Update to “An unstable thermojet engine” post: thanks to the advice from this sub, here is a modified, stable propane thermojet (full video link in comments)

Update to “An unstable thermojet engine” post: thanks to the advice from this sub, here is a modified, stable propane thermojet (complete video link in remarks)

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About the Author: livescience


  1. Nice –

    I was an impressed cow when I watched your 2nd video. Good creativity using flow impingement and beam calcs to estimate flow rate.

    Also – super important – safety glasses anytime you are using something that could hurt your eyes. Melty stuff, pressurized things, springs, drill chips – all very dangerous to eyes and easily preventable with safety squints.

    In college, we would really struggle to find easy ways to measure air volumes and flow. Flow is often disrupted by measurement – and using other techniques (e.g. hanging your plenum and seeing how much angle it can push itself up to, then converting the force into a flow rate) also has major accuracy problems.

    There is a piece of equipment called a flow bench and many times companies are happy to support engineering students with educational projects. But this is time consuming and probably overkill for what you’re trying to do. (It will also make way more sense after you have your fluids class – I also see your solid works flow simulation shows your gas is moving really close to the speed of sound. That’s extremely suspicious…)

    The good news is you don’t need to be super accurate knowing your flowrate because you can tune your system – by mapping an ideal valve angle to a fan speed.

    The bad news is that you will see some delay with your systems – because air compresses and the fan takes time to spool up, and your gas valve will also have delays. (And an apparently jumpy servo – have you checked your PWM signal with an oscilloscope?)

    All that said, I think your lowest hanging fruit is your combustion chamber and nozzle. I’m not an expert here, but the key to developing thrust is to accelerate the gas as much as possible. Getting a more serious material, especially at the business end would really spice things up. I see you have a lot more velocity now with the office plate, but anything you can do to get a smoother transition will help. I don’t think you have enough oompf for a supersonic nozzle, so you might try out a solid brass garden hose nozzle.

    PS – there are some off the shelf parts that might show promise:

    PPS – I’ve always wanted to try to make a flamethrower out of a motorcycle carburetor and a fan, but you need to be super super careful playing with actual gasoline.

    Can’t wait to see what you come up with next!

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