The pulsejet engine is beautiful in it’s simplicity. Tangible examples of it’s use through history are rare, leaving it to esoteric aviation applications like the world war two German “Buzz-Bomb,” and military drones (more like fancy model airplanes) from long before the age of electronic artificial intelligence, and pilots sitting in air-conditioned boxes thousands of miles away.

The German “Buzz-Bomb” technical drawing.

The pulsejet represented an emerging understanding of how power can be harnessed through internal combustion with the least moving parts, and remains a somewhat poetic gesture in the speed-hunter’s timeline. With a pulsejet there is no rotational energy to convert into forward motion, no fan blades or compression, just the inertia of air and some understanding of harmonic high and low pressure areas.

While little practical application exists for these engineering wonders, enthusiasts like Bob Maddox of Maddox Jets, has harnessed the thrust potential of these engines and brought them to market for some creative repurposing. Oregon-based Maddox grew up in an era when jet-age technology was really starting to capture the minds of people. Military aircraft and airliners filled the skies in the 1960s and transformed the way we thought about travel and speed.

“Ever since I was born I was fascinated with airplanes, in the 1960s the Vietnam War was going on and there were sonic booms all the time. I started messing around with little jet engines and rocket engines when I was 9 or 10, I remember reading in an encyclopedia about the Buzz-Bomb engine. In the early nineties I started sky-diving and I was thinking (in freefall) I need a jet or rocket on my chest to go faster!” explained Maddox.

As a carpenter and artist, Maddox had a solid set of hands-on fabrication skills at his disposal. Without an engineering background he set out to recreate full scale versions of the model airplane using photographs, magazine articles and toy examples. Some serious trial and error later he finished his first functioning prototype.

“I started with a 50-pound thrust engine, it took a couple of months to figure out what I was doing. I re-invented the wheel a whole lot over a couple months. I mounted it on a board and jumped out of a plane with it three times to see if I could free-fall with it. I decided, never to run it because of the hazards,” Maddox recalled.

Randy Grubb fabricated the art-deco aluminum body for Maddox’s lakester.

This success opened the flood gates for Maddox, and he began experimenting and refining the design. Rolling 304 stainless sheet metal, and learning to TIG weld, Maddox enlisted some help through machinists, a water jet operator, and a famed hot-rodder to make his more complex designs a reality. “I met a guy called Randy Grubb, I ran into him at my water jet shop, he did the aluminum work on my landspeed car and crazy bike. Becoming friends with him opened me up more to design and build whatever I want,” Maddox relayed.

Getting deeper into the design of these engines, we learned that Maddox had to take some creative approaches to develop a reliable platform. His desire to run on liquid gasoline rather than propane forced him to devise a unique fuel delivery system to atomize the gasoline, and an ignition system to tolerate the environment.

“The little tiny model airplane engines from the ’50s have a reed-valve in them that looks like a little flower. My first 50-pound thrust engine that I made had a head like that. But they had a venturi, you blew air into them to start them up and created suction in the nose which drew fuel into the engine. I couldn’t see having a system like that because I was wanting to jump out of an airplane with it at the time, so I decided I wanted to inject it,” Maddox explained.

“I went to a wrecking yard and got a $5 fuel injection pump, all of my engines from the 50-pound thrust to the 900-pound thrust don’t scale very well in the pipe. You can’t just take a little one and make it 100 percent bigger and have it work, but they especially don’t scale in the fuel delivery system,” he continued.

There are two basic types of pulsejet engine, those that use reed valves like those found in a two-stroke engine, and valve-less variants that rely on a tuned length of tubing to draw a fresh charge into the combustion chamber upon each high-velocity pulse. “I’ve always been a valved-engine guy, until about three months ago I’d never built a valveless-engine,” Maddox asserted.

The reed valves on this type of engine are unique and require machining, they are not off the shelf parts just slapped together. “The aluminum part of the reed cage is made out of 6061-T6 and they are CNC-milled. The way I built my heads is they just stack up — a 50-pound thrust head has four reed cages in it and a 100-pound has eight, up to my biggest heads which are about 64. The reed valves themselves are .010-inch thick blued spring steel,” Maddox detailed.

A valve-less Maddox engine, notice the forward facing intake tubes, and lack of reed valve head.

Maddox illustrated that delivering fuel into the stream of air can be a tricky proposition, it needs to be dispersed without contacting the chamber walls where it will condense back into liquid form and upset the engine. Because of the high fuel-consumption and rich environment, a normal spark plug or glow-plug ignition system would not suffice.

“It’s a spark plug, but because my engines are injected-gasoline and take quite a bit of gas it’s pretty wet inside when you go to start them. You’re blowing air in and then the spray comes out and it’s really easy to foul a spark plug. What I do is open the spark plug gap really wide and use a 50-million volt stun gun as an ignition coil. It’s really cracking loud, that’s what you’re looking for,” Maddox explained. Ah, the mad scientist comes out!

Once the engine is running, fuel flow to the tune of 75 percent is necessary just to sustain the engine at idle. However, similarly to conventional turbine engines, the additional fuel flow required to reach maximum thrust only accounts for an additional 25 percent. The concept of idle speed and redline are non-intuitive compared to a piston engine. The operation is much more like a pipe-organ, each tube has a resonant frequency, you can increase the volume of output with more air and fuel but the frequency stays roughly the same.

“Every tube has it’s own frequency, if you lengthen it out it will drop a little bit. The way pulsejets work — lets say we’re running a V1 Buzz-Bomb size engine, they cycle at about 43 times per second. The difference between idle and full throttle will only be about three cycles per second, it’s not like an rpm that goes up and down,” Maddox said.

Despite the small change in frequency, these engines are fully throttle-able. “Most of my 100-pound thrust engines idle down at about 15 pounds of thrust, the increase in cycles is hardly noticeable, when it’s at full throttle it’s pulling in a lot more air and fuel,” he continued.

Maddox is in the process of bringing these engines into more mainstream recreational usage, devising a festival of sorts dedicated to thrust.

“I’m starting a yearly event, probably starting out as three or four days and hopefully eventually a week on a dry lake bed in Cedarville, California. I think it will grow into something like Burning Man, but all thrust. There is no place to run anything if it’s thrust powered, the drag strips won’t let you run them because they’re loud, out on the salt they don’t have any thrust classes. The lake bed is privately owned, two miles wide and 20 miles long, and the city is really interested in me coming in and doing something to bring commerce to the area,” Maddox outlined.

We hope his event gets off the ground, these engines serve as great learning tools for engine builders of all sorts to better understand some of the fundamental fluid dynamics of high and low pressure, velocity and combustion. As simple as these engines are, they produce impressive performance and lend themselves to creative applications. Check out Maddox Jets for more information.