Some kind souls have responded to the ball bearing motor post.
Their excellent information led me to published papers that explain the way the motor works.
And some that mis-understand the way the motor works, and still got published.
This situation reminded me of papers that mis-understand the deceptively simple pop-pop boat.
Pop-pop boats get thrust from a water-filled metal tube with one end closed and one open, pointing backwards underwater (see the sketch), positioned in a boat where the propeller would normally be.
If you make one with a bit of 3mm copper tube and a candle for heat, after a minute or so it will start to move forward – at a respectable speed for motoring up and down in a bath tub.
In practice, to make filling the ‘boiler’ possible, the engine is made from a length of brass or copper tube bent so that both ends point backwards underwater. Rather than a simple U-shape, a one-and-a-half turn loop is formed at the hot end to act as the boiler.
Two things make the boat go forward, one easy to explain and one frequently misunderstood.
Thing one, the easy bit, is that the heated water-filled tube forms a mechanical oscillator.
Some water in the boiler eventually becomes steam and pushes some water out of the tube – giving a bit of thrust like a rocket does.
In practice, no steam emerges, but steam does get into the cooler part of the tube.
After a couple of hundred milliseconds, steam in the cooler part of the tube condenses, sucking water back into the pipe, which cools more steam, sucking in more water.
Any water getting as far as the boiler now flashes into steam, and the cycle repeats at a few Hertz.
It is a little mechanical oscillator that has the effect of pushing water in and out of a rear-facing
That is the easy bit. I could not have predicted this oscillation would happen, but I can see how it works after seeing it happen.
Bit two, the tricky bit, is that water oscillating in and out of the nozzle produces forward motion.
If I was in charge of physics, the boat would not move because any forward motion imparted to the boat when water was ejected from the tube would be cancelled out by rearward motion when the water was sucked back into the tube.
Luckily for pop-pop boats, something else happens – one more reason they will not let me be a mechanical engineer.
Wikipedia now seems to have a pop-pop boat page.
The momentum-based argument that I favour, but have not yet fully understood, is near the bottom of the Wikipedia page.
It mentions Richard Feynman’s sucking-an-immersed-lawn-sprinkler problem as a similar question – one that has puzzled greater minds than I.
The alternative argument is hinted at near the top of the Wikipedia page (second image down) – to do with water leaving in a jet, but entering in a hemisphere.
Mechanical engineers V Sharadha and Professor Jaywant Arakeri went to some lengths, including a 500frame/s camera and tiny hydrogen bubbles, to find a mechanism based on similar reasoning, publishing a paper in journal Resonance.
A bit of searching on the Resonance site also revealed a second paper which describes the thermodynamic process within the pop-pop engine, and a more detailed account of the propulsion mechanism.
I am suspecting that one answer is correct and one is not, but I have no way of telling which one.
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