Ramjet Engine

Around 1963, when I was a teenager, I became fascinated with Ramjet engines. Where standard jet engines are very complex, where hundreds of precise components spin at extremely high speed to create power, the concept of a RamJet engine is wonderfully simple!

It was invented around the 1940s when jet engine technology was first being created. Rather than needing to use spinning compressor blades to create the needed compression ratio for efficient burning of the jet fuel, a RamJet relies on its high speed of forward motion, It has no moving parts, and it is possible to look completely through the whole thing!

Experimental Ramjet engines were created which were able to operate at airspeeds above 200 mph. This was because the oncoming air had to be collected and carefully funneled to generate the high compression pressure that was needed to get the fuel burning efficiently. Note that no RamJet engine could ever start out from the ground! Either a Rocket-Assist first had to get the aircraft moving faster than that airspeed or it had to be towed up into the air by a bigger aircraft before the Ramjet engine could ignite.

As a teenaged kid who was fascinated with Physics, I thought I could improvise an experimental Ramjet engine. My father had a ratty old flatbed trailer that he never used any more. So I found sturdy steel and created what is called an exponential horn, which I bolted to that trailer, with the large opening facing forward. The front opening was about three feet in diameter, and it funneled down to become a two-inch iron water pipe. There are a couple details which I choose to not describe here, to try to avoid any other kid trying to duplicate my Ramjet experiment!

But my theory was that if I towed the trailer down the highway at 70 mph, the air would be forced through the exponential horn by the velocity, which both would speed up the air going into the two-inch pipe and also cause that air to be compressed as needed.

A fuel-injector nozzle was mounted in the sidewall of the two-inch pipe, and a spark plug to start the fire. Then the fuel injector would constantly feed gasoline in, it would mix with the compressed air and ignite and burn, and a very powerful jet exhaust would go out the back end (at close to supersonic speed!).

Once I got this thing built and bolted to the trailer, there was a problem! I had not yet gotten a Driver's License, so I could not drive my father's car to pull the trailer. And my 'friends' were very skeptical of my idea and none wanted to pull it with their cars, as well as the fact that teenaged boys tended not to have trailer hitches on the back of their cars!

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I did not really have any good response to their common question regarding lighting a lot of gasoline to burn (and essentially explode) just a few feet behind their cars! While I was trying to figure out who I might be able to trick into towing my Ramjet experiment, I did some calculations. I calculated the approximate thrust of my experimental Ramjet engine, and I got some enormous amount. I no longer remember the exact amount, but it was certainly over 1000 horsepower.

It only then dawned on me that if a car with a 90 horsepower engine was towing a trailer that suddenly might produce over a thousand horsepower of forward thrust, the trailer might easily then try to suddenly PUSH the car, big time! And only then I realized that if my experiment actually worked as I hoped, the 70 mph towing car might easily be hit from behind into an extremely serious accident. And I remember smiling at thinking about how such an accident might be explained to a Traffic Cop!

In any case, once I realized how dangerous my Ramjet experiment might easily be, I gave up the idea and took it apart and never actually tried to test it after that!


This presentation was first placed on the Internet in August 2012.

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago