, these plugs are removed, and the empty rocket bodies serve as combustion chambers [source: Ward].

Looking ahead, crossing the Mach 5 threshold into hypersonic speeds will likely involve scramjets (supersonic combusting ramjets). Unlike other ramjets, scramjets don't need to slow down air to subsonic speeds within their combustion chambers. To ignite and expand within the minuscule 0.001 seconds before the pressurized air exits the exhaust, scramjets typically use hydrogen fuel, which has a high specific impulse (a change in momentum per unit mass of propellant), ignites across a wide range of fuel/air ratios, and releases an enormous amount of energy when burned [sources: Bauer; Encyclopaedia Britannica; NASA].

Before recent decades, scramjets were mostly theoretical, and much of the work remains experimental. In November 2004, NASA's eight-year, $230-million Hyper-X Program produced a scramjet that achieved Mach 9.6 on its final flight. Some experts believe this technology could eventually reach Mach 15-24, though air travel at hypersonic speeds would require overcoming forces far beyond what the fastest supersonic crafts experience. In short, we are still a long way from being able to commute from New York to Los Angeles in 12 minutes [sources: Bauer; DARPA; Fletcher; NASA].