A question that comes up a lot is what is the best “first pulse jet engine”.

This depends on a few factors. Some of the engines that are easiest to build, are the most difficult to run. The converse is also true, some of the ones that have the best running characteristics are the most difficult to build. Typically there are two engines I recommend which have the best balance of ease of construction, ease of starting, and good performance.

Focused Wave Engine

pje mini fwe

Mini Advanced FWE with Dual Thrust Augmenter Tubes

The Focused Wave Engine has a really cool name that sounds like it must have amazing performance. It is by no means the most advanced or highest performing, though it is about one of the simplest pulse jet engines you can build that still starts fairly easily and runs under a wide range of conditions. Originally designed by Larry Cottrill, the FWE makes for a fun project build and a great little engine for demonstrations & science projects.

The FWE can be made in a linear configuration, where you can see down through the engine length wise. In this configuration the engine can be made from only 4 main pieces: the intake, the transition plate, the combustion chamber, and the exhaust pipe. If you’re looking for a unique engine, where ease of construction is paramount, the linear FWE is your best bet.

The standard FWE layout is slightly more difficult to build, as it has effectively 5 sections, and the intake can be more tricky to cut and weld, especially where it meets the combustion chamber. By following the ever important process of making sure your parts fit well together before you weld them, you will minimize the difficulty.

Both of these designs still require a puff of compressed air to start, along with a spark source for easy reliable starting. Once the FWE is started, it runs over a very wide throttle range, and in poor environmental conditions.

It also can be improved upon by adding thrust augmenters, shown above, which are simple specially shaped ducts that harness the reactive force produced by the momentum of the pulsed exhaust, exhaust heat, and aerodynamic forces.

The Thermojet

pje thermojet 4

A high performance thermojet variant displaying a unique exhaust pattern.

The Thermojet is a design from around the 70’s. There have been a lot of  reincarnations of the Thermojet style engine, typically most have between 2 and 4 intakes. The Thermojet can have very good thrust output, though sometimes at the expense of a bit of fuel efficiency. But you’re probably not going to care about that unless you’re trying to build some kind of jet powered Prius.

The design in the above configuration consists of 6 main parts, and while the intakes are straight and easy to form, welding them to the transition cone can be a little tricky because of how tight the space can be. Other than that, building one is a breeze, as 4 of the pieces could be made from straight cut pieces of prefabricated tubing. If you are looking for an engine that has the best range of performance characteristics, and super easy starting at the expense of a bit of construction difficulty, then you want to go with a thermojet.

When properly built, starting the thermojet is as easy as turning on the spark ignition source, and turning on the fuel. Compressed air is not necessary as with almost every other pulse jet design.

If you follow the plans and have good quality of construction you will have an engine that is capable of absurdly wide throttle range, extremely easy starting, and high thrust output. These factors make it one of the best engines for demonstrations and science projects, as you don’t need to lug any heavy air compressors or blowers around to reliably start the engine.

pje injector engine

A thermojet with injection ports / pressure tubes.

A well constructed thermojet will also tolerate a lot of strange modifications. Sometimes they impact the performance or make starting more difficult, but most of the time the engine just keeps chugging along. The engine also can be upgraded by thrust augmenter ducts on both the intakes and exhaust.

pje mini thermo

Mini Thermojet with Dual Stage Augmentation

 

Conclusion:

If you’re looking for ease of construction, the FWE is your best option. If you’re looking for the easiest starting engine, the thermojet is the clear choice.

You cant really go wrong with either though, as both are overall very simple to build and generally tolerate sloppy construction and other conditions that are not ideal. If you have a quality build, you’ll have a great little engine either way.

If you’re looking to build your own, this series of pages will help you enormously:

www.pulsejetengines.com/what-is-a-pulse-jet

 

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