Answer Geek: How Scramjet and Ramjet Engines Work

<br> -- 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.