With 'Scramjet,' NASA Shoots for Mach 10

By Guy Gugliotta Washington Post Staff Writer Wednesday, November 10, 2004; Page A01

HAMPTON, Va. -- They call it a "scramjet," an engine so blindingly fast that it could carry an airplane from San Francisco to Washington, D.C., in about 20 minutes -- or even quicker. So fast it could put satellites in space. So fast it could drop a cruise missile on an enemy target, almost like shooting a rifle.

Next week, NASA plans to break the aircraft speed record for the second time in 7 1/2 months by flying its rocket-assisted X-43A scramjet craft 110,000 feet above the Pacific Ocean at speeds close to Mach 10 -- about 7,200 mph, or 10 times the speed of sound.

The flight will last perhaps 10 seconds and end with the pilotless aircraft plunging to a watery grave 850 miles off the California coast. But even if the X-43A doesn't set the record, it has already proved that the 40-year-old dream of "hypersonic" flight -- using air-breathing engines to reach speeds above Mach 5 (3,800 mph) -- has become reality.

Unlike rockets, which must carry oxygen with them as a "combustor" to ignite the fuel supply, scramjets take oxygen from the atmosphere, offering a huge savings in aircraft weight, and researchers around the world would like to take advantage.

In northeast Australia, a scramjet team funded by the U.S. and Australian armed forces will try for Mach 10 next year as a first step in using a scramjet to put satellites in space. The U.S. Air Force hopes to demonstrate within five years a scramjet-driven cruise missile fast enough to drive explosives deep into hardened targets. Other projects are moving forward in France and Japan.

If we and you are only really concerned with ten seconds or so why use a scram jet to begin with as such a propulsion system relies on a rocket to bring the craft up to speed to allow operation of the scramjet? Rockets work fine for short distances where velocity is crucial such as shooting down an aircraft or bringing down a missile but for larger, slower targets in the atmosphere, there is the cruise missile. Air breathing, hypervelocity aircraft are still a research program and will be for some time to come.

What's the advantage to moving air through the engine faster - does one get higher thrust out of it?

Your question really needs to be rephrased, more along the lines of "How can an engine be designed to work on an aircraft that travels at very high speeds.?"

A typical turbojet engine with a compressor and turbine needs to slow the air entering the engine down to subsonic speeds before it enters the compressor or the engine will suffer serious problems. Also, there is an upper limit to the speed at which a turbojet can operate because there is a point where the air entering the engine can't be slowed down to subsonic speeds.

On ramjets and scramjets, there are no moving parts to cause problems because the air entering the engine is compressed by passing through shockwaves. On a ramjet the air flow behind the shockwave (which is external to the engine) is subsonic and at a higher pressure than in front of the shockwave. It passes into the combustion chamber where it is mixed with the fuel and ignited. The resulting expanding gases provide the thrust. Like the turbojet, there is an upper limit to its speed.

In a scramjet, as in the ramjet, the incoming air is compressed by shockwaves (the shockwaves can be external or internal to the engine). The difference in the scramjet is that the airflow is supersonic all the way through the engine. The (theoretical) advantage of the scramjet is that the top speed is limited by heating and structural issues instead of velocity.

How does one start one of these - wouldn't the air have to already be moving through the engine at that speed before turning it on?

Correct, neither ramjets or scramjets produce static thrust.

Wasn't the German "buzz bomb" of the 40's considered a ramjet? In any case it was the first "cruise missile".

First "cruise missile", yes.

But the power was provided by a "pulse-jet", a kind of two-cycle engine that had a compression and exhaust cycle.

To my knowledge, no other aircraft has ever employed such an engine.

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.