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How Ramjets Work


Ramjets: Making Mock of Mach
The Lockheed SR-71A Blackbird reconnaissance aircraft prepares for flight. The Blackbird parked at the Steven F. Udvar-Hazy Center once flew from Los Angeles to Washington, D.C., in one hour, four minutes and 20 seconds.
The Lockheed SR-71A Blackbird reconnaissance aircraft prepares for flight. The Blackbird parked at the Steven F. Udvar-Hazy Center once flew from Los Angeles to Washington, D.C., in one hour, four minutes and 20 seconds.
© George Hall/Corbis

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].