How Ramjets Work

Jets run on controlled explosions. That sounds strange until you realize that most car engines do, too: Pull in air, compress it, mix it with fuel, ignite it and bang! You've pushed a piston. But whereas gasoline and diesel engines involve cyclical or intermittent combustion, jets entail continuous combustion, in which fuel and air mix and burn nonstop. Either way, burning more rubber means guzzling more gas, and that means sucking in more oxygen to get the mixture right. Souped-up cars do this with superchargers; in jet engines, it's more complicated [source: Encyclopaedia Britannica].

The first operational jet aircraft zoomed into combat near the end of World War II using turbojet engines, a straightforward but ingenious design based on the Brayton (or Joule) Cycle: As the plane flies, air streams through an intake into a diffuser, a chamber that slows airflow and inhibits shock waves. It then passes through a series of bladed disks: spinning rotors, which force air backward, and stationary stators, which guide airflow. Together, they act as a compressor that pumps up pressure within the jet's combustion chambers. There, fuel mixes with pressurized air and ignites, blasting temperatures into the 1800-2800 F (980-1540 C) range or higher [sources: Encyclopaedia Britannica; Krueger; Spakovszky].

Pressure rises with temperature, so this explosion creates a lot of force with nothing to do but seek a quick exit. As the exhaust shoots through the rear nozzle it generates thrust to move the aircraft. En route to this nozzle, the exhaust also shoots through a turbine connected to the rotors by a torque shaft. As the turbine spins, it transfers energy to the compressor blades in front, completing the cycle.

In airplanes with turboprops or helicopters with turboshaft engines, the turbines also transfer power to a propeller or helicopter rotor via a series of gears.

Turbojets pack a lot of power but struggle at low speeds. Consequently, in the 1960s and 1970s, low-supersonic aircraft began trending toward the turbofans that most private jets and commercial airliners still use. A turbofan is the turducken of engines -- essentially a turbojet wrapped in a larger cowling with a big fan slapped on its front. The fan pulls in more air, which the engine then splits into two streams: Some air moves through the nested turbojet, while the rest flows through the empty space around it. The two streams reunite when redirected cooler air mixes with the turbojet's exhaust and slows it down, creating a larger, slower thrust stream that is more efficient at low speeds [sources: Encyclopaedia Britannica; Krueger].

Meanwhile, around the time that turbofans came into their own, research into ramjet aircraft was finally hitting its stride. It had been a long road.

Whoever said that you have to walk before you can run never met Frenchman René Lorin. He saw the possibilities of ram-pressure propulsion as early as 1913, when pilots were still flying glorified wooden kites. Aware of the design's uselessness at subsonic speeds, he instead designed a ramjet-assisted flying bomb. The French military waved him off. Hungarian engineer Albert Fono, another ramjet pioneer, pursued a similar idea in 1915 and received a comparable reception from the Austro-Hungarian Army [sources: Gyorgy; Heiser and Pratt; Wolko].

Ramjets designs enjoyed a short vogue between world wars. Soviet engineers made early strides in rocket-based ramjets (see next section), but interest burned out before 1940. The German occupation interrupted French engineer René Leduc's early work, but his persistence and secrecy paid off on April 21, 1949, when his Lorin-inspired 010 model made its first powered flight of a ramjet aircraft. Carried aloft atop a Languedoc 161 airliner, it flew for 12 minutes and reached 450 mph (724 kph) at half power [sources: Siddiqi; Ward; Wolko; Yust et al.].

And, for a while, that was that. Despite Leduc's success, lack of funds ended official support for his research in 1957 [sources: Siddiqi; Ward; Wolko; Yust et al.]. The ramjet was beginning to look like an invention with no application. Meanwhile, World War II had ushered in the first generation of operational turbojets: 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 war ended and the Cold War heated up, it became clear that turbojets and turbofans presented more practical subsonic and low-supersonic solutions than ramjets. Thereafter, most U.S. and Soviet work in ramjets focused on building intercontinental missiles. In 1950, American engineer William H. Avery and the Johns Hopkins University Applied Physics Laboratory produced Talos, the U.S. Navy's first ramjet missile. Future generations would refine and streamline the design, introducing hybrid ramrockets capable of achieving high supersonic speeds (Mach 3-5) (see next section) [sources: Hoffman; Kossiakoff; Ward].

Despite intriguing 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 languished until the 1964 debut of the Lockheed SR-71 Blackbird. The fastest manned aircraft until its retirement in 1989, the Mach 3+ Blackbird also used a hybrid engine, sometimes called a turboramjet [sources: National Museum of the USAF; Smithsonian; Ward].

We'll dive into the SR-71 and other ramjet hybrids and subtypes in the next section.

If ramjets are so fiddly, then why bother? Well, at the pressures and temperatures generated at Mach 2.5+, most jet engines become hugely impractical -- and utterly pointless. Even if you could make one work, doing so would combine the hazards of running a windmill in a hurricane with the pointlessness of hauling a wave machine to Oahu's North Shore.

Ramjets take the basic principles of other jets and crank them up to 11, all without major moving parts. Air enters a ramjet's diffuser at supersonic speeds, assaulting it with shock waves that help build ram pressure. A diamond-shaped center body in the intake further squeezes the air and slows it to subsonic speeds to more efficiently mix with fuel and combust. Combustion occurs in an open chamber akin to a giant afterburner, where liquid fuel is injected or solid fuel is ablated from the chamber's sides [sources: Ashgriz; Encyclopaedia Britannica; SPG; Ward].

Ramjets' speed limitations gradually inspired hybrid engines that could fly at lower speeds and accelerate to supersonic velocities. The most famous example, the SR-71 Blackbird, used a turbojet-ramjet hybrid called, appropriately, a turboramjet. Such engines work like an afterburning turbojet until well past Mach 1, after which ducts bypass the turbojet and redirect the ram-compressed airflow into the afterburner, making the engine behave like a ramjet [source: Ward].

Missile designs, meanwhile, gradually did away with boosters by moving them inside the ramjet itself, creating ramrockets, aka integral rocket ramjets. During rocket acceleration, plugs temporarily seal the ramjet's intake and fuel injectors. Once the rockets are spent and the ramjet is up to speed, these pop off, and the empty rockets act as combustion chambers [source: Ward].

Looking forward, crossing the Mach 5 line into hypersonic speeds will likely entail scramjets (supersonic combusting ramjets). Unlike other ramjets, scramjets do not need to slow air to subsonic speeds in their combustion chambers. To pull off ignition and expansion in the 0.001 seconds before the pressurized air shoots out the exhaust, scramjets typically use hydrogen fuel, which has a high specific impulse (change in momentum per unit mass of propellant), ignites over a wide range of fuel/air ratios and releases a huge burst of energy when burned [sources: Bauer; Encyclopaedia Britannica; NASA].

Scramjets remained theoretical before the past few decades, and work remains mostly experimental. In November 2004, NASA's eight-year, $230-million Hyper-X Program produced a scramjet that reached Mach 9.6 on its final flight. Some analysts believe the technology could reach Mach 15-24, but air travel at hypersonic speeds means overcoming forces unlike those faced by even the fastest supersonic craft. In short, we have a long way to go before we can commute from New York to Los Angeles in 12 minutes [sources: Bauer; DARPA; Fletcher; NASA].