"A good lightweight design that a beginner can build uses a tubular steel tailpipe and intake welded to a combustion chamber," Cottrill says. "The chamber is a long cone (the chamber wall) and a blunt cone on the front end (which I call the chamber dome). Both are made of mild sheet steel, thin enough to form by hand with the simplest tools."

Start with a well-proven design, like Cottrill's focused wave pulse jet engine. This is a valveless pulse jet designed so that the combustion chamber consists of nothing but a long cone flowing into the tailpipe. Get the plans here.

Materials:

- 22 gauge weldable mild steel sheet

- M10x1.0 metric hex nut

- NGK Model CM-6 spark plug

- Tailpipe Material: 1.25-inch OD "antenna mast" tubing, from Menard's

Tools:

- Pre-designed cone patterns (glued onto manila folder and cut out)

- Fine point PERMANENT marking pen

- Rubbing alcohol

- Leather work gloves

- Eye protection

- Hand operated tin snips - OR - Air-driven hand shears - OR - Air-driven "nibbler"

- Steel pipe mandrel

- Light ball peen hammer or small steel mallet

- Small sandbag

- Electric hand drill OR (much better) small drill press

- Small "half round" file

- Welding outfit and 1/16-inch mild steel filler rod

- Steel tape measure

- Hack saw

- Small "half round" file

- Scrap block of 2x4 hardwood with one rounded edge

- Welding rig and 1/16-inch mild steel filler rod

Step-By-Step:

Build a Chamber

Cut 22-gauge mild sheet steel, tracing a pattern with aircraft snips, to fabricate a two-piece combustion chamber. Using a pipe anvil and mallet, roll a long open cone. Use the mallet to form a blunt dome to cap the cone. Weld each cone's seam. Drill a hole in the dome's tip and weld on a spark-plug mount nut.

Make a Tailpipe

Cut and file an exhaust tailpipe and an intake pipe from lengths of thin-walled tubing. Use a ball-peen hammer to flare both end. "Take a hardwood scrap block and use a file or Surform tool to round one edge," Cottrill says. "The radius of this round can be pretty small, 1/4-inch or even less." Fit the narrow end of the chamber cone into the exhaust.

Connect It

Weld the dome and the tailpipe onto each end of the combustion cone. Cut a hole in the cone side for a port to fit the intake pipe.

Mount and Fire

Fit the rig onto a Bukowsky mount. Arrange a propane torch so the flame goes across the tailpipe opening. Light it and blow air in.

Warnings:

Cutting Metal: "Working with sheet steel itself can be hazardous," Cottrill says. "Do not try to hold a sheet by hand while drilling! A friend of mind got his hand severely cut (incuding nerve damage) when a drill caught and spun a piece out of his control. The safest way to drill sheet metal is on a drill press at low speed with the sheet clamped firmly to the drill press table." Cottrill always files all edges smooth after cutting. "It takes a few minutes, but after that you can handle the pieces without worry of injury." When cutting sheet metal, sweep up the little crescent-shaped scrap pieces—they are sharp as razor blades.

Running Your Pulse Jet: "Always double-check that your engine is solidly mounted before trying to start it," Cottrill says. "Make sure all your propane connections are tight before you start. Make sure there aren't flammables nearby, especially behind your engine. Plain steel engines can throw sparks occasionally while running—they come out like little white-hot bullets—so make sure you're not going to cause a grass fire behind your engine."

Noise: "Pulse jets can damage your hearing permanently if you are within a few feet of it," Cottrill says. He wears foam plugs and hard shell muffs as hearing protection while testing or running an engine. Some communities have strict noise ordinances, and you can expect to get into trouble if you violate it with your engine. Check with local authorities to find a suitable location or a noise-friendly time of day.

Glossary:

Bukowsky Mount: A simple rack that uses a hose clamp to secure the tailpipe to a rear bracket while the spark plug connects to a front bracket. The mount prevents the engine from moving when ignited, and a built-in heat shield protects the operator. This was designed in 2004 by a high school student, Steve Bukowsky, for his original build of the prototype focused wave valveless pulse jet.

Combustion Chamber: The main engine space in which explosions take place. It is always the location of maximum positive and negative pressure swings in the engine, while in most of the chamber the gas velocities will remain relatively low. Typically, explosion gases are replaced with fresh air and fuel mixture throughout only a small part of the chamber.

Deflagration: This is the proper technical term for the type of explosions inside a running pulse jet. The term "detonation" is incorrect for pulse jets; it indicates a much more violent and rapid explosive action. The term "explosion" is acceptable for ordinary discussion, though it is technically too general a term.

Flame-Out: Even after you get your engine running well, it is possible to open up the propane so much that the engine can't breathe enough air to keep up with it. When this happens, the engine "flames out" - it stops roaring, and instead of impressive blue flame flying out of the tailpipe, you'll suddenly have a large, billowing yellow (possibly sooty) flame that can actually envelop the whole engine. If you're running your engine and it suddenly stops roaring, you MUST shut down propane flow immediately! And, always have an A-B-C type fire extinguisher handy while testing, just in case something gets ignited.

Flare (on Intake or Tailpipe): An important action of the intake and tailpipe is to acquire outdoor air as rapidly as possible once the engine interior pressure starts to drop in each cycle. This action is optimized by bending the outer edges of the ducts smoothly outward, similar to the flared mouth of a musical wind instrument or horn. Basically, the flare greatly reduces the air drag that a simple cut-off end would have. For most valveless engines to tun well, the intake flare must be fairly large. Tailpipe flares can be small, and are not needed at all on some engine designs.

Intake: The intake duct (or intake pipe) is a much smaller volume duct than the tailpipe. Its main function is to deliver the correct volume of fresh air and fuel mixture into the combustion chamber to create the next explosion cycle. Unlike the tailpipe, all residual air is blown out of the intake after each explosion, and this is followed by complete replacement of the explosion gases with the next charge of fresh air from outside.

Tailpipe: The largest outlet duct in the engine, where a relatively large mass of air and combustion product gas is moved rearward. After the pressure wave from the explosion cycle exits the tailpipe, the pressure drops and cold air is pulled in at the rear, forming the next mass that will be pushed out. This cold mass motion (due to the next explosion) is the main source of reaction momentum in the engine.

Watch Cottrill and Colleagues Test a Pulse Jet:

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