Roger Scramjet

In a wind tunnel in Hampton, Virginia , on the 30th of May this year, a new kind of cruise missile engine, called a scramjet, was fired up. Just like any other cruise missile engine, it used conventional liquid hydrocarbon fuel, but this one was a mite different. In simulated hypersonic conditions, this engine reached MACH 6.5 speeds at 90,000 feet altitude.

This is fast. In fact it is extraordinarily fast...over six times the speed of sound.*

All rocket motors (we'll confine ourselves to good old-fashioned chemical rockets for now, like sky rockets, Saturn Vs, Space Shuttle engines, and ICBMs and forget about more exotic kinds) work by burning propellant, which typically consists of a fuel and an oxidizer. The point here is that rockets carry both their fuel and oxidizer with them. They don't get their oxidizer from the air like typical engines do. This is why rocket engines work in space… where there is no air.

Jet engines carry only fuel, and get their oxygen from their air intakes. All jet engines have to get the air to the fuel, ignite it, and expel the hot exhaust gases out the back. To get the stuff to burn hot enough (so the gases will leave fast enough to generate enough thrust to move the aircraft) you've got to compress the air. Turbojets compress the air by using big, spinning turbine blades, so they don't have to be moving fast (or even at all) when you start them up - the blades spin, the air gets compressed, the fuel gets injected, the fuel and the compressed air burn, and the exhaust goes blazing out the back (cool, huh?). All jet airliners and combat jets use either turbojets or turbofans.

But, if you're going fast enough, you can compress the air without the fans just by forcing it through a narrow neck. The air gets rammed together as it moves toward the combustion chamber, and so these engines are called ramjets (get it?). The downside, of course, is that you have to get the ramjet moving before it lights off, and so ramjet-powered craft have typically been either carried aloft by other planes, or kicked up to a high speed by a turbojet or rocket booster. Once it gets going fast enough, then the ramjet takes over.

Ramjets can achieve very high speeds, but a new, enhanced kind of ramjet, the scramjet, can go even faster. Scramjet is a semi-acronym meaning supersonic combustion ramjet. It works basically like a ramjet, but it's designed to handle some strange and tricky combustion dynamics - with air moving faster than the speed of sound - at supersonic speeds. The ramjet can reach Mach 4 with interior air slowed down to subsonic speeds. A scramjet will fly faster than Mach 5, and can achieve this with combustion of air moving at supersonic speeds.

But as ONR science officer Gil Graff points out, "Nobody has ever flown a scramjet - yet."

The new engine just tested will be used in the joint Defense Advance Research Projects Agency DARPA)/Office of Naval Research (ONR) Flight Demo program called HyFly. HyFly will flight-test a missile able to cruise at speeds of up to Mach 6 to a range of 600 nautical miles. The engine is being developed by The John's Hopkins University Applied Physics Laboratory under ONR's Hypersonic Weapon Technology Program.

"Ultimately, the missile will be designed to launch from ships, submarines, and Navy and Air Force aircraft," says Graff. "We'll get to powered flights at Mach 4 in November 2004, with Mach 6 flights starting a year later. Obviously, the faster we can get a missile on target anywhere in the world, the less likely an enemy will be able to intercept it on the way in."

* A supersonic vehicle is one that goes faster than the speed of sound. Sound travels at different speeds through different media, but a standard measure used is the speed of sound at sea level through dry air at a temperature of 32 degrees Fahrenheit. Mach 1 is the speed of sound, Mach 2 is twice the speed of sound, etc. Definitions vary, but NASA defines hypersonic flight as flight at speeds of Mach 5 and higher.

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