Custom Turbojet Engine Build

by AeroEngineer20 | April 6, 2018 | (8) Posted in Projects

[UPDATE] My friend and I attempted the first run on April 15 and it did not self-sustain. However, we did learn a lot that I'll be able to put toward getting this thing running.  The engine itself came out of the test in nearly perfect condition, so I'll be able to continue working on it. If you'd like to see the video and learn more about what we learned, feel free to check out the forum post I made about this project at the link below. Thanks!

Hello all!

To give a little background before the project description, I'm currently a second-year mechanical engineering student in Illinois. Since April 2017 I've been designing a turbojet engine and building it from scratch. I will be testing the engine in the next few days, but I decided to post here before the test in case the engine decides to destroy itself in the process (don't worry too much, I'll be faaaaar away from it before trying to start it). I have done stress calculations for the rotating components, but even so, I wouldn't necessarily suggest anyone try this for themselves unless they have experts they can ask for help, like at a university. 

I have been interested in jet propulsion in the form of jet and rocket engines since I was very little, so it seems logical to me that I would one day try my hand at making my own. It was once I got to university that I realized I may have an actual chance of making and finishing an engine, given my new access to CNC machining equipment. I also had quite a bit of time in researching the general theory of operation, as well as particular jet engines, namely the General Electric J79 and the Orenda 14.  AgentJayZ on YouTube is an expert on servicing those engines, so if you want to learn more, check out his YouTube channel.

FliteTest article rules state to add lots of detail, so if you just want to see the nearly-finished product, you can skip to the end. Just be aware you're missing lots of fun CNC and work-in-progress photos!

I started with a very rough design, creating the major components to be semi-realistic and possible for me to potentially make. I based my whole design off of existing and proven engines, such as the full scale Fairchild J44 and RC turbojet engines. 

The design was pretty rough, but hopefully functional. I had no time to waste, so I started work machining the diffuser out of 0.5" aluminum plate on a 3 axis CNC mill. I chose this part simply because it was the easiest to machine. I got the aluminum plate from a scrap cabinet for free, so I was alright testing the CNC waters, as it were, with this simple piece.

Once cleaned up and cut out of the large piece used to hold it during machining, it started to resemble something neat and, more importantly, usable.

This was just the beginning. For the next piece, I set up the 4th axis attachment for the mill, getting ready to mill the compressor outlet guide vanes. The purpose was to redirect the swirling air from the compressor to a more linear path, making combustion more stable. This piece also functioned as a frame element onto which the intake, main frame, and some case parts would be bolted.

The path of the air goes from right to left in this view:

While the 4th axis was still attached, I redesigned some of the high temperature components and machined them out of mild steel. This was not an ideal material, but I had no access or capability to machine Inconel or other nickel alloys. This is the turbine inlet guide vane:

And the turbine bladed disk:

Parts were starting to come together! The diffuser is bolted to the compressor outlet guide vanes, which is bolted to the main frame and turbine inlet guide vanes. The main frame houses the bearings and holds the other main pieces together.  Knee bone to the thigh all started to look like something.

Shortly thereafter, I machined the compressor wheel. I had originally intended to cast this piece, but upon learning the complexities of casting such a part in an internship I had over the summer, I decided to machine it out of aluminum round. While the compressor was running on the CNC, I also machined the main shaft. This would complete the main rotating components.

The engine parts as they stood at the time:

At this point I was nearly finished with all of the internal components. I moved on to spend more time on the casing parts, made from rolled 1mm stainless steel sheet I got from my high school. I had absolutely no experience working with sheet metal, so it was a wild ride trying to learn the best way to make certain parts. The parts I made are not pretty necessarily, but they should function.

I cut out the templates on a band saw and rolled the cylindrical and conical components. The first assembly I made was the annular combustor, where the fuel and air would be burned. I decided on using propane as the fuel due to the ease of flow control and availability.

Right around that time I decided I should work on the fuel system, so I printed a small bracket that would hold a servo onto the propane valve for fuel control. For a quick test, I just used masking tape to form a seal between the torch nozzle and a brass fuel manifold ring I made. The fuel was controllable and could be easily shut off with the flip of a switch.

I continued with the sheet metal work, starting some external casing parts. I made the exhaust nozzle and rear case by welding a few parts together and spending some quality time with the bench grinder. 

I also received a piece of 304 stainless round from a local machine shop, and went to work boring the 3.25" round to 3" inside diameter. This piece would become the turbine shroud, which will serve as both air containment and blast containment, should I have a rapid unplanned disassembly. It took many hours, and I almost gave up on the process, but I stuck with it and eventually finished it. I am very glad, however, that I will not have to do that again.

With the design, casting, and machining of the intake out of aluminum, I was in the home stretch!

The final components came together relatively quickly. All I had to do was weld a few components together and drill a few holes (simple, right?).

The setup to weld the case together:

Alignment of the combustor in the case:

And now we get to the nearly finished product! All that's left to do is connect fuel, a starter, and a way to hold it down during the test. I should be testing the engine within a week.

If I have accomplished nothing else, I have learned a ton about machining, metalworking, design, and reality. While it's not necessarily the prettiest engine ever built, I can say I built every part of it myself. Even if it doesn't run, it's been a great year of design and manufacture. I cannot wait to see what this can do!


Zephyr1 on April 7, 2018
WOW!! This is truly impressive. As a second year aero engineering student in Ohio, I am super jealous. Nicely done! I really hope it works!
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AeroEngineer20 on April 7, 2018
Thank you very much! Obviously everyone has a different situation, and I was afforded many privileges in my life, but it can't hurt to make friends with the person in charge of any metalworking shop you may have at your university if you haven't already.
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PenguinWings on April 8, 2018
YES! I've been hoping somebody would give this a shot! It looks incredible and I can't wait to see if it works. Would you consider starting a build thread on the forum? I have a couple questions, and I'm sure I'm not the only one! Good luck!
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AeroEngineer20 on April 8, 2018
I did start an introductory thread on the forum here:

If there's interest, I could start a formal build thread specifically for questions. Whichever is more convenient for people!
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PenguinWings on April 8, 2018
Awesome, thank you!
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Air-headed Aviator on April 22, 2018
What an undertaking. I've always wondered about doing this myself, tho at a much smaller level, and years on down...
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Jackson T on September 11, 2018
Is there any reason why model gas turbines use only centrifugal compressors?
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AeroEngineer20 on October 10, 2018
My apologies for the late reply. I believe the popularity of the centrifugal compressor is due to the relatively high pressure ratio you can get from a single stage. In an axial compressor, you need several stages of blades to get the same pressure ratio as a single stage of a centrifugal compressor. I would think that would make it much cheaper to produce, both in engineering and manufacturing costs. It'd also be easier to maintain with one compressor part instead of tens to hundreds. Hope that helps.
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Jackson T on October 23, 2018
That helped a lot! Thanks!
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Custom Turbojet Engine Build