Answer Geek: How Scramjet and Ramjet Engines Work

-- Q U E S T I O N: So, you explained how a gas turbine engine works. Pretty good job. Now try explaining how the Sterling cycle engine works? — Rob M.

Q U E S T I O N: Okay, so we’ve covered the Otto, the Wankel, the diesel, the ion, and the gas turbine engines. So let’s really round this out . . . hehe. How does the rotary engine work? — Matt

Q U E S T I O N: To continue on with your engine topics I believe I have one more engine for you. How does a scramjet or ramjet engine work?

— Nate L.

A N S W E R: Thanks for the compliment, Rob. Pretty good job of asking a question, too, except that you spelled “Sterling” wrong. It’s “Stirling,” named for Robert Stirling, the Scottish minister who invented that particular external combustion engine in 1816. Don’t worry, though, more than half of the dozen or so people who wrote to ask about the Stirling engine spelled it incorrectly.

One more thing, Rob. I’ve already answered that question. See Revving Up Two More Engines, which looks at both Stirling and Miller Cycle engines. And as for you, Matt, … hehe … the Wankel is a rotary engine. Take a look at Will the Wankel Engine Last? for more details.

That’s probably enough time spent enough making fun of well-meaning question writers. And for the record, last week’s column on jet engines elicited an unusually heavy influx of queries, the vast majority of which were interesting, intelligent, and worth answering. I’ll get to many of them in the coming weeks. But since we’re on the subject of engines, I thought I’d start by looking at ramjets and scramjets.

The Need for Speed

Ramjets are the engines of choice if your goal is supersonic speed. Developed beginning in the 1950s, the ramjet flew for the first time in 1957 in an experimental aircraft developed by the French government. That plane set a speed record in 1959 of 1,020 miles per hour, and eventually hit Mach 2.19 or about 1,400 miles per hour. The SR71 Blackbird — a sleek, dark, high-flying spy plane — tops out at better than Mach 3. And in theory, at least, ramjets can achieve speeds approaching Mach 6, which translates to a stunning 4,000 miles per hour.

But the truly amazing thing about ramjets is they achieve this feat with no working parts. It turns out that once you reach supersonic speeds the compressors and turbines essential to conventional jet engines aren’t required. Instead, the sheer force of forward movement does most of the work of compressing the incoming air.

In simplest terms, a ramjet engine is a long tube with a torpedo-shaped object at the front opening, which is called an inlet. Air rushing through the inlet is compressed between the sides of the tube and the torpedo-shaped body, whose nose is pointed at the front. Its wider part, which sits inside the engine, slows incoming air down to subsonic speeds. Fuel is then sprayed into the air stream, and the mixture is ignited as it flows past a flame inside the combustion chamber. The resulting hot gasses are then ejected out of a nozzle in the back, yielding plenty of thrust.

Ramjets have a couple of important limits. One is that they don’t work until the engine is moving at high speeds, so you need a way to get the plane moving at Mach speeds in the first place. One way to do that is to combine a ramjet and a turbine engine in the same housing. Then you run the turbine portion of the engine until you are moving through the air fast enough for the ramjet to work efficiently. Once you reach that speed, you can shut the turbines down and let the ramjet do all the work.

The other problem is that as you approach speeds of Mach 6 or so, the air flowing into the inlet is moving so quickly that it creates a supersonic shock wave as it is compressed inside the engine. At best, the shock wave stops the ignition of the air-and-fuel mixture in the combustion, shutting the engine down. At worst, pressure and heat from the shock wave tear the engine to bits.

Solution: Scramjet

The solution is a supersonic combustion engine or scramjet — a ramjet with the “s” from supersonic and the “c” from combustion added to the front.

In scramjets, the engine inlet is designed so it doesn’t create as much compression as in a ramjet, allowing the air to zip through the engine at supersonic speeds. This reduces shockwave problem, somewhat. Even so, when fuel is injected into the onrushing air, small shock waves are created, so the combustion chamber must be able to withstand the pressure. And at supersonic speeds, fuel injection and combustion have to be accomplished in mere milliseconds. Making sure that the fuel burns while still inside the engine and not after it is ejected from it is one of the main challenges for scramjet builders, and it is such a thorny proposition that a multi-billion-dollar NASA program to build a supersonic combustion ramjet in the mid-1990s was cancelled.

A new program, called Hyper-X, was launched by NASA in 1997. The goal is to build working jet airplanes that use a clever combination of ramjet, scramjet, and rocket engines to reach sufficient speeds to achieve orbit around the earth. These planes would use conventional rocket engines to get started, then switch over to ramjet power at Mach 2 or 3, and then scramjet power at about Mach 5. The scramjet engines would accelerate the plane to as fast as Mach 18 (nearly 12,000 miles per hour). The rockets would kick back in to take the vehicle the rest of the way into orbit.

The first Hyper-X plane, called the X-43A, is scheduled for a test flight this month over the Pacific Ocean. About 12 feet long and pilotless, the X-43 will be launched from the wing of a B-52. A rocket engine will boost the X-43 to Mach 7, about 4,600 miles per hour. At that point, NASA scientists hope, the scramjet engine will kick in. If all goes well, test flights planned for next year will see the X-43 reach speeds of Mach 10-a hasty 6,600 miles per hour. After that, NASA will look at the possibility of building a scramjet large enough to take off from a runway with a pilot at the controls.

Advancing Technology

Launching a scramjet-powered plane into orbit has huge advantages over the standard rocket-powered spacecraft. Today, rockets have to carry their own oxygen with them, which adds a great deal of weight-and expense-to the task of placing objects in space. The space shuttle, for example, carries more than a million pounds of liquid oxygen with it when it lifts off. Because scramjets get their oxygen from the passing air, they could play a major role in helping bring the cost of reaching earth orbit down from thousands of dollars per pound of payload to just hundreds per pound.

Among other things, that could open up the door to space tourism. Just imagine, instead of saving your money for your annual winter trip to Hawaii, you could save it for a trip into space. And you know how you diet before that Hawaii vacation so you’ll look good in a bathing suit? Well you could diet for this trip, too, because every pound you shed would save you a couple hundred bucks.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.