The Leduc ramjet
I ran across this gizmo from looking at Yann LeCun’s google plus profile, and wondering at the preposterous gadget sitting next to him at the top. Figuring out what it was, I realized the genius of the metaphor. Yann builds things (convolutional networks and deep learning in general) which might very well be the Leduc ramjets of machine learning or even “AI” if we are lucky. Unmistakably Phrench, as in both French and physics-based, original in conception, and the rough outlines of something that might become common in the future, even if the engineering of the insides eventually changes.
Rene Leduc was working on practical ramjet engines back in the 1930s. His research was interrupted by the war, but he was able to test his first ramjet in flight in 1946. The ramjet seems like a crazy idea for a military aircraft; ramjets don’t work until the plane is moving. A ramjet is essentially a tube you squirt fuel into which you light on fire. The fire won’t propel the engine forward unless there is already a great deal of air passing through. It isn’t that crazy if you can give it a good kick to get it into motion. If we stop to think about how practical supersonic aircraft worked from the 1950s on; they used afterburners. Afterburners to some approximation, operate much like inefficient ramjets; you squirt some extra fuel in the afterburning component of the engine and get a nice increase in thrust. Leduc wasn’t the only ramjet guy in town; the idea was in the proverbial air, if not the literal air. Alexander Lippisch (a German designer responsible for the rocket powered Komet, the F-106 and the B-58 Hustler) had actually sketched a design for a supersonic coal burning interceptor during WW-2, and his engine designer was eventually responsible for a supersonic ramjet built by another French company. The US also attempted a ramjet powered nuclear cruise missile, the SM-64 Navaho, which looks about as bizarre as the Leduc ramjets.
In fact, early nautical anti-aircraft missiles such as the Rim-8 Talos used ramjets for later stages as well. The bleeding edge Russian air to air missile, the R-77, also uses ramjets as does a whole host of extremely effective Russian antiship missiles. Ramjets can do better than rockets for long range missilery as they are comparably simple, and hydrocarbon ramjets can have longer range than rockets. Sticking a man in a ramjet powered airframe isn’t that crazy an idea. It works for missiles.
The Leduc ramjets didn’t quite work as a practical military technology, in part due to aerodynamic problems, in part because they needed turbojets to get off the ground anyway, but they were important in developing further French fighter planes. They were promising at the time and jam packed with innovative ideas; the first generation of them was much faster in both climb and final speed than contemporary turbojets.
Ultimately, their technology was a dead end, but what fascinates about them is how different, yet familiar they were. They look like modern aircraft from an alternate steampunk future. Consider a small detail of the airframe, such as the nose. The idea was a canopy bubble would cause aerodynamic drag. Since ramjets operate best without any internal turbulence, the various scoops and side inlets you see in modern jets were non starters. So they put the poor pilot in a little tin can in the front of the thing. The result was, the earliest Leduc ramjet (the 0.10) looked like a Flash Gordon spaceship. The pilot was buried somewhere in the intake and had only tiny little portholes for visibility.
Later models incorporated more visibility by making a large plexiglass tube for the pilot to sit in. Get a load of the look of epic Gaulic bemusement on the pilot’s “avoir du cran” mug:
The later model shown above, the Leduc 0.22, actually had a turbojet which got it into the air. It was supposed to hit Mach-2, but never did. In part because the airframe didn’t take into account the “area rule” effect which made supersonic flight practical in early aircraft. But also in part because the French government withdrew funding from the project in favor of the legendary Dassault Mirage III; an aircraft so good it is still in service today.
The Leduc designs are tantalizing in that they were almost there. They produced 15,000 lbs of thrust, which was plenty for supersonic flight. A later ramjet fighter design, the Nord Griffon actually achieved supersonic flight, more or less by using a more conventional looking airframe. Alas, turbojets were ultimately less complex (and less interesting looking) so they ended up ruling the day.
As I keep saying, early technological development and innovative technology often looks very interesting indeed. In the early days people take big risks, and don’t really know what works right. If you look at a radio from the 1920s; they are completely fascinating with doodads sticking out all over the place. Radios in the 50s and 60s when it was down to a science were boring looking (and radios today are invisible). Innovative technologies look a certain way. They look surprising to the eye, because they’re actually something new. They look like science fiction because, when you make something new, you’re basically taking science fiction and turning it into technology.