Considering the many folk tales of giants and dwarves, featuring in all sorts of cartoons, or toy trucks and model trains I played with a kid, it's interesting to think scaling in real life works very poorly - even going beyond such simple principles as the square-cube law, if you think about stuff like a pressure vessel with a certain wall thickness that needs to hold 100 bar - the thickness needed is the same regardless you have something the size of a golfball or a swimming pool.
This is imo why scaling down combustion engines beyond a certain point makes little sense - you don't gain anything in terms of weight since the wall thicknesses are determined by the pressures the engine has to endure which is the same - this is why model engines suck - they're not only less powerful than big ones, but less powerful per pound.
This helps define some of the challenges with making very very small turbine engines. We have electrical (lithium) powered drones but they are heavy and have low energy density compared to what a liquid fuel + turbine could provide. But could we make a 2 inch diameter turbine engine reliably? Maybe!
This reminds me of the explanation of why the US primarily uses light water reactors.
Heavy water reactors (CANDU in the case of the explanation) don't scale down past a certain point - the water flow ends up with weird dead spots and that affects the reaction and cooling of the fuel rods. Since they don't scale down well enough, they aren't suitable for portable use, such as in submarines.
I'm not sure if it's still the case, but apparently one of the largest sources of nuclear engineers/scientists in the US was the US navy.
This also lead into the story about Jimmy Carter (nuclear sub naval officer at the time) coming to Chalk River in the 50s to help with the NRX meltdown...
This week I was wondering how long it would take a pilot light to deplete a tank of LP fuel (the kind people use for grilling.) Several months? A year? For no particular reason, I wondered what the limitations would be on shrinking the pilot light. Could a small tank keep a flame going for 10 years? 100 years? I sense one challenge would be machining a small scale nozzle for laminar flow, and carefully filtering both fuel and air inputs to ensure the tiny nozzle didn't clog, for instance, with a grain of sand, or a piece of pollen. At a small scale, what are the limits of flame?
A pilot light is tricky: in typical designs, it needs to heat a thermocouple enough to produce enough current to drive a solenoid to allow the rest of the flame to ignite. Thermocouples are outrageously inefficient.
Considering the many folk tales of giants and dwarves, featuring in all sorts of cartoons, or toy trucks and model trains I played with a kid, it's interesting to think scaling in real life works very poorly - even going beyond such simple principles as the square-cube law, if you think about stuff like a pressure vessel with a certain wall thickness that needs to hold 100 bar - the thickness needed is the same regardless you have something the size of a golfball or a swimming pool.
This is imo why scaling down combustion engines beyond a certain point makes little sense - you don't gain anything in terms of weight since the wall thicknesses are determined by the pressures the engine has to endure which is the same - this is why model engines suck - they're not only less powerful than big ones, but less powerful per pound.
Reminds me of Feynman's 'There's Plenty of Room at the Bottom'. He discusses miniaturizing ICE engines in page 5 - bottom left paragraph.
https://calteches.library.caltech.edu/1976/1/1960Bottom.pdf (8 pages, PDF)
This helps define some of the challenges with making very very small turbine engines. We have electrical (lithium) powered drones but they are heavy and have low energy density compared to what a liquid fuel + turbine could provide. But could we make a 2 inch diameter turbine engine reliably? Maybe!
> could we make a 2 inch diameter turbine engine reliably?
https://en.wikipedia.org/wiki/Capstone_Green_Energy
http://lambert-modellturbinen.de/html/english.html
55mm diameter
This reminds me of the explanation of why the US primarily uses light water reactors.
Heavy water reactors (CANDU in the case of the explanation) don't scale down past a certain point - the water flow ends up with weird dead spots and that affects the reaction and cooling of the fuel rods. Since they don't scale down well enough, they aren't suitable for portable use, such as in submarines.
I'm not sure if it's still the case, but apparently one of the largest sources of nuclear engineers/scientists in the US was the US navy.
This also lead into the story about Jimmy Carter (nuclear sub naval officer at the time) coming to Chalk River in the 50s to help with the NRX meltdown...
This week I was wondering how long it would take a pilot light to deplete a tank of LP fuel (the kind people use for grilling.) Several months? A year? For no particular reason, I wondered what the limitations would be on shrinking the pilot light. Could a small tank keep a flame going for 10 years? 100 years? I sense one challenge would be machining a small scale nozzle for laminar flow, and carefully filtering both fuel and air inputs to ensure the tiny nozzle didn't clog, for instance, with a grain of sand, or a piece of pollen. At a small scale, what are the limits of flame?
This article scratched an itch.
A pilot light is tricky: in typical designs, it needs to heat a thermocouple enough to produce enough current to drive a solenoid to allow the rest of the flame to ignite. Thermocouples are outrageously inefficient.
The entire section on large and small flying insects has been debunked:
https://arstechnica.com/science/2026/03/leading-explanation-...