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How critical are the dimensions of a jtube

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I have a J tube I built 2 years ago That has a bell over the riser tube with soft 15mm copper tube 15 m long coiled around it. It has done stirling service but now the steel riser is starting to disintegrate,hence my decision to build a new one.The firebox was larger and taller feeding a 6 “ pipe as burn chamber and riser.  I suspect that this is why it didn’t get as hot as a 6” rocket can get probably fortunately as I have always had a concern about the copper pipe melting,hence my reason for a new design and my question “How critical are the correct dimensions?regards Jock
Rocket Scientist
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Location: latitude 47 N.W. montana zone 6A
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Hi Jock;
The dimensions of the feed tube and burn tunnel are critical to good rocket performance.  The height of the riser can be taller than specs.  The barrel gap between riser and lid should be no less than 2.5" but can be 12" or more. Side gap between the side of the riser and barrel should be no less than 2" preferably more.

We moved your post to the wood burning stove / rocket stove forum.
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